The Action Potential in Chara coraliina II.* Two Activation-Inactivation Transients in Voltage Clamps of the Plasmalemma

Abstract

Voltage-clamp experiments over a wide range of clamp potentials were made with cells of C. corallina using a fast voltage-clamp apparatus. Clamps of the plasmalemma potential in the range - 170 mV to - 70 mV revealed a large transient inward current which followed the usual chloride current transient hitherto described in the literature. This additional component decreased in amplitude and occurred earlier in the clamp as the clamp potentials were made more positive up to ~ -70 mV. For clamp potentials > - 50 mV, a large, prompt, outward current appeared. The additional current transients could not be observed in vacuolar potential clamp experiments, except at clamps ¿ - 10 mV. The effects of changes in the external Cl- and Ca²+ concentrations with plasmalemma clamps at various potentials suggest that the additional transient here reported consists of a flow of Ca²+ ions and also reaffirms the identification of the other transient with a flow of Cl- ions. Reversal of the additional current transient when the clamp potential is sufficiently large (i.e. at large depolarization), places the equilibrium potential of the ionic component responsible for this transient at - 50 mV (± 20 mV). Estimates, based on this equilibrium potential, of the cytoplasmic Ca²+ concentration yield unrealistic values. Possible answers to this dilemma are discussed.