Alterations of insulin secretion from mouse islets treated with sulphonylureas:: perturbations of Ca2+ regulation prevail over changes in insulin content

1 To determine how pretreatment with sulphonylureas alters the beta cell function, mouse islets were cultured (18-20 h) without (controls) or with (test) 0.01 mu M glibenclamide. Acute responses to glucose were then determined in the absence of glibenclamide. 2 Test islets were insensitive to drugs (sulphonylureas and diazoxide) acting on K+-ATP channels, and their [Ca2+](i) was already elevated in the absence of stimulation. 3 Insulin secretion was increased in the absence of glucose, and mainly stimulated between 0-10 instead of 7-20 mM glucose in controls. The maximum response was halved, but this difference disappeared after correction for the 45% decrease in the islet insulin content. 4 The first phase of glucose-induced insulin secretion was abrogated because of a paradoxical decrease of the high basal [Ca2+](i) in beta cells. The second phase was preserved but occurred with little rise of [Ca2+](i). These abnormalities did not result from alterations of glucose metabolism (NADPH fluorescence). 5 In islets cultured with 50 mu M tolbutamide, glucose induced biphasic increases in [Ca2+](i) and insulin secretion. The decrease in the secretory response was matched by the decrease in insulin content (45%) except at maximal glucose concentrations. Islets pretreated with tolbutamide, however, behaved like those cultured with glibenclamide if tolbutamide was also present during the acute functional tests. 6 In conclusion, treatment with a low glibenclamide concentration causes long-lasting blockade of K+-ATP channels and rise of [Ca2+](i) in beta cells. Glucose-induced insulin secretion occurs at lower concentrations, is delayed and is largely mediated by a modulation of Ca2+ action on exocytosis. It is suggested that glucose regulation of insulin secretion mainly depends on a K+-ATP channel-independent pathway during in vivo sulphonylurea treatment.