Glucagon-like peptide-1-(7-36) amide (GLP-1) and glucose-dependent insulinotropic peptide (GIP) are known incretin hormones, released from enteroendocrine cells in response to food, that enhance insulin secretion, but only in the presence of elevated blood glucose. We used a rat insulinoma cell line, RIN 1046-38, to study the mechanisms underlying the interaction of incretins and glucose. We measured insulin secretion using RIA and the reverse hemolytic plaque assay. GLP-1 stimulates insulin secretion, with a half-maximal concentration of 34 pM. GLP-1 is approximately 2 orders of magnitude more potent than GIP. GLP-1 and GIP have additive effects at submaximal concentrations, but probably not at maximal concentrations, suggesting a common signal transduction pathway. The glucose requirement for GLP-1 action can be replaced by cell membrane depolarization (20 mM KCl in the extracellular medium), suggesting that a rise of intracellular Ca2+ may be an early step required for GLP-1 action. GLP-1 stimulates insulin secretion by significantly increasing the maximum rate of insulin secretion from 10.3 +/- 2.25 to 25.2 +/- 2.94 ng insulin/mg protein.h. GLP-1 acts by recruiting 1.5-fold more cells to secrete insulin as well as enhancing insulin secretion by individual cells. Combinations of stimuli, such as glucose, cell membrane depolarization, and GLP-1, can recruit 90% of RIN 1046-38 cells to secrete insulin.