Changes in gating kinetics of the Ca²⁺-activated BK channel by intracellular Mg²⁺ in cultured human proximal tubule cells

Abstract

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 (P_o) of the channel was raised by adding [Mg²⁺]_i (1 - 10 mM), which shifted the P_o - V_m relationship to the negative voltage direction without change in the value of gating charge. At constant V_m, increasing [Mg²⁺]_i to 10 mM raised P_o and enhanced sigmoidicity of the P_o - [Ca²⁺]_i relationship with elevation of the Hill coefficient. These would suggest that the [Mg²⁺]_i-induced changes in P_o resulted from changes in [Ca²⁺]_i-sensitivity. However, increasing [Mg²⁺]_i from 0 to 10 mM at 10⁻⁵ M [Ca²⁺]_i raised P_o by extension of the open time, whereas increasing [Ca²⁺]_i from 10⁻⁵ to 10⁻⁴ M without [Mg²⁺]_i raised P_o mainly by shortening the closed time, even though these two maneuvers induced similar elevation of P_o. 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 P_o 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)]