Characteristics of the anion conductance of muscle fibres from green sunfish have been determined. The membrane conductance of fibres from sunfish acclimated to 25 and 7 degrees C was linearly related to the extracellular chloride concentration. The chloride conductance exhibited a pH dependence which was adequately described by the titration of an acidic site with a pKa of 5.3 in 25 degrees C-acclimated fibres and 6.4 in 7 degrees C-acclimated fibres. The anion current-voltage (I-V) relationship of warm-acclimated fibres exhibited constant-field rectification, while the I-V relationship of cold-acclimated fibres was linear. In Ringer solutions containing elevated calcium concentrations (33 and 115 mmol 1(−1)), the I-V relationship of warm-acclimated fibres was similar to the control situation. However, the I-V relationship of cold-acclimated fibres showed a calcium concentration-dependent curvature in the direction expected for constant-field rectification. The voltage-dependence of the time constant of chloride current inactivation was shifted along the voltage axis by about 40 mV in the negative direction in 7 degrees C-acclimated fibres as compared to 25 degrees C-acclimated fibres. The results can be adequately described by a model of constant-field rectification with the inclusion of a term for the membrane surface potential. A simple hypothesis which can qualitatively account for many of these observations is that temperature acclimation alters the density of fixed negative surface charges in the vicinity of chloride channels.

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