EXENDIN-(9-39) IS AN INVERSE AGONIST OF THE MURINE GLUCAGON-LIKE PEPTIDE-1 RECEPTOR - IMPLICATIONS FOR BASAL INTRACELLULAR CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE LEVELS AND BETA-CELL GLUCOSE COMPETENCE

The effect of exendin-(9-39), a described antagonist of the glucagon-l ike peptide-1 (GLP-1) receptor, was evaluated on the formation of cAMP - and glucose-stimulated insulin secretion (GSIS) by the conditionally immortalized murine beta TC-Tet cells. These cells have a basal intra cellular cAMP level that can be increased by GLP-1 with an EC50 of app roximately 1 nM and can be decreased dose dependently by exendin-(9-39 ). This latter effect was receptor dependent, as a beta-cell Line not expressing the GLP-1 receptor was not affected by exendin-(9-39). It w as also not due to the endogenous production of GLP-1, because this ef fect was observed in the absence of detectable preproglucagon messenge r RNA levels and radioimmunoassayable GLP-1. Importantly, GSIS was sho wn to be sensitive to this basal level of cAMP,as perifusion of beta T C-Tet cells in the presence of exendin-(9-39) strongly reduced insulin secretion. This reduction of GSIS, however, was observed only with gr owth-arrested, not proliferating, beta TC-Tet cells; it was also seen with nontransformed mouse beta-cells perifused in similar conditions. These data therefore demonstrated that 1) exendin-(9-39) is an inverse agonist of the murine GLP-1 receptor; 2) the decreased basal cAMP lev els induced by this peptide inhibit the secretory response of beta TC- Tet cells and mouse pancreatic islets to glucose; 3) as this effect wa s observed only with growth-arrested cells, this indicates that the me chanism by which cAMP leads to potentiation of insulin secretion is di fferent in proliferating and growth-arrested cells; and 4) the presenc e of the GLP-1 receptor, even in the absence of bound peptide, is impo rtant far maintaining elevated intracellular cAMP levels and, therefor e, the glucose competence of the beta-cells.