Physiological and pharmacological aspects of adrenergic receptor classification

Abstract

The questions raised are: what is the physiological or pharmacological basis for the differentiation into β 1- and β 2-, and α 1- and α 2-adrenergic receptors?; and do the neurotransmitter norepinephrine and the hormone epinephrine differ in their receptors? On the basis of a preference of β 2- and α 2-adrenergic receptors for epinephrine, the hormone, and of β 1- and α 1-receptors for norepinephrine, the neurotransmitter, it was postulated that the α 2- and β 2-receptors are predominantly epinephrinergic in nature and located extrajunctionally and presynaptically whereas the α 1- and β 1-receptors are predominantly norepinephrinergic in nature and located postsynaptically in the sympathetic terminal junction. The α 2- and β 2-character of the presynaptic receptors matches that of the corresponding extrajunctional receptors. This indicates that a circulating catecholamine, namely epinephrine, is involved in the regulation of adrenergic transmitter release.