Characterization of alpha-adrenoceptor subtypes in smooth muscle of equineileum

Objective-To determine the concentration and binding characteristics of alp ha -adrenoceptor subtypes in smooth muscle cell membranes of equine ileum. Sample Population-Segments of longitudinal and circular smooth muscle from the ileum of 8 male and 8 female adult horses. Procedure-Distribution of alpha -adrenoceptor subtypes was assessed by use of radioligand binding assays incorporating [H-3]-prazosin and [H-3]-rauwol scine, highly selective alpha (1)- and alpha (2)-adrenoceptor antagonists, respectively. Characterization of adrenoceptor subtypes was performed by us e of binding inhibition assays. Results-On the basis of binding affinity for specific radioligands, low- an d high-affinity alpha (1)- and alpha (2)-adrenoceptors were detected. Conce ntration of low-affinity alpha (2)-adrenoceptors was significantly greater in male horses, compared with females. Competition studies confirmed the sp ecificity of the radioligands used in the binding assays. alpha (1)-Adrenoc eptors of both subtypes in male and female horses had a higher affinity for prazosin than phentolamine, whereas yohimbine did not compete with the rad ioligand for binding. For alpha (2)-adrenoceptors regardless of subtype, po tency of inhibition elicited by each drug varied between sexes. In males, y ohimbine was a more potent inhibitor than phentolamine, which was more pote nt than prazosin, In females, yohimbine was more potent than prazosin, whic h was more potent than phentolamine. Conclusions and Clinical Relevance-High- and low-affinity alpha (1)- and al pha (2)-adrenoceptors were detected in smooth muscle of equine ileum. Becau se alpha -adrenoceptor subtypes, particularly alpha (2)-adrenoceptors, are involved in the regulation of gastrointestinal tract function, characteriza tion of these receptors may represent the basis for development of new ther apeutic strategies for the control of gastrointestinal disturbances in hors es.