Voltage-activation of high-conductance K+ channel in the insulin-secreting cell line RINm5F is dependent on local extracellular Ca2+ concentration.

Patch-clamp single-channel current recording experiments have been carried out on intact insulin-secreting RINm5F cells. Voltage-activation of high-conductance K+ channels were studied by selectively depolarizing the electrically isolated patch membrane under conditions with normal Ca2+ concentration in the bath solution but with or without Ca2+ in the patch pipette solution. When Ca2+ was present in the pipette, 40 mV to 120 mV depolarizing pulses (100 ms) from the normal resting potential (-70 mV) regularly evoked tetraethylammonium-sensitive large outward single-channel currents and the average open state probability during the pulses varied from about 0.015 (40 mV pulses) to 0.1 (120 mV pulses). In the absence of Ca2+ in the pipette solution the same protocol resulted in fewer and shorter K+ channel openings and the open-state probability varied from about 0.0015 (40 mV pulses) to about 0.03 (120 mV pulses). It is concluded that Ca2+ entering voltage-gated channels raises [Ca2+]i locally and thereby markedly enhances the open-state probability of tetraethylammonium-sensitive voltage-gated high-conductance K+ channels.