Voltage-gated calcium and Ca2+-activated chloride channels and Ca2+ transients: voltage-clamp studies of perfused and intact cells of Chara

The voltage-clamp technique was used to study Ca2+ and Cl− transient currents in the plasmalemma of tonoplast-free and intact Chara corallina cells. In tonoplast-free cells [perfused medium with ethylene glycol bis(2-aminoethyl ether)tetraacetic acid] long-term inward and outward currents through Ca channels consisted of two components: with and without time-dependent inactivation. The voltage dependence of the Ca channel activation ratio was found to be sigmoid-shaped, with about −140-mV activation threshold, reaching a plateau at V>50 mV. As the voltage increased, the characteristic activation time decreased from approximately 103 ms in the threshold region to approximately 10 ms in the positive region. The positive pulse-activated channels can then be completely deactivated, which is recorded by the Ca2+ tail currents, at below-threshold negative voltages with millisecond-range time constants. This tail current is used for fast and brief Ca2+ injection into tonoplast-free and intact cells, to activate the chloride channels by Ca2+ . When cells are perfused with EDTA-containing medium in the presence of excess Mg2+, this method of injection allows the free submembrane Ca2+ concentration, [Ca2+]c, to be raised rapidly to several tens of micromoles per liter. Then a chloride component is recorded in the inward tail current, with the amplitude proportional to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \left[ {{\text{Ca}}^{{\text{2 + }}} } \right]_{\text{c}}^2 $$\end{document}. When Ca2+ is thus injected into an intact cell, it induces an inward current in the voltage-clamped plasmalemma, having activation–inactivation kinetics qualitatively resembling that in EDTA-perfused cells, but a considerably higher amplitude and duration (approximately 10 A m−2 and τinact~0.5 s at −200 mV). Analysis of our data and theoretical considerations indicate that the [Ca2+]c rise during cell excitation is caused mainly by Ca2+ entry through plasmalemma Ca channels rather than by Ca2+ release from intracellular stores.