Involvement of pituitary adenylate cyclase-activating peptide in opossum internal anal sphincter relaxation

Despite its widespread distribution and significance in the gut, the role of pituitary adenylate cyclase-activating peptide (PACAP) in internal anal sphincter (IAS) relaxation has not been examined. This study examined the role of PACAP in nonadrenergic noncholinergic (NANC) nerve-mediated relaxation of IAS smooth muscle. Circular smooth muscle strips from the opossum IAS were prepared for measurement of isometric tension. The influence of PACAP and vasoactive intestinal peptide (VIP) antagonists and tachyphylaxis on the neurally mediated IAS relaxation was examined either separately or in combination. The release of these neuropeptides in response to NANC nerve stimulation before and after the nitric oxide (NO) synthase inhibitor Nomega-nitro-L-arginine and NO was also investigated. Both PACAP and VIP antagonists caused significant attenuation of IAS relaxation by NANC nerve stimulation. The combination of the antagonists, however, did not have an additive effect on IAS relaxation. VIP tachyphylaxis caused significant suppression of IAS relaxation by NANC nerve stimulation. PACAP and VIP were found to be released by NANC nerve stimulation and exogenous NO. The data suggest the involvement of PACAP in IAS relaxation primarily by the activation of PACAP1/VIP receptor and lack of its independent role in the relaxation. Furthermore, NO may regulate the presynaptic release of PACAP and VIP.