抄録
Effects of intracellular Mg2+ ([Mg2+]i) on gating kinetics of the native Ca2+-activated and voltage-sensitive BK channel in cultured human proximal tubule cells (RPTECs) were analyzed using the inside-out mode of the patch-clamp technique. At constant intracellular Ca2+ ([Ca2+]i, 10−5 - 10−4 M), open probability (Po) of the channel was raised by adding [Mg2+]i (1 - 10 mM), which shifted the Po - Vm relationship to the negative voltage direction without change in the value of gating charge. At constant Vm, increasing [Mg2+]i to 10 mM raised Po and enhanced sigmoidicity of the Po - [Ca2+]i relationship with elevation of the Hill coefficient. These would suggest that the [Mg2+]i-induced changes in Po resulted from changes in [Ca2+]i-sensitivity. However, increasing [Mg2+]i from 0 to 10 mM at 10−5 M [Ca2+]i raised Po by extension of the open time, whereas increasing [Ca2+]i from 10−5 to 10−4 M without [Mg2+]i raised Po mainly by shortening the closed time, even though these two maneuvers induced similar elevation of Po. Moreover, adding 10 mM [Mg2+]i at a low [Ca2+]i (10−5.5 M) extended the closed time as well as the open time. We conclude that the major action of raising [Mg2+]i on the BK channel in RPTECs is to extend the open time, which would be distinct from the changes in Po by voltage or [Ca2+]i. The extended closed time at the low [Ca2+]i might result from reduction of the Ca2+-sensitivity probably due to a competitive inhibition of Mg2+ to high affinity Ca2+-binding sites. [Jpn J Physiol 54 Suppl:S128 (2004)]
