Conductance properties and voltage dependence of an anion channel in amphibian skeletal muscle

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Abstract
Single anion-selective channels were studied in excised membrane patches of adult frog toe muscle. The conductance γ and the probabilityp o of the main open state were determined for various ionic compositions of the extra-and intracellular solutions. γ=280 pS in symmetrical 110 mM NaCl, pH 7.4 solutions at 15°C. Higher γ values were found at elevated internal or external NaCl concentrations, in 70 mM external CaCl2 and at lower extracellular pH. Thep o(E) curve declined steeply with hyperpolarization and was shifted towards more negative potentials at increased internal ionic strength and at higher external pH. Positive shifts were induced by extracellular Ca. The results show that the anion channel saturates at Cl concentrations >110 mM, that the potential profile of an open channel is almost symmetrical and that channel gating is affected by neighboring channels. It is suggested that the anion channel has a voltage sensor (effective gating charge 4.3) and a similar collection of local fixed charges on the extra- and intracellular sides as voltage-gated cation channels.