Interplay between cytoplasmic Ca2+ and the ATP/ADP ratio: a feedback control mechanism in mouse pancreatic islets.
In pancreatic beta cells, the increase in the ATP/ADP ratio that follows a stimulation by glucose is thought to play an important role in the Ca2+-dependent increase in insulin secretion. Here we have investigated the possible interactions between Ca2+ and adenine nucleotides in mouse islets. Measurements of both parameters in the same single islet showed that the rise in the ATP/ADP ratio precedes any rise in the cytoplasmic free-Ca2+ concentration ([Ca2+]i) and is already present during the initial transient lowering of [Ca2+]i produced by the sugar. Blockade of Ca2+ influx with nimodipine did not prevent the concentration-dependent increase in the ATP/ADP ratio produced by glucose and even augmented the ratio at all glucose concentrations which normally stimulate Ca2+ influx. In contrast, stimulation of Ca2+ influx by 30 mM K+ or 100 microM tolbutamide lowered the ATP/ADP ratio. This lowering was of rapid onset and reversibility, sustained and prevented by nimodipine or omission of extracellular Ca2+. It was, however, not attenuated after blockade of secretion by activation of alpha2-adrenoceptors. The difference in islet ATP/ADP ratio during blockade and stimulation of Ca2+ influx was similar to that observed between threshold and submaximal glucose concentrations. The results suggest that the following feedback loop could control the oscillations of membrane potential and [Ca2+]i in beta cells. Glucose metabolism increases the ATP/ADP ratio in a Ca2+-independent manner, which leads to closure of ATP-sensitive K+ channels, depolarization and stimulation of Ca2+ influx. The resulting increase in [Ca2+]i causes a larger consumption than production of ATP, which induces reopening of ATP-sensitive K+ channels and arrest of Ca2+ influx. Upon lowering of [Ca2+]i the ATP/ADP ratio increases again and a new cycle may start.
Detimary
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Unit of Endocrinology and Metabolism, University of Louvain Faculty of Medicine, UCL 55.30, Avenue Hippocrate 55, B-1200 Brussels, Belgium.
Gilon
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Henquin
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D000244:Adenosine Diphosphate / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D003593:Cytoplasm / Q000378:metabolism*; D005260:Female; D005947:Glucose / Q000378:metabolism; D007004:Hypoglycemic Agents / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D007515:Islets of Langerhans / Q000378:metabolism*; D051379:Mice; D009553:Nimodipine / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D014044:Tolbutamide / Q000494:pharmacology
D002121:Calcium Channel Blockers; D007004:Hypoglycemic Agents; D007328:Insulin; D009553:Nimodipine; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D014044:Tolbutamide; D005947:Glucose; D011188:Potassium; D002118:Calcium