European Journal of Cell Biology 101 (2022) 151208
Contents lists available at ScienceDirect
European Journal of Cell Biology
journal homepage: www.elsevier.com/locate/ejcb
Review
Metabolic regulation and dysregulation of endothelial small conductance
calcium activated potassium channels
Shawn Kant, Frank Sellke, Jun Feng *
Division of Cardiothoracic Surgery, Rhode Island Hospital, Alpert Medical School of Brown University, Providence, RI, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
EDHF
Endothelial dysfunction
SK channel
Diabetes
Hypertension
Ischemia-reperfusion
The vascular endothelium is an important regulator of vascular reactivity and preserves the balance between
vasoconstrictor and vasodilator tone during normal physiologic conditions. Example endothelial-derived vaso­
constrictors include endothelin-1 and thromboxane A2; example vasodilators include nitric oxide and prosta­
cyclin. A growing body of evidence points to the existence of a non-nitric oxide, non-prostacyclin endotheliumderived vasodilatory factor of currently unclear identity, often referred to as endothelium-derived hyper­
polarizing factor (EDHF). Recent research testifies to the significance of EDHF in endothelium-dependent
vascular smooth muscle relaxation. Special emphasis has been placed on the role of small conductance
calcium-activated potassium channels (SK) in facilitating the endothelial and vascular responses to EDHF across
the microcirculation, including coronary, mesenteric, and pulmonary vascular beds. Meanwhile, decreased ac­
tivity of endothelial SK channel activity has been implicated in the pathology of a variety of disease states that
alter the balance between vasodilator and vasoconstrictor tone. Hence the primary goal of this review is to
characterize the physiology of endothelial SK channels in the microvasculature under normal and pathological
conditions. Themes of regulation and dysregulation of SK channel activity through the action of protein kinases,
reactive oxygen species, and byproducts of intermediary metabolism provide unifying principles to tie together
vascular pathology in altered metabolic states ranging from hypertension to diabetes, to ischemia-reperfusion. A
comprehensive understanding of SK channel pathophysiology may provide a foundation for development of new
therapeutics targeting SK channels, particularly SK channel potentiators, that may have widespread application
for many chronic disease states.
1. Introduction
Consisting of a single layer of cells lining the intima of blood vessels,
the vascular endothelium influences multiple important physiological
processes, including metabolism, vascular permeability, vascular tone,
hemostasis, and inflammation. Preservation of balance, be it between
thrombosis and fibrinolysis, between promotion and inhibition of
leukocyte extravasation, and most importantly, between vasodilation
and vasoconstriction, is a key principle that unifies the many functions
of the vascular endothelium (Krüger-Genge et al., 2019). With respect to
regulation of vascular tone, the endothelium produces a variety of short
and long-acting factors that have important effects on vascular smooth
muscle. Notable endothelium-derived vasoconstrictors include
endothelin-1 and thromboxane A-2 (Kvietys and Granger, 2014).
Notable endothelium-derived vasodilators include nitric oxide (NO),
prostacyclin (PGI2) and endothelium-derived hyperpolarizing factor
(EDHF),
Most vasoconstrictors, like endothelin-1, act on cell surface receptors
that promote vascular smooth muscle depolarization. For example,
endothelin binding to ETA receptors on vascular smooth muscle cells
activates a phospholipase C-protein kinase C driven pathway that ulti­
mately elevates intracellular calcium levels (through release from
sarcoplasmic reticulum stores) to trigger smooth muscle contraction
(Kowalczyk et al., 2014). In contrast, most endothelium-derived vaso­
dilators act either by (1) inhibiting depolarization-induced smooth
muscle contraction or (2) inducing endothelial and vascular smooth
muscle hyperpolarization. For an example of (1), consider NO. NO
produced by endothelial nitric oxide synthase (eNOS) stimulates
vascular smooth muscle guanylyl cyclase, which converts GTP to cyclic
GMP (cGMP) (Chen et al., 2008). cGMP activates protein kinase G,
which can promote closure of vascular smooth muscle membrane
voltage gated calcium channels and promote calcium uptake by
* Correspondence to: Cardiothoracic Surgery Research Laboratory, Rhode Island Hospital, 1 Hoppin Street, Coro West Room 5229, Providence, RI 02903, USA.
E-mail address: jfeng@lifespan.org (J. Feng).
https://doi.org/10.1016/j.ejcb.2022.151208
Received 27 December 2021; Received in revised form 6 February 2022; Accepted 7 February 2022
Available online 8 February 2022
0171-9335/© 2022 Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
S. Kant et al.
European Journal of Cell Biology 101 (2022) 151208
BK channels (KCa1.1) (Kshatri et al., 2018; Yang et al., 2015; Lee and
Cui, 2010).
sarcoplasmic reticulum calcium-ATPases (SERCA) (Chen et al., 2008;
Cohen et al., 1999).
Reduced intracellular calcium levels prevent calcium-induced
smooth muscle contraction during depolarization. PGI2 acts in a
similar fashion by activating adenylyl cyclase, causing increased con­
version of ATP to cyclic AMP (cAMP), after which cAMP activates pro­
tein kinase A (PKA). PKA promotes sarcoplasmic reticulum calcium
uptake and inhibits myosin light chain kinase via phosphorylation,
leading to smooth muscle relaxation and vasodilation (Majed and Khali,
2012).
For an example of (2), consider EDHF. Although its specific identity
remains a matter of intense debate, extensive research has confirmed the
presence of a non-NO, non-PGI2 inducer of vasodilation that acts on
vascular beds by increasing potassium conductance through opening of
endothelial and smooth muscle potassium channels (Bryan et al., 2005;
Busse et al., 2002; Félétou and Vanhoutte, 2006; Feletou and Vanhoutte,
2007). Application of potassium channel agonists to blood vessels
mimics the effects of EDHF, and pharmacological blockade of multiple
distinct types of potassium channels, in particular calcium-activated
potassium channels, prevents non-NO, non-PGI2 dependent vasodila­
tion (Grgic et al., 2009). Efflux of potassium along its electrochemical
gradient results in cell membrane hyperpolarization that can be
communicated from endothelial cells to vascular smooth muscle cells
via gap junctions; this has been called the “myoendothelial coupling”
hypothesis (Yamamoto et al., 1999; Sandow et al., 2004). Alternatively,
EDHF may stimulate inward rectifier potassium channels (KIR) and the
sodium potassium ATPase on vascular smooth muscle cells which could
also promote membrane hyperpolarization (Coleman et al., 2004).
Altogether, most putative explanations of the action of EDHF
include, in some capacity, the activation of endothelial calciumactivated potassium channels (KCa) as critical mediators of the final
vasodilatory pathway. Two broad categories of calcium-activated po­
tassium channels have been identified: small/intermediate conductance
(SK/IK) and large conductance (BK) calcium activated potassium
channels. SK channel-mediated vasodilation plays significant roles
across many vascular beds, most notably in the microcirculation. This
review will examine the specific roles of calcium-activated potassium
channels in endothelial function, with particular emphasis on the SK and
IK channels that are most heavily implicated in endothelium-dependent
vasodilation. We will consider aspects of normal physiology and phar­
macology before turning to pathophysiology, more specifically the
dysregulation of SK/IK channels in conditions that alter metabolic ho­
meostasis, such as diabetes, hypertension, and hypoxia-reperfusion
injury.
2.2. Endothelial localization of calcium-activated potassium channels
Extensive research has confirmed that SK and IK channels are
constitutively expressed on vascular endothelial cells in humans and a
variety of animals (Félétou, 2009; Dalsgaard et al., 2009; Steffensen
et al., 2017; Bychkov et al., 2002; Burnham et al., 2002). Of the different
subtypes of SK channels, KCa2.3 (SK3) and KCa3.1 (IK) are particularly
abundant in endothelia (Kohler et al., 1996; Sandow et al., 2006; Ber­
tuccio et al., 2018; Dora et al., 2008). More specifically, KCa2.3 and
KCa3.1 have been found in rat and pig carotid arteries, rat mesenteric
arteries, human small pulmonary arteries, and human coronary arteries
(Takai et al., 2013; Kroigaard et al., 2012; Burnham et al., 2002; Sandow
et al., 2006; Dora et al., 2008; Köhler et al., 2001; Wulff and Köhler,
2013; Liu et al., 2015).
BK channels have been definitively identified in the vascular smooth
muscle of arteries and arterioles, where they contribute to vasodilation
and relaxation (Yang et al., 2011; Dopico et al., 2018; Wu and Marx,
2010). There, BK channels trigger a negative feedback loop in response
to depolarization induced calcium influx and smooth muscle contrac­
tion, thereby helping regulate the degree of myogenic tone (Dopico
et al., 2018). However, when it comes to endothelial cells, the presence
of BK channels becomes more complicated. Although most studies have
not found BK channels in the vascular endothelium, some electrophys­
iology and molecular biology experiments have detected BK channels in
certain human endothelial cell lines, more specifically umbilical and
microvascular endothelia (Grgic et al., 2005; Papassotiriou et al., 2000;
Zyrianova et al., 2021). Nonetheless, most work to date has focused on
SK and IK channels in the context of endothelial function, given their
confirmed presence; hence, the focus of this review.
2.3. Structure and activation of SK and IK channels
Having completed an exploration of the localization of endothelial
SK channels in the circulation, we will move on to discuss some prin­
ciples governing their molecular structure. The general structure of SK
channels resembles voltage-gated potassium channels, insofar as the
framework is a tetrameric assembly (Nam et al., 2017). Detailed analysis
from protein purification assays show that the SK tetramer is symmetric,
with a length of approximately 95 angstroms and a width of 120 ang­
stroms in the plane of the plasma membrane (Lee and MacKinnon,
2018). Different genes encode the SK alpha subunits for the various SK
channel subtypes. KCNN1, KCNN2, and KCNN3 genes encode the SK
alpha subunits of SK1, SK2, and SK3 respectively (Kohler, 1996). Each
SK alpha subunit consists of six hydrophobic transmembrane alpha he­
lical domains (S1-S6). A pore forming p-loop between the S5 and S6
domains governs the potassium selectivity of the overall channel (Fig. 1)
(Nam et al., 2017). Moreover, the pore-forming regions of the alpha
subunits always face the channel center. Amino (N) and carboxy (C)
terminal domains of the alpha subunits are cytosolic, and calmodulin
binding domains lie at the proximal C terminal end of S6.
An important feature that distinguishes SK and IK channels from BK
channels is their voltage-independence, as they do not possess any
voltage sensor domains. Instead, the chief regulator of SK and IK channel
activity is calcium, acting through a calmodulin-dependent mechanism.
Therefore, it should not be surprising that many SK channels are located
in close proximity to intracellular calcium stores and cell membrane
calcium channels (Stocker, 2004; Adelman et al., 2012). Note that the
lack of voltage gating suggests that SK channels will not inactivate at
negative membrane potentials. Thus, if left open without modulation,
they will facilitate a hyperpolarization response that moves cell mem­
brane potential towards the potassium equilibrium potential.
Anywhere from two to eight calmodulin molecules can bind to any
one SK channel (Halling et al., 2014). Meanwhile, only one calmodulin
2. Structure, Localization, and Function of SK channels
2.1. Categories calcium-activated potassium channels
The first part of this review will present the localization and funda­
mental cellular/molecular characteristics of endothelial SK channels.
We begin with a general overview of the different subcategories of SK
channels. The fundamental role of calcium-activated potassium chan­
nels is to couple changes in intracellular calcium concentrations to cell
membrane hyperpolarization. This in turn allows for a physiological
“brake” on runaway excitation that might otherwise occur in the pres­
ence of persistently elevated intracellular calcium. As mentioned before,
there are two broad families of calcium-activated potassium channels:
the small/intermediate conductance (SK/IK) and the large conductance
(BK) channels (Wei et al., 2005). Within the SK/IK family, four subtypes
of channels have been identified: SK1 (KCa2.1), SK2 (KCa2.2), SK3
(KCa2.3), and SK4 (KCa3.1). The latter, SK4, is sometimes referred to as
IK1 due to its higher unitary conductance (up to 40–45 pS) relative to
SK1–3 (all in the range of 5–15 pS) (Maylie et al., 2004; Grgic et al.,
2009; Kshatri et al., 2018). However, all the SK/IK channels exhibit
lower unitary conductance when compared to the 100–300 pS range of
2
S. Kant et al.
European Journal of Cell Biology 101 (2022) 151208
Fig. 1. General SK Channel structure. Four pore-forming alpha subunits form the SK channel complex (right). Each alpha subunit consists of 6 transmembrane
domains (S1-S6) (left), with a pore forming loop (P-loop) between S5 and S6. S4 is an important calcium sensing domain. Note N and C termini, with a calmodulin
(CaM) binding domain at the C terminus of each alpha subunit. Created with BioRender.com.
can bind to each SK subunit, resulting in a maximum of four bound
calmodulins per tetramer (Lee and MacKinnon, 2018). Curiously, ex­
periments aiming to purify SK channels from human cells also copurified
calmodulin in the presence and absence of calcium, suggesting that
calcium is not necessary per se for calmodulin-SK channel binding
(which appears to be constitutive). Indeed, research shows that calcium
binding to the N-lobes of calmodulin triggers a structural change in the
bound calmodulin-SK channel-calmodulin-binding-domain conforma­
tion (Lee and MacKinnon, 2018). This conformational shift alters the
arrangement of the alpha helices that form the SK channel pore, leading
to channel opening and permitting the flow of potassium.
2000). Therefore, even though some studies (as mentioned earlier) al­
lude to the presence of endothelial BK channels, results such as those
produced by Kohler et al. support the notion that BK channels do not
have a significant role in endothelial hyperpolarization.
Turning to genetic knockout models, deletion of the gene encoding
IK channels in mice results in significantly impaired KCa currents and
impaired hyperpolarization in the carotid endothelium (Si et al., 2006).
In addition, deletion of endothelial SK3 channel genes in mice produces
impaired endothelium-derived hyperpolarization, impaired vaso­
relaxation, and altered calcium dynamics (Yap et al., 2016a, 2016b).
Furthermore, treatment with doxycycline, shown to alter KCa2.3
channel expression and activity, also virtually abolishes KCa currents in
mouse models (Grgic et al., 2009). On the other hand, overexpression of
SK3 in transgenic mouse models produces significantly elevated KCa
currents and vascular hyperpolarizing responses which result in mark­
edly reduced myogenic tone (Taylor et al., 2003).
2.4. Function of SK and IK channels in endothelium-dependent
hyperpolarization
2.4.1. Pharmacological and genetic studies
The second part of this review will explore the physiologic function
of endothelial SK channels in the human body and their associated
signaling pathways in normal/healthy conditions. We begin by consid­
ering an assortment of pharmacologic and genetic studies that have
examined the effects of altered SK channel activity and expression on
endothelial hyperpolarization. First, application of highly selective
KCa3.1 inhibitors 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole
(TRAM-34) and 2-(2-chlorophenyl)-2,2-diphenylacetonitrile (TRAM39) abolish IK-mediated potassium currents in response to acetylcholine
in rat carotid endothelia (Eichler et al., 2003). Moreover, application of
TRAM-34 alone or TRAM-39 plus SK3 channel inhibitor apamin blocks
nitric oxide and prostacyclin independent vasodilation (Eichler et al.,
2003). These findings have been replicated in the context of human
coronary artery endothelial cells and the human skeletal muscle
microvasculature (Liu et al., 2015, 2008). In addition, application of the
selective SK/IK channel activator NS309 increases endothelial SK/IK
currents and promotes arteriolar vasodilation in the coronary and
skeletal microvasculature (Liu et al., 2015, 2008). It is also worthwhile
to note that application of iberiotoxin, a selective KCa1.1 (BK) channel
blocker, appears to have little, if any, effect on endothelial hyperpo­
larization when tested in human mesenteric arteries (Köhler et al.,
2.4.2. EDHF and SK/IK channel activity
In the introduction, we introduced the concept of EDHF, a NOindependent, PGI2-independent vasodilator that acts in a variety of
vascular beds through activation of SK and IK channels. (Félétou, 2009;
Garland et al., 2011). While the specific identity of EDHF remains un­
clear, several potential candidates, such as potassium itself, ananda­
mide,
hydrogen
peroxide,
C-natriuretic
peptide,
and
epoxyeicosatrienoic acids (EETs), have been proposed based on current
theories regarding the mechanism of EDHF-driven vascular smooth
muscle relaxation (Bellien et al., 2008). A synthesis of the existing
literature results in the following proposal (Goto et al., 2018; Bellien
et al., 2008; Luksha et al., 2004; Luksha et al., 2009; Ozkor and
Quyyumi, 2011). Essentially, endothelium specific agonists, such as
acetylcholine, bradykinin, and substance P, bind to receptors on the
vascular endothelium, mobilize release of calcium from intracellular
stores into the cytosol, and potentially induce synthesis of EDHF.
Acetylcholine acts on endothelial cells via muscarinic M1 or M3 re­
ceptors, which are G-protein coupled receptors (Tangsucharit et al.,
2016; Walch et al., 2001; Furchtott and Furchgott and Zawadzki, 1980).
Binding of acetylcholine to M1 or M3 receptors leads to activation of Gq
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S. Kant et al.
European Journal of Cell Biology 101 (2022) 151208
and phospholipase-C (PLC) mediated signal transduction cascade. PLC
cleaves plasma membrane bound PIP2 to IP3 and DAG; IP3 moves
through the cytosol to the smooth endoplasmic reticulum and induces
release of stored calcium into the cytosol. Note, however, that M3 re­
ceptor activity on endothelial cells also stimulates endothelial nitric
oxide synthase in addition to promoting elevated intracellular calcium;
hence acetylcholine may also promote non-EDHF mediated
vasorelaxation.
Bradykinin stimulates endothelial bradykinin B2 receptors, which
increase free cytosolic calcium levels and promotes release of prosta­
cyclin, NO, and potentially EDHF (Gryglewski et al., 2002; Hornig and
Drexler, 1997). Like acetylcholine, bradykinin receptor activation in­
creases cytosolic calcium through a PLC-mediated pathway. Two addi­
tional points are worth noting. First, among the all the various
endothelial agonists, bradykinin has been shown to be a particularly
potent stimulator of intracellular calcium release. Two, like acetylcho­
line, bradykinin also promotes non-EDHF pathways of vasorelaxation (e.
g., through endothelial nitric oxide synthase stimulation) (Hornig and
Drexler, 1997). Meanwhile, substance P mobilizes intracellular calcium
stores and promotes calcium influx into endothelial cells, potentially
through action of endothelial neurokinin 1 receptors, although the
precise details of this process remain murky (Greeno et al., 1993;
Sharma and Davis, 1995; Kuroiwa et al., 1995). Akin to bradykinin and
acetylcholine, it is also possible that besides increasing intracellular
calcium, substance P promotes synthesis of a separate EDHF substance.
Vessel wall shear stress due to blood flow impinging on the endo­
thelium, and the resulting mechanical stretch, can also trigger endo­
thelial signaling pathways, including laminar shear-stress induced
endothelial nitric oxide synthase phosphorylation and activation, stim­
ulation of COX-2 synthesis of PGI2, and EDHF-mediated vasodilation
(Lu and Kassab, 2011; Bellien et al., 2008). The combination of elevated
intracellular calcium and increasing synthesis of EDHF work to stimu­
late calcium activated potassium channels through a few potential
pathways.
For example, EDHF itself may passively diffuse from endothelial cells
to vascular smooth muscle cells and activate vascular smooth muscle BK
channels to trigger hyperpolarization (Bellien et al., 2008). Alterna­
tively, EDHF itself may further elevate endothelial cell calcium levels,
such as through stimulation of transient receptor potential (TRP) V4
channels that promote calcium influx from the extracellular environ­
ment (Ozkor and Quyyumi, 2011). This would elevate intracellular
calcium levels beyond the action of the previously discussed endothelial
agonists, resulting in enhanced calcium-induced opening of endothelial
SK and IK channels (e.g., via the calcium-calmodulin binding mecha­
nism to SK/IK channel C termini). Release of potassium through endo­
thelial SK and IK channels produces the characteristic current observed
in SK/IK channel stimulation experiments, and results in endothelial
hyperpolarization.
This hyperpolarization is then directly conducted to vascular smooth
muscle via myoendothelial gap junctions to trigger smooth muscle hy­
perpolarization. Or, the efflux of potassium from endothelial cells into
the subendothelial space can itself stimulate vascular smooth muscle
inward rectifying potassium channels or sodium-potassium ATPase
pumps that trigger smooth muscle hyperpolarization. Ultimately, as is
self-evident, all these different EDHF pathways converge on endothelial
SK channel activation and smooth muscle hyperpolarization that leads
to closure of voltage-gated calcium channels on vascular smooth muscle
cells (Ozkor and Quyyumi, 2011; Bellien et al., 2008; Garland et al.,
2011). Closure of voltage gated calcium channels lowers smooth muscle
cell intracellular calcium levels, inhibiting vasoconstriction and pro­
moting vasodilation.
While it is well established that EDHF is a critical component of this
framework for endothelial hyperpolarization, uncertainty regarding the
specific place of EDHF in these pathways contributes a great deal to the
controversy surrounding its identity. For example, if EDHF activates SK
channels through increasing intracellular calcium, then molecules like
EETs are strong candidates for EDHF because they can stimulate cell
membrane TRPV4 channels that mediate calcium influx. Alternatively,
because potassium released through SK channels can itself trigger
endothelial-dependent vascular smooth muscle relaxation, it is possible
that so-called “EDHF” is, in fact, simply, potassium.
2.5. Physiologic regulation of endothelial SK/IK channel activity
Any discussion of SK channel function would be incomplete without
mentioning the many regulatory mechanisms that modulate SK channel
activity. First and foremost is calcium. Calcium is the most important
ionic regulator of SK channels. Hence the name “calcium sensitive”
potassium channels, and the instrumental role of cytosolic calcium in
triggering activation of SK channels via mechanisms detailed earlier.
Studies have shown co-localization of SK channels with L-type voltage
gated cell membrane calcium channels, such as Cav1.3 and Cav1.2,
lending credence to the notion of a close link between voltage gated
calcium channel opening and SK channel activity (Lu et al., 2007).
Experimental manipulation of intracellular calcium also affects SK
channel trafficking, as shown by increased trafficking of SK channels to
the plasma membrane of rabbit pulmonary veins in response to rising
intracellular calcium (Ozgen et al., 2007). However, calcium is not the
only ion that affects SK channel activity—other divalent cations do so as
well. Indeed, this stems from the ability of other divalent cations, such as
magnesium, to bind to charged inner pore residues of the SK channel S6
transmembrane domain, which are important regulators of SK channel
sensitivity and open probability (Li and Aldrich, 2011).
EDHF-SK/IK channel induced vasodilation may be inhibited by nitric
oxide, itself a strong vasodilator. For example, experiments on pig cor­
onary arterioles show attenuated EDHF-mediated dilation after treat­
ment with nitric oxide donors, while endothelium-independent
vasodilation was preserved (Bauersachs et al., 1996). Other experiments
with nitric oxide synthase inhibitors in dog coronary arterioles produced
comparable results (Nishikawa et al., 2000). Curiously, some evidence
suggests that activation of SK3 and IK channels may promote endothelial
nitric oxide synthase activation. Indeed, the IK channel activator NS309
has been shown to both produce endothelial and smooth muscle hy­
perpolarization AND increase nitric oxide formation in human endo­
thelial cells and porcine retinal arterioles (Balut et al., 2012; Dalsgaard
et al., 2010; Schmidt et al., 2010). This may be a built-in mechanism of
SK channel autoregulation to mitigate against the adverse effects of
runaway activity.
The specific means by which SK/IK channel activity bolster nitric
oxide synthesis remain a matter of investigation; one hypothesis may be
a paradoxical increase in the ion driving force for calcium across
endothelial cell membrane calcium channels and coupling of calcium to
eNOS stimulation. However, there is conflicting data on this matter
(Dalsgaard et al., 2010). Overall, the interaction between nitric oxide
and EDHF is an unresolved matter, and further work needs to be done to
better characterize their relationship.
Protein kinase C has also demonstrated some ability to modulate SK
channel activity. Experiments aiming to isolate SK2/SK3 channel bind­
ing proteins via mass spectrometry, binding assays, and coimmunoprecipitation revealed catalytic and regulatory subunits of
protein kinase CK2 and protein phosphatase 2 A constitutively bind to
the cytoplasmic C and N termini of the SK channel protein (Bildl et al.,
2004). Recall that calmodulin constitutively binds to the SK channel
protein C terminus. Protein kinase CK2 phosphorylation of SK
channel-bound calmodulin at the threonine 80 residue produces an
almost 5-fold reduction in SK channel calcium sensitivity, leading to
channel inhibition (Bildl et al., 2004; Allen et al., 2007).
Note that in-vitro assays testing protein kinase CK2 interaction with
SK2 have shown that protein kinase CK2 only appears to phosphorylate
calmodulin when complexed with the SK channel calmodulin binding
domain and does not phosphorylate the SK channel calmodulin binding
domain alone without bound calmodulin (Allen et al., 2007).
4
S. Kant et al.
European Journal of Cell Biology 101 (2022) 151208
Furthermore, with respect to SK2 channels, the N terminal domain
lysine 121 residue has been shown to activate SK2-bound protein kinase
CK2, which suggests that some intrinsic activation/feedback mecha­
nisms may be at play between the channel and its bound protein kinase
CK2 (Allen et al., 2007). Meanwhile, activity of bound protein phos­
phatase 2A to dephosphorylate SK channel-bound calmodulin reverses
the inhibitory effects of protein kinase CK2 on calcium sensitivity,
shifting the channel’s state towards a heightened open probability. Note
that most of the aforementioned experiments have been performed on
SK2 and SK3 channels. Therefore, verifying these findings for SK1 and IK
channels will assist with drawing firmer generalizations about the
definitive place of protein kinase C in SK channel regulation.
circulating levels of inflammatory markers such as CRP, TNF-alpha, and
IL-6 (Piga et al., 2007; Festa et al., 2000).
Hyperglycemia also blunts endothelial relaxation responses across
different vascular beds in animal models and humans with type 1 and
type 2 diabetes (Vriese et al., 2000). This may occur through several
mechanisms, including elevated levels of oxygen-derived free radicals,
reduced expression of endothelial nitric oxide synthase (eNOS), reduced
phosphorylation of the eNOS active site Ser1177, or increased phos­
phorylation of the eNOS inhibitory site Thr495 (Shi and Vanhoutte,
2009; Matsumoto et al., 2014). Conversely, diabetes may promote
increased responsiveness to endothelial constricting factors, such as
endothelin-1, thereby increasing arterial stiffness (Tabit et al., 2010).
Thus, diabetes disrupts the balance between endothelial relaxation and
constriction in favor of constriction, leading to impaired vasomotor
tone.
Diabetes may also hamper vasorelaxation through direct dysregula­
tion of electrochemical signaling, and it is here that potentially pivotal
roles for SK channels and EDHF become apparent (Burnham et al., 2006;
Weston et al., 2008). Reduced EDFH-mediated vascular smooth muscle
relaxation and reduced SK/IK channel currents have been noted in an
assortment of rodent models of type 1 (Morikawa et al., 2005; Hosoya
et al., 2010; Ding et al., 2005; Csanyi et al., 2007; Matsumoto and
Wakabayashi et al., 2004; Shi, 2006; Zhu et al., 2010; Leo et al., 2011;
Makino et al., 2000; Wigg et al., 2001; Absi et al., 2013; Ma et al., 2013;
Sotnikova et al., 2011a, 2011b; De Vriese et al., 2000; Gokina et al.,
2015) and type 2 (Burnham et al., 2006; Brøndum et al., 2009; Weston
et al., 2008; Schach et al., 2014a, 2014b; Yin et al., 2017a, 2017b,
2017c; Cotter et al., 2010; Matsumoto et al., 2010; Oniki et al., 2013;
Oniki et al., 2006; Matsumoto et al., 2006; Matsumoto et al., 2008;
Matsumoto et al., 2009; Zhang et al., 2020; Xing et al., 2021; Song et al.,
2020; Park et al., 2008) diabetes (Tables 1–4). Similar results have been
found across a variety of different vascular beds in these rodent models,
including the mesenteric circulation (Burnham et al., 2006; Brøndum
et al., 2009; Weston et al., 2008; Schach et al., 2014a, 2014b; Morikawa
et al., 2005; Hosoya et al., 2010; Ding et al., 2005; Leo et al., 2011;
Makino et al., 2000; Wigg et al., 2001; Absi et al., 2013; Ma et al., 2013;
Sotnikova et al., 2011a, 2011b; Matsumoto et al., 2010; Oniki et al.,
2013; Oniki et al., 2006; Matsumoto et al., 2006; Matsumoto et al., 2008;
Matsumoto et al., 2009; Shi, 2006), renal arcuate arteries (Yin et al.,
2017a, 2017b, 2017c; Cotter et al., 2010; De Vriese et al., 2000), aortic
endothelial cells ( Matsumoto and Wakabayashi et al., 2004), carotid
arteries (Shi, 2006), femoral arteries (Shi, 2006), cavernous tissue
endothelial cells (Zhu et al., 2010), and uterine vasculature (Gokina
et al., 2015).
In other vascular beds, the effect of diabetes on SK channel activity
and EDHF responsiveness is less clear, at least based on observations in
rodent models. For instance, Mishra et al. report preserved cremaster
skeletal muscle arteriolar function in their type 2 diabetic GK rats
(Mishra et al., 2021a, 2021b). Next, while several studies report that SK
channel activity, along with endothelium-dependent vascular relaxa­
tion, decreases in coronary microvessels of genetically modified type 2
diabetic mice (Zhang et al., 2020; Xing et al., 2021; Song et al., 2020),
others report no significant changes (Park et al., 2008). Likewise, Kaji­
kuri et al. report preserved EDHF responsiveness in LAD coronary ar­
teries of OLETF type 2 diabetic rats (Kajikuri et al., 2009). Importantly,
the coronary microcirculation is one of the few vascular beds where SK
channel investigations have also been done using human tissue, more
specifically atrial microvessels (Table 4)—and those studies universally
demonstrate decreased SK channel currents and decreased responsive­
ness to EDHF-mediated relaxation in diabetic vs nondiabetic coronary
arterioles (Liu et al., 2015, 2018, 2020).
Evidence for altered endothelial SK channel gene or protein expres­
sion in diabetes is extremely mixed. In some experiments, SK gene/
protein expression paradoxically increased even as overall EDHF
responsiveness decreased in diabetic rodent mesenteric vessels (Burn­
ham et al., 2006; Weston et al., 2008; Schach et al., 2014a, 2014b). Some
3. SK channel pathology and endothelial dysfunction
In the final section of this review, we move from SK channel physi­
ology to pathophysiology, with specific attention paid to endothelial
dysfunction in disease states involving altered metabolism. Endothelial
dysfunction, in the form of micro and macrovascular complications, is a
hallmark of many disease processes. Earlier, we mentioned several ho­
meostatic functions of the vascular endothelium (thrombosis and
coagulation, regulation of vascular tone, angiogenesis, immune modu­
lation). Pathological changes in these homeostatic functions occur with
endothelial dysfunction, characterized by impaired vasodilation,
vascular leakage, promotion of thrombosis, and promotion of inflam­
mation (Rajendran et al., 2013). Although not routinely done in the
clinic, one can measure degree of endothelial dysfunction by assessing
the ability of vessels to dilate or constrict in response to drug adminis­
tration (e.g., phenylephrine).
Characterizing the molecular basis of endothelial dysfunction is an
active focus of research. Given that endothelial dysfunction is itself a
feature of many other systemic conditions rather than an isolated finding
of its own, the initiating factors for endothelial dysfunction may differ
under different circumstances. For example, endothelial dysfunction in
bacterial sepsis is often driven by interactions between lipopolysac­
charide, TNF-alpha, and endothelial toll-like receptors that promote
reactive oxygen species (ROS) production, oxidative stress, endothelialimmune activation, and apoptosis (Peters, 2003). Alternatively, in
atherosclerosis, sudden rupture of plaques can alter blood vessel he­
modynamics, activating the endothelium into a more pro-thrombotic,
pro-inflammatory phenotype (e.g., through heightened NF-kB expres­
sion, exposure of subendothelial collagen, and platelet activation)
(Gimbrone and García-Cardeña, 2016; Davignon and Ganz, 2004).
Although the precipitating events of endothelial dysfunction may be
different in different situations, a growing body of literature suggests
that there are shared phenotypic features that underpin endothelial
dysfunction across several diseases, chiefly cardiovascular diseases that
involve altered metabolic phenotypes. One such feature would be
changes in the expression/function of endothelial cell membrane SK/IK
channels. Hence the remainder of this review will elaborate on the roles
of SK/IK channel dysfunction in three major pathological states that all
involve aberrant or disrupted metabolic processes: diabetes, hyperten­
sion, and hypoxia-reperfusion.
3.1. Diabetes and SK channel dysfunction
The first major metabolic disease state highlighted here is hyper­
glycemia in the context of diabetes, obesity, and metabolic syndrome.
Cardiovascular complications of uncontrolled/untreated hyperglycemia
are well-known contributors to the increased morbidity and mortality
rates of diabetic patients in comparison to the general population (Shi
and Vanhoutte, 2017; Wilson et al., 2005). First, diabetes compromises
endothelial barrier integrity by downregulating cell junction protein
expression, leading to altered vascular permeability (Li et al., 2016;
Chapouly et al., 2015). Second, diabetes induces a systemic inflamma­
tory state, for hyperglycemia can activate the endothelium and elevate
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European Journal of Cell Biology 101 (2022) 151208
Table 1
SK channels in zucker obese/fatty rat model of type 2 diabetes.
Animal Model
Tissue and Cell Type
SK Protein Expression
in Diabetes
SK Channel Gene
Expression
Vascular Function/ EDHF
responsiveness
SK/IK Channel
Recording
References
Zucker Diabetic Fatty
Rat, type 2 DM
Zucker Diabetic Fatty
rat, Type 2 DM
Zucker Diabetic Fatty
rat, Type 2 DM
Zucker Diabetes fatty
rat (ZDF)
Zucker Diabetic Fatty
Rats
Mesenteric artery
endothelial cells
Mesenteric artery
endothelial cells
Mesenteric artery
endothelial cells
Mesenteric artery
endothelial cells
Renal arcuate artery
endothelial cells
N/A
SK3: Increased
IK: No change
N/A
Small mesenteric arteries:
decreased
Small mesenteric arteries:
decreased
Decreased
IK: No change
SK: Decreased
N/A
Burnham et al. (2006)
Brondum et al., 2010
N/A
Weston et al., 2008
decreased
N/A
Schach et al. (2014)
Renal arcuate arteries:
Decreased
N/A
Yin et al. (2017);
Cotter et al. (2010)
No significant
differences
IK: Increased
IK: Increased
Increased
IK increased, SK no
change
N/A
N/A
Table 2
SK channels in variations of the streptozotocin-induced type 1 diabetic rat model.
Animal Model
Tissue and Cell Type
SK Protein
Expression in
Diabetes
SK Channel
Gene
Expression
Vascular Function/ EDHF
responsiveness
SK/IK
Channel
Recording
References
Streptozotocininduced APOE
deficient rats
Streptozotocininduced APOE
deficient rats
Streptozotocininduced diabetic rats
Mesenteric artery
endothelial cells
N/A
N/A
Small mesenteric arteries:
decreased
N/A
Morikiawa et al., 2005;
Hosoya et al. (2010)
Mesenteric artery
endothelial cells
N/A
Small mesenteric arteries:
decreased
N/A
Ding et al. (2005)
Aorta endothelial cells
N/A
SK2 and SK3:
Decreased
IK: No change
N/A
Thoracic aorta: Decreased
N/A
Streptozotocin
induced diabetic rats
Carotid, femoral,
mesenteric artery
endothelial cells
Cavernous tissue
endothelial cells
Mesenteric artery
endothelial cells
N/A
N/A
N/A
SK3: Decreased
IK1: Decreased
N/A
SK3: Decreased
IK1: Decreased
N/A
More significantly decreased
EDHF in carotid and femoral vs
mesenteric arteries
Cavernous tissue: Decreased
Csanyi et al. (2007);
Matsumoto, Wakabayashi
et al. (2004)
Shi (2006)
N/A
Zhu et al. (2010)
Small mesenteric arteries:
decreased
N/A
Mesenteric artery
endothelial cells
Mesenteric artery
endothelial cells and aorta
Renal microcirculation
endothelial cells
Uterine vasculature
SK3: Decreased
N/A
N/A
N/A
N/A
N/A
N/A
Small mesenteric arteries:
decreased
Small mesenteric arteries and
aorta: decreased
Renal microvessels: decreased
Leo et al. (2011); Makino
et al. (2000); Wigg et al.
(2001); Absi et al. (2013)
Ma et al. (2013)
N/A
Sotnikova et al. (2011a,
2011b)
De Vriese et al. (2000)
N/A
N/A
Uterine microvessels:
decreased
Decreased
Gokina et al. (2015)
Streptozotocininduced diabetic rats
Streptozotocininduced diabetic rats
Streptozotocininduced diabetic rats
Streptozotocininduced diabetic rats
Streptozotocininduced diabetic rats
Streptozotocininduced diabetic
pregnant rats
N/A
Table 3
SK channels in GK and OLETF type 2 diabetic rats.
Animal Model
Tissue and Cell Type
SK Protein
Expression in
Diabetes
SK Channel
Gene Expression
Vascular Function/
EDHF responsiveness
SK/IK Channel
Recording
References
T2D GotoKakizaki rats
cremaster skeletal
muscle resistance
arteries
Mesenteric artery
endothelial cells
N/A
N/A
preserved
N/A
Mishra et al. (2021a, 2021b)
N/A
N/A
Small mesenteric
arteries: decreased
Matsumoto et al. (2010); Oniki
et al. (2013)
Mesenteric artery
endothelial cells
N/A
N/A
Small mesenteric
arteries: decreased
Reduced SK and IK
channel responses to
stimulation
N/A
Mesenteric artery
endothelial cells
N/A
N/A
Small mesenteric
arteries: decreased
Reduced endothelial SK/
IK activity
LAD coronary arteries
N/A
N/A
LAD coronary arteries:
preserved
N/A
Goto-Kakizaki
type 2 diabetic
rats
Goto-Kakizaki
type 2 diabetic
rats
OLETF type 2
diabetic rats
OLETF type 2
diabetic rats
experiments report decreased SK gene/protein expression in diabetic
rodent mesenteric arteries (Ding et al., 2005; Ma et al., 2013) and
cavernous tissue endothelial cells (Zhu et al., 2008). To complicate
matters further, other studies report no changes in SK gene/protein
Oniki et al. (2006)
Matsumoto et al. (2006);
Matsumoto et al. (2008);
Matsumoto et al. (2009)
Kajikuri et al. (2009)
expression in diabetic rat mesenteric arteries (Brøndum et al., 2009),
diabetic rat coronary arterioles (Zhang et al., 2020; Xing et al., 2021;
Song et al., 2020), and human coronary arterioles (Liu et al., 2015,
2018, 2020).
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European Journal of Cell Biology 101 (2022) 151208
Table 4
SK channels in other models of diabetes/metabolic syndrome.
Animal Model or Human Tissue
Tissue and Cell
Type
SK Protein
Expression in
Diabetes
SK Channel Gene
Expression
Vascular Function/
EDHF responsiveness
SK/IK Channel
Recording
References
Mouse, genetically modified type
2 DM
Heart endothelial
cells
No change
N/A
Coronary microvessel:
Decreased
Decreased
Genetically modified type 2
diabetic mice
Human, type 2 DM
Coronary arterioles
N/A
N/A
N/A
Atrial
No change
No change
Pig, coronary endothelial cells
cultured in hyperglycemic
media
Coronary artery
endothelial cells
N/A
N/A
Coronary microvessels:
Indeterminate
Coronary microvessel:
Decreased
Coronary microvessels:
decreased
Zhang et al. (2020); Xing
et al. (2021); Song et al.,
2020
Park et al. (2008)
Decreased
Liu et al. (2015); Liu et al.
(2018); Liu et al. (2020)
Tsai et al. (2011)
N/A
underpin obesity-induced EDH dysfunction.
Coronary arteriolar SK/IK channel dysfunction in diabetes is of
particular interest for two reasons. First, the coronary endothelium ex­
presses a considerable number of SK channels, and SK channels are
crucial mediators of coronary arteriolar relaxation (Grgic et al., 2009;
Ledoux et al., 2006). Second, many people with diabetes experience
more severe coronary artery disease than non-diabetics, and so targeting
coronary SK channels in diabetic patients may provide some therapeutic
benefit. Hence a considerable amount of work has been done attempting
to understand the molecular basis of SK channel dysfunction in diabetic
coronary arteries (Fig. 2); it is possible that similar mechanisms may be
at play in other vascular beds as well, although more specific studies will
Wide variation across different studies, particularly in animal
models, may be a function of differences in the extent of diabetes-related
vascular pathology in alternative animal models and the specific pro­
tocols used by individual groups concerning when to test vascular
function. Indeed, different studies test for diabetes-induced changes in
vascular function at different time points in the development of diabetes
in their model system of choice. Thus, it may be that early on in diabetes,
SK channel expression increases to compensate for disturbances in
vascular function due to growing hyperglycemia; however, this
attempted compensation is inadequate since vascular function almost
always decreases anyway. As the disease process continues and vascular
damage increases, gene/protein expression patterns change, and SK
channel expression levels revert to baseline or even fall. Regarding the
studies by Liu et al. on the human coronary microcirculation, the lack of
changes in SK channel gene/protein expression alongside reduced SK
channel currents suggests that the pathophysiology at play in this
circumstance is most likely a post-translational or cytosol-to-membrane
SK channel protein trafficking issue.
Obesity often accompanies diabetes, especially type 2 DM; hence it
bears mention in any discussion of diabetes/metabolic syndrome. In
diet-induced models of obesity using Sprague-Dawley rats, EDHmediated relaxation of small mesenteric arteries is significantly
decreased (Haddock et al., 2011; Gradel et al., 2018) (Table 5). The type
of diet used to generate rat models may have an impact on the presence
or absence of altered SK/IK channel expression. For example, feeding
rats high fat chow alone to induce obesity resulted in upregulated IK
channel protein expression despite overall diminished EDH activity
(Haddock et al., 2011). In contrast, for rats fed a combination of high-fat
chow and high-fructose drinking water to produce obesity, SK and IK
channel mRNA expression were both downregulated (Gradel et al.,
2018). Meanwhile, unlike with mesenteric arteries, vascular relaxation
responses in the saphenous artery of rats fed a cafeteria-style diet for
obesity induction were preserved, and IK channel protein expression
increased (Chadha et al., 2010) Additional work is required to better
characterize phenotypes of obesity and any molecular changes that
Fig. 2. Proposed mechanisms for diabetic inactivation of endothelial SK/IK
channels in human coronary endothelial cells. The dotted line reflects inhibi­
tion, and the solid black lines reflect enhancement. PKC = Protein Kinase C;
NOX = NADPH oxidase.
Table 5
SK channels in animal models of obesity.
Animal Model
Tissue and Cell
Type
SK Protein
Expression in
Diabetes
SK Channel Gene Expression
Vascular Function/
EDHF responsiveness
SK/IK Channel
Recording
References
Sprague Dawley rat model,
diet-induced obesity
Sprague-Dawley rats, high
fat and high fructose
model of obesity
Cafeteria-style diet-induced
obese rats
Obese Zucker rats
Mesenteric artery
endothelial cells
Mesenteric artery
endothelial cells
IK: Upregulated
SK: No change
N/A
N/A
Small mesenteric
arteries: Decreased EDH
Small mesenteric
arteries: Decreased EDH
IK: Increased
SK: Decreased
N/A
Haddock et al.
(2011)
Gradel et al.
(2018)
Saphenous artery
endothelial cells
Coronary artery
endothelial cells
IK: Increased
SK3: No change
SK3 and IK:
Increased
Saphenous artery:
preserved
Coronary artery
endothelial cells:
preserved
N/A
Chadha et al.
(2010)
Climent et al,
2014
SK and IK channels:
Downregulated in high fat and
high fructose group
N/A
N/A
7
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S. Kant et al.
European Journal of Cell Biology 101 (2022) 151208
be required.
First, diabetes has been shown to increase the levels of NADH
(nicotinamide adenine dinucleotide hydrogen) and the NADH/NAD+
(nicotinamide adenine dinucleotide) ratio in human coronary artery
endothelial cells and the human myocardium (Liu et al., 2020).
Increased intracellular NADH resulted in decreased endothelial SK
channel currents while increased intracellular NAD+ increased endo­
thelial SK currents (Liu et al., 2020). The basic concept of pyridine
nucleotide regulation of ion channels is not new-other studies have
shown that high NADH/NAD+ ratios decrease cardiac sodium currents,
and high intracellular NADH downregulates BK currents in rabbit pul­
monary artery smooth muscle cells (Liu et al., 2009; Lee et al., 1994).
Therefore, given that a hallmark of diabetes is generalized impairment
of mitochondrial metabolism and elevated intracellular NADH, it is
possible that NADH facilitates diabetic inhibition of coronary SK chan­
nels to produce diminished electrical signaling.
Meanwhile, reactive oxygen species (ROS) may also play a role in
diabetic SK channel dysfunction. The mitochondria are major sources of
enhanced ROS production in diabetes. Oxidative stress is a key mediator
of cell injury and, in extreme cases, apoptosis (Kizhakekuttu et al., 2012;
Cho et al., 2013). Note that increased ROS production will affect the
redox status of key enzymes, such as eNOS and its cofactor tetrahy­
drobiopterin (Tabit et al., 2010). Experiments on diabetic mice have
shown that administration of mito-Tempo, an inhibitor of mitochondrial
ROS
production,
significantly
improved
coronary
endothelium-dependent relaxation responses to ADP and NS309, and
resulted in increased endothelial SK currents in comparison to diabetic
mice that did not receive treatment (Xing et al., 2021, Fig. 3). It is
possible that oxidation or nitration of SK channels due to the action of
free radicals may impair their calcium sensitivity/overall function;
however, further investigation will be necessary to identify specific
mechanisms by which mitochondrial ROS may inhibit endothelial SK
channels in diabetes.
Finally, enhanced activity of protein kinase C may contribute to
altered SK/IK channel activity in uncontrolled diabetes. Earlier, we
described at length how protein kinase C beta inhibits SK channel
activity by reducing channel calcium sensitivity. Hyperglycemia has
been shown to increase activation of protein kinase C and diacylglycerol
(DAG), with preferential activation of beta and delta isoforms in the
diabetic vasculature (Koya and King, 1998). DAG levels rise due to
overabundance of the glycolytic intermediate dihydroxyacetone phos­
phate, which is rapidly reduced to glycerol-3-phosphate; this in turn
increases de-novo DAG biosynthesis (Xia et al., 1994). Given that DAG is
a powerful activator of PKC, increased production of DAG facilitates
increased PKC activation.
PKC overactivity leads to several harmful effects on endothelial cells.
For example, PKC degrades endothelial tight junctions, increasing
endothelial permeability to large macromolecules, such as albumin
(Geraldes and King, 2010). In addition, PKC alters bioavailability of
crucial endothelial-derived vasoactive factors, decreasing activity/pro­
duction of vasodilators like eNOS and prostacyclin while increasing
production of vasoconstrictors like thromboxane A-2 and endothelin-1
(Geraldes and King, 2010). Furthermore, studies in bovine aortic
endothelial cells suggest that hyperglycemia, through elevated PKC,
interferes with gap junction mediated cell-cell communication. This
effect is abolished with PKC inhibition (Inoguchi et al., 1995). This could
have relevance for SK channel activity modulation if viewed through the
lens of the myoendothelial coupling hypothesis of EDHF activity,
wherein SK channel hyperpolarization of endothelial cells is transmitted
to vascular smooth muscle cells via gap junctions. Reduced gap junction
signaling would compromise endothelial SK-channel induced vascular
smooth muscle relaxation, accounting for impaired responses observed
in diabetes.
It is also possible that increased PKC activity simply enhances the
ability of PKC to reduce SK channel calcium sensitivity through
heightened phosphorylation of SK-channel bound calmodulin. However,
these are merely hypotheses; additional studies will be needed to
determine any potential mechanisms. One last point with respect to
diabetes and PKC-metformin has been shown to inhibit the PKC/extra­
cellular signal-regulated kinase (ERK) pathway in a rat model of type 2
diabetes, and this inhibition was linked to increased SK2 protein
expression in atrial tissue (Liu et al., 2018a). Curiously, SK3 expression
in metformin-treated diabetic rats decreased. Such findings suggest that
if PKC is involved with SK channel synthesis/trafficking, specific effects
may differ based on the specific SK channel subtype in question, an
important complication that bears consideration for future studies.
3.2. Hypertension and SK channels
Beyond diabetes, another major altered-metabolic disease state that
may involve SK channel pathology is hypertension. Any discussion of
hypertension must begin with a brief survey of the sheer size and scope
of the problem at hand. Hypertension is a leading cause of cardiovas­
cular morbidity and mortality around the world, with an estimated
global adult prevalence of over 30% (Mills et al., 2020). In the United
States alone, in 2018 hypertension was implicated in over half a million
deaths as either the primary or an important contributing factor (Centers
for Disease Control and Prevention, March, 2020). Moreover, about 37
million adults in the United States have uncontrolled hypertension,
defined as blood pressures greater than or equal to 140/90 (Centers for
Disease Control and Prevention, Hypertension Prevalence, Treatment,
and Control Estimates, 2019). Unsurprisingly, the costs of hypertension
to the American healthcare system are staggering, in-excess of $131
billion year on year (Kirkland et al., 2018).
Hypertension imposes a wide array of adverse effects on every organ
system. These include increased risk of hemorrhagic or ischemic stroke,
kidney disease, retinopathy, cardiac valvular disease, ischemic heart
disease, and left ventricular dysfunction (Mensah et al., 2002). All these
disparate pathologies stem from macrovascular and microvascular
damage due to persistent uncontrolled hypertension. At the macro­
vascular level, hypertension can induce arteriolosclerosis in the small
arteries and arterioles (Balakrishnan et al., 2015; Kono et al., 2016;
Fig. 3. NADH, mROS, and SK Channels in Diabetes. This figure depicts NADHinhibited SK/IK channel activity via PKC and mROS activation in endothelial
cells in the context of diabetes.
8
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European Journal of Cell Biology 101 (2022) 151208
Gavornik and Galbavy, 2001). In benign essential hypertension, arte­
riolosclerosis is hyaline, characterized by simple thickening of vessel
walls secondary to leakage of plasma proteins into the vessel media. In
severe hypertension, hyperplastic arteriolosclerosis may be seen, char­
acterized by extensive myointimal cell hyperplasia and medial vascular
smooth muscle proliferation that resembles an onion skin on histology.
Thickening of vessel walls further reduces systemic arterial compliance,
exacerbating already high systolic blood pressures. Hence the heart must
work increasingly hard to maintain adequate organ perfusion, elevating
the risk of left ventricular remodeling and left ventricular failure.
At the microvascular level, hypertensive vessels exhibit extensive
endothelial dysfunction. For example, hypertension accelerates the
progression of atherosclerosis (Schiffrin, 2002; Ning et al., 2018; Alex­
ander, 1995). Mechanical injury to the endothelium due to increasingly
turbulent blood flow/shear stress in hypertension may enhance intimal
permeability to LDL and may also promote increased entry of monocytes
into vessel intimae (Ning et al., 2018). In addition, increased levels of
circulating angiotensin II may be found in hypertensive patients, which
enhances atherosclerosis and the risk of aortic aneurysms in several
animal models (Ning et al., 2018; Weiss et al., 2001).
A growing body of evidence suggests that alterations in endothelial
regulation of vasomotor tone have a significant role in microvascular
changes induced by hypertension. Indeed, the normal endothelial
response to increased shear stress involves increased release of vaso­
dilatory factors such as nitric oxide and prostacyclin, along with
heightened endothelium-dependent hyperpolarization. Studies have
shown that hypertension reduces the bioavailability of endothelial nitric
oxide and increases production of endothelium-derived contracting
factors, chiefly vasoconstrictive arachidonic acid metabolites such as
thromboxane A2 (Goto et al., 2018; Vanhoutte et al., 2005). Dysregu­
lation of SK channels in peripheral vascular and microvascular endo­
thelial dysfunction during hypertension has been extensively studied.
SK channels have an important role in regulation and control of
blood pressure. For example, one of the most common forms of endo­
crine hypertension is primary hyperaldosteronism, and blockade of SK
channels in human adrenocortical cells (e.g., with apamin) stimulates
aldosterone secretion (Yang et al., 2016). With respect to endothelial
cells, experiments with hypertensive connexin-40 deficient mice have
found that stimulation of endothelial SK4/IK channels with SKA-31
reduced blood pressure by over 30 mmHg, providing additional evi­
dence of a role for SK channels in blood pressure modulation (Radtke
et al., 2013). Moreover, spontaneously hypertensive mice show
impaired responsiveness to the SK/IK channel activator NS309 in
mesenteric arteries, along with reduced overall protein expression of SK
and inward rectifier potassium channels (Weston et al., 2010).
Others have also reported downregulation of SK channel expression
in mesenteric arteries of rat models of spontaneously induced hyper­
tension and angiotensin induced hypertension (Seki et al., 2017; Hilgers
and Webb, 2007). Alongside SK channel downregulation, Seki et al.
report decreased expression of endothelial TRPV4 channels in their
mouse model (Seki et al., 2017). This is of interest because calcium
influx through TRPV4 channels may be involved in downstream acti­
vation of SK/IK channels (Seki et al., 2017). Hence reduced endothelial
SK channel currents in hypertension may be the result of a combination
of at least two factors: decreased overall SK channel expression and
reduced activation of available SK channels due to TRPV4
downregulation.
Approaching animal models of hypertension from a different angle,
SK3 and IK1 knockout mice exhibit markedly impaired EDHF-mediated
vasodilation and elevated arterial blood pressures (Bra¨hler et al., 2009;
Yap et al., 2016a, 2016b; Taylor et al., 2003). Endothelial SK channel
deficiency has also been linked with reduced nitric oxide mediated
vasodilator responses in transgenic mouse models (Köhler and Ruth,
2010). Moreover, beyond displaying hypertensive phenotypes, IK
channel deficient mice also exhibit mild left ventricular hypertrophy,
mimicking a well-known myocardial consequence of long-standing
uncontrolled hypertension in humans (Si et al., 2006).
Curiously, some studies have found increased IK channel expression
in mesenteric and renal arteries of spontaneously hypertensive rats
alongside decreased SK channel expression, albeit with overall dimin­
ished endothelial cell responsiveness to NS309 (Giachini et al., 2009;
Simonet et al., 2012). Meanwhile, a different rodent model of hyper­
tension, Dahl salt-sensitive hypertensive rats (DS-H rats), showed
increased EDHF-like relaxation, after treatment with nitric oxide syn­
thase inhibitors (Goto et al., 2012). However, this particular EDHF-like
response appears to be driven by BK channel upregulation rather than
changes to SK/IK channels, given that iberiotoxin blockade of BK
channels attenuated the relaxation response (Goto et al., 2012).
Specific molecular mechanisms underpinning differences in SK and
IK channel regulation in hypertension remain unelucidated. One possi­
bility may be differences in subcellular localization, with SK channels
largely localized to endothelial cell membranes and IK channels largely
localized to the myoendothelial projections that link endothelial cells
with vascular smooth muscle cells (Goto et al., 2018). It is also important
to note that nearly all mouse model studies examining SK channel
function in hypertension thus far have been done on the peripheral
circulation. Future research will need to examine SK channel changes in
other vascular beds, such as the coronary, pulmonary, or cerebral mi­
crocirculations, before drawing conclusions about broad systemic im­
plications of SK channel dysfunction in hypertension. Furthermore, few
studies have examined hypertensive human blood vessels to verify the
changes in endothelial SK channel activity/expression seen in mouse
models (Grgic et al., 2009). If these findings are confirmed, endothelial
SK channel modulation, particularly SK3 and IK stimulation, may pre­
sent a novel therapeutic approach to improve endothelium-dependent
vasodilation and lower blood pressure in individuals with hypertension.
3.3. Hypoxia-reperfusion and SK channels
The final pathological condition featured in this review is hypoxia/
ischemia-reperfusion injury, another prototypical example of an
altered metabolic disease state in which SK channel dysfunction may
play a crucial role. All organ systems rely on aerobic metabolism as a
main source of cellular energy production; hence maintenance of oxygen
homeostasis is crucial for survival. While some organs, like skeletal
muscle, are more resistant to fluctuations in oxygen levels, other organs,
most notably the heart and the brain, are exquisitely vulnerable to ox­
ygen perturbations and cease to function normally within minutes of
oxygen deprivation. Ischemia may also occur in medical settings such as
cardiac surgery involving cardioplegia/cardiopulmonary bypass (CP/
CPB), where the heart is temporarily stopped, and a heart-lung machine
takes over the role of maintaining the body’s circulation. Despite ad­
vances in cardioprotective strategies over the years, ischemiareperfusion injury still often occurs during/after CP/CPB, hallmarks of
which include reduced coronary endothelial function and impaired
relaxation that result in coronary vasospasm and myocardial malper­
fusion (Feng and Sellke, 2016; Robich et al., 2010).
A vast array of deleterious consequences result from ischemia (Yang
et al, 2016; Crossman, 2004; Chen et al., 2020). First, oxidative phos­
phorylation ceases, triggering a shift to anaerobic glycolysis that ele­
vates intracellular lactate levels. Increased lactic acid lowers
intracellular pH, leading to suboptimal enzyme function and clumping
of nuclear chromatin. Ultimately, anaerobic glycolysis cannot generate
enough ATP to fuel metabolic needs. For example, an immediate
consequence of low ATP is decreased activity of the cell membrane Na-K
ATPase. Decreased Na-K ATPase activity leads to increased sodium
influx, potassium efflux, and altered cellular resting membrane poten­
tials. Over time, disrupted membrane potential leads to calcium influx.
Rising levels of intracellular calcium activate enzymes such as phos­
pholipases, proteases, and nucleases that degrade plasma membrane
phospholipids, cytoskeletal proteins, and nuclear DNA. Calcium can also
trigger destruction of mitochondrial membranes (via phospholipase A2)
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European Journal of Cell Biology 101 (2022) 151208
and opening of mitochondrial permeability transition pores (MPTPs)
that allow leakage of mitochondrial enzymes, like the pro-apoptotic
cytochrome c, into the cytosol. In the case of hypoxia, where cellular
oxygen levels are low but NOT zero, these effects occur more gradually
over a longer period.
The main treatments for tissue hypoxia/ischemia depend on the
cause. If the problem is poor blood flow to a specific location, targeted
reperfusion is the primary approach. If the problem is hypoxemia, then
pulmonary interventions (e.g., supplemental oxygen, mechanical
ventilation) may be necessary. Unfortunately, reperfusion is not without
some adverse effects (Slegtenhorst et al., 2014; Cowled and Fitridge,
2011). First, rapid restoration of blood to oxygen-starved tissue can lead
to oxygen overload that itself generates reactive oxygen species (ROS),
while the antioxidant scavenging capacity of ischemic cells is insuffi­
cient to compensate. Unchecked ROS can damage already injured
mitochondria in reperfused tissue, increasing MPTP formation and
release of mitochondrial contents into the cytosol, causing cell swelling
and death (Slegtenhorst et al., 2014). Meanwhile, ROS can stimulate
inflammation by promoting formation of inflammasomes and activation
of pro-inflammatory cytokines such as IL-1 beta (Gross et al., 2011;
Tschopp and Schroder, 2010). Finally, reperfusion may also lead to
calcium overload that can facilitate cellular apoptosis.
Endothelial dysfunction, most often manifesting as increased endo­
thelial activation and altered vasomotor tone, is an important aspect of
pathology in hypoxia/ischemia-reperfusion due to disease or CP/CPB.
For example, human umbilical vein endothelial cells exhibit increased
ICAM-1 expression and enhanced monocyte adhesion following hypoxia
(Liang et al., 2019). Hypoxia also contributes to depletion of BH4 and
L-arginine, the former being an important cofactor of and the latter
being a key substrate for endothelial nitric oxide synthase (eNOS)
(Janaszak-Jasiecka et al., 2021). This results in two major consequences.
First, eNOS stops generating normal levels of nitric oxide, leading to
impaired vascular smooth muscle relaxation. Second, through depletion
of BH4 and L-arginine, hypoxia elicits eNOS uncoupling as eNOS begins
to produce superoxide anions instead of nitric oxide, which contributes
to endothelial cell oxidative stress.
In addition, ischemia-reperfusion has been shown to increase
vascular sensitivity to the endothelial-derived vasoconstrictor
endothelin-1 in dog coronary arteries and the coronary microvascula­
ture (Miura, 1996). Experiments in human coronary arterioles following
cardioplegic ischemia-reperfusion during CP/CPB show reduced
responsiveness to the endothelial-derived vasoconstrictor thromboxane
A2 (Feng et al., 2011). Curiously, human coronary arterioles exhibit
reduced contractile responses to endothelin-1 following CP/CPB, con­
trary to results seen in some animal models of ischemia-reperfusion (e.
g., Miura, 1996) (Feng et al., 2010).
Altered SK channel-EDHF activity gives rise to endothelial dysfunc­
tion following hypoxia/ischemia-reperfusion. At first, early animal
models of ischemia-reperfusion suggested that EDHF responses were
heightened after ischemia-reperfusion. For instance, rat middle cerebral
arteries showed increased EDHF-mediated vasodilation following
ischemia-reperfusion (Marrelli et al., 2001). Likewise, coronary arteri­
oles of dogs subjected to transient occlusion and reperfusion of the left
circumflex coronary artery showed increased EDHF responses (Chan and
Woodman, 1999).
However, more recent studies paint a different picture. Human cor­
onary arterioles harvested from atrial tissue after CP/CPB display
markedly reduced relaxation responses to the SK/IK channel activator
NS309 compared with coronary arterioles harvested from atrial tissue
before CP/CPB (Feng et al., 2008). Decline in SK channel activity is even
more pronounced in coronary arterioles of diabetic patients following
CP/CPB (Liu et al., 2018). In addition, mouse small coronary artery
endothelial cells exposed to CP hypoxia-reoxygenation exhibit signifi­
cantly attenuated endothelial SK channel activity, along with intracel­
lular calcium overload that strongly correlated with decreased SK
channel currents (Song et al., 2021). Pretreatment of mouse and human
coronary artery endothelial cells with NS309 before CP
hypoxia-oxygenation conferred significant protection of coronary
endothelial function after hypoxia-reperfusion compared to control
groups that received no pre-treatment (Zhang et al., 2020).
In the specific context of CP, the type of cardioplegic solution used
during hypoxia-reperfusion may impact the extent of EDHF/SK channel
dysfunction. Although depolarizing (hyperkalemic) cardioplegia is
currently the most common method for myocardial preservation during
cardiac surgery, hyperpolarizing cardioplegia (e.g., through use of ATPsensitive potassium channel activators) has been shown to better pre­
serve EDHF-mediated vascular relaxation (He et al., 2004; He and Yang,
1997). Indeed, coronary microvascular responses to the EDHF stimu­
lator bradykinin are markedly attenuated during hyperkalemic car­
dioplegia, while hyperpolarizing cardioplegia results in less significant
attenuation (He et al., 2004; He and Yang, 1997).
Other models of hypoxia that do not involve CP/CPB provide similar
results of blunted EDHF activity. For example, porcine coronary artery
endothelial relaxation responses to SK/IK channel activators bradykinin
or 1-ethyl-2-benzimidazolinone are decreased following in-vitro expo­
sure to hypoxia (Yang et al., 2013; Dong et al., 2005; Ziberna et al.,
2009; Wang et al., 2017). Furthermore, rat models of chronic
hypoxia-induced
pulmonary
hypertension
show
diminished
endothelial-derived hyperpolarization in small pulmonary arteries
(Kroigaard et al., 2013). Note also that experiments on the pulmonary
vasculature in isolated rat lungs have shown that inhibition of SK
channel activity with substances such as apamin strengthens pulmonary
vasoconstriction in response to hypoxia (Dumas et al., 1999).
Potential causes of altered SK/IK channel activity following hypoxia/
ischemia-reperfusion remain somewhat of a mystery, although several
theories abound. One study by Wang et al. observed pretreatment of pig
coronary artery endothelial cells with the TRPC3 channel activator 1oleoyl-2-acetyl-sn-glycerol (OAG) restores SK channel currents and
EDHF vasodilation after hypoxia-reperfusion (Wang et al., 2017). Given
that hypoxia inhibits TRPC3-mediated cellular calcium entry and TRPC3
trafficking to the endothelial cell surface, it is possible that dysregulated
TRPC3 trafficking may be implicated in hypoxic SK/IK channel
dysfunction (Huang et al., 2011).
There is debate in the literature as to whether endothelial SK/IK
channel levels decrease following hypoxia/ischemia-reperfusion, with
some studies suggesting no (e.g., Feng et al., 2008; Zhang et al., 2020)
and others suggesting yes (more specifically, decreased SK4/IK channel
expression as reported by Yang et al., 2013; Kroigaard et al., 2013).
Curiously, Kroigaard et al. observed upregulated SK3 channel protein in
small pulmonary arteries in their chronically hypoxic rat models of
pulmonary hypertension; nonetheless, EDH-type relaxation was overall
reduced (Kroigaard et al., 2013). A decrease in overall SK/IK channel
levels following hypoxia-reperfusion would directly explain the reduced
currents. However, if overall SK/IK channel levels do not change, then
the molecular mechanisms responsible for observed SK/IK channel
dysfunction must be post-translational (Feng et al., 2008; Zhang et al.,
2020). Potential possibilities in this scenario include (1)
ischemia-reperfusion induced activation of protein kinases that inhibit
SK/IK channel activation and open probability, (2) free radical (ROS)
interference with SK channel activity, or (3) alterations in SK/IK channel
trafficking from cytosol to cell membrane. Fig. 4 illustrates how several
of these proposed mechanisms may come together to result in altered
SK/IK channel activity in the context of cardioplegic
hypoxia/reperfusion.
Evidence exists that supports each of these hypotheses. First,
increased protein kinase C (PKC) levels have been observed in
myocardial tissue following CP/CPB (Sodha et al., 2008). As discussed
earlier, PKC inhibits SK channel activity by phosphorylating SK
channel-bound calmodulin, decreasing its responsiveness to calcium and
reducing overall channel calcium sensitivity. However, the main isoform
of PKC implicated in SK channel regulation is the beta isoform, and this
study only noted upregulation of delta and epsilon isoforms (Sodha
10
S. Kant et al.
European Journal of Cell Biology 101 (2022) 151208
calcium-mediated SK channel activity) as opposed to aggregate protein
levels.
Higher cytosolic vs cell membrane SK3 expression has been found
following CP hypoxia-reperfusion in human coronary artery endothelial
cells and mouse heart endothelial cells; SK4 channels, however, display
no alterations in subcellular distribution (Zhang et al., 2020). Hence
CP-hypoxia/reperfusion may induce SK3 channel internalization, which
would explain diminished overall endothelial responses to SK activators
like NS309. However, few studies exist replicating these findings, and
details surrounding the processes by which SK3 internalization occurs
remain unclear. Additional work is needed to elucidate the specific
molecular processes at play.
3.4. Final thoughts: clinical and translational perspectives and questions
Strong evidence of a role for SK channel dysfunction in the patho­
genesis of microvascular endothelial dysfunction in the alteredmetabolic disease states discussed in this review raises several impli­
cations for clinical and translational applications. Many such applica­
tions were mentioned in the preceding sections on diabetes,
hypertension, and ischemia-reperfusion; here, the major highlights are
summarized and emphasized. Beginning with diabetes, work done by
Liu et al. demonstrating decreased SK currents in human diabetic cor­
onary microvessels, and Xing et al., which showed that the mROS in­
hibitor mito-Tempo improved coronary endothelium-dependent
relaxation responses and endothelial SK currents in diabetic mice, was
discussed at length Liu et al., 2018, 2020, 2015; Xing et al., 2021). This
lends credence to the possibility that mROS inhibition may augment SK
channel function in diabetic humans, who also exhibit high levels of
cellular oxidative stress, thereby serving as a therapeutic tool for pro­
tecting coronary microvascular function.
Animal models of hypertension and ischemia-reperfusion also
display diminished SK channel currents, and treatment with SK channel
activators such as NS309 or mROS inhibitors like mito-Tempo appears to
improve/protect endothelial function in these circumstances (Song
et al., 2021; Zhang et al., 2020; Seki et al., 2017; Goto et al., 2018).
Hence there is potential for pharmacologic stimulation of SK channels to
also be useful in treating disordered vasomotor tone in hypertensive
vascular disease, and in protecting the microcirculation from
ischemia-reperfusion damage during situations such as cardiopulmo­
nary bypass. However, although many experiments have been done
using coronary microvascular tissue harvested from human atria, there
remains a general dearth of research investigating the impact of phar­
macologic modulation of SK channels in human tissue more broadly (e.
g. other non-coronary human microcirculations). Even with respect to
animal models of diabetes, hypertension, and ischemia-reperfusion,
nearly all major studies of SK channel modulation have been done on
mesenteric and coronary microvessels.
Future research will need to address the impact of SK channel
dysfunction in other vascular beds, such as the pulmonary or renal mi­
crocirculations, in humans and animal models of diabetes/
hypertension/ischemia-reperfusion. Moreover, even though there is
relative consensus concerning diminished SK channel activity in these
disease states, it remains hotly contested whether SK channel expression
levels themselves change and if so, how; this is another area ripe for
future investigations. Finally, it will be important to eventually consider
how SK channel modulators could be delivered to patients in a clinical
setting. Indeed, most major experiments so far have utilized infusions of
SK channel activators in living mouse models or application of SK
channel activators in-vitro to isolated microvessels. Many questions
remain regarding potential side effects of systemic SK channel activator
infusion beyond the microcirculation (as SK channels are not only pre­
sent on endothelial cells) that may complicate clinical use. In addition, it
may be worthwhile to investigate whether SK channel activators can be
delivered via oral formulations (e.g. a pill), and compare the pharma­
codynamic and pharmacokinetic profile with IV and IM routes of
Fig. 4. SK channel dysfunction during cardioplegic hypoxia/reoxygenation
(CP-H/R). Increased NOX or NADH during CP-H/R activates PKC and induces
mROS production, resulting in decreased SK channel activity, coronary endo­
thelial dysfunction, and reduced coronary arteriolar relaxation.
et al., 2008). PKC beta inhibition has been demonstrated to improve SK
channel currents in other disease processes (e.g., diabetes, discussed
earlier). Additional studies are required to examine whether PKC beta
expression or activity levels are altered in endothelial cells during
hypoxia-reperfusion.
Next, endogenous antioxidants such as glutathione peroxidase, su­
peroxide dismutase, and catalase have all been shown to attenuate
myocardial injury caused by ischemia-reperfusion (Dhalla, 2000).
Moreover, altered mitochondrial membrane potentials, in particular
mitochondrial membrane hyperpolarization, have been shown to in­
crease mitochondrial ROS production in the setting of endothelial injury
(Kluge et al., 2013). This is thought to be due in part to an increase in the
NADH/NAD+ ratio and a decrease in electron flow rates through com­
plex I and complex III of the mitochondrial electron transport chain,
which prolong the lifespan of reactive intermediates and promote su­
peroxide anion formation from oxygen reduction (Kluge et al., 2013).
With respect to ischemia-reperfusion, inhibition of mitochondrial
reactive oxygen species using the antioxidant Mito-Tempo enhances
relaxation responses in mouse coronary artery endothelial cells
following CP/CPB, compared to untreated control groups (Song et al.,
2021). Mito-Tempo also reduced the magnitude of calcium overload,
providing a potential mechanistic link between reduced mitochondrial
ROS production and improved SK channel activity (Song et al., 2021).
Here, it is important to recall that while calcium is required to activate
SK channels, it only does so at certain levels—the theme, as often is the
case in biological systems, is balance. Indeed, Song et al. demonstrate
that free calcium concentrations of around 400 nM evoke strong in-vitro
SK currents in their mouse heart endothelial cells. However, when free
calcium concentrations rise to 2uM, SK channel currents drop to levels
akin to those under 0 calcium conditions, suggesting that excess calcium
is detrimental to SK channel activity. Overall, Mito-Tempo treatment
resulted in stronger SK channel currents following CP
hypoxia-reperfusion, albeit without any changes in SK channel protein
expression pre/post CP, supporting the notion that mitochondrial ROS
may affect SK channel activity (e.g., through tighter regulation of
11
European Journal of Cell Biology 101 (2022) 151208
S. Kant et al.
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4. Conclusion
SK channel activity is instrumental in endothelial function and ho­
meostasis, particularly endothelium-dependent hyperpolarization.
Activation of SK channels and subsequent potassium efflux is an inde­
pendent mediator of vascular smooth muscle relaxation in the absence
of nitric oxide, prostacyclin, and endothelium-independent vasodilators.
A variety of factors regulate SK channel activity under normal physio­
logic circumstances, including but not limited to calcium itself, cellular
NADH/NAD+ ratios, and certain isoforms of protein kinase C.
Conversely, SK channel dysfunction/dysregulation has been implicated
as an important driver of vascular pathology in a variety of altered
metabolic disease states, including diabetes mellitus/metabolic syn­
drome, hypertension, and hypoxia/ischemia-reperfusion. In each of
these conditions, SK channel activity decreases, disrupting the balance
between vasodilation and vasoconstriction and increasing the risk of
generalized endothelial dysfunction. Increased protein kinase C activity,
increased NADH/NAD+ ratios, increased ROS production, and altered
SK channel localization all appear to promote SK channel pathology,
suggesting that unifying themes of pathophysiology may exist across
different disease states. Future studies will need to expand on the work
done in animal models by exploring different vascular beds and,
whenever possible, human tissue to verify the principles explored in this
review. Ultimately, improved understanding of the role of SK channels
in normal physiology and pathophysiology may lead to novel targeted
treatments aimed at restoring endothelial function.
Data availability
No data was used for the research described in the article.
Acknowledgments
None.
Funding
This research project was mainly supported by the National Heart
Lung and Blood Institute (NHLBI) of 1R01HL127072-01A1, 1R01
HL136347-01, and 3R01HL136347-04S1; and National Institute of
General Medical Science (NIGMS) of 5P20-GM103652 (Pilot Project) to
J.F.
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Further reading
Carneiro, Fernando, Lima, Victor, Carneiro, Zidonia, Dorrance, Anne, Webb, Clinton,
Tostes, Rita, 2009. Up-regulation of intermediate calcium-activated potassium
channels (IKCa), counterbalances the impaired endothelium-dependent vasodilation
in SHRSP. Translational research: the journal of laboratory and clinical medicine 154
(4), 183–193. https://doi.org/10.1016/j.trsl.2009.07.003.
Carvalho-de-Souza, J.L., Varanda, W.A., Tostes, R.C., Chignalia, A.Z., 2013. BK channels
in cardiovascular diseases and aging. Aging Dis. 4 (1), 38–49.
Félétou, M., Köhler, R., Vanhoutte, P.M., 2011. Nitric oxide: orchestrator of endotheliumdependent responses. Ann. Med. 44 (7), 694–716. https://doi.org/10.3109/
07853890.2011.585658.
McNeish, A.J., Sandow, S.L., Neylon, C.B., Chen, M.X., Dora, K.A., Garland, C.J., 2006.
Evidence for involvement of both IK CA and SK ca channels in hyperpolarizing
responses of the rat middle cerebral artery. Stroke 37 (5), 1277–1282. https://doi.
org/10.1161/01.str.0000217307.71231.43.
Tajbakhsh, N., Sokoya, E.M., 2014. Compromised endothelium-dependent
hyperpolarization-mediated dilations can be rescued by NS309 in obese Zucker rats.
Microcirculation 21 (8), 747–753. https://doi.org/10.1111/micc.12157.
Ziegler, O., Anderson, K., Liu, Y., Ehsan, A., Fingleton, J., Sodha, N., Feng, J., Sellke, F.
W., 2020. Skeletal muscle microvasculature response to β-adrenergic stimuli is
diminished with cardiac surgery. Surgery 167 (2), 493–498. https://doi.org/
10.1016/j.surg.2019.07.018.
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