Two sites of glucose control of insulin release with distinct dependence on the energy state in pancreatic B-cells.
The energy state of pancreatic B-cells may influence insulin release at several steps of stimulus-secretion coupling. By closing ATP-sensitive K+ channels (K(+)-ATP channels), a rise in the ATP/ADP ratio may regulate the membrane potential, and hence Ca2+ influx. It may also modulate the effectiveness of Ca2+ on its intracellular targets. To assess the existence of these two roles and determine their relative importance for insulin release, we tested the effects of azide, a mitochondrial poison, on mouse B-cell function under various conditions. During stimulation by glucose alone, when K(+)-ATP channels are controlled by cellular metabolism, azide caused parallel, concentration-dependent (0.5-5 mM), membrane repolarization, decrease in cytosolic Ca2+ concentration [Ca2+]i and inhibition of insulin release. When K(+)-ATP channels were closed pharmacologically (by tolbutamide in high glucose), azide did not repolarize the membrane or decrease [Ca2+]i, and was much less effective in inhibiting insulin release. A similar resistance to azide was observed when K(+)-ATP channels were opened by diazoxide, and high K+ was used to depolarize the membrane and raise [Ca2+]i. In contrast, azide similarly decreased ATP levels and increased ADP levels, thereby lowering the ATP/ADP ratio under all conditions. In conclusion, lowering the ATP/ADP ratio in B-cells can inhibit insulin release even when [Ca2+]i remains high. However, this distal step is much more resistant to a decrease in the energy state of B-cells than is the control of membrane potential by K(+)-ATP channels. Generation of the signal triggering insulin release, high [Ca2+]i, through metabolic control of membrane potential requires a higher global ATP/ADP ratio than does activation of the secretory process itself.
Detimary
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Unité d'Endocrinologie et Métabolisme, University of Louvain Faculty of Medicine, Brussels, Belgium.
Gilon
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P
Nenquin
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M
Henquin
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JC
D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001386:Azides / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D004140:Dinitrophenols / Q000494:pharmacology; D004734:Energy Metabolism; D005260:Female; D005947:Glucose / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism* / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D009840:Oligomycins / Q000494:pharmacology
D001386:Azides; D004140:Dinitrophenols; D007328:Insulin; D009840:Oligomycins; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D005947:Glucose; D002118:Calcium