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The characteristics of Ca ++ ‐activated Cl – channels of the salt‐tolerant charophyte Lamprothamnium
Author(s) -
BEILBY MARY J.,
SHEPHERD VIRGINIA A.
Publication year - 2006
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1111/j.1365-3040.2005.01437.x
Subject(s) - reversal potential , conductance , chemistry , membrane potential , biophysics , patch clamp , analytical chemistry (journal) , biochemistry , chromatography , biology , physics , receptor , condensed matter physics
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 × 10 −7 to 0.17 × 10 −7 ms −1 in 19 min), with the positive half activation PD, V 50+ , shifting from +35 to −65 mV, and the negative half activation PD, V 50– , 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 I s 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.