INTERACTION OF SECRETIN WITH PANCREATIC MEMBRANES

125I-labeled secretin bound rapidly and specifically to membranes from cat pancreas. Binding of labeled hormone was competitively inhibited by unlabeled secretin in the same range of concentrations that stimulated pancreatic adenylate cyclase in these membranes. The dissociation constant of the membrane binding sites for unlabeled secretin as evaluated by these displacement experiments was 4.1 .cntdot. 10-9 M and the number of binding sites 1.0 pmol/mg of membrane protein. Studies using different concentrations of [125I]secretin (at a constant ratio of labeled to unlabeled hormone) revealed a similar value of 4.4 .cntdot. 10-9 M for the Kd. Both the association and dissociation rate constants of [125I]secretin binding were temperature sensitive; the dissociation rate constant increased more rapidly with increase in temperature. The ratio k-1/k+1 (at 22.degree. C) gave a dissociation constant of 3.7 .cntdot. 10-9 M which agrees closely with the figure obtained from equilibrium data. 125I-labeled secretin and unlabeled secretin probably bind to the same binding site on pancreatic membranes, with high affinity. Unlabeled secretin stimulated pancreatic adenylate cyclase with an apparent Km of 8.4 .cntdot. 10-9 M, while [125I]secretin apparently did not stimulate the adenylate cyclase. Together with the binding data this might suggest that different portions of the secretin molecule are responsbile for binding and adenylate cyclase activation. In studies on the specificity of [125]secretin binding carried out with various peptide hormones (glucagon, human gastrin, pancreozymin and caerulein) which are all inefficient in stimulating pancreatic fluid secretion, these hormones had no influence on the binding of [125I]secretin. Vasoactive intestinal polypeptide, which stimulates pancreatic fluid and bicarbonate secretion, inhibited competitively secretin binding to the plasma membrane preparation.