β₃‐adrenoceptors and intestinal motility

Early substantial evidence of the low susceptibility to beta-adrenoceptor antagonists of non alpha-adrenergic responses reducing gut motility and tone was reluctantly accepted as indicating a third beta-receptor subtype different from the beta 1 and beta 2. This applied likewise to lipolysis until new selective "lipolytic" beta-agonists poorly effective at established beta-receptors were introduced. Shortly afterwards these "lipolytic" as well as certain newer and even more selective beta-adrenoceptor agonists were shown to be potent inhibitors of intestinal motility. The latter are the "gut-specific" phenylethanolaminotetralins whose availability as pure isomers attested to the stringent stereochemical requirements for selectivity at non-beta 1, non-beta 2 beta-adrenoceptors. Acceptance of the functionally based concept of a beta 3-adrenoceptor was boosted on structural grounds by molecular biology studies. Sequence analysis indicated the existence in humans and rodents of genes coding for a third subtype of beta-receptor that, when expressed in transfected heterologous cells, had a pharmacological profile distinct from the previously established subtypes. Finally, aryloxypropanolaminotetralins have been prepared as the first selective antagonists of beta 3-adrenoceptors, thus providing unambiguous conclusive evidence of the distinctive functional features of those abundant in the rat colon. The therapeutic potential in gastroenterology of the newer compounds targetable on the beta 3-adrenoceptor is suggested by their potent intestinal action in vivo in animal models without any of the cardiovascular or other unwanted effects of conventional beta-adrenoceptor agonists and antagonists, and by the clinically confirmed importance of beta-adrenergic control of motor function throughout the alimentary canal. However, open questions include the incidence of species-related differences in beta 3-adrenoceptors, and as yet there are no data on gastrointestinal functions in humans under the influence of drugs designed to act selectively at these receptors.