Evidence against cholinergic mediation of the effect of angiotensin II on the guinea pig ileum

Abstract

The belief that the smooth muscle contracting activity of angiotensin II (angiotensin) in the guinea pig ileum is partly mediated by release of acetylcholine was reexamined, with the following results. Atropine did not reduce the maximum contraction produced by angiotensin, although it caused a shift to the right of the log dose-response curve (dose ratio =2.2). A similar shift was observed with histamine, bradykinin and BaCl₂. A moderate potentiation of angiotensin by eserine was also observed, which was similarly found for the other agonists. A previous report that atropine blocks the fast (phasic) component of the isometric response of the ileum to angiotensin was not confirmed. The disappearance of the phasic component was found to be due to a tachyphylactic change in the response. Depolarization by high doses of nicotine, and inhibition of acetylcholine synthesis by hemicholinium, did not affect the response to angiotensin. Ilei in which the intramural ganglia had been destroyed by incubation at 4° C for 48–56 h responded maximally to angiotensin, without loss of the phasic component of the response. It is concluded that the available evidence does not support the participation of a cholinergic mechanism in the effect of angiotensin upon the guinea pig ileum.