Potassium loss from hypoxic myocardium: influence of external K concentration

The influence of external potassium Ko and tetraethylammonium on the cellular K content of hypoxic myocardium was investigated. Perfused rabbit hearts were submitted to 60 min hypoxia in medium containing 5 mM K throughout or either low (1.5 mM) or high (10 mM) K during the last 20 min of hypoxia. Paced electrical activity (2.5 Hz) was kept throughout the experiments. Tissue samples excised from the left ventricle were analyzed for total water, inulin space, and Na and K content. Lowering Ko to 1.5 mM increased both K loss and Na accumulation. Addition of 3.5 mM RbCl under these conditions reversed Na accumulation to levels found for hypoxia in normal medium but did not modify the cellular K loss. Tetraethylammonium (10 mM) did not alter Na accumulation but partly prevented the decrease in K content produced by hypoxia. A similar effect was observed by increasing Ko to 10 mM. At this high Ko prolongation of hypoxia did not enhance K loss. Abolition of electrical activity by TTX in a high K solution prevented K loss and reduced the sodium content. These results are consistent with the view that voltage-dependent channels are implicated in the K loss induced by hypoxia or ischemia. Furthermore, they indicate that the K loss may be modulated by external K because of the influence of the electrochemical gradient on passive K efflux and thus provide an explanation for the existence of a plateau in the early extracellular K accumulation observed during cardiac ischemia.