DIFFERENCES BETWEEN LONGITUDINAL AND CIRCULAR SMOOTH-MUSCLE IN BETA-ADRENERGIC CONTROL OF MOTILITY OF ISOLATED EQUINE ILEUM

Objective-To identify beta-adrenoceptor subtypes involved in motility inhibition of circular and longitudinal smooth muscle layers of equine ileum. Sample Population-Isolated strips of equine ileum circular smo oth muscle and membrane preparations from circular and longitudinal mu scle layers. Procedure-Functional assays of circular muscle preparatio ns and radioligand binding assays and measurements of cAMP production in smooth muscle membranes from circular and longitudinal layers. Resu lts-Selective beta-adrenergic agonists exerted inhibitory effects on c ircular muscle preparations. Binding studies of cell membranes indicat ed that the density and distribution of 3 beta-adrenoceptor subtypes d id not differ between longitudinal and circular muscle layers. Measure ment of cAMP production in membrane preparations of longitudinal and c ircular muscle after selective beta-stimulation confirmed presence of the 3 adenylate cyclase-coupled beta-adrenoceptor subtypes; however, p reparations from the 2 layers had differing cAMP production efficacy. Conclusions-The data may partly explain the differing functional respo nses between circular and longitudinal muscle preparations. Clinical R elevance-Findings support the important role of beta-atypical adrenoce ptors in the inhibitory regulation of equine ileum motility.