Characterization of the Beta-adrenergic Inhibition of Motility in Cat Colon Strips

The inhibition of spontaneous contractile activity in isolated cat colon strips by isoprenaline, prenalterol and terbutaline was studied. Isoprenaline produced a concentration-dependent complete inhibition with an EC50 of 6.18 nM. Prenalterol and terbutaline produced a concentration-dependent partial inhibition of 68 and 75% respectively, with EC50 of 0.52 and 0.43 microM respectively. The inhibitory effects of the three agonists were blocked by metoprolol and IPS 339 in such a way as to suggest that the prenalterol effect was mediated by beta 1-adrenoceptors and the terbutaline effect was mediated by beta 2-adrenoceptors, whereas the effect of isoprenaline appeared to be mediated by both types of receptors. Thus, the results indicate that both beta 1- and beta 2-adrenoceptors are involved in the adrenergic inhibition of spontaneous contractile activity in the cat colon strip. Qualitative effects of prenalterol and terbutaline on the spontaneous contractile activity in the cat colon strip indicated that the beta 1-adrenoceptor-mediated effects may be related to inhibition of the activity in cholinergic neurons, whereas the beta 2-adrenoceptors may be acting on the smooth muscle cells per se.