Excitatory and inhibitory actions of pituitary adenylate cyclase-activating peptide (PACAP) in the internal anal sphincter smooth muscle: sites of actions.

Unlike its effects on the rest of the GI tract, the effects of pituitary adenylate cyclase-activating peptide (PACAP) on the internal anal sphincter (IAS) are not known. We examined the actions of PACAP-38 (here PACAP) and PACAP-27 on the basal IAS tone of circular smooth muscle strips before and after the administration of different neurohumoral antagonists. PACAP caused a concentration-dependent fall in the basal tone of the IAS. Interestingly, however, at higher concentrations, PACAP caused a biphasic response: an initial contraction followed by a relaxation. Both the contractile and the relaxant responses were insensitive to atropine, guanethidine, apamin or tetrodotoxin. Both the contractile and the relaxant effects were inhibited by PACAP 6-38 (a selective antagonist of PACAP), vasoactive intestinal polypeptide 10-28 (a vasoactive intestinal polypeptide antagonist) and PACAP tachyphylaxis. The nitric oxide synthase inhibitor Nomega-nitro-L-arginine attenuated the inhibitory but not the excitatory effect of PACAP. Conversely, the contractile but not the relaxant effect of PACAP on the IAS was nearly obliterated by the substance P antagonist spantide. The N-type Ca++-channel blocker omega-conotoxin caused significant suppression of both the contractile and the inhibitory actions of PACAP. We conclude that in the IAS, PACAP has a dual effect: a contraction followed by a relaxation. The contraction of IAS by PACAP is speculated to occur via the activation of PACAP receptor at the substance P-containing nerve terminals. PACAP-induced IAS relaxation, on the other hand, appears to be mediated in large part by its direct action at the smooth muscle cells and in part by its action at the nerve terminals of the myenteric inhibitory neurons.