Protons inhibit the BK(Ca) channel of rat small artery smooth muscle cells

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 pD(2) 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 NH(4)Cl 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.