sensitive large conductance (big) K ϩ channels (BK channels) have been reported in the cell membranes of a variety of tissues [1,2]. An elevation of cytoplasmic Ca 2ϩ concentration ([Ca 2ϩ ] i ) and depolarization of the membrane commonly stimulates activity of the BK channels [1][2][3]. It has already been proved by the use of cloned BK channels that the channel protein possesses the Ca 2ϩ -binding sites [4,5] and the voltagesensing domain [6,7]. Besides Ca 2ϩ and voltage, other factors such as ATP [8-11], pH [8, 9, 12-21], and Mg 2ϩ [19,[22][23][24] were also reported to modulate BK channels. Among these factors, intracellular pH (pH i ) is one of the significant factors that affect channel activity [8,9,[12][13][14][15][16][17][18][19][20][21], i.e., channel activity is inhibited by intracellular acidification and stimulated by its alkalization. Although the mechanism for pH i effects on channel activity was not precisely understood, two different mechanisms have been proposed. One is that an increased cytoplasmic H ϩ can compete with Ca 2ϩ in binding to the same sites, thereby inhibiting channel activity [14][15][16], whereas the other is that the pH i -mediated change in channel activity is not competitive with Ca 2ϩ [17]. Moreover, it was demonstrated that the conductance of some BK channels was reduced by lowering pH i and enhanced by raising it [9,15,20].In the renal tubules, BK channels have been reported in cultured rabbit proximal tubule cells [25]