Ouabain-Like Na⁺, K⁺-ATPase Inhibitory Activity of a Plasma Extract in Normal Pregnancy and Pregnancy Induced Hypertension

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To estimate the participation of a Na⁺, K⁺-ATPase-inhibiting plasma foctor in pregnancy induced hypertension (PIH), the inhibitory activity and the characteristics of plasma extract eluted with ethanol through a C8 column were examined in normotensive non-pregnant women (N) and women with normal pregnancy (NP) and PIH. There were no differences among the 3 groups in the Na⁺, K⁺-ATPase activity of erythrocyte ghosts. The heat- and acid-stable plasma extract dose-dependently inhibited Na⁺, K⁺-ATPase activity with a pattern similar to that of ouabain, but different from that of vanadate. The inhibitory activity of plasma extract was not influenced by polyclonal digoxin antibody which almost completely prevented digoxin-induced inhibition and slightly but significantly reduced the ouabain-induced one. The results indicate that the plasma extract has ouabain-like inhibitory activity on Na⁺, K⁺-ATPase and that it is not endogenously synthesized digoxin itself, but a substance differing in structure from digoxin. Furthermore, the ouabain-like Na⁺, K⁺-ATPase inhibitory activity in NP plasma was significantly lower than that in PIH plasma, which was similar to that in N plasma. There were significant relationships between the ouabain-like Na⁺, K⁺-ATPase inhibitory activities in plasma and the diastolic and systolic blood pressures in NP and PIH groups. The results suggest that the lower ouabain-like Na⁺, K⁺-ATPase inhibitory activity in plasma probably participates in maintaining the blood pressure within the normal range during pregnancy and its failure may be involved in the genesis of PIH.