Ouabain effect on myocardial mechanical function and sodium pump in the fetus.

Abstract

Effects of ouabain (0.75, 1, 1.5, 2.5 or 5 X 10(-6) M) on the mechanical function and sodium pump were studied in the isolated arterially perfused fetal and newborn rabbit heart. Measurement of myocardial 86Rb+ active uptake was used as a marker of the sodium pump activity. The inotropic effect of ouabain in the fetus was not significantly different from that in the newborn; 2.5 X 10(-6) M ouabain caused mechanical toxicity (decrease in +dT/dtmax and increase in resting tension) in the fetus but not in the newborn. After ouabain infusion, both the inhibition of Rb+ uptake and the significant decrease in tissue potassium content were similar in the fetus and the newborn. In the fetal muscle perfused with low calcium (0.5 mM) solution, mechanical toxicity of ouabain was significantly less than in the control (1.5 mM Ca2+) solutions. High extracellular calcium (30 mM) per se caused mechanical toxicity in the fetus but not in the newborn. These data indicate that, in the isolated arterially perfused heart preparation, mechanical toxicity in the fetus is observed at lower ouabain concentrations than in the newborn. This difference in mechanical toxicity may not be explained by the age-related difference in sodium pump activity. The greater calcium toxicity in the fetus may be the reason for the increased ouabain toxicity in this age group.