Distribution of Potassium, Sodium, and Chloride in Canine Purkinje and Ventricular Tissues

Cellular resting and action potentials and total tissue concentrations of K+, Na+, and Cl− were measured in canine myocardial and Purkinje tissues. The results with myocardium were consistent with those of others: the total tissue concentration of K+ was greater than that of Na+ of Cl− In Purkinje tissue, the total tissue concentration of K+ was relatively low, while that of Na+ and of Cl− was high. Extracellular space was calculated on the basis of four different assumptions about the intracellular concentration of one of the ions: (1) the intracellular concentrations of Cl− is zero; (2) the electrochemical equilibrium potential of Cl− equals the cellular resting potential; (3) the electrochemical equilibrium potential of K+ equals the cellular maximum diastolic potential; (4) the electrochemical equilibrium potential of Na+ equals the reversal of the cellular action potential. In the myocardium, the lowest estimate of extracellular space was 14±2%, and the highest was 18±2%. In the Purkinje tissue, the minimum extracellular space: (1) permitting adequate intracellular K+ to account for the maximum diastolic potential was 45±3%; (2) consistent with passive distribution of Cl− was 61±4%; (3) yielding low enough intracellular concentration of Na+ to account for the reversal of the action potential was 63±10%. These results suggest that in cardiac Purkinje tissue: (1) the extracellular space is unusually large with respect to that of myocardium and skeletal muscle; (2) intracellular activities of ions do not necessarily equal concentrations; or (3) intracellular space is not necessarily a single compartment.