Effects of ion transport inhibitors on D <inf>2</inf> O induced action potential in Characeae

Abstract

Substitution of the solvent H 2 O, with D 2 O, induced excitation in the Chara cell. The phenomenon. 'D 2 O-induced action potential' (DAP), was characterised by abrupt membrane depolarisation with incomplete repolarisation. Electrophysiological studies on the subcellular level (perfused plasmalemma and reconstituted channels in BLM) revealed a mechanism of channel activation similar to the electrically evoked action potential. In the present study the origin of DAP was analysed pharmacologically on intact internodal cells and thus the previous findings obtained on subcellular systems could be checked. The first pre-depolarisation DAP phase was significantly prolonged by depletion of external Ca 2+ with EGTA. The DAP spike was diminished with C1 - channel inhibitors, 9-anthracene-carboxilic acid and ethacrynic acid. The repolarisation phase was prolonged with tetraethylammonium chloride and Cs + (K + channel inhibitors) or could be abolished by less specific cation transport inhibitors, La 3+ and diethylstilbestrol. Thus, it was suggested, that the C1 - current was the main ion flux, underlying the DAP spike, and that it was activated by a Ca 2+ inward current (i.e. rise in [Ca 2+ ](i)), which was the first membrane response to D 2 O. In fact, D 2 O-induced rise in [Ca 2+ ](i) was observed by fura-2 fluorescence measurements. The K + channels could give the main contribution to the repolarisation phase, but they were apparently partially blocked by D 2 O.