The receptors for vasoactive intestinal peptide (VIP) present in dispersed acini and membranes from rat and guinea-pig pancreas differed in selectivity pattern, i.e. in the displacement of [125I]iodo-VIP by parent peptides, as revealed by a VIP:secretin IC50 ratio at least ten times higher in rat than in guinea-pig preparations. The molecular properties of these VIP receptors were therefore investigated. 2. When comparing six succinimidyl ester cross-linkers, bis[2-(succinimidooxycarbonyloxy)ethyl]sulfone proved to be the most universal [125I]iodo-VIP cross-linker for all pancreatic preparations. 3. In intact rat acini the main labeled peptide had an Mr of 80,000, whereas the main labeled peptide in intact guinea-pig acini was a smear of Mr 160,000. In both rat and guinea-pig pancreatic membranes, the main labeled peptide ([125I]iodo-VIP-binding-protein complex) had an Mr of 66,000. In addition, variable proportions of an Mr-80,000 peptide and an Mr-83,000 peptide were visualized in, respectively, rat and guinea-pig membranes. The labeling of all peptides was suppressed by VIP and by GTP. Reducing conditions allowed only a better resolution, making the presence of intermolecular disulfide bridges unlikely. 4. Taking into account the Mr of VIP it is thus plausible that the main native Mr-77,000 VIP-binding site present in rat acini could be easily converted to an Mr-63,000 peptide during membrane preparation, while in guinea-pig acini Mr-80,000 and/or Mr-63,000 VIP-binding sites were often closely associated with another membrane component in the native state. These molecular differences between VIP receptors in intact rat and guinea-pig acini are in keeping with functional differences.
