The effect of exendin-(9–39), a described antagonist of the glucagon-like peptide-1 (GLP-1) receptor, was evaluated on the formation of cAMP- and glucose-stimulated insulin secretion (GSIS) by the conditionally immortalized murine βTC-Tet cells. These cells have a basal intracellular cAMP level that can be increased by GLP-1 with an EC₅₀ of approximately 1 nm and can be decreased dose dependently by exendin-(9–39). This latter effect was receptor dependent, as aβ -cell line not expressing the GLP-1 receptor was not affected by exendin-(9–39). It was also not due to the endogenous production of GLP-1, because this effect was observed in the absence of detectable preproglucagon messenger RNA levels and radioimmunoassayable GLP-1. Importantly, GSIS was shown to be sensitive to this basal level of cAMP, as perifusion of βTC-Tet cells in the presence of exendin-(9–39) strongly reduced insulin secretion. This reduction of GSIS, however, was observed only with growth-arrested, not proliferating, βTC-Tet cells; it was also seen with nontransformed mouse β-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 levels induced by this peptide inhibit the secretory response of βTC-Tet cells and mouse pancreatic islets to glucose; 3) as this effect was observed only with growth-arrested cells, this indicates that the mechanism by which cAMP leads to potentiation of insulin secretion is different in proliferating and growth-arrested cells; and 4) the presence of the GLP-1 receptor, even in the absence of bound peptide, is important for maintaining elevated intracellular cAMP levels and, therefore, the glucose competence of theβ -cells.