Intracellular Mg2+ influences both open and closed times of a native Ca2+- activated BK channel in cultured human renal proximal tubule cells

Effects of intracellular Mg2+ on a native Ca2+ -and voltage-sensitive large-conductance K+ channel in cultured human renal proximal tubule cells were examined with the patch-clamp technique in the inside-out mode. At an intracellular concentration of Ca2+ ([Ca2+](i)) of 10(-5)-10(-4) M, addition of 1-10 mM Mg2+ increased the open probability (P-o) of the channel, which shifted the P-o -membrane potential (V-m) relationship to the negative voltage direction without causing an appreciable change in the gating charge (Boltzmann constant). However, the Mg2+-induced increase in P-o was suppressed at a relatively low [Ca2+](i) (10(-5.5)-10(-6) M). Dwell-time histograms have revealed that addition of Mg2+ mainly increased P-o by extending open times at 10(-5) M Ca2+ and extending both open and closed times simultaneously at 10(-5.5) M Ca2+. Since our data showed that raising the [Ca2+](i) from 10(-5) to 10(-4) M increased P-o mainly by shortening the closed time, extension of the closed time at 10(-5.5) M Ca2+ would result from the Mg2+-inhibited Ca2+-dependent activation. At a constant V-m, adding Mg2+ enhanced the sigmoidicity of the P-o-[Ca2+](i) relationship with an increase in the Hill coefficient. These results suggest that the major action of Mg2+ on this channel is to elevate P-o by lengthening the open time, while extension of the closed time at a relatively low [Ca2+](i) results from a lowering of the sensitivity to Ca2+ of the channel by Mg2+, which causes the increase in the Hill coefficient.