Expression of functional receptors for vasoactive intestinal peptide in freshly isolated and cultured gastric muscle cells.

Vasoactive intestinal peptide (VIP) receptors were characterized in freshly isolated and cultured smooth muscle cells from guinea pig stomach by radioligand binding and by measurement of relaxation in single isolated and cultured cells. 125I-VIP bound to both freshly isolated and cultured muscle cells: binding was rapid, specific, saturable and temperature-dependent, and was inhibited in a concentration-dependent fashion by VIP, VIP10-28, PHI and secretin, in this order. Competition curves for VIP could be resolved into high- and low-affinity components, yielding similar binding constants in freshly isolated and cultured cells (high-affinity Kd 0.11 and 0.22 nM; low-affinity Kd 59 and 37 nM; high-affinity binding sites: 1183 and 1021 per cell, representing about 1% of total binding sites). VIP10-28 inhibited 125I-VIP binding completely and acted as potent competitive antagonist of VIP-induced relaxation (Ki 0.5 nM). PHI and secretin, however, inhibited partly 125I-VIP binding: the pattern of inhibition implied that VIP interacts with VIP-preferring receptors that are recognized by PHI and secretin as well as with VIP-specific receptors. The pattern of binding is consistent with recent evidence indicating that VIP activates two signalling pathways, a VIP-specific, nitric oxide/cGMP-dependent pathway and a common cAMP-dependent pathway shared by all three peptides. PHI and secretin were relatively more potent as relaxant agents than as inhibitors of 125I-VIP binding raising the possibility that PHI and secretin could interact additionally with PHI- and secretin-preferring receptors in mediating relaxation.