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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.
Vick et al. (Sat,) studied this question.