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 Ca2- concentration ([Ca2-]i), membrane depolarization raised channel open probability (Po), and lowering pHi shifted the Po-membrane potential (Vm) relationship to the positive voltage direction. However, the value of the gating charge was not affected by changes in pHi, suggesting that the effects of pHi on Po were not due to an alternation of the voltage sensitivity. At constant Vm, lowering pHi suppressed the [Ca2+]i-dependent channel activation and shifted the Po-[Ca2+]i relationship in the direction of higher [Ca2-]i with a reduction of maximal Po. Furthermore, both the mean open and mean closed times of the BK channels at pHi 6.3 in the presence of 10-4M [Ca2-]i were shorter than those at pHi 7.3 in the presence of 10-5 M [Ca2+]i, even though these two different conditions gave a similar Po. The data indicate that cytoplasmic H- suppresses Po of the BK channel in RPTECs, which involves the mechanism independent of Ca2+ activation. Our preliminary kinetic analysis also supported this notion.
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