Specific binding sites for GIP have been characterized in a insulin-secreting pancreatic tumor cell line, In 111. The specific binding of 125I-GIP is time, temperature and cells concentration dependent. Under steady state conditions (2 hours at 13 degrees C) specific binding of 125I-GIP (0.3 nM) is competitively inhibited by increasing concentrations of native GIP from 10(-10) to 10(-6) M. Scatchard analysis reveals the presence of two types of sites: a high affinity (KD = 7 nM)/low capacity (3000 sites/cell) site and a low affinity (KD = 800 nM)/high capacity (150,000 sites/cell) site. No other peptide structurally related or not to GIP, interacts with GIP receptors. GIP (10(-10) to 10(-6) M) is able to potently stimulate insulin release in In 111 cells. At 37 degrees C, the stimulation is rapid and reaches a maximum from 30 minutes of incubation. Half-maximal stimulation is elicited by 10 nM GIP and maximal effect reaches 3 times the basal level of insulin release. Concomitantly, GIP (10(-10) - 10(-6) M) increases the basal cyclic AMP level in the cells. Half-maximal stimulation is observed in the presence of 30 nM GIP, maximal stimulation induced by 10(-6) M peptide increases up to 4 times the basal cyclic AMP production. In conclusion, our data provide the first description of a functional GIP receptor in an insulin-secreting pancreatic beta cell.