Catecholamines act at α2-adrenoceptors to cause contraction of circular smooth muscle of guinea-pig stomach

Abstract

Circular smooth muscle strips taken from the body region of the guinea-pig stomach responded to dopamine and noradrenaline with contraction at lower concentrations followed by relaxation at higher concentrations. A (β-adrenoceptor-mediated relaxation response was excluded by propranolol treatment and this allowed the remaining α-adrenoceptor involvement with relaxation and contraction to be incisively differentiated in terms of two distinct α-adrenoceptor mechanisms. Thus, the relaxation responses to the catecholamines were mimicked by phenylephrine and antagonized by prazosin, phentolamine but not by yohimbine or rauwolscine. In contrast, the catecholamine-induced contractions were mimicked by clonidine and antagonized by yohimbine and phentolamine but not by prazosin. It is therefore concluded that the α mechanisms via which dopamine and noradrenaline are able to relax and contract the circular smooth muscle from the body region of guinea-pig stomach are of the α1- and α2-type respectively.