The effects were studied of hypoxia on intracellular ion activities in sheep heart Purkinje fibres. The intracellular pH (pHi), surface pH (pHs), intracellular potassium activity (aki), and intracellular sodium activity (aNai) of the cells were recorded using liquid ion exchanger-filled microelectrodes. Various methods of inhibiting oxidative phosphorylation were compared for their effect on pHi. These methods were the use of hypoxia, anoxia or NaCN (2 mM). Hypoxia was produced by degassing solutions under reduced pressure then bubbling with 100% nitrogen gas. Anoxia was produced in a similar manner but with the addition of the reducing agent sodium dithionite (0.5 mM) to remove all traces of oxygen from the solutions. Anoxia caused the most marked changes. Concentration of sodium dithionite between 0.1 and 1 mM produced similar maximum rates of acidification. High concentrations (5 or 20 mM) could produce larger intracellular acidifications apparently unrelated to anoxia. The effects of hypoxia and NaCN were similar. Inhibition of Na(+)-H+ exchange with amiloride (1 mM) had little effect on the pH changes produced by hypoxia. Periods of hypoxia exceeding 1 h still resulted in rapid, readily reversible changes in pHi. Hypoxia caused a rise in aNai, the effect being larger in anoxic conditions. The intracellular K+ activity decreased in hypoxia with further decreases in anoxic conditions. The intracellular ion changes produced during hypoxia are discussed in terms of the production of lactic acid by the cells and changes in the ATP supply to the Na(+)-K+ pump.

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