Why Calcium?
Double positive charge provides increased affinity f or negatively c harged proteins
but lower affinity than l arger divalent cations suc h as Cu, Zn, or Mn. Th e
coordination c hemistry of Ca is higher and more flexible tha n for Mg.
The fact that Ca complexes wi th i norganic compound s and to proteins s uggests
that the maintenance of low [Ca] intracellularly w ould require less energy than for
other cations. The maintenance of a large transmembrane gradient is critical for
a second messanger ion.
This l arge transmembrane gradient p rovides the signal-to-noise ratio required for
efficient signal transduction.
Resting intracellular [Ca] is ~100 nM versus mM extracellular [Ca] or a
concentra tion gradient of ~10,000.
NCE
mM [Ca]
NMDA, AMPA rec
nAChR
CNG
VR/temp rec
Stretch receptor
TRP
VDAC
mM [Ca]
SOC
IP3R
SERCA
ROC
VGCC
PMCA
mM [Ca]
RyR
SERCA
100 nM [Ca]
Sarcoplasmic Reticulum (SR) / T
Tubule System
Twitch
Summation
Tetanus
Role of Ca++ in contraction
T-tubule
A
B
T-tubule
RyR1
RyR2
SR
SR
VDCC
C
VDCC
T-tubule
Ca release
FKBP12/12.6
RyR
SR
Muscle contraction
RyR Accessory proteins
FKBP12/12.6
Kinases
Phosphatases
Adaptor/anchoring/targeting proteins
Sorcin
S100s
Triadin
Junctin
Annexin VI
Calmodulin
vertebrate
skeletal
muscle
contraction
Excitation-contraction coupling
Ryanodine receptors
Properties of C alcium Release Channels
Ryanodine Receptors
Size (amino a cids)
of monomer
Size (daltons) of
monome r
Sedimentation
coefficient of
tetramer
Stoichiometry of
FKBP/RyR
Single channe l
conductance
in Ca 50 mM
Single channe l
conductance
in Cs 250 mM
Endogenous
modulators
M Ca activates
mM C a inhibits
mM Mg inhibits
kinases
phosphata ses
DHPR in teraction
calmodulin
adenine
nucleotides
MgATP
NO
RyR1
RyR2
RyR3
5,037
4,970
4,870
~565,000
~560,000
~560,000
30 S
30 S
30 S
4
4
4
~120 pS
~120 pS
~100 pS
~540 pS
~540 pS
?
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
?
yes
yes
yes
yes
?
?
?
yes
yes
yes
yes
yes
yes
yes
yes
yes
?
Pharmacology of Ryanodine Receptors
RyR1
RyR2
RyR3
Site of action
Xanthines
(caffeine)
Ryanodine
/ryanoids
activates
subconductance
state
activates
subconductance
state
activates
subconductanc
e state
ruthenium red
inhibits
inhibits
inhibits
Ca activation sites
carboxy
terminus
Ca binding
site(s)/channel
pore
activates
activates
activates
?
activates
activates
activates
FKBP12/12.6
activates
activates
activates
ATP binding sites
inhibits
inhibits
inhibits
calmodulin b inding
site
inhibits
inhibits
inhibits
inhibits
inhibits
?
?
?
activates
activates
?
?
Activates/inhibits
Activates/inhibits
?
?
Antra quinones
(doxorubicin)
FK506 &
rapamyci n
Purinergic
agonists/
antagonists
(adenosine)
Calmodulin
antagonist
Local anesthetics
(tetracaine)
Dantrolene
Pheno l d erivatives
(4-chloro-mcresol)
NO generating
compounds
RyR
500
1000
NH2
PP1 MH/CCD
1500
PP2A
2000 DHPR/RyR1
FKBP
MH/CCD
2500
Ca-Activation
3000
3500
Calmodulin
Ryanodine
4000
Ca-Inactivation
mAKAP
RII-PKA
4500
Cytosol
SR
Membrane
M2
M3
PKA/CamKII
PO4 site
5000
CO2H
RyR2
GIG
M1
P
M4
Lumen
MH/CCD
Voltage-gated ion channels
2AR
1AR
AC
P

 
cAMP
Ca2+
RyR2
P
Regulation of EC
coupling by adrenergic
signaling -“fight or flight”
SR
Lumen
Ca2+
P
RyR2 macromolecular complex
RyR macromolecular complexes are
held together by
leucine/isoleucine zippers
RyR2/calcium release channel phosphorylated
by addition of cAMP alone
PKA
phosphorylation
activates RyR2
and induces
subconductance
states
RyR2 open probability
Iso CHF
ECC gain
Ca transient
contractility
[Ca]
Cytosolic
[Ca]
DADs - SCD
Time
IP3R
RyR
Skel muscle
+
+++
Smooth m.
+++
+
Neurons
+++
+++
IP3
Activates
None
Ryanodine
None
Locks
open/closes
Caffeine (5 mM) Inhibits
Activates
Ca2+
IP3R1 biphasic
biphasic
IP3R2/3 - opens
RR
None
inhibits
Heparin
inhibits
activates
Excitation-secretion coupling
Inositol 1,4,5-trisphosphate
receptors
IP3 Receptors
Size (amino acids)
of monomer
Size (daltons) of
monomer
Stoichiometry of
FKBP/RyR
Endogenous
modulators
IP3
nM Ca activates
M Ca inhibits
ATP <2 mM
ATP >2 mM
tyrosine kinases
phosphatases
IP3R1
IP3R2
IP3R3
2,749
2,691
2,685
313,000
~300,000
~300,000
4
4
?
activates
yes
yes
activates
inhibits
activates
?
activates
yes
yes
activates
inhibits
?
?
activates
yes
no
activates
inhibits
?
?
Pharmacology of IP3 Receptors
(caffeine)
heparin
2-APB
xestospongin
IP3R1
inhibits
inhibits
inhibits
inhibits
IP3R2
inhibits
inhibits
inhibits
inhibits
IP3R3
inhibits
inhibits
inhibits
inhibits
Site of action
Ca activation sites
?
?
?
IP3 signaling
Stable transfectant
Jurkat T cells do not
express IP3R1
IP3R1
Activation of IP3-gated Ca2+ channel by fyn
Calcium signaling during apoptosis
Growth Factor
PIP2
Plasma Membrane
Rec
PLC-
Cytoplasm
P
IP3
Ca2+
ER
Bcl-2
IP3R
P
14-3-3
Bad
Bad
Degradation
SERCA 2+
Akt
Ca
+X?
Calcineurin
Cytosolic T
F
Bcl-2
Bad Bcl-2
DNA
CAD
Digestion
Nuclear
Caspases
TF
Cytochrome C M
Nucleus
Survival
Bcl-2
Bcl-x
M
Cytoskeletal Breakdown
Cell Death
Survival
NCE
mM [Ca]
mitochondria
nAChR
CNG
Stretch receptor
VDAC
Endoplasmic
reticulum
mM [Ca]
PMCA
IP3R
SERCA
100 nM [Ca]
T-tubule
mM [Ca]
VGCC
RyR
Sarcoplasmic
reticulum
SERCA