The effect of Cu2+ on ion transport systems of the plant cell plasmalemma

Changes in plasmalemma permeability caused by excessive Cu2+ levels were examined in cells of a freshwater alga (Nitella flexilis) using a conventional microelectrode voltage-clamp technique. A rapid Cu2+-induced increase of plasmalemma conductance starting from 5 mu M Cu2+ was shown. Cu2+-induced plasmalemma conductance (CIGm) was nonselective and potential-independent, resembling the conductance of nonselective ionic leakage of the plasmalemma. The K+ channel conductance was shown to be unaltered by Cu2+, and a decrease in plasmalemma Cl- channel conductance at Cu2+ concentrations above 5 mu M was found. The depression of Cl- channels and CIGm were time-, dosage-, and Ca2+-dependent processes, revealing a great similarity in all parameters, with Ca2+ causing the preventive effect by shifting the effective Cu2+ concentrations to higher levels. This phenomenon may be explained by the same Cu2+-modified target on the plasmalemma both for CIGm and Cl- channel depression. In addition, a reversible, inhibitory effect of Cu2+ (> 10 mu M) on the light-stimulated H+-ATPase electrogenic pump in the plasmalemma was demonstrated. This effect was Ca2+ independent, which made it possible to distinguish it from CIGm. Therefore, the Cu2+-induced dramatic alterations in plant cell plasmalemma permeability are caused mainly by nonselective conductance increases and electrogenic pump inhibition.