Voltage-gated proton channels help regulate pHi in rat alveolar epithelium

Voltage-gated proton channels are expressed highly in rat alveolar epithelial cells. Here we investigated whether these channels contribute to pH regulation. The intracellular pH (pH(i)) was monitored using BCECF in cultured alveolar epithelial cell monolayers and found to be 7.13 in nominally HCO3--free solutions [at external pH (pH(o)) 7.4]. Cells were acid-loaded by the NH4+ prepulse technique, and the recovery was observed. Under conditions designed to eliminate the contribution of other transporters that alter pH, addition of 10 muM ZnCl2, a proton channel inhibitor, slowed recovery about twofold. In addition, the pHi minimum was lower, and the time to nadir was increased. Slowing of recovery by ZnCl2 was observed at pH(o) 7.4 and pH(o) 8.0 and in normal and high-K+ Ringer solutions. The observed rate of Zn2+-sensitive pH(i) recovery required activation of a small fraction of the available proton conductance. We conclude that proton channels contribute to pHi recovery after an acid load in rat alveolar epithelial cells. Addition of ZnCl2 had no effect on pHi in unchallenged cells, consistent with the expectation that proton channels are not open in resting cells. After inhibition of all known pH regulators, slow pHi recovery persisted, suggesting the existence of a yet-undefined acid extrusion mechanism in these cells.