Evidence for and Characterization of Specific High Affinity Binding Sites for the Gastric Inhibitory Polypeptide in Pancreaticβ-Cells*

Abstract

High affinity binding sites have been found in membrane preparations from hamster β-cell tumors by using radiolabeled gastric inhibitory polypeptide (¹²⁵I-GIP). HPLC of ¹²⁵I-GIP resulted in two major peaks (A III and B III), with identical specific binding. It was verified that peaks A III and B HI stimulate insulin release from the isolated perfused rat pancreas to an extent at least equal to that obtained with unlabeled GIP at 10⁻⁹m. Natural GIP competitively inhibited the binding of ¹²⁵I-GIP in the range of 10⁻¹⁰–10⁻⁶m and half-maximal inhibition was observed at 1.9 ± 0.19 x 10⁻⁹m GIP. The number of high affinity sites was 219 ± 8 fmol/mg protein and the dissociation constant was 2.05 ± 0.1 × 10⁻⁹m. None of 10 regulatory peptides tested exhibited any effect on the ¹²⁵I-GIP binding at concentrations in the range of 10⁻⁶–10⁻⁴m. Consequently, saturable, high affinity and specific binding sites for the GIP have been found and characterized in the plasma membranes of β-cells. This model can be of use in studying the interaction of GIP with its preponderant target tissue. (Endocrinology115: 1324–1331, 1984)