Mitochondrial uncoupler FCCP activates proton conductance but does not block store-operated Ca current in liver cells

Uncouplers of mitochondrial oxidative phosphorylation, including carbonilcyanide p-triflouromethoxyphenylhydrazone (FCCP) and carbonilcyanide m-cholorophenylhydrazone (CCCP), are widely used in experimental research to investigate the role of mitochondria in cellular function. Unfortunately, it is very difficult to interpret the results obtained in intact cells using FCCP and CCCP, as these agents not only inhibit mitochondrial potential, but may also affect membrane potential and cell volume. Here we show by whole-cell patch clamping that in primary rat hepatocytes and H4IIE liver cells, FCCP induced large proton currents across the plasma membrane, but did not activate any other observable conductance. In intact hepatocytes FCCP inhibits thapsigargin-activated store-operated Ca²⁺ entry, but in patch clamping under the conditions of strong Ca²⁺ buffering it has no effect on store-operated Ca²⁺ current (I_SOC). These results indicate that there is no direct connection between mitochondria and activation of I_SOC in liver cells and support the notion of indirect regulation of I_SOC by mitochondrial Ca²⁺ buffering.