Intracellular ionic activities in the EDL muscle of the mouse

The intracellular ionic concentrations of sodium, potassium and chloride in the mouse EDL muscle were measured by chemical analysis using inulin as the extracellular marker. Cellular concentrations of 157 +/- 8, 38 +/- 3, 44 +/- 5 mmol kg-1 intracellular water were estimated for potassium, sodium and chloride respectively. The resting membrane potential was measured by a conventional microelectrode filled with 3 mol KCl and found to be -76 +/- 0.5 mV. Ion-selective microelectrodes were used to measure the intracellular ionic activities of potassium, sodium and chloride. The activities measured were 117 +/- 5, 16 +/- 2, 5 +/- 0.1 mmol l-1 for potassium, sodium and chloride respectively. Apparent activity coefficients for the intracellular ions were calculated. The observed discrepancies between the extracellular activity coefficient and the calculated apparent intracellular activity coefficients for sodium and chloride might be explained in terms of the binding to cellular macromolecules and/or the compartmentalisation of these ions. Potassium appears uniformly distributed throughout the cellular water. Intracellular chloride activity was similar to that predicted by the Donnan distribution and it is concluded, therefore, that chloride is distributed at electrochemical equilibrium.