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

Abstract

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.

The effects of Ni2+ on electrical signaling of Nitellopsis obtusa cells

“…Excitation threshold in Characeaen cells is determined not only by membrane voltage but by calcium channel activation as well (Beilby, 2007). It is known that the amplitude of the Cl − current is proportional to the square of the cytoplasmic Ca 2+ concentration in plants (Berestovsky and Kataev 2005). Reversibility of excitation threshold enhancement in our data points to Ni 2+ /Ca 2+ competition or Ni effect on calcium channels from external side.…”

Transient removal of alkaline zones after excitation ofCharacells is associated with inactivation of high conductance in the plasmalemma

“…[22][23][24] This may explain why the large inactivation of plasma membrane conductance in alkaline regions remained undiscovered until now. The pH band formation in characean internodes requires irradiance above the threshold level ~5 μE m -2 s -1 .…”

“…The conductances of formerly acid and alkaline cell regions became almost even in darkness (Table 1), while longer dark incubation (1-2 h) might be needed to attain completely uniform G m distribution. 16 The possible reason for similar effects of excitatory stimulation and darkening on membrane conductance is that both treatments elevate the cytoplasmic Ca 2+ level, 24,27 which might affect ion permeabilities. The combined influence of AP-associated cytosolic Ca 2+ increase and cytoplasm alkalinization upon the blockade of G m and H + influx in the alkaline cell areas might underlie the effect of AP on chloroplast functioning (Fig.…”

Effects of hydrolysable tannins on native and artificial biological membranes

“…Currents (I = I Ca + I Cl ) evoked by voltage steps were measured using a classical four electrode techniques in a voltage clamp mode on a 2 mm long working space of the alga [29][30][31]. A cell diameter ranged from 0.6 to 0.9 mm.…”

“…By the time chloride current reaches its maximum, calcium current is already inactivated [29,31]. There fore, calcium current could not impact a peak value of chloride current.…”