Catecholamines such as epinephrine and norepinephrine regulate physiological processes via their interactions with a variety of cellular receptors. Ahlquist (5) originally classified these receptors as cy-and, 6-adrenergic receptors on the basis of the relative potency of various agonists in stimulating responses through these receptors. Later it became clear that at least two major subtypes of both p-and a-adrenergic receptors-the &-and &-(138) and cyl-and cy2-(24, 139) adrenergic receptors-can be distinguished by a variety of pharmacological criteria. These receptor subtypes are discussed in some detail below.

Both PI-and p2-adrenergic receptors stimulate the membrane-bound enzyme adenylate cyclase; this action leads to the intracellular accumulation of adenosine 3’: 5’-cyclic phosphate [cyclic AMP (CAMP)], the second messenger of@-adrenergic action in virtually all tissues examined to date (252). In some systems cY,-adrenergic receptors inhibit adenylate cyclase (NO), whereas cyladrenergic receptors are not clearly linked to the adenylate cyclase system.