Oscillation of Membrane Currents During the Action Potential in Chara corallina: Modification and Significance for Repolarisation of the Membrane Potential and Salt Sensitivity

Abstract

Depolarising voltage-clamp steps in C. corallina induced membrane currents which differ from those of C. inflata in two aspects: (1) The absence of a 'hump', i.e. a transient outward current,I_o(max) which is present in C. inflata, and (2) the presence in C, corallina of a voltage-dependent current oscillation, i.e. a succession of decaying peaks. The peaks of the oscillating transient inward current, I_i(max), were voltage dependent and sensitive to block with 9-anthracenecarboxylic acid (9-AC). The oscillating current is carried by C1- and its time course is determined by the activation and inactivation kinetics of C1- channels. Extracellular NaCl delayed current activation, induced a voltage-dependent increase in I_i(max) and a decrease in the steady-state outward K+ current, I_s. NaCl increased the occurrence of oscillation and enhanced the amplitude of the oscillating current. Extracellular sorbitol induced an overall reduction in I_i(max) and had virtually no effect on I_s. I suggest that the enhancement of the oscillating transient inward CI- current, I_i(max), by NaCl is due to ionic effects of NaCl rather than to its osmotic effects.