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Articles in PresS, published online ahead of print January 2, 2002
Am J Physiol Endocrinol Metab, 10.1152/ajpendo.00347.2001
Submitted on August 1, 2001
Accepted on December 18, 2001
1 Unite d'Endocrinologie et Metabolism, University of Louvain Faculty of Medicine, Brussels, Brussels, Belgium
* To whom correspondence should be addressed. E-mail: Gilon{at}endo.ucl.ac.be.
Thapsigargin (TG), a blocker of Ca2+ uptake by the endoplasmic reticulum (ER), was used to evaluate the contribution of the organelle to the oscillations of cytosolic Ca2+ concentration ([Ca2+]c) induced by repetitive Ca2+ influx in mouse pancreatic ß-cells. Because TG depolarized the plasma membrane in the presence of glucose alone, extracellular K+ was alternated between 10 and 30 mM in the presence of diazoxide to impose membrane potential (MP) oscillations. In control islets, pulses of K+ mimicking regular MP oscillations elicited by 10 mM glucose induced [Ca2+]c oscillations whose nadir remained higher than basal [Ca2+]c. Increasing the depolarization phase of the pulses while keeping their frequency constant (to mimic the effects of a further rise of the glucose concentration on MP), caused an upward shift of the nadir of [Ca2+]c oscillations that was reproduced by raising extracellular Ca2+ (to increase Ca2+ influx) without changing the pulse protocol. In TG-pretreated islets, the imposed [Ca2+]c oscillations were of much larger amplitude than in control islets and occurred on basal levels. During intermittent trains of depolarizations, control islets displayed mixed [Ca2+]c oscillations characterized by a summation of fast oscillations on top of slow ones, whereas no progressive summation of the fast oscillations was observed in TG-pretreated islets. In conclusion, the buffering capacity of the ER in pancreatic ß-cells limits the amplitude of [Ca2+]c oscillations and may explain how the nadir between oscillations remains above baseline during regular oscillations or gradually increases during mixed [Ca2+]c oscillations, two types of response observed during glucose stimulation.
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