Interaction of porcine vasoactive intestinal peptide with dispersed pancreatic acinar cells from the guinea pig. Binding of radioiodinated peptide.

We have used 125I-labeled vasoactive intestinal peptide (VIP) to study the kinetics, stoichiometry, and chemical specificity with which the labeled peptide binds to dispersed acinar cells prepared from guinea pig pancreas. Binding of 125I-VIP to pancreatic acinar cells was moderately rapid, reversible, specific, saturable, and depended on incubation temperature. Deterioration of 125I-VIP incubated with pancreatic acinar cells at 37 degrees was reflected in a decrease in acid-precipitable radioactivity and in the amount of tracer which could bind to fresh acinar cells. On the other hand, 125I-VIP bound to pancreatic acinar cells appeared to be protected from deterioration. VIP and secretin but not glucagon or COOH-terminal octapeptide of cholecystokinin inhibited binding of 125I-VIP to pancreatic acinar cells. The dose-response curve for inhibition of 125I-VIP binding by VIP or secretin was biphasic and suggested that pancreatic acinar cells have two classes of binding sites: (a) a relatively small number of sites with a high affinity for VIP and a low affinity for secretin, and (b) a relatively large number of sites with a low affinity for VIP and a high affinity for secretin. The difference between the relative affinities of VIP and secretin for the high affinity VIP binding sites appears to be primarily attributable to the NH2-terminal portions of these molecules since synthetic COOH-terminal fragments VIP 14-28, VIP 15-28, and secretin 14-27 were equipotent in inhibiting 125I-VIP binding. On the other hand, secretin 5-27, [6-tyrosine] secretin and native secretin were equipotent in inhibiting binding of 125I-VIP to its high affinity site, and these three peptides were 5 times more potent than secretin 14-27 but 10,000 times less potent than native VIP.