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 ∼ 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 ∼2.5 s and summation of [Ca²⁺]_i occurred whenever depolarizations were applied with an interval of <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.