Alpha- and beta-adrenergic receptor subtypes properties, distribution and regulation

Abstract

The effects of catecholamines in the central and peripheral nervous systems appear to be mediated through interactions with 2 major classes of receptor: alpha-adrenoceptors and beta-adrenoceptors. Subtypes of both alpha- and beta-adrenoceptors exist. In the periphery, alpha 1-receptors are located postsynaptically, mediating the excitatory effects of catecholamines at alpha-receptors. alpha 2-Adrenoceptors, on the other hand, are autoreceptors involved in the regulation of noradrenaline (norepinephrine) release. In the central nervous system, both alpha 1- and alpha 2-receptors exist on postsynaptic cells; there are also 2 principal subtypes of beta-adrenoceptors. beta 1-Receptors have a high affinity for both noradrenaline and adrenaline (epinephrine) and are found in the heart, brain, and adipose tissue. beta 2-Receptors have a low affinity for noradrenaline and are involved in mediation of relaxation of vascular and other smooth muscles and in many of the metabolic effects of catecholamines. A variety of effector systems have been implicated in the actions of catecholamines. Most, though not all, of the effects of catecholamines at beta-receptors are mediated through activation of adenyl cyclase and increases in cyclic AMP accumulation. The effects of catecholamines at alpha-receptors generally involve other second messenger systems. Thus, in at least some systems, stimulation of alpha 1-adrenoceptors mediates increases in phosphoinositide breakdown, while alpha 2-adrenoceptors appear to act through inhibition of adenyl cyclase activity. The pharmacological effects of alpha- and beta-adrenoceptors were initially characterised by measuring responses observed in intact preparations. The advent of the use of radioligand binding techniques has allowed direct approaches to the characterisation of receptor properties. The use of radioligands makes it possible to determine the affinities of receptors for specific ligands, and it is possible to determine the density of receptors in a tissue. Finally, in vitro assays serve as a means through which receptors can be followed during solubilisation, isolation, and reconstitution. Several ligands are now available for the study of alpha- and beta-adrenoceptors. In general, relatively selective radioligands are available for the study of alpha-receptors. Thus, 3H-WB 4101 and 3H-prazosin are selective ligands for alpha 1-receptors; the ligand 125I-IBE 2254 also shows high selectivity for alpha 1-receptors. 3H-Yohimbine and 3H-rauwolscine are selective antagonists for the labelling of alpha 2-receptors and 3H-clonidine is a selective agonist used for studying alpha 2-receptors.(ABSTRACT TRUNCATED AT 400 WORDS)