Membrane potential dependent modulations of calcium oscillations in insulin-secreting INS-1 cells

This study was undertaken to examine the role of K(+) channels on cytosolic Ca(2+) (Ca(2+)](i)) in insulin secreting cells. Ca(2+)](i) was measured in single glucose-responsive INS-1 cells using the fluorescent Ca(2+) indicator Fura-2. Glucose, tolbutamide and forskolin elevated Ca(2+)](i) and induced Ca(2+)] oscillations. Whereas the glucose effect was delayed and observed in 60% and 93% of the cells, in a poorly and a highly glucose-responsive INS-1 cell clone, respectively, tolbutamide and forskolin increased Ca(2+)](i) in all cells tested. In the latter clone, glucose induced Ca(2+)](i) oscillations in 77% of the cells. In 16% of the cells a sustained rise of Ca(2+)](i) was observed. The increase in Ca(2+)](i) was reversed by verapamil, an L-type Ca(2+) channel inhibitor. Adrenaline decreased Ca(2+)](i) in oscillating cells in the presence of low glucose and in cells stimulated by glucose alone or in combination with tolbutamide and forskolin. Adrenaline did not lower Ca(2+)](i) in the presence of 30mM extracellular K(+), indicating that adrenaline does not exert a direct effect on Ca(2+) channels but increases K(+) channel activity. As for primary b-cells, Ca(2+)](i) oscillations persisted in the presence of closed K(ATP) channels; these also persisted in the presence of thapsigargin, which blocks Ca(2+) uptake into Ca(2+) stores. In contrast, in voltage-clamped cells and in the presence of diazoxide (50mM), which hyperpolarizes the cells by opening K(ATP) channels, Ca(2+)](i) oscillations were abolished. These results support the hypothesis that Ca(2+)](i) oscillations depend on functional voltage-dependent Ca(2+) and K(+) channels and are interrupted by a hyperpolarization in insulin-secreting cells.