TABLE 32-3
The Catecholamine Receptors
Receptor
Relative
Subtype-Specific
Subtype-Specific
Physiologic
Effect
Subtype1
Affinity2
Agonist
Antagonist
Post-receptor Events
Tissue
Ligand3
D1
DA»E=NE
Metoclopramide
Gs: AC stimulation
Vascular smooth muscle (kidney,
DA
Relaxation (low levels)
Increased cAMP
coronary, mesenteric)
Inhibition of prolactin release
D2
DA»E= NE
Bromocriptine
Gi: AC inhibition
Lactotrope (anterior pituitary)
DA
Decreased cAMP
a l
4
E>NE»I
Methoxamine
Phentolamine
Gp:PLC:IP3:DAG
Vascular smooth muscle (skin)
NE
Constriction
Phenylephrine
Phenoxybenzamine
Ca
+2
-Protein Kinase
Vascular smooth muscle (arterioles)
NE
Constriction
Vascular smooth muscle (abdominal
NE
Constriction
viscera except liver)
Intestinal smooth muscle
NE
Relaxation
Intestinal sphincters
NE
Contraction
Urinary smooth muscle
NE
Increased ureter tone, contraction of
(ureter, sphincter of bladder)
sphincter
Iris (radial muscle)
NE
Contraction (mydriasis (pupil dilation))
Skin (piloerector muscle)
NE
Contraction (pilorection)
Skin (sweat gland) (palm of hands,
NE
Secretion
a 2 6
sole of feet
)5
Hypothalamus
NE
Stimulation of ADH release
E> N E » I
Clonidine
Yohimbine
Gi:AC inhibition
ß
cell Islet of Langerhans
NE, E
Inhibition of insulin secretion
Decreased cAMP
Vascular smooth muscle (skin)
E
Constriction (vasoconstriction)
Vascular smooth muscle (veins)
NE, E
Constriction
Hypothalamus
NE
Decreased SRIH and/or increased
GRH release (results in increased
GH release from pituitary)
ß C
I>E= NE
Dobutamine
Metoprolol
Gs: AC stimulation
Heart (SA node, AV node,
NE, E
Increased firing rate; increased
Atenolol
Increased cAMP
His-Purkinjie system, ventricles)
conduction velocity; increased
contractility, increased heart rate,
increased cardiac output
Juxtaglomerular appartus of kidney
NE
Stimulation of renin release
Adipose tissue (white)
E
Lipolysis
ß 2
I>E»NE
Terbutaline
Butoxamine
Gs: AC stimulation
Vascular smooth muscle (skeletal
E
Relaxation (vasodilation)
Salbutamol
Soterenol
Increased cAMP
muscle)
Vascular smooth muscle (coronary)
E
Relaxation (vasodilation)
Bronchiolar smooth muscle
E
Relaxation (bronchodilation)
Skeletal muscle
E
Glycogenolysis; Increased contractility?
a cells Islets of Langerhans
Intestinal smooth muscle
NE, E
Stimulation of glucagon release
Genitourinary smooth muscle
Liver
E
Glycogenolysis; Gluconeogenesis
ß 3
1= NE>E, Low
Fenoterol
none
8
Gs: AC stimulation
White adipose tissue
NE, E
Lipolysis
affinity receptor
Increased cAMP
(visceral > subcutaneous)
Brown adipose tissue
NE(andE) Thermogenesis
'cd and a 2 each have 3 subtypes, information about which can be found in: Bylund (1992) Subtypes of al- anda2-adrenergic receptors. FASEB J 6:832-839.
2DA = dopamine; E = epinephrine; NE = norepinephrine; I = isoproterenol (a non-selective /3-adrenergic agonist; stimulates all 3 subtypes of /3-adrenoceptors).
3NE = norepinephrine as neurotransmitter released from sympathetic nerve endings at synaptic junctions, acting upon postjunctional adrenergic receptors; E = plasma-bome epinephrine that bind to extrajunctional adrenergic receptors that are not
associated with nerve terminals.
4Localized entirely postsynaptically
5Most of the sweat glands are innervated by
c h o lin e r g ic
sympathetic fibers that release AChol at synaptic junctions and respond to sympathetic stimulation with an increase in secretion (increased sweating).The exceptions are the sweat glands in
the palm of the hands and sole of the feet, which are innervated by adrenergic fibers that release NE at synaptic junctions. Emotional excitement activates these adrenergic nerves, and causes sweating in the hands and feet (“adrenergic sweating”).
Emotional excitement usually has no effect on sweating in other regions because the cholinergic sympathetic nerves are mainly controlled by the thermoregulatory center of the hypothalamus, and are not affected by behavioral reactiveness. Note,
however, that all sweat glands haveal adrenoceptors, and are therefore capable of responding to plasma epinephrine (and NE?) with an increase in sweat secretion. This would result in a more generalized sweating.
6Located both pre- and postsynaptically.
Primarily involved in neuronal functions.
8Metoprolol is a non-selective antagonist.
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