Membrane potential fluctuations in Chara australis: a characteristic signature of high external sodium.

We have studied fluctuations in membrane PD in Chara australis at frequencies between 1 and 500 mHz, by classical noise analysis and by inspection of the PD time-course. The former shows (1) a quasi-Lorentzian (1/f (2)) rise of noise power as frequency falls, and (2) a marked increase in noise power when the cell is exposed to high salinity (Chara australis is a salt-sensitive species). The latter shows that, as well as initiating depolarization, exposure to 50 mM Na as either chloride or sulfate usually initiates a continuous but random series of small depolarizations which gives rise to the increase in noise and whose mechanism is discussed.

Mechano-perception in Chara cells: the influence of salinity and calcium on touch-activated receptor potentials, action potentials and ion transport.

This paper investigates the impact of increased salinity on touch-induced receptor and action potentials of Chara internodal cells. We resolved underlying changes in ion transport by current/voltage analysis. In a saline medium with a low Ca(2+) ion concentration [(Ca(2+))(ext)], the cell background conductance significantly increased and proton pump currents declined to negligible levels, depolarizing the membrane potential difference (PD) to the excitation threshold [action potential (AP)(threshold)]. The onset of spontaneous repetitive action potentials further depolarized the PD, activating K(+) outward rectifying (KOR) channels. K(+) efflux was then sustained and irrevocable, and cells were desensitized to touch. However, when [Ca(2+)](ext) was high, the background conductance increased to a lesser extent and proton pump currents were stimulated, establishing a PD narrowly negative to AP(threshold). Cells did not spontaneously fire, but became hypersensitive to touch. Even slight touch stimulus induced an action potential and further repetitive firing. The duration of each excitation was extended when [Ca(2+)](ext) was low. Cell viability was prolonged in the absence of touch stimulus. Chara cells eventually depolarize and die in the saline media, but touch-stimulated and spontaneous excitation accelerates the process in a Ca(2+)-dependent manner. Our results have broad implications for understanding the interactions between mechano-perception and salinity stress in plants.

The characteristics of Ca -activated Cl- channels of the salt-tolerant charophyte Lamprothamnium.

The dependence of the Ca++-activated Cl- channels on potential difference (PD) was extracted from current-voltage (I/V) profiles recorded at the time of hypotonic regulation while the large conductance (G) K+ channels were blocked by tetraethylammonium (TEA). The total clamp current (I) was dominated by the Cl- I, i(Cl), with small contribution from the background I (i(background)). The i(Cl) was fitted by the Goldman-Hodgkin-Katz (GHK) model with enhanced PD dependence simulated by Boltzmann probability distributions. The i(background) was modelled by an empirical equation. The i(Cl) responded to PD changes within tens of milliseconds. The G maxima were located between -20 and -150 mV. The Cl- channel number and channel permeability parameter, N(Cl)P(Cl), decreased as a function of time in a hypotonic medium (from 0.45 x 10(-7) to 0.17 x 10(-7) ms(-1) in 19 min), with the positive half activation PD, V50+, shifting from +35 to -65 mV, and the negative half activation PD, V50-, shifting from -134 to -310 mV. The fitted Cl- concentration [Cl-]cyt at the time of hypotonic regulation indicated rapid equalization of vacuolar and cytoplasmic concentrations. Excellent data obtained under similar experimental conditions in a previous study enabled us to infer [Ca++]cyt influences on the Cl- channel characteristics. Thick sulphated polysaccharide mucilage, found on Lamprothamnium cells acclimated to more saline media, eliminated the activation of the i(Cl) at the time of the hypotonic regulation. This effect was reversed by the application of the enzyme heparinase. The characteristics of the i(Cl) were found to be consistent with a component of the excitation Is at the time of the action potential (AP). The short duration of the excitation transients was contrasted with that of the hypotonic regulation. The mechanisms for Cl- channel activation (and hence the Ca++ channel activation) were considered.