Intracellular chloride activity in the extensor digitorum longus (EDL) muscle of the rat

Measurement of intracellular chloride activity in the extensor digitorum longus (EDL) muscle of the rat using liquid ion-exchanger microelectrodes gave an apparent resting value of 10 to 11 mmol liter-1. If chloride ions were distributed passively across the muscle fiber membrane the predicted value would be 4 mmol liter-1. In experiments in which the bathing fluid was changed by (a) reduction of external chloride, or incubation at low external chloride followed by a return to normal concentrations, (b) an increase or reduction in external potassium, (c) alteration in potassium and chloride in the bathing medium so as to maintain a constant [K] X [Cl] product, or in other experiments in which the membrane potential was caused to change by anoxia, or by addition of ouabain to the medium, changes in intracellular chloride activity were invariably consistent with the hypothesis that this ion is passively distributed. Measurements of intracellular chloride activity with recessed-tip solid-state Ag/AgCl electrodes gave a value of 4.6 mmol liter-1. Since the liquid ion-exchanger is known to be poorly selective for chloride, it is concluded that the chloride ion is passively distributed in rat EDL muscle.