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

Depolarization-activated outward currents ranging in amplitude from 100-1000 pA were studied in cultured, insulin-secreting 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 near EK. The K currents of HIT cells dialyzed with internal solutions containing 0.1-10 mM EGTA with no added calcium (Ca), or 10 mM EGTA with 2 mM added Ca, activated rapidly with depolarization. However, the stronger Ca buffer BAPTA (5 mM; 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 (5 mM, no added Ca) or high EGTA (10 mM with 2 mM Ca) blocked the rapid Ca-activated K current observed when cells were filled with 0.1 to 1 mM 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.