Protons Inhibit the BK_Ca Channel of Rat Small Artery Smooth Muscle Cells

Abstract

The regulation of the activity of calcium-activated potassium (BK_Ca) channels by intracellular proton ions (pH_i) was investigated using the patch-clamp technique in smooth muscle cells freshly isolated from rat tail small arteries. Single-channel conductance and voltage dependence of activation were not different at pH_i 7.0, 7.4 and 7.8. However, the membrane potential at which channel open probability reached 0.5 was 74 ± 5 mV (n = 6) (mean ± SE) at pH_i 7.4 and 54 ± 2 mV (n = 4) at pH_i 7.8 under conditions of pCa 5.9, and 30 ± 5 mV (n = 5) at pH_i 7.4 and 62 ± 4 mV (n = 5) at pH_i 7.0 under conditions of pCa 5.4. Furthermore, at a membrane potential of 0 mV, the pD₂ for intracellular calcium ions was 5.19 ± 0.04 (n = 26) (mean ± SD) at pH_i 7.8, 5.02 ± 0.05 (n = 28) at pH_i 7.4, and 4.82 ± 0.05 (n = 30) at pH_i 7.0. In addition, an alteration of pH_i resulted in a profound change in the amplitude of BK_Ca currents in intact cells; it reversibly attenuated the current-voltage relationship decreasing the current by 55 ± 3% (n = 7) (p < 0.001) at 70 mV after lowering the extracellular NH₄Cl concentration to decrease the calculated pH_i from 7.2 to 6.8. Thus, alterations of pH_i in the range from 7.0 to 7.8 did not affect single-channel conductance and voltage dependence of activation but markedly altered single BK_Ca channel activity as well as intact cell BK_Ca current amplitude, where an increase of the intracellular proton concentration inhibited this channel.

Modulation of the Ca2+-Activated Large Conductance K+ Channel by Intracellular pH in Human Renal Proximal Tubule Cells.

“…1B), suggesting that a cytoplasmic H ϩ dose not interfere with K ϩ conduction through the pore at the pH range we used (between pH 6.3 and 7.8). These results are consistent with other previous papers [12,14,[16][17][18][19]21]. The difference of the pH i effects on channel conductance may be due to the molecular variant in individual BK channels.…”

“…3B. This suggests that the inhibitory effect of pH i on P o is independent of voltage-sensitivity of the channel, which is similar to BK channels in several animals [13,17,21].…”

“…The effect of changes in pH i on the Ca 2ϩ -sensitivity of BK channels was reported in various tissues [14][15][16][17][18]21]. Regarding this mechanism, several investigators have proposed that Ca 2ϩ and H ϩ could compete for binding to the Ca 2ϩ -binding sites [14][15][16]18].…”

“…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.…”

Multiple targets of chemosensitive signaling in locus coeruleus neurons: role of K+ and Ca2+channels

“…pH i sensitivity has been observed in members of the K ir (12,39), the K Ca (31,49), and the K v (6) channel family. pH o sensitivity has been observed in members of the TASK channel family (5,22).…”