Expression of a rapid, low-voltage threshold K current in insulin-secreting cells is dependent on intracellular calcium buffering

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Abstract
Depolarization-activated outward currents ranging in amplitude from 100–1000 pA were studied in cultured, insulinsecreting HIT cells and mouse B-cells using the whole-cell patch clamp. Outward current was identified as a K current since it was blocked by K channel blockers and its tail current reversed nearE K. The K currents of HIT cells dialyzed with internal solutions containing 0.1–10mm EGTA with no added calcium (Ca), or 10mm EGTA with 2mm added Ca, activated rapidly with depolarization. However, the stronger Ca buffer BAPTA (5mm; no added Ca) blocked the rapidly activating current to reveal an underlying more slowly activating K current. With intracellular EGTA, application of the Ca channel blocker cadmium mimicked the effect of intracellular BAPTA. These data suggest that the rapid K current was mediated by low-voltage threshold, Ca-activated K channels while the slower K current was mediated by high threshold delayed rectifier K channels. Mouse B-cells also had both K current components. Dialyzing these cells with either BAPTA (5mm, no added Ca) or high EGTA (10mm with 2mm Ca) blocked the rapid Ca-activated K current observed when cells were filled with 0.1 to 1mm EGTA. It is concluded that the extent of Ca-activated K current activation in either HIT or adult mouse B-cells depends on the degree of intracellular Ca buffering.