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

Glucose-induced insulin secretion is classically attributed to the cooperation of an ATP-sensitive potassium (KATP) channel-dependent Ca2+ influx with a subsequent increase of the cytosolic free Ca2+ concentration ([Ca2+](c)) (triggering pathway) and a KATP channel-independent augmentation of secretion without further increase of [Ca-2](c) (amplifying pathway). Here, we characterized the effects of glucose in beta-cells lacking KATP 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 KATP 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 KATP channel-independent amplifying pathway in glucose-induced insulin secretion, and show that KATP channels are not the only possible transducers of metabolic effects on the triggering Ca2+ signal. (Endocrinology 150: 33-45, 2009)