SIGNAL TRANSDUCTION BY
ADRENERGIC AND CHOLINERGIC
RECEPTORS
Andy Catling Ph.D.
Department of Pharmacology Room 5238
acatli@lsuhsc.edu
568-4740
Adrenergic Receptors

Alpha Receptors:
1: Contraction of vascular and genitourinary smooth muscle.
 2: Contraction of vascular smooth muscle; decreased insulin secretion;

aggregation of platelets; pre-synaptic inhibition of NE.

Beta Receptors:


1: Positive inotropic and chronotropic effects on the heart.
2: Relaxes vascular, bronchial, gastrointestinal and genitourinary
smooth muscle; stimulates glyconenolysis and gluconeogenesis in
the liver.

3: Lipolysis in adipose tissue.
Cholinergic Receptors

Nicotinic Receptors:



NM (muscle): Depolarization of NMJ; Skeletal muscle contraction.
NN (neuronal): Activation of post ganglionic neurons.
Muscarinic Receptors:



M1: Depolarization of autonomic and CNS neurons
M2: Negative inotropic and chronotropic effects on
the heart.
M3: Stimulates sweat, bronchial, salivary and gastric acid
secretions; Increased NO production from vascular
endothelium and vasorelaxation.
How does this work?

Different receptors can have reinforcing or opposing functions:
e.g. 1 adrenergic receptors stimulate contraction of vascular smooth
muscle whereas 2 adrenergic and muscarinic M3 receptors both cause
relaxation.
e.g. 1 adrenergic receptors stimulate contraction of heart muscle,
whereas muscarinic M2 receptors inhibit myocardial contraction.
Goal of these lectures: begin to understand the signal transduction
mechanisms by which specificity is established. Note that this is still a
work-in-progress: the human body is complex (!) and there are both
gaps in our knowledge and exceptions to general rules
Components of Signal Transduction
Signal transduction within cells is accomplished by
combinations of:
 1st Messenger (extracellular signals e.g. epinephrine,
acetylcholine)
 Receptor
 Effectors (e.g. adenylyl cyclase, phospholipases, kinases,
ion channels etc)
 2nd messengers (cAMP, cGMP, inositol triphosphate,
diacylgycerol, Ca2+ etc)
 Downstream effectors required for specific functional
outputs (e.g. muscle contraction, secretion)
How does this work?
Specificity results from:
 Differential expression and localization (junctional vs extra
junctional) of receptors



Different receptors couple to different signal transducers
Signal transducers/2nd messengers couple to different
effectors in different tissues
Integration of reinforcing and antagonistic signals
 Adrenergic Receptors


 receptors differ in their location and sensitivity to Epinephrine and
Norepinephrine (simplified!):
1
Myocardium
E=NE
2
Smooth muscle
E (essentially no affinity for NE)
3
Adipose tissue
NE>E
i.e. tissue response to agonist is governed by expression of receptor
subtypes and ligand present
All three  adrenergic receptors function through a major class of signal
transducer: G-proteins

G-proteins couple  adrenergic receptors to adenylyl cyclase:
 agonists increase intracellular cyclic AMP levels and protein kinase A
activity, which in turn regulate downstream effectors
G-protein Activation-Deactivation
Cycle
GDP 
GDP + 
GTP
Pi
GTP + 
Effectors
GDP
Effectors
GPCR*
Adrenergic Receptors

Beta Receptors

 1 Receptors




Gs (stimulatory): Activation of adenylyl cyclase and increased
cAMP levels.
Positive inotropic and chronotropic effects on the heart; speeds
conduction across the AV node.
Agonist:
Antagonist:
Dobutamine
Atenolol
1 adrenergic receptors function through Gs to
stimulate the effector adenylyl cyclase to
produce the 2nd messenger cyclic AMP
Activated Gs:
- stimulates adenylyl cyclase to produce
cAMP
- enhances activation of voltage gated Ca2+
channels in the plasma membrane
cAMP:
- activates protein kinase A, which directly
phosphorylates proteins (e.g. troponin I)
essential for cardiac muscle contraction
- stimulates sodium/potassium influx which
opens voltage-gated Ca2+ channels
- inhibits uptake of Ca2+ into cellular stores
- cAMP hydrolyzed by phosphodiesterases
Overall effect: increased intracellular Ca2+ concentration and phosphorylation of
contractile proteins. Result: cardiac muscle cells expressing 1 receptors
contract in response to epinephrine or norepinephrine.
Adrenergic Receptors

Beta Receptors

2 Receptors
 Gs: Activation of adenylyl cyclase and increased cAMP levels.



Relaxes vascular, bronchial, gastrointestinal and genitourinary
smooth muscle, stimulates the uptake of potassium into
skeletal muscle, stimulates glycogenolysis and gluconeogenesis
in the liver.
Agonist:
Terbutaline
Antagonist:
Propranolol
Why does 1 stimulation cause contraction in cardiac muscle while 2
stimulation causes relaxation of smooth muscle – both elevate cAMP?
Different downstream effectors: different
responses
EPI, 1, cardiac muscle
EPI, 2, smooth muscle
protein kinase A
Phosphorylation of contractile
machinery proteins: e.g. Troponin I
CONTRACTION
Troponin I absent from smooth
muscle. PKA phosphorylation
of a different target, myosin light
chain kinase, inhibits myosin
function.
RELAXATION
Different downstream effectors: different
responses
SMOOTH MUSCLE
EPI, 2, Gs
RELAXATION
CONTRACTION
Adrenergic Receptors

Beta Receptors

 3 Receptors


Activate Gs protein, stimulates adenylate cyclase and
increases cAMP levels. cAMP activates PKA which
stimulates the lipase activity i.e. another context-specific
effector
Adipose tissue: Lipolysis.
Adrenergic Receptors

Beta Receptors: summary

 Receptors



1, 2 and 3 ALL activate Gs which stimulates adenylyl
cyclase and increases cAMP levels.
cAMP activates protein kinase A
Outcome depends on what PKA phosphorylates: e.g.
Troponin in cardiac muscle (contraction); MLCK in smooth
muscle (relaxation); lipase in adipose tissue
Adrenergic Receptors

Alpha Receptors:


1: Contraction of vascular and genitourinary smooth muscle.
2: Contraction of vascular smooth muscle but also indirect
effects that lead to vasodilation. Also decreased insulin
secretion, aggregation of platelets.
Adrenergic Receptors
1 and 2 receptors both signal through G-proteins, yet
can cause opposite effects on the same tissue (e.g.
genitourinary smooth muscle).
How?
1 and 2 signal through different G-proteins
 Subunits
olf
s
12
13
s Family
12 Family
t1
1
2
 Subunits
4
3
5
t2
gust
11
o1
1
o2
i1
i Family
Scone
10
i3
5
i2
12
z
7
15
2
16
4
11
q
14
q Family
3
Solf
 Subunits
Adrenergic Receptors

1 Receptors:







1 receptors coupled to Gq not Gs
Gq activates phospholipase C (PLC) causing production of
inositol triphosphate (IP3) and diacyglycerol (DAG) from
inositol phospholipids
Gq-linked receptor operated calcium channel
Overall effect is to increase intracellular calcium
Calcium-calmodulin stimulates myosin light chain kinase
activity and hence contraction of vascular and genitourinary
smooth muscle
Agonist:
Antagonist:
Phenylephrine
Prazosin
Epi, NE
Interstitial fluid
DAG
Contraction of
vascular and
genitourinary
smooth muscle
PLC
Gq

GTP GDP
PIP2

IP3

Ca2+
IP3
IP3R
Intracellular calcium pools
1
Different downstream 2nd messengers and
effectors: different responses
EPI, 1, Gq
e.g. vascular or genitourinary
smooth muscle
EPI, 2, Gs
RELAXATION
CONTRACTION
IMPORTANT…..direction of response depends
upon ligand concentration
e.g. in vascular smooth muscle
1
1
2
Gs, cAMP,
VASODILATION
2
1
2
Gq, Ca2+, overcomes
cAMP effects,
VASOCONSTRICTION
Adrenergic Receptors
2 adrenergic receptors on vascular smooth muscle cause
contraction
How?
Adrenergic Receptors

direct effect on vascular smooth muscle is
contraction mediated by extra-junctional 2
receptors:




NE or Epi stimulation of 2 engages Gi/o classes of G-protein
Gi/o inhibits adenylyl cyclase thus decreasing cAMP levels
Gi/o increases Ca2+ influx
Decrease in cAMP allows calcium-calmodulin stimulation of
MLCK activity, causing contraction
Gi and Gs have opposite effects on adenylyl
cyclase activity
Epi, 2 adrenergic
receptor
Direct effect of 2 on vascular smooth
muscle: contraction
Vascular smooth muscle
Epi, 2
Gi
CONTRACTION
Adrenergic Receptors
BUT 2 adrenergic receptors also can cause vasodilationon
How?
Indirect effect of 2 on vascular smooth
muscle: relaxation and vasodilation
Presynaptic Receptors
(Autoreceptors)
Adrenergic Receptors

Pre-synaptic 2 receptors: indirectly cause
vasodilation




stimulation of pre-synaptic 2 receptors by NE or EPI
inhibits release of NE at the synapse
NE concentration in the adrenergic synapse is reduced
decreasing stimulation of post-synaptic 1 receptors
Less post-synaptic 1/Gq activation, translates into less
calcium-calmodulin stimulation of MLCK
Relaxation
Cholinergic Receptors

Muscarinic Receptors:
M1: Depolarization of autonomic and CNS neurons
 M2: Negative inotropic and chronotropic effect on the heart.
 M3: Smooth muscle contraction with ONE EXCEPTION: cause
vascular smooth muscle relaxation and vasodilation;
Glandular secretion
Also M4 and M5.


Nicotinic Receptors:


NM (muscle): Depolarization of NMJ; Skeletal muscle contraction.
NN (neuronal): Activation of post ganglionic neurons.
Multiple acetylcholine-mediated effects:
how?

Different subtypes of cholinergic receptors in
different tissues.
Cholinergic Receptors

Muscarinic Receptors:



M1: Autonomic ganglia, CNS, some secretory glands. Cause
depolarization of autonomic and CNS neurons
M2: Heart, CNS. Cause negative inotropic and chronotropic effects
on the heart
M3: Smooth muscle; vascular endothelium and secretory glands.
Cause smooth muscle contraction; glandular secretion; BUT
also vasodilation
i.e. as for adrenergic responses, tissue response is governed by
expression of specific receptor subtypes
What accounts for the differences in
Acetylcholine-mediated effects?


Different subtypes of cholinergic receptors in
different tissues.
Different receptors are coupled to different Gproteins and hence different effectors.
Different muscarinic receptors couple to different Gproteins

Muscarinic Receptors: all G-protein linked



M1: Gq/11 Gastric secretion in parietal cells and depolarization
of autonomic and CNS neurons
M2: Gi
Negative inotropic and chronotropic effect on the heart.
M3: Gq/11 Stimulates smooth muscle contraction; sweat, bronchial
and salivary secretions; paradoxical vasodilation.
Cholinergic Receptors

Muscarinic Receptors

M1 Receptors


Gq/11: Activation of phospholipase C generates DAG and
IP3; IP3 increases intracellular calcium
i.e. M1 and 1 have similar signaling mechanism
Acetylcholine
Interstitial fluid
DAG
Neurons:
Depolarization of
autonomic and
CNS neurons
PKC
PLC
Gq

GTP GDP
PIP2

IP3

Ca2+
IP3
IP3R
Intracellular calcium pools
M1
Cholinergic Receptors

Muscarinic Receptors

M2 Receptors

Gi: inhibition of adenylyl cyclase and decreased cAMP

M2 and 2 have similar signaling mechanism

Reduced PKA phosphorylation of troponin I, negative inotropic
and chronotropic effect on the heart (i.e. antagonistic to 1)
Cardiac Muscle
Cholinergic Receptors

Muscarinic Receptors

M3 Receptors



Gq/11: Activation of PLC, hydrolysis of IP3 and increased
intracellular calcium, similar to M1.
Secretion (bronchial, sweat and salivary glands, gastric
acid); contraction of most smooth muscle
Paradoxical relaxation of vascular smooth muscle and
vasodilation: result of increased synthesis of NO and PGI2
in vascular endothelium
Indirect effect of M3 stimulation on vascular
smooth muscle: vasodilation
M3
Gq/11
Guanylyl
cyclase
COX – cyclo-oxygenase
PCS – prostacyclin synthase
Gs
Vasodilation
Simplistically: the endothelial cell converts a Gq response (increased Ca2+) to a
Gs response (increased cyclic AMP)
Cholinergic Receptors

Nicotinic Receptors: Cation Channels


NM: Depolarization of NMJ; Skeletal muscle contraction.
NN: Activation of post ganglionic neurons.
Cholinergic Receptors

Nicotinic Receptors: Cation Channel

NN Type



Autonomic ganglia: Activation of post
ganglionic neurons in autonomic ganglia.
Agonist: Nicotine
Antagonist: Trimethaphan
Ach
Ach
Binding
Ach
K+ Na+
Ach
K+ Na+
Open
NN nicotinic receptors: Heteropentamers of  and  subunits or Homopentamers
of  subunits.
Cholinergic Receptors

Nicotinic Receptors: Cation channel

NM Type




Neuromuscular Junction: Depolarization of
NMJ; Skeletal muscle contraction.
Heteropentamers of ,,and d subunits
Agonist: Acetylcholine
Antagonist: Tubocurarine
Nicotinic
Receptor (NM)