Characterization of specific pancreatic polypeptide receptors on basolateral membranes of the canine small intestine.

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Abstract
We have identified specific binding sites for pancreatic polypeptide (PP) on the mucosal lining of canine small intestine. The present study was undertaken to further characterize these binding sites (receptors) on purified intestinal membranes and to establish their location on the brush border or basolateral surface of the intestinal enterocyte. Basolateral and brush border membranes were prepared by sorbitol density centrifugation. PP receptors were localized predominately to the vascular surface, and thus binding of PP 125I-labeled on Tyr-27 to the basolateral preparation was used to evaluate receptor characteristics. Binding of PP was calcium, time, temperature, and pH dependent. Maximum specific binding of labeled PP occurred after an 8-hr incubation at 4.degree.C with 5 mM calcium at pH 6.8. Data analysis by Scatchard plot showed high-and low-affinity binding sites with relative affinities of 1.5 .times. 10-9 M and 2.6 .times. 10-8 M and with corresponding binding capacities of 0.23 pmol/mg and 0.84 pmol/mg of protein, respectively. This receptor was specific for PP since peptide YY and neuropeptide Y, peptides of the PP family, cross-reacted by < 3%, as judged from comparisons of half-maximal displacement of label. Structurally dissimilar peptides, insulin and glucagon, did not compete for binding. Specific 125-I-labeled PP binding was localized primarily to basolateral membranes (9.8 .+-. 0.8%) with little binding by brush border membranes (0.8 .+-. 0.2%). Thus, we have identified highly specific receptors for PP, located predominately on the vascular surface of the small intestinal mucosa. These data suggest that the mucosal lining of the small intestine is a target for PP and that PP participates in the hormonal regulation of fuel metabolism and substrate transport in the small intestinal mucosa.