Characteristics of receptors for VIP in rat peritoneal macrophage membranes

Abstract

Vasoactive intestinal peptide (VIP) receptors were investigated in rat peritoneal macrophage membranes (RPMM) using [ 125I]VIP as ligand. The receptor binding was rapid, reversible, saturable, specific, and dependent on time, temperature, and membrane concentration. The Scatchard analysis of binding data was consistent with the existence of two classes of VIP binding sites with K d values of 0.60 ± 0.08 and 275 ± 39 n M and binding capacities of 580 ± 71 and 72,500 ± 810 fmol VIP/mg protein, respectively. The interaction showed a high degree of specificity, as suggested by competitive displacement experiments with several peptides structurally or not structurally related to VIP. These pharmacological studies showed the following order of potency: VIP (IC 50 = 1 n M) > rGRF (IC 50 = 13 rmn M) > PHI (IC 50 = 421 n M) ⪢ secretin. Glucagon, somatostatin, insulin, octapeptide of cholecystokinin [CCK(26–33)], and pancreastatin were ineffective at concentrations up to 1 μ M. Binding of [ 125I]VIP to membranes is markedly reduced by increasing the ionic strength of incubation medium. Treatment of membranes with dithiothreitol, trypsin, and phospholipases A 2 and C resulted in a loss of the ability of these membranes to bind VIP. However, treatment with phospholipase D did not affect binding of VIP by membranes. The molecular characterization of VIP receptors in RPMM was performed after [ 125I]VIP cross-linking to membranes using the cross-linker dithiobis (succinimidyl propionate). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of membrane proteins revealed specific [ 125I]VIP-protein complexes of M r 55,000 ± 1700, 35,000 ± 900, and 22,000 ± 500.