Contribution of the endoplasmic reticulum to the glucose-induced [[[Ca.sup.2+]].sub.c] response in mouse pancreatic islets

Thapsigargin (TG), a blocker of [Ca.sup.2+] uptake by the endoplasmic reticulum (ER), was used to evaluate the contribution of the organelle to the oscillations of cytosolic [Ca.sup.2+] concentration ([[[Ca.sup.2+]].sub.c]) induced by repetitive [Ca.sup.2+] influx in mouse pancreatic [beta]-cells. Because TG depolarized the plasma membrane in the presence of glucose alone, extracellular [K.sup.+] was alternated between 10 and 30 mM in the presence of diazoxide to impose membrane potential (MP) oscillations. In control islets, pulses of [K.sup.+], mimicking regular MP oscillations elicited by 10 mM glucose, induced [[[Ca.sup.2+]].sub.c] oscillations whose nadir remained higher than basal [[[Ca.sup.2+]].sub.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 [[[Ca.sup.2+]].sub.c] oscillations that was reproduced by raising extracellular [Ca.sup.2+] (to increase [Ca.sup.2+] influx) without changing the pulse protocol. In TG-pretreated islets, the imposed [[[Ca.sup.2+]].sub.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 [[[Ca.sup.2+]].sub.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 [beta]-cells limits the amplitude of [[[Ca.sup.2+]].sub.c] oscillations and may explain how the nadir between oscillations remains above baseline during regular oscillations or gradually increases during mixed [[[Ca.sup.2+]].sub.c] oscillations, two types of response observed during glucose stimulation. cytosolic [Ca.sup.2+] concentration oscillations; endoplasmic reticulum; pancreatic [beta]-cell; thapsigargin