Two types of receptor sites are theorized to explain adrenergic effects.
- Alpha-receptors are associated mainly with increased contractibility of vascular smooth muscle and intestinal relaxation.
- 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.
- The alpha2 receptor site is located presynaptic on axon terminals to inhibit the release of nor-epinephrine (the transmitter). The effects of alpha2 stimulation results in relaxation of the intestinal tract--motility and tone are decreased.
- 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 and lipolysis.
- Beta2 Stimulation of beta2 receptor sites causes bronchodilation, relaxation of blood vessels (usually in skeletal muscles), and muscle glycogenolysis.
Alpha Receptor Site: Important features of the site include in order of importance:
Beta Receptor Site: Important features of the site include in order of importance - also see the graphic on the left: