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Unité d'Endocrinologie et Métabolisme, University of Louvain Faculty of Medicine, UCL 55.30, B-1200 Brussels, Belgium
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 
-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 -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.
cytosolic Ca2+ concentration oscillations; endoplasmic
reticulum; pancreatic -cell; thapsigargin
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