REGULATION OF THE DESCENDING RELAXATION PHASE OF INTESTINAL PERISTALSIS BY PACAP

The presence of pituitary adenylate cyclase-activating peptide (PACAP), a homologue of vasoactive intestinal peptide (VIP), in enteric neurons suggests that it may be involved in the regulation of the descending relaxation phase of the peristaltic reflex. The role of PACAP was evaluated by measurement of PACAP release and by immuno-neutralization with specific monoclonal antibodies to PACAP-27 and PACAP-38, and an antibody to VIP. Electrical field stimulation at 4 Hz caused a 12-fold increase in PACAP release that was inhibited by 53 +/- 6% (P < 0.01) by the nitric oxide synthase inhibitor, N-G-nitro-L-arginine (L-NNA). Orad stretch of colonic segments elicited descending relaxation and PACAP release in proportion to the degree of stretch. PACAP release induced by 10-g stretch was inhibited by 67 +/- 10% (P < 0.01) by L-NNA. Previous studies (Am. J. Physiol., 264 (1993) G334-G340) showed that orad stretch elicits also VIP release and nitric oxide (NO) production and that VIP release is inhibited (59%) by L-NNA. Preincubation of the segments with PACAP-27 plus PACAP-38 antibodies (50 mu g/ml each), or with VIP antibody (1:60) inhibited descending relaxation at all degrees of stretch (inhibition with PACAP antibodies: 90 +/- 8% with 2-g and 22 +/- 5% with 10-g stretch). Preincubation with both PACAP and VIP antibodies virtually abolished descending relaxation. A similar pattern was observed with the antagonists, PACAP6-38 and VIP10-28, alone and in combination. Studies in dispersed colonic muscle cells showed that (i) VIP- and PACAP-induced relaxation was inhibited by each antagonist and by L-NNA, and (ii) selective receptor protection with each peptide preserved the response to all three peptides, implying interaction of PACAP and VIP with a common receptor coupled to NO generation in muscle cells. We conclude that PACAP and VIP are the main determinants of descending relaxation and that their effect reflects interplay with NO in neurons and muscle cells.