In vitro effects of 5-hydroxytryptamine and cisapride on the circular smooth muscle of the jejunum of horses

Objective-To determine effects of cisapride and 5-hydroxytryptamine (5-HT) on the jejunum of horses. Sample Population-Jejunal muscle strips from 8 horses. Procedure-Muscle str ips were suspended in isolated muscle baths. Isometric stress responses to 5-HT and cisapride, with and without specific antagonists, were determined. Results-Muscle strips incubated with atropine and tetrodotoxin responded to 5-HT and cisapride with an increase in contractile force. The 5-HT caused a concentration-dependent increase in contractile amplitude, with a maximum response (E-max) of 1,151 +/- 214 g/cm(2) and a molar concentration that i nduces contractile force equal to 50% of maximum response (EC50) of 0.028 /- 0.002 muM. Prior incubation with the 5-HT2 antagonist ketanserin decreas ed the E-max (626 +/- 147 g/cm(2)) and potency (EC50, 0.307 +/- 0.105 mum) of 5-HT. Prior incubation with the 5-HT3 antagonist tropisetron decreased t he efficacy (E-max, 894 +/- 184 g/cm2) to 5-HT. Cisapride also caused a con centration-dependent increase in contractile amplitude, with an E-max of 33 1 +/- 82 g/cm(2) and an EC50 of 0.302 +/- 0.122 muM. Prior incubation with ketanserin decreased the E-max (55 +/- 17 g/cm(2)) and potency (EC50, 0.520 +/- 0.274 muM) of cisapride. Conclusion and Clinical Relevance-Stimulatory effects of 5-HT and cisapride on circular smooth muscle of equine jejunum are mediated primarily through a noncholinergic effect. The effects of 5-HT are mediated, at least partia lly, by 5-HT2 and 5-HT3 receptors, whereas the effects of cisapride are med iated primarily by 5-HT2 receptors. This may impact treatment of horses wit h postoperative ileus. (Am J Vet Res 2000;61:1561-1565).