Dependence of the membrane potential of Chara cells on external pH in the presence or absence of internal adenosinetriphosphate.

The dependence of the membrane potential (Em) and the membrane resistance (Rm) of Chara australis R. Brown on the pH of the external medium (pH0) was studied by controlling the activity of the plasmamembrane H(+) pump under both light and dark conditions. The activity of the pump was controlled by regulating the internal ATP or Mg(2+) concentration in tonoplast-free cells prepared by vacuolar perfusion. In these cells, which contained Mg · ATP (mgATP cells), Em and Rm were very sensitive to pH0, as in normal cells. Em was more negative in light than in the dark at all pH0 values tested. Tonoplast-free cells with very low [ATP]i (-ATP cells) or [Mg(2+)]i (-Mg cells) showed very weak dependence of Em and Rm on pH0. Thus, the active and not the passive component of Em was sensitive to pH0. At the same time, the high permeability of the plasma membrane to H(+) was questioned. In both-ATP cells and-Mg cells, Em was scarcely affected and Rm markedly decreased on illumination.