Characterization of the Ca²⁺-dependent Cl^- efflux in perfused chara cells

The mechanism of the Ca²⁺-dependent Cl^- efflux was studied in tonoplast-free cells, in which the intracellular chemical composition can be freely controlled. Tonoplast-free cells were prepared by perfusing the cell interior of internodal cells of Chara corallina with a medium that contained EGTA. The Ca²⁺-induced Cl^- efflux was measured together with the membrane potential during continuous intracellular perfusion. The dependencies of Cl^- efflux and the membrane potential on the intracellular Ca²⁺ or Cl^- concentrations were analyzed. When perfusion was started with medium that contained Ca²⁺ ions, Cl^- efflux and membrane depolarization were induced. The amount of Cl^- efflux varied considerably among individual cells. The rate of efflux decreased exponentially but a residual efflux remained detectable. The Cl^- efflux was induced at concentrations of Ca²⁺ ions above 1 μM and reached a maximum at 1 mM. By contrast, the membrane depolarization reached a maximum at about 10 μM Ca²⁺. The rate of Cl^- efflux increased linearly with logarithmic increases in the intracellular Cl^- concentrations. These findings suggest that more than two kinds of Ca²⁺-dependent Cl^- channel might be present in the plasma membrane.Addition of ATP or its removal from the perfusion medium did not affect the Ca²⁺-dependent Cl^- efflux. Calmodulin antagonists slightly inhibited the Ca²⁺-dependent Cl^- efflux.