The imidazoline RX871024 increased basal-and glucosestimulated insulin release in vitro and in vivo. The compound inhibited activity of ATP-sensitive K ؉ channels as well as voltage-gated K ؉ channels, which led to membrane depolarization, an increase in the cytosolic Ca 2؉ concentration ([Ca 2؉ ] i ), and insulin release. Importantly, RX871024 also enhanced the insulinotropic effect of glucose in cells with clamped [Ca 2؉ ] i but in the presence of high ATP and Ca 2؉ concentration inside the cell. We believe that the latter effect on insulin exocytosis was at least in part mediated by a rise in diacylglycerol, which then activated protein kinase C (PKC) and increased the generation of arachidonic acid (AA) metabolites. Activation of both the PKC and AA pathways resulted in potentiation of glucose effects on insulin secretion. Unlike RX871024, the novel imidazoline BL11282 did not block ATP-dependent K ؉ channels, but similarly to RX871024, it stimulated insulin secretion in depolarized or permeabilized islets. Accordingly, BL11282 did not influence glucose and insulin levels under basal conditions either in vitro or in vivo, but it markedly enhanced the insulinotropic effects of glucose. BL11282 restored the impaired insulin response to glucose in islets from spontaneously diabetic GK rats. We conclude that BL11282 belongs to a new class of insulinotropic compounds that demonstrate a strong glucose-dependent effect on insulin exocytosis. Diabetes 51 (Suppl. 3):S448 -S454, 2002 G lucose stimulates insulin secretion mainly by activation of two mechanisms: increasing the cytosolic Ca 2ϩ concentration ([Ca 2ϩ ] i ) (1) and directly promoting insulin exocytosis (2). It has been demonstrated that sulfonylureas increase insulin secretion by simulating the effect of glucose on [Ca 2ϩ ] i (3). However, recent data suggest that sulfonylureas also moderately enhance the direct effect of glucose on exocytosis (4,5). The direct insulinotropic effect of sulfonylureas, observed already at normal or moderately increased blood glucose concentrations, may provoke pronounced and prolonged hypoglycemia in patients treated with these drugs (6). Therefore, drugs that solely enhance the exocytotic component of the insulinotropic effect of glucose by influencing transport and fusion of insulin granules could provide a better alternative to sulfonylureas in treatment of type 2 diabetes. It has been shown that glucagon like peptide-1 (GLP-1), a peptide from the gut, shows a glucose-dependent insulinotropic effect and exerts a strong antidiabetic effect (7). However, the necessity of injecting this peptide during the treatment led to the idea to search for small organic compounds possessing glucose-dependent insulinotropic activity similar to GLP-1. Among those compounds, of special interest are novel imidazoline compounds (8,9).Phentolamine, an ␣-adrenergic blocking agent with an imidazoline ring, stimulated basal and glucose-induced insulin release in humans and animals (10 -12). These data were initially interpreted as an indicati...