DIVALENT-CATION AND ANION CURRENTS ARE ACTIVATED DURING THE DARK-INDUCED TRANSIENT HYPERPOLARIZATION OF THE PLASMA-MEMBRANE OF THE GREEN-ALGA EREMOSPHAERA-VIRIDIS

Darkening after illumination induces a transient hyperpolarization of the plasma membrane of the unicellular green alga Eremosphaera viridis de Bary. With electrophysiological methods, in particular the two electrode voltage-clamp technique, we investigated the ion fluxes involved in this transient potential change (TP). The question was whether other ion currents besides those carried by the known Ca2+-dependent K+ channel take part in this action potential-like, but hyperpolarizing, response. At maximum hyperpolarization voltage-clamp measurements resulted in 'N-shaped' I/V curves, known from other botanical systems, The different instantaneous current components of the N-shaped I/V curves occurred at different times during a single transient potential change (TP). Substitution of alkali metal cations in the bathing solution by NMG/NO3 showed that the inward currents in the I/V curves were not carried by an influx of K+ into the cytoplasm. The voltage amplitude of the TP not only depended on the external K+ concentration, but also on the Mg2+ concentration in the bathing solution. Increasing Mg2+ concentrations shifted the membrane potential in the top of the TP in the direction of the Nernst potential of Mg2+ and resulted in an increased inward current component of the N-shaped I/V curves. Another current component was found to be carried not by cations but by an efflux of anions. It was a voltage-dependent component with a maximum current amplitude at voltages of about -220 to -240 mV, and was blocked by the anion channel inhibitors anthracen-9-carboxylic acid (A9C), (5-nitro-2-3-phenylpropylamino) benzoic acid (NPPB) and ZnCl2. Based on these data a model is proposed which explains the N-shape of the I/V curves observed during the transient potential change of the alga E. viridis by the combination of an inward cation current with an inward anion current and the outward cation current carried by the Ca2+-dependent K+ channels.