Affiliation:
1. Unit of Endocrinology and Metabolism (M.A.R., M.N., J.-C.H.), University of Louvain Faculty of Medicine, B-1200 Brussels, Belgium
2. Division of Cellular and Molecular Medicine (T.M., S.S.), Kobe University Graduate School of Medicine, Kobe 650-0017, Japan
Abstract
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 [Ca2+]c (amplifying pathway). Here, we characterized the effects of glucose in β-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 β-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.Glucose can stimulate insulin secretion from beta cells by increasing Ca2+ influx, cytosolic Ca2+ concentration, and Ca2+ action independently of ATP-sensitive K channels.
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