Modulation of the Ca²⁺-activated large conductance K⁺ channel by intracellular pH in human renal proximal tubule cells

The Ca²⁺-activated and voltage-sensitive large conductance K^- channel (BK channel) with a slope conductance of about 300 pS is present in the surface membrane of cultured human renal proximal tubule epithelial cells (RPTECs). In this study we examined the effects of cytoplasmic pH (pHi) on activity and gating kinetics of the BK channel by using the inside-out configuration of the patch-clamp technique. At a constant cytoplasmic Ca^2- concentration ([Ca^2-]_i), membrane depolarization raised channel open probability (P_o), and lowering pH_i shifted the P_o-membrane potential (V_m) relationship to the positive voltage direction. However, the value of the gating charge was not affected by changes in pH_i, suggesting that the effects of pH_i on P_o were not due to an alternation of the voltage sensitivity. At constant V_m, lowering pH_i suppressed the [Ca²⁺]_i-dependent channel activation and shifted the P_o-[Ca²⁺]_i relationship in the direction of higher [Ca^2-]_i with a reduction of maximal P_o. Furthermore, both the mean open and mean closed times of the BK channels at pH_i 6.3 in the presence of 10^-4M [Ca^2-]_i were shorter than those at pH_i 7.3 in the presence of 10^-5 M [Ca²⁺]_i, even though these two different conditions gave a similar P_o. The data indicate that cytoplasmic H^- suppresses P_o of the BK channel in RPTECs, which involves the mechanism independent of Ca²⁺ activation. Our preliminary kinetic analysis also supported this notion.