Calcium effects on electrogenic pump and passive permeability of the plasma membrane of Chara corallina

Removal of Ca²⁺ from the medium results in depolarization of the Chara internodal cell and an increase in membrane conductance (G_m). The increase in conductance is associated with an increase in K⁺ conductance, as judged by Ca²⁺ effects on the K⁺ dependence of clamp current. The voltage dependence of G_m is also affected by Ca²⁺, as is the time course of the response of clamp current to a step change in voltage. Mg²⁺ restores the low conductance and the fast response to a voltage change, but not hyperpolarization at neutral pH, suggesting that there is an additional, independent effect on the electrogenic pump. The membrane does not show the normal ability to increase proton conductance at high pH in the absence of Ca²⁺; this is also restored by Mg²⁺ as well as by Ca²⁺.