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.