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 Ca(2+) 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 10(3) 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 Ca(2+) tail currents, at below-threshold negative voltages with millisecond-range time constants. This tail current is used for fast and brief Ca(2+) injection into tonoplast-free and intact cells, to activate the chloride channels by Ca(2+) . When cells are perfused with EDTA-containing medium in the presence of excess Mg(2+), this method of injection allows the free submembrane Ca(2+) concentration, [Ca(2+)](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 [see text]. When Ca(2+) 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 tau(inact)~0.5 s at -200 mV). Analysis of our data and theoretical considerations indicate that the [Ca(2+)](c) rise during cell excitation is caused mainly by Ca(2+) entry through plasmalemma Ca channels rather than by Ca(2+) release from intracellular stores.
Berestovsky
Genrikh N
GN
Institute of Cell Biophysics, Russian Academy of Sciences, 142290 Pushchino, Moscow Region, Russia. gberest@icb.psn.ru
Kataev
Anatoly A
AA
D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002478:Cells, Cultured; D044784:Chara / Q000187:drug effects / Q000502:physiology*; D018118:Chloride Channels / Q000187:drug effects / Q000502:physiology*; D002713:Chlorine / Q000378:metabolism; D003198:Computer Simulation; D004492:Edetic Acid / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D008954:Models, Biological; D018408:Patch-Clamp Techniques; D010477:Perfusion
D015220:Calcium Channels; D018118:Chloride Channels; D002713:Chlorine; D004492:Edetic Acid; D002118:Calcium