Electrical tolerance (breakdown) of theChara corallina plasmalemma: I. Necessity of Ca2+

Summary The relationship between the external Ca²⁺ concentrations Ca²⁺]₀ and the electrical tolerance (breakdown) in theChara plasmalemma was investigated. When the membrane potential was negative beyond –350–400 mV (breakdown potential, BP), a marked inward current was observed, which corresponds to the so-called punch-through (H.G.L. Coster,Biophys. J. 5:669–686, 1965). The electrical tolerance of theChara plasmalemma depended highly on Ca²⁺]₀. Increasing Ca²⁺]₀ caused a more negative and decreasing it caused a more positive shift of BP. BP was at about –700 mV in 200 M La³⁺ solution. Mg²⁺]₀ depressed the membrane electrical tolerance which was supposed to be due to competition with Ca²⁺ at the Ca²⁺ binding site of the membrane. Such a depressive effect of Mg²⁺ was almost masked when the Ca²⁺]₀/Mg²⁺]₀ ratio was roughly beyond 2.