Specific gastric inhibitory polypeptide (GIP) receptors were characterized in human benign insulinoma plasma membranes employing [mono-[125I]iodo-Tyr10]-GIP (125I-GIP) as the radioligand. GIP 1-42 inhibited 125I-GIP binding with an IC50 value of 10(-9) M. Scatchard analysis showed two classes of binding sites: a high-affinity site (Kd = 2.23 x 10(-10) M; Bmax = 24 fmol/mg protein) and a low-affinity site (Kd = 8.39 x 10(-9) M; Bmax = 118 fmol/mg protein). A synthetic replicate of human GIP 1-31 inhibited 125I-GIP binding with an IC50 value of 10(-8) M. The GIP binding sites of human insulinoma were coupled to adenylate cyclase stimulation. GIP 1-31 regulated the adenylate cyclase activity to the same extent as GIP 1-42. The concentrations of GIP required for maximal activity ranged from 10(-9) to 10(-8) M for either GIP 1-42 or GIP 1-31. The existence of functional GIP receptors in human insulinoma substantiates our recent reports demonstrating the presence of GIP binding sites in transplantable hamster insulinoma and indicates that GIP could exert a direct control of the beta-cell function in humans through a purely endocrine pathway.

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High affinity binding sites have been found in membrane preparations from hamster beta-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 III stimulate insulin release from the isolated perfused rat pancreas to an extent at least equal to that obtained with unlabeled GIP at 10(-9) M. Natural GIP competitively inhibited the binding of 125I-GIP in the range of 10(-10) -10(-6) M and half-maximal inhibition was observed at 1.9 +/- 0.19 X 10(-9) M GIP. The number of high affinity sites was 219 +/- 8 fmol/mg protein and the dissociation constant was 2.05 +/- 0.1 X 10(-9) M. None of 10 regulatory peptides tested exhibited any effect on the 125I-GIP binding at concentrations in the range of 10(-6) -10(-4) M. Consequently, saturable, high affinity and specific binding sites for the GIP have been found and characterized in the plasma membranes of beta-cells. This model can be of use in studying the interaction of GIP with its preponderant target tissue.

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