Calcium and plant action potentials

Summary.The mechanism of Cl--channel activation in the plasmalemma of Nitellopsis obtusa was studied by measuring both the transient inward current under voltage clamp and Ct efflux during the action potential. 9-anthracenecarboxylic acid (A-9-C) at 1.0 mM inhibited both the transient inward current and the CIefflux, but did not uncouple the sudden cessation of the cytoplasmic streaming. Since this excitation-cessation coupling is caused by a transient increase in the cytoplasmic Ca ~* concentration, these results suggest that A-9-C inhibited not the Ca 2+ channel but specifically the CI-channel. The following results were found between the Ca2+-channel activation and the Cl--channel activation: (1) The Ca2+-channel blocker La 3+ uncoupled the excitation-cessation coupling and inhibited both the transient inward current and the CI-efflux, although the CI-channel blocker A-9-C did not affect the excitation-cessation coupling.(2) The C1-efflux was greatly reduced by depletion of Ca 2+ from the external solution and restored by an increase in the external Ca 2+ concentration. (3) An increase in the external ionic strength which increases Ca 2+ entry (T. Shiina & M. Tazawa, J. Membrane Biol. 96:263-276, 1987) enhanced the CI-efflux. (4) Mg 2+, which cannot pass through the Ca z+ channel, reduced both the transient inward current and the CI efflux. (5) Although Sr 2T can pass through the plasmalemma Ca 2+ channel, CI -channel activation by Sr 2+ was only partial. These findings support the hypothesis that voltage-dependent Ca;+-channel activation, which increases the free Ca 2+ concentration in the cytoplasm, is necessary for the subsequent Cl--channel activation.Key Words CaZ+-activated CI channel 9 Ca 2. channel 9 Characeae 9 C1-efflux 9 membrane excitation . Nitellopsis obtusa

Section: Ca 2+ Hypothesismentioning

confidence: 93%

Section: Ca 2+ Hypothesismentioning

confidence: 96%

Ca2+-activated Cl− channel in plasmalemma ofNitellopsis obtusa

Shiina

Tazawa

1987

“…Studies in diverse systems utilizing ion channel blockers indicate that Ca ?+ influx contributes to the depolarizing current (Weisenseel & Ruppert, 1977;Minorsky & Spanswick, 1989), although concomitant modulation of other transport processes, such as inhibition of the plasma membrane ATPase (Hanson, Rincon, & Rogers, 1986;Felle, 1988) or stimulation of CI efflux through anion channels (Beilby, 1984;Schroeder & Hagiwara, 1989) may also be required. Perpetuation of the signal transduction cascade, however, appears to be primarily dependent upon the elevation ofcytosolic free Ca 2+, which can actuate the ensuing physiological response through biochemical pathways involving calcium binding proteins (Marm6, 1989), protein phosphorylation (Blowers & Trewavas, 1989) and/or the regulation ofgene expression (Guilfoyle, 1989;Braam & Davis, 1990).…”

Section: Introductionmentioning

confidence: 98%

Cytosolic calcium regulates a potassium current in corn (Zea mays) protoplasts

Ketchum

Poole

1991

The voltage- and time-dependent K+ current, IK+ out, elicited by depolarization of corn protoplasts, was inhibited by the addition of calcium channel antagonists (nitrendipine, nifedipine, verapamil, methoxyverapamil, bepridil, but not La3+) to the extracellular medium. These results suggested that the influx of external Ca2+ was necessary for K+ current activation. The IC50, concentration of inhibitor that caused 50% reduction of the current, for nitrendipine was 1 microM at a test potential of +60 mV following a 20-min incubation period. In order to test whether intracellular Ca2+ actuated the K+ current, we altered either the Ca2+ buffering capacity or the free Ca2+ concentration of the intracellular medium (pipette filling solution). By these means, IK+out could be varied over a 10-fold range. Increasing the free Ca2+ concentration from 40 to 400 nM also shifted the activation of the K+ current toward more negative potentials. Maintaining cytoplasmic Ca2+ at 500 nM with 40 nM EGTA resulted in a more rapid activation of the K+ current. Thus the normal rate of activation of this current may reflect changes in cytoplasmic Ca2+ on depolarization. Increasing intracellular Ca2+ to 500 nM or 1 microM also led to inactivation of the K+ current within a few minutes. It is concluded that IK+out is regulated by cytosolic Ca2+, which is in turn controlled by Ca2+ influx through dihydropyridine-, and phenylalkylamine-sensitive channels.

“…We cannot understand this discrepancy between the measured Em and the calculated Eci-. The same discrepancy was also observed in Ca > channel of Characean plasmalemma (Lunevsky et al, 1983;Beilby, 1984;Shiina & Tazawa, 1987a). Since there is a considerably large resistance along the cell (Rs), the exact value of R,~ could not be measured.…”

Section: Resultsmentioning

confidence: 54%

Ca2+-dependent Cl− efflux in tonoplast-free cells ofNitellopsis obtusa

Shiina

Tazawa

1988

Summary. In order to demonstrate the presence of a Ca'--activated Cl--channel in the Nitellopsis plasmalemma, tonoplastfree cells were prepared and their intracellular Ca -'+ concentration was modified by internal perfusion. An increase in the Ca -'+ concentration caused a large CI efflux with a concomitant depolarization of the membrane potential. These changes were for the most part reversible. The critical Ca 2+ concentration was about 4.0/*M. Neither the CI-efflux nor the membrane depolarization showed a time-dependent inactivation. A C1 -channel blocker, A-9-C (9-anthracenecarboxylic acid) reduced both the C1 efflux and the magnitude of the membrane potential depolarization. A small increase in the intracellular Ca > concentration, which is caused by membrane excitation of tonoplast-free cells is not sufficient to activate this Ca2+-dependent Cl--channel.