Distinct receptors mediate pituitary adenylate cyclase-activating peptide- and vasoactive intestinal peptide-induced relaxation of rat ileal longitudinal muscle

Relaxant responses to pituitary adenylate cyclase-activating peptide (PACAP)-27, PACAP-38 and vasoactive intestinal peptide (VIP) were examined in rat ileal longitudinal muscle. PACAP-27 was much more potent than PACAP-38 and VIP, with PACAP-38 and VIP being equipotent. The relaxation induced by each of the peptides was unaffected by pretreatment with NG-nitro-L-arginine methyl ester (L-NAME) (10(-4) M), tetrodotoxin (10(-6) M) or atropine (10(-6) M). Pretreatment with apamin (10(-6) M) abolished the relaxations induced by PACAP-27, but not those induced by PACAP-38 or VIP. Pretreatment with neuropeptide Y (NPY) (10(-7) M) inhibited relaxations induced by VIP, but not those induced by PACAP-27 or PACAP-38. No cross-desensitization between PACAP-27 and VIP could be revealed. In conclusion, distinct receptors mediate PACAP- and VIP-induced relaxations of rat ileal longitudinal muscle. At least three different types of receptors may exist: (1) a PACAP-27 preferring receptor coupled to apamin sensitive Ca2+-dependent K+ channels, (2) a PACAP specific receptor activated by both PACAP-27 and PACAP-38 but not by VIP and (3) a VIP specific receptor regulated by NPY by yet unknown mechanisms. (C) 1997 Elsevier Science B.V.