Beta-adrenoceptors regulate myoelectric activity in the small intestine of rats: stimulation by beta 2 and inhibition by beta 3 subtypes.

Using beta-adrenergic agonists and antagonists this study investigated the importance of three different adrenoceptor subtypes for the regulation of migrating myoelectric complexes (MMCs) of the upper small intestine in conscious, naive rats. After a control period of 60 min with four activity fronts, agonists were given as an intravenous infusion for another 60 min. The non-selective beta-adrenoceptor agonist isoprenaline (1 microgram kg-1 min-1) inhibited MMCs and induced irregular spiking during the infusion period. This effect was blocked by intravenous administration of a bolus dose of either the non-selective beta-adrenoceptor antagonist propranolol (1 mg kg-1), or the beta 2-antagonist ICI 118 551 (1 mg kg-1), both given prior to isoprenaline. However, acebutolol (1 mg kg-1), a selective beta 1-antagonist, failed to antagonize the effect of isoprenaline. Furthermore, prenalterol, a selective beta 1-agonist (12.5-800.0 micrograms kg-1 min-1), had no effect on the MMC pattern, whereas the beta 2-selective agonist ritodrine (25-100 micrograms kg-1 min-1) induced a myoelectric pattern similar to one induced by isoprenaline. The partial beta 3-adrenoceptor agonist D7114 (50-100 micrograms kg-1 min-1), disrupted the MMCs and induced quiescence. Neither of the antagonists, i.e. propranolol (1 mg kg-1), acebutolol (1 mg kg-1) nor ICI 118 551 (1 mg kg-1), given alone induced changes in the MMC pattern. In conclusion, beta 2-adrenoceptors in particular but also beta 3-adrenoceptors seem to be of importance in the regulation of small intestinal motility by disrupting the regular MMC pattern in rats.