High-affinity binding sites for VIP in renal cortical membranes: Possible role of VIP in renal transport

High-affinity binding sites for VIP in renal cortical membranes: Possible role of VIP in renal transport. We studied binding and degradation of vasoactive intestinal peptide (VIP) by highly purified brush border and basolateral membranes from rabbit kidney cortex. Brush border and basolateral membranes were capable of 73 and 49% degradation of VIP after 20 minutes, and the degradation was totally prevented by bacitracin. There was 66 and 87% specific binding of 125 I-VIP to brush border and basolateral membranes, respectively. 125 I-VIP binding to renal membrane was displaced in a dose dependent fashion by unlabeled VIP with half maximal displacement at 2 × 10 -7 M. Other related peptides failed to displace VIP. Scatchard analysis showed one single class of receptors for VIP in both membranes with similar Kd (0.5 × 10 -7 M) but higher number of binding sites (B max ) in the basolateral membranes than in the brush border membranes (22.0 vs. 4.4 pmol/mg protein), respectively. Forty-eight percent of VIP binding to brush border membranes could be explained by cross contamination of these membranes with basolateral membranes. We examined the effect of VIP on Na-H antiporter, Na-dependent glucose uptake and Na-dependent phosphate uptake by isolated proximal tubule suspension. In acid loaded proximal tubules VIP (10 -6 M) inhibited total and amiloride-sensitive 22 Na uptake by 35 and 75%, respectively, as compared to control. On the other hand VIP failed to inhibit Na-dependent methyl α -14 C-glucopyranoside and Na-dependent 32 phosphate uptake. VIP failed to stimulate cyclic AMP generation by proximal tubule suspension while PTH snowed the expected stimulation. Our results demonstrate the presence of specific binding for VIP in highly purified cortical membranes and suggest an effect of VIP to inhibit the Na-H antiporter by a mechanism independent of cyclic AMP.