CYTOSOLIC PH REGULATES MAXI K+ CHANNELS IN NECTURUS GALLBLADDER EPITHELIAL-CELLS

1. The patch clamp technique was used to study the effects of internal and external pH on the Ca2+- and voltage-activated maxi K+ channel present in the apical membrane of Necturus gall-bladder epithelial cells. 2. When the pH of the solution bathing the cytosolic side of inside-out patches (pH(i)) was lowered from 7.9 to 6.9, with internal free Ca2+ concentration ([Ca2+]i) buffered below saturation levels for the channel gating sites, channel open probability (P(o)) decreased. At saturating Ca2+ concentrations, P(o) was near 1.0, and unaffected by pH(i). The results are consistent with a competitive interaction between Ca2+ and H+ at regulatory binding sites. Kinetic analysis assuming competitive binding yields a Hill coefficient for H+ of 1.3. 3. At sub-maximal [Ca2+]i, changing the pH of the solution bathing the extracellular surface of the patch (pH(o)) between 8 and 7, had no effect on maxi K+ channel P(o), but lowering pH(o) to 6 or 5 significantly reduced P(o). At saturating [Ca2+]i, P(o) was independent of pH(o). 4. There were no effects of either pH(i) or pH(o) on single-channel conductance. 5. Inasmuch as reductions in either pH(o) or pH(i) decrease maxi K+ channel P(o), changes in maxi K+ channel activity account in part for the reduction of apical membrane K+ conductance elicited by acidification of the bathing medium. The dominant effect of pH on maxi K+ channels is on the cytosolic surface of the membrane. 6. The change in P(o) elicited by small changes in [H+]i (delta-P-o/delta-[H+]i) is -7.6-mu-M-1, compared to delta-P-o/delta-[Ca2+]i = 2.6-mu-M-1, both at V(m) = -30 mV and at physiological intracellular [H+] and [Ca2+]. This implies that [H+]i and [Ca2+]i have opposite effects on channel P(o) at physiological levels and underlines the importance of pH(i) in channel gating.