Inhibitory effects of SR 58611A on canine colonic motility: evidence for a role of β 3 ‐adrenoceptors

1 In order to clarify whether atypical or β‐adrenoceptors can modulate canine colonic motility in vivo , we studied the effects of SR 58611A (a selective agonist for atypical β‐adrenoceptors) alone and after pretreatment with β‐adrenoceptor antagonists on colonic motility in the conscious dog. The gastrocolonic response (postprandial increase in motility) was monitored by means of electrodes and strain‐gauge force transducers chronically implanted along the distal colon. In some experiments, heart rate was also measured. The possible role of β 3 ‐adrenoceptors in mediating the effects of SR 58611A was also tested in vitro in circular muscle strips taken from the canine distal colon. 2 Intravenous infusion of SR 58611 A, ritodrine or isoprenaline at doses inducing the same degree of tachycardia inhibited the gastrocolonic response to a different extent, with SR 58611A and ritodrine being more effective than isoprenaline. 3 In a dose‐response study, SR 58611A was more potent in inhibiting colonic motility than in inducing tachycardia: the ED 35 values for inhibition of colonic motility and induction of tachycardia were 23 and 156 μg kg −1 , i.v., respectively. 4 The inhibitory effect of SR 58611A 100 μg kg −1 , i.v., on the gastrocolonic response was reversed by alprenolol (non‐selective β‐adrenoceptor antagonist), but resistant to CGP 20712A (β 1 ‐adrenoceptor antagonist) or ICI 118551 (β 2 ‐adrenoceptor antagonist). 5 In vitro , SR 58611A concentration‐dependently relaxed circular muscle strips, an effect that was competitively antagonized by alprenolol with a pA 2 value of 7.1, but resistant to CGP 20712A (100 nM), ICI 118551 (100 nM) or tetrodotoxin (1 μ m ). 6 The present study provides strong functional evidence for a role of atypical or β 3 ‐adrenoceptors in the modulation of canine colonic motility both in vivo and in vitro by an inhibitory effect most likely at the smooth muscle level.