Ca2+-induced activation and irreversible inactivation of chloride channels in the perfused plasmalemma of Nitellopsis obtusa.

Experiments were carried out on the algal cells with removed tonoplast using both continuous intracellular perfusion and voltage clamp on plasmalemma. The transient plasmalemma current induced by depolarizatio n disappeared upon perfusion with the Ca2+-chelating agent, EGTA, since the voltage-dependent calcium channels lost their ability to activate. Subsequent replacement of the perfusion medium containing EGTA by another with Ca2 + for clamped plasmalem­ ma (-100 mV) induced an inward Cľ current which showed both activation and inactivation. The maximal amplitude of the current at (CI )in = 15 mmol/1 (which is similar to that in native cells) was approximately twice that in electrically excited cell in vivo. The inactivation of CI channels in the presence of internal Ca2+ was irreversible and had a time constant of 1—3 min. This supports our earlier suggestion (Lunevsky et al. 1983) that the inactivation of CI channels in an intact cell (with a time constant of 1—3 s) is due to a decrease in Ca 2+ concentration rather than to the activity of their own inactivation mechanism. The CI channel selectivity sequence was following: Crs>CH 3 SOl~K + i>S 04 _ (PK/Pso4~10). Activation of one half the channels occurs at a Ca2+ concentration of 2 x 10 5 mol /l. Sr2+ also (though to a lesser extent) activated CI channels but had to be present in a much more higher concentration than Ca2+ .Mg2+ and Ba2+ appeared ineffecti­ ve. Ca2+ activation did not, apparently, require participation of water-soluble intermediator including ATP. Thus, CI channel functioning is controlled by Ca 2+-, Sr2+-sensitive elements of the subplasmalemma cytoskeleton.