CHARACTERIZATION OF VASOACTIVE-INTESTINAL-PEPTIDE RECEPTORS ON RAT ALVEOLAR MACROPHAGES

In view of the evidence that vasoactive intestinal peptide (VIP) may modulate acute inflammatory injury in the lung, we investigated the presence and characteristics of VIP receptors on alveolar macrophages (AMs). We examined the binding of monoiodinated [Tyr(I-125)(10)]-labeled VIP (I-125-VIP) to rat AMs (>96% pure), obtained from Sprague-Dawley rats by bronchoalveolar lavage (BAL). At 23 degrees C, the interaction of I-125-VIP With AMs was rapid, reversible, saturable, and linearly proportional to the number of cells. At equilibrium, the binding was competitively inhibited by 10(-11)-10(-6) M of native peptide [half-maximal inhibition (IC50) = 0.53 +/- 34 nM, n = 8], with evidence for two classes of binding sites: one with a high affinity (K-d = 0.20 +/- 0.09 nM) and a low capacity (1,190 +/- 640 sites/cell) and another with a low affinity (K-d = 43.2 +/- 13.8 nM) and a high capacity (51,700 +/- 14,000 sites/cell). VIP-related peptides inhibited the binding with the order of potency: VIP > peptide histidine isoleucine > helodermin >> secretin; glucagon was ineffective. In the presence of 3-isobutyl-1-methylxanthine, VIP dose dependently stimulated adenosine 3',5'-cyclic monophosphate accumulation in intact AMs, with maximal stimulation (6.3 times basal level) at 1 nM, and half-maximal accumulation at 0.23 +/- 0.11 nM VIP (K-d for high-affinity sites). For determination of the mass of the VIP receptor, I-125-VIP was covalently bound to AMs with the cross-linking agent dithiobis succinimidyl propionate. Autoradiographic studies after sodium dodecyl sulfate/polyacrylamide gel electrophoresis of solubilized affinity-labeled cells revealed a single major band of M(r) 76,400. We conclude that VIP binds to specific receptors on rat AMs that are coupled to adenylate cyclase, through which VIP may modulate inflammatory responses within the lung.