Effect of hypoxia on ouabain inhibition of sodium pump in newborn rabbit myocardium.

The effects of hypoxia and reoxygenation on the inotropic effect of ouabain and sodium pump were studied in the isolated, arterially perfused newborn rabbit heart. Myocardial 86Rb+ active uptake was used as a marker of the sodium pump activity. During reoxygenation after mild hypoxia, 86Rb+ active uptake and ouabain effect were not significantly different from control. After severe hypoxia (with glucose) and reoxygenation, the inotropic effect of ouabain was not different from control, but mechanical toxicity occurred sooner. Severe hypoxia (with glucose-free solution) and reoxygenation caused an increase in tissue calcium, an attenuated inotropic effect of ouabain, and earlier mechanical toxicity. 86Rb+ active uptake was similar in all severe hypoxia groups and was significantly less than control values. Ouabain inhibition of 86Rb+ uptake in the severe hypoxia groups was not significantly different from control. These data indicate that severe hypoxia and reoxygenation depress the sodium pump but the effect of ouabain on the sodium pump in these muscles is similar to control. The decreased inotropic effect and increased toxicity of ouabain during reoxygenation after severe hypoxia may be due to an increase in cytoplasmic calcium and decreased tissue ATP.