Excitatory and Inhibitory Actions of Pituitary Adenylate Cyclase-Activating Peptide ( PACAP ) in the Internal Anal Sphincter Smooth Muscle : Sites of Actions 1

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 Nv-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 v-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. The role of VIP as an inhibitory neurotransmitter in the internal anal sphincter is well known (Biancani et al., 1985; Nurko and Rattan, 1988). Recently, another neuropeptide PACAP closely related to VIP has been identified in the central as well as the peripheral nervous systems (Miyata et al., 1990; Suda et al., 1992; Shen et al., 1992; Uddman et al., 1991; Ny et al., 1995; Portbury et al., 1995; Rawlings, 1994). PACAP is a 38-amino acid peptide with amidation at the C-terminus (PACAP-38). Endogenously, PACAP may also occur in the form of PACAP-27, a 27-amino acid form (Miyata et al., 1990). PACAP has been shown to be distributed in the myenteric neurons throughout the GI tract from the esophagus (Uddman et al., 1991; Ny et al., 1995) to the small and large intestines in animals (Portbury et al., 1995) as well as humans (Shen et al., 1992). Furthermore, PACAP has been shown to be released from the myenteric neurons of the GI tract (Grider et al., 1994; Katsoulis et al., 1996). Like VIP, PACAP has been shown to be a potent and direct relaxant of different smooth muscles of the gut (Katsoulis et al., 1993a; Grider et al., 1994; Ny et al., 1995; Katsoulis et al., 1996). It has also been suggested that PACAP plays a significant role in the inhibitory neurotransmission of the gut (Grider et al., 1994; Jin et al., 1994; Katsoulis et al., 1996). Moreover, the inhibitory actions of PACAP and VIP may share a common