The effects of adrenaline, noradrenaline and isoprenaline on inhibitory α‐ and β‐adrenoceptors in the longitudinal muscle of the guinea‐pig ileum

Abstract

Summary: Two preparations, a segment of the ileum and the myenteric plexus‐longitudinal muscle preparation, have been used for an analysis of the inhibitory effects of adrenaline, noradrenaline and isoprenaline on the contractor responses of the longitudinal muscle to acetylcholine or to electrical, coaxial or field, stimulation. Since the inhibitory effects of adrenaline, noradrenaline and isoprenaline on the acetylcholine‐induced contractions were not affected by phenoxybenzamine but were antagonized by propranolol, it is concluded that β‐adrenoceptors are present on the muscle cells. The responses to electrical stimulation were suppressed by adrenaline or noradrenaline but only partly inhibited by isoprenaline. Propranolol antagonized the effect of isoprenaline and, to some extent, that of noradrenaline, but scarcely affected the action of adrenaline. Phenoxybenzamine, on the other hand, antagonized most of the effect of adrenaline and, to some extent, that of noradrenaline; it usually potentiated the effect of isoprenaline. The output of acetylcholine evoked by electrical stimulation was diminished by adrenaline or noradrenaline but was not affected by isoprenaline. The depressant effect on acetylcholine release was antagonized by phenoxybenzamine but not affected by propranolol; therefore these effects of adrenaline and noradrenaline are mediated by α‐adrenoceptors. It may be assumed that α‐adrenoceptors in situ are stimulated mainly by circulating adrenaline and possibly noradrenaline and thus cause a prejunctional inhibition at the nerve‐smooth muscle junction.