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Department of General Physiology and Biophysics, Panum Institute, University of Copenhagen, Denmark.

1. 36Cl- efflux rates were measured in depolarized fibre bundles (predominantly fast fibres) from m. semitendinosus of Rana temporaria in order to investigate the influence of chloride concentration (20-400 mM) and external pH (pHo, 5.5-11.6) on chloride self-exchange. Usually, the bundles were depolarized to a membrane potential of virtually zero ([Cl-]i = [Cl-]o = [Cl-]) in 140 mM-K+ and 20 mM-Cl- combined with either 60 mM-SO4(2-) (+52 mM-sucrose) or 120 mM-methyl sulphate, gluconate or glucuronate. Various values of [Cl-] were obtained by adding KCl to the medium, keeping constant the concentration of relatively impermeant solutes and thus the fibre volume. 2. The chloride permeability, PCl, at pH 7.5 decreased with increasing [Cl-] above 50 mM. At [Cl-] less than 50 mM, PCl was about 4 X 10(-6) cm s-1. 3. PCl at pHo 7.5 and 5.6 was a hyperbolic function of [Cl-1] (greater than or equal to 50 mM), consistent with saturation kinetics of chloride self-exchange. 4. Methyl sulphate inhibited chloride self-exchange and may enter depolarized fibre membranes slowly. Comparison of PCl estimates in SO4(2-), gluconate, and glucuronate suggested that SO4(2-) and gluconate may interfere with chloride self-exchange at low [Cl-], and that glucuronate interferes the least. 5. A change from chloride equilibrium to an inward KCl gradient increased the 36Cl- efflux rate in acid media (pH less than 6.5) and decreased the efflux rate at pHo greater than or equal to 7. The effects were not due to changes of membrane potential and demonstrate a change of transport mode consistent with single-filing at high pH and dominant exchange diffusion at low pH. This result makes interpretation of the saturation kinetics difficult. 6. At constant [Cl-], PCl showed a bell-shaped dependence on pHo with a maximum around pH 8.5. The decrease of chloride permeability as the pH is decreased below 8.5 showed an apparent pK of about 7. PCl fell at very alkaline pH with an apparent pK of about 10. 7. The results are in agreement with previous observations of an external pH-sensitive control of PCl (pK approximately 7) and suggest that, in conditions close to chloride equilibrium, this control includes a transition between a dominant conductive channel conformation at high pH and a dominant non-conductive conformation at low pH.(ABSTRACT TRUNCATED AT 400 WORDS)