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Department of Cardiac Medicine, National Heart and Lung Institute, London.

1. Intracellular Na+ activity (aiNa) has been measured in Purkinje fibres from sheep heart and in ventricular muscle from rabbit heart during hypoxia and metabolic inhibition. The aiNa was measured using liquid sensor ion-sensitive microelectrodes. 2. Hypoxia, produced by replacement of O2 with N2 in the superfusate, produced an increase in aiNa. This increase was larger if sucrose replaced glucose in the superfusing Tyrode solution. The increase in aiNa was accompanied by a small depolarization. Upon reoxygenation aiNa decreased and cells rapidly repolarized. 3. When oxidative phosphorylation was inhibited by application of 2 mM-cyanide, aiNa increased. This increase was also accompanied by a small depolarization. Upon removal of cyanide, aiNa and membrane potential recovered to control levels. 4. After inhibiting glycolysis, by replacing glucose with 2-deoxy-D-glucose, inhibition of oxidative phosphorylation (by addition of cyanide or exposure to hypoxia) produced a much more rapid increase in aiNa and a large contracture. The rise in aiNa and the occurrence of a contracture could not be inhibited by application of amiloride (1 mM) or tetrodotoxin (1 microgram ml-1). Removal of cyanide or reoxygenation and replacement of glucose resulted in a rapid relaxation of the contracture and a slower decrease in aiNa. 5. The relative rates of increase in aiNa during metabolic inhibition were compared with the rate observed when Na+-K+-ATPase was inhibited by application of 10 mumols l-1 of the cardio-active steroid strophanthidin. The rate of increase of aiNa when both oxidative phosphorylation and glycolysis were inhibited was approximately twice that observed with only oxidative phosphorylation inhibited and approximately half that observed in the presence of 10 microM-strophanthidin. 6. Cyanide, applied when aiNa had been elevated (i.e. during exposure to 10 microM-strophanthidin to inhibit Na+-K+-ATPase), did not produce a contracture. The contracture observed in the presence of cyanide and 2-deoxy-D-glucose still occurred when Ca2+ was removed from the superfusate.

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