Uptake and release of Ca²⁺ by the endoplasmic reticulum contribute to the oscillations of the cytosolic Ca²⁺ concentration triggered by Ca²⁺ influx in the electrically excitable pancreatic B-cell

The role of intracellular Ca²⁺ pools in oscillations of the cytosolic Ca²⁺ concentration ([Ca²⁺](c)) triggered by Ca²⁺ influx was investigated in mouse pancreatic B-cells. [Ca²⁺](c) oscillations occurring spontaneously during glucose stimulation or repetitively induced by pulses of high K⁺ (in the presence of diazoxide) were characterized by a descending phase in two components. A rapid decrease in [Ca²⁺](c) coincided with closure of voltage-dependent Ca²⁺ channels and was followed by a slower phase independent of Ca²⁺ influx. Blocking the SERCA pump with thapsigargin or cyclopiazonic acid accelerated the rising phase of [Ca²⁺](c) oscillations and increased their amplitude, which suggests that the endoplasmic reticulum (ER) rapidly takes up Ca²⁺. It also suppressed the slow [Ca²⁺](c) recovery phase, which indicates that this phase corresponds to the slow release of Ca²⁺ that was taken up by the ER during the upstroke of the [Ca²⁺](c) transient. Glucose promoted the buffering capacity of the ER and amplified the slow [Ca²⁺](c) recovery phase. The slow phase induced by high K⁺ pulses was not affected by modulators of Ca²⁺- or inositol 1,4,5-trisphosphate-induced Ca²⁺ release, did not involve a depolarization- induced Ca²⁺ release, and was also observed at the end of a rapid rise in [Ca²⁺](c) triggered from caged Ca²⁺. It is attributed to passive leakage of Ca²⁺ from the ER. We suggest that the ER displays oscillations of the Ca²⁺ concentration ([Ca²⁺](ER)) concomitant and parallel to [Ca²⁺](c). The observation that thapsigargin depolarizes the membrane of B-cells supports the proposal that the degree of Ca²⁺ filling of the ER modulates the membrane potential. Therefore, [Ca²⁺](ER) oscillations occurring during glucose stimulation are likely to influence the bursting behavior of B-cells and eventually [Ca²⁺](c) oscillations.