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.
The Journal of Membrane Biology – Springer Journals
Published: Jan 1, 2005
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