Temporal fluctuations of voltage-gated proton currents in rat spinal microglia via pH-dependent and -independent mechanisms

Voltage-gated proton (H+) channels are unique mechanisms to extrude a massive amount of H+, and are proposed to regulate intracellular pH of microglia during respiratory bursts. Temporal variations of the H+ current were studied in rat spinal microglia cultivated on the glial cell layer using the voltage-ramp protocol. Repetitive applications of the large and long-lasting depolarization decreased the amplitudes of the H+ current transiently and reversibly. This decrease was accompanied by a shift of the reversal potential to a more positive direction, indicating that a drop in the transmembrane pH gradient (Delta pH) by the H+ efflux through the channel reduced the current. The decline of the H+ current during depolarizations was also observed in a rat microglial cell line (GMI-R). An increase in the extracellular buffer suppressed the reduction of the current, suggesting that H+ secreted into the extracellular space contributed to the drop in Delta pH. On the other hand, the amplitudes of the H+ current often fluctuated greatly at intervals of 5-20 min without changes in Delta pH. These results suggest that the H+ current of microglia is tuned via both Delta pH-dependent and -independent mechanisms, which may regulate both microglial behavior and the pH environments of the surrounding neural tissue. (C) 2000 Elsevier Science Ireland Ltd and the Japan Neuroscience Society.