Two Generations of Insulinotropic Imidazoline Compounds

Abstract

The imidazoline RX871024 increased basal- and glucose-stimulated 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 Ca²⁺ concentration ([Ca²⁺]_i), and insulin release. Importantly, RX871024 also enhanced the insulinotropic effect of glucose in cells with clamped [Ca²⁺]_i but in the presence of high ATP and Ca²⁺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.