Glucose controls cytosolic Ca2+ and insulin secretion in mouse islets lacking ATP-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 a KATP channel-dependent Ca(2+) influx with subsequent rise of the cytosolic free Ca(2+) concentration ([Ca(2+)]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 of the pore-forming subunit Kir6.2. Islets from 1-year and 2-week-old Kir6.2KO mice were used freshly after isolation and after 18h culture to measure glucose effects on [Ca(2+)]c and insulin secretion. Kir6.2KO islets were insensitive to diazoxide and tolbutamide. In fresh adult Kir6.2KO islets, basal [Ca(2+)]c and insulin secretion were marginally elevated, and high glucose increased [Ca(2+)]c only transiently, so that the secretory response was minimal (10% of controls) despite a functioning amplifying pathway (evidenced in 30mM KCl). Culture in 10mM glucose increased basal secretion and considerably improved glucose-induced insulin secretion (200% of controls), unexpectedly because of a rise in [Ca(2+)]c with modulation of [Ca(2+)]c oscillations. Similar results were obtained in 2-week-old Kir6.2KO islets. Under selected conditions, high glucose evoked biphasic increases in [Ca(2+)]c and insulin secretion, by inducing KATP channel-independent depolarization and Ca(2+) influx via voltage-dependent Ca(2+) channels. In conclusion, Kir6.2KO beta-cells down-regulate insulin secretion by maintaining low [Ca(2+)]c but culture reveals a glucose-responsive phenotype mainly by increasing [Ca(2+)]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 Ca(2+) signal.