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+ ([Ca 2+]i) of 10-5-10-4 M, addition of 1-10 mM Mg2+ increased the open probability (Po) of the channel, which shifted the Po -membrane potential (Vm) relationship to the negative voltage direction without causing an appreciable change in the gating charge (Boltzmann constant). However, the Mg 2+-induced increase in Po 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 Po by extending open times at 10-5 M Ca 2+ and extending both open and closed times simultaneously at 10 -5.5 M Ca2+. Since our data showed that raising the [Ca2+]; 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 Mg 2+-inhibited Ca2+-dependent activation. At a constant Vm, adding Mg2+ enhanced the sigmoidicity of the P o-[Ca2+]; relationship with an increase in the Hill coefficient. These results suggest that the major action of Mg2+ on this channel is to elevate Po 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.
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