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⁺ ([Ca ²⁺]_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 Mg ²⁺-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 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 V_m, 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 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.