Region-specific antiproliferative effect of VIP and PACAP-(1-38) on rabbitenteric smooth muscle

The ability of neuropeptides to modulate enteric smooth muscle proliferatio n was examined in primary explant cultures of rabbit gastric antrum and col on smooth muscle. Cell proliferation was determined by [H-3]thymidine incor poration measurements and cell counting. Subcultured rabbit antrum and colo n myocytes (passages 2-6) preserved a smooth muscle phenotype, as verified by immunohistochemistry for a-smooth muscle act;in and electron microscopy. Both vasoactive intestinal polypeptide (VIP) and pituitary adenylate cycla se-activating peptide-(1-38) [PACAP-(1-38)] concentration dependently(10(-1 0) to 10(-6) M) inhibited the serum-induced [H-3]thymidine incorporation [i n colon, 48.2 +/- 5.8 and 55.6 +/- 9.3% of control with 10(-6) M VIP and 10 (-7) M. PACAP-(1-38)] and inhibited increase in cell numbers in cultures de rived from the colon but not in those from the antrum. Effects of VIP and P ACAP-(1-38) were mimicked by forskolin (10(-7) to 10(-6) M) but not by 8-br omo-cGMP, whereas theophylline enhanced the effects of VIP. Inhibition of n itric oxide synthase with N-G-nitro-L-arginine methyl ester (10(-3.5) M) di d not alter the effects of VIP. Substance P, motilin, calcitonin gene-relat ed peptide, and somatostatin had no effect. A single class of I-125-labeled VIP binding sites was found in antrum and colon myocyte cultures with an e qual affinity for VIP and PACAP-(1-38) [dissociation constant (K-d) in antr um = 3.4 +/- 0.8 nM for VIP and 2.0 +/- 1.0 nM for PACAP-(1-38); Kd in colo n = 2.0 +/- 1.0 nM for VIP and 2.8 +/- 1.6 nM for PACAP-(1-38)]. Density of binding sites in the antrum was higher than in the colon. In disease state s such as inflammatory bowel disease, inhibition of myocyte proliferation b y VIP and PACAP may serve to control smooth muscle hyperplasia in the colon but not in the antrum.