Glucose controls cytosolic Ca2+ and insulin secretion in mouse islets lacking adenosine triphosphate-sensitive K+ channels owing to a knockout of the pore-forming subunit Kir6.2

Abstract

Glucose-induced insulin secretion is classically attributed to the cooperation of an ATP-sensitive potassium (K ATP) channel-dependent Ca2+ influx with a subsequent increase of the cytosolic free Ca2+ concentration ([Ca2+]c) (triggering pathway) and a K ATP channel-independent augmentation of secretion without further increase of [Ca2+]c (amplifying pathway). Here, we characterized the effects of glucose in beta-cells lacking K ATP channels because of a knockout (KO) of the pore-forming subunit Kir6.2. Islets from 1-yr and 2-wk-old Kir6.2KO mice were used freshly after isolation and after 18 h culture to measure glucose effects on [Ca2+]c and insulin secretion. Kir6.2KO islets were insensitive to diazoxide and tolbutamide. In fresh adult Kir6.2KO islets, basal [Ca2+]c and insulin secretion were marginally elevated, and high glucose increased [Ca2+]c only transiently, so that the secretory response was minimal (10% of controls) despite a functioning amplifying pathway (evidenced in 30 mm KCl). Culture in 10 mm glucose increased basal secretion and considerably improved glucose-induced insulin secretion (200% of controls), unexpectedly because of an increase in [Ca2+]c with modulation of [Ca2+]c oscillations. Similar results were obtained in 2-wk-old Kir6.2KO islets. Under selected conditions, high glucose evoked biphasic increases in [Ca2+]c and insulin secretion, by inducing K ATP channel-independent depolarization and Ca2+ influx via voltage-dependent Ca2+ channels. In conclusion, Kir6.2KO beta-cells down-regulate insulin secretion by maintaining low [Ca2+]c, but culture reveals a glucose-responsive phenotype mainly by increasing [Ca2+]c. The results support models implicating a K ATP channel-independent amplifying pathway in glucose-induced insulin secretion, and show that K ATP channels are not the only possible transducers of metabolic effects on the triggering Ca2+ signal.