Glucose regulation of insulin secretion independent of the opening or closure of adenosine triphosphate-sensitive K+ channels in β cells

Abstract

Two major pathways are implicated in the stimulation of insulin secretion by glucose. The K+-ATP channel-dependent pathway involves closure of these channels, depolarization of the β-cell membrane, acceleration of Ca2+ influx, and a rise in cytosolic free Ca2+ ([Ca2+](i)). The K+-ATP channel-independent pathway potentiates the stimulation of exocytosis by high [Ca2+](i). To determine whether this second pathway is influenced by the configuration of the channel, we compared the effects of glucose on [Ca2+](i) and insulin secretion in mouse islets under three conditions. First, in the presence of 20, 25, and 30 mM K+, i.e. without pharmacological action on K+-ATP channels, [Ca2+](i) and insulin secretion were already elevated at 3 mM glucose. High glucose (20 mM) caused a transient decrease in [Ca2+](i) followed by an ascent to slightly above control levels, and rapidly stimulated insulin secretion. Second, opening of K+-ATP channels with diazoxide did not influence [Ca2+](i) and insulin secretion at 3 mM glucose and high K+. However, high glucose now caused a sustained lowering of [Ca2+](i) accompanied by a slow increase in secretion that augmented with the K+ concentration. Third, when K+-ATP channels were blocked and β-cells depolarized by high concentrations of tolbutamide or glibenclamide, [Ca2+](i) and insulin secretion were elevated even in low glucose. High glucose transiently lowered [Ca2+](i), which then increased to or slightly above control levels, while insulin secretion was rapidly stimulated. Under all conditions the correlation between [Ca2+](i) and insulin secretion was excellent at low and high glucose levels, and high glucose increased release at all [Ca2+](i). The potentiation of Ca2+-induced exocytosis by glucose is thus independent of the closed or open state of K+-ATP channels. It is only when the channels are opened by diazoxide that the increase in release is a strict amplification of the action of Ca2+. When the channels are closed (sulfonylureas) or still closable (high K+ alone), the effect of glucose on secretion also comprises a slight increase in [Ca2+](i) and, in the latter case, is not strictly K+-ATP channel independent.