Cytoplasmic calcium transients due to single action potentials and voltage‐clamp depolarizations in mouse pancreatic B‐cells.

Changes in the cytoplasmic free calcium concentration ([Ca2+]i) in pancreatic B‐cells play an important role in the regulation of insulin secretion. We have recorded [Ca2+]i transients evoked by single action potentials and voltage‐clamp Ca2+ currents in isolated B‐cells by the combination of dual wavelength emission spectrofluorimetry and the patch‐clamp technique. A 500–1000 ms depolarization of the B‐cell from −70 to −10 mV evoked a transient rise in [Ca2+]i from a resting value of approximately 100 nM to a peak concentration of 550 nM. Similar [Ca2+]i changes were associated with individual action potentials. The depolarization‐induced [Ca2+]i transients were abolished by application of nifedipine, a blocker of L‐type Ca2+ channels, indicating their dependence on influx of extracellular Ca2+. Following the voltage‐clamp step, [Ca2+]i decayed with a time constant of approximately 2.5 s and summation of [Ca2+]i occurred whenever depolarizations were applied with an interval of less than 2 s. The importance of the Na(+)‐Ca2+ exchange for B‐cell [Ca2+]i maintenance was evidenced by the demonstration that basal [Ca2+]i rose to 200 nM and the magnitude of the depolarization‐evoked [Ca2+]i transients was markedly increased after omission of extracellular Na+. However, the rate by which [Ca2+]i returned to basal was not affected, suggesting the existence of additional [Ca2+]i buffering processes.