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 (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.
Ravier
Magalie A
MA
Unit of Endocrinology and Metabolism, University of Louvain, Faculty of Medicine, Brussels, Belgium.
Nenquin
Myriam
M
Miki
Takashi
T
Seino
Susumu
S
Henquin
Jean-Claude
JC
D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D005260:Female; D005947:Glucose / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D054086:KATP Channels / Q000172:deficiency*; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D018345:Mice, Knockout; D009991:Oscillometry; D024661:Potassium Channels, Inwardly Rectifying / Q000172:deficiency* / Q000502:physiology*; D000099161:Kcnj11 Channel
D001786:Blood Glucose; D002118:Calcium; D005947:Glucose; D007328:Insulin; D054086:KATP Channels; D024661:Potassium Channels, Inwardly Rectifying; D000099161:Kcnj11 Channel