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.