Contribution of the endoplasmic reticulum to the glucose-induced [Ca²⁺]_c response in mouse pancreatic islets

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 ir mouse pancreatic β-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⁺], 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) with. out 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 summa. tion of fast oscillations on top of slow ones, whereas nc progressive summation of the fast oscillations was observed in TG-pretreated islets. In conclusion, the buffering capacity of the ER in pancreatic β-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.