Differences between longitudinal and circular smooth muscle in β-adrenergic control of motility of isolated equine ileum

Abstract

SUMMARY Objective To identify β-adrenoceptor subtypes involved in motility inhibition of circular and longitudinal smooth muscle layers of equine ileum. Sample Population Isolated strips of equine ileum circular smooth muscle and membrane preparations from circular and longitudinal muscle layers. Procedure Functional assays of circular muscle preparations and radioligand binding assays and measurements of cAMP production in smooth muscle membranes from circular and longitudinal layers. Results Selective β-adrenergic agonists exerted inhibitory effects on circular muscle preparations. Binding studies of cell membranes indicated that the density and distribution of 3 β-adrenoceptor subtypes did not differ between longitudinal and circular muscle layers. Measurement of cAMP production in membrane preparations of longitudinal and circular muscle after selective β-stimulation confirmed presence of the 3 adenylate cyclase-coupled β-adrenoceptor subtypes; however, preparations from the 2 layers had differing cAMP production efficacy. Conclusions The data may partly explain the differing functional responses between circular and longitudinal muscle preparations. Clinical Relevance Findings support the important role of β-atypical adrenoceptors in the inhibitory regulation of equine ileum motility. (Am J Vet Res 1997;58:1422–1426)