7-3. RECEPTOR SITE THEORY OF ADRENERGIC TRANSMISSION
Two types of receptor sites are theorized to explain adrenergic effects.
a. Alpha-Receptors. Alpha-receptors are associated mainly with increased
contractibility of vascular smooth muscle and intestinal relaxation.
(1) Alpha1. The alpha1 is located at postsynaptic effector sites to stimulate
transmitter release in smooth muscle. For example, the smooth muscle of peripheral
blood vessels is contracted in alpha1 stimulation.
(2) Alpha2. The alpha2 receptor site is located presynaptic on axon
terminals to inhibit the release of norepinephrine (the transmitter). The effects of alpha2
stimulation results in relaxation of the intestinal tract--motility and tone are decreased.
b. Beta-Receptors. Beta-receptors are associated with vasodilation and
relaxation of nonintestinal smooth muscle and cardiac stimulation.
Beta1. Stimulation of beta1 receptor sites results in cardiac stimulation
(2) Beta2. Stimulation of beta2 receptor sites causes bronchodilation,
relaxation of blood vessels (usually in skeletal muscles), and muscle glycogenolysis.
7-4. PHARMACOLOGICAL EFFECTS PRODUCED BY ADRENERGIC
a. Certain Types of Smooth Muscle. The adrenergic effect on certain types of
smooth muscle--especially the blood vessels of the skin, mucous membranes, and
salivary glands--is constriction. This is an alpha1 effect.
b. Other Types of Smooth Muscle. The adrenergic effect on other types of
smooth muscle varies according to the receptor site. The wall of the gut is relaxed
through inhibition--this is an alpha2 effect. The bronchial smooth muscle is dilated--this
is a beta2 effect. The blood vessels supplying skeletal muscle are dilated--this is a beta2
c. Cardiac Stimulation. Cardiac stimulation is a beta1 effect. Such stimulation
results in increased heart rate and increased force of contraction by the heart.
d. Metabolic Effects. Beta2 stimulation causes glycogenolysis in liver and
muscle tissue. Beta1 stimulation causes liberation of free fatty acids (lipolysis) from