The Ouabain-Binding Site of the α2 Isoform of Na,K-ATPase Plays a Role in Blood Pressure Regulation During Pregnancy

The four catalytic α isoforms of Na,K-ATPase, α1, α2, α3, and α4, have been identified, exhibiting unique tissue distribution, substrate affinity, and physiological roles.1,2 The cardiotonic steroid/ouabain-binding site in the α subunit of Na,K-ATPase is highly conserved among different species, suggesting physiological importance of this binding site. In order to study the significance of the ouabain-binding site of α2 Na,K-ATPase, we developed genetically modified mice with the mutations of L111R and N122D, designated as the α2R/R mice that are relatively resistant to inhibition by ouabain compared to the ouabain-sensitive wild-type α2S/S mice.3 We previously demonstrated the ouabain-binding site of the α2 Na,K-ATPase is involved in adrenocorticotropic hormone (ACTH)–induced hypertension.4,5 The α2S/S mice developed hypertension after 5 days of ACTH treatment, whereas the α2R/R mice did not exhibit an increase in blood pressure. This suggests that during ACTH treatment, endogenous cardiotonic steroids are released and interact with the ouabain-sensitive cardiotonic steroid–binding site in the α2S/S mice. Moreover, there is some evidence that endogenous cardiotonic steroids regulate cardiovascular function, renal homeostasis, and hemodynamics by their direct interaction with the α subunit of Na,K-ATPase.3,6–10 Normal pregnancy involves a number of maternal physiological changes to meet the demands of fetal development.11,12 Particularly, a continuous increase in extracellular fluid and plasma volume, and a significant decrease in systemic vascular resistance are essential physiological adaptations that begin in the first trimester.13–15 The gestational plasma volume expansion is characterized by renal Na+ and fluid retention. During normal pregnancy, natriuresis in response to atrial natriuretic peptide is remarkably reduced in rats16 and goats.17,18 The upregulation of aquaporin 2, a water channel expressed in the renal collecting duct, and a reduction of the osmotic thresholds for the vasopressin secretion and the perception of thirst during gestation also result in increased water intake and retention.14,19,20 Interestingly, in normotensive human pregnancy, atrial natriuretic peptide maintains its natriuretic effect, suggesting species differences in the renal response to atrial natriuretic peptide after conception.21 Despite the substantial volume expansion during pregnancy, blood pressure of pregnant females is significantly lower compared to their preconceptional blood pressure.22–24 This is attributed to peripheral vasodilation causing a considerable fall in systemic vascular resistance. As a result of the gestational decline in systemic vascular resistance, a compensatory increase in cardiac output is profound in both mid- and late gestation compared to nonpregnant animals.25 The physiological significance of cardiotonic steroids in the cardiovascular system during pregnancy has been implicated in the pathogenesis of a pregnancy-induced hypertensive disorder, pre-eclampsia. Pre-eclampsia is characterized by the onset of hypertension and proteinuria after midgestation, and a leading cause of maternal and fetal morbidity and mortality, affecting 3–8% of all pregnancies.26 In pre-eclampsia rats and human subjects, renal excretion of a cardiotonic steroid, marinobufagenin, and plasma marinobufagenin level increased remarkably compared to the normal pregnancy.22,27 Administration of the antimarinobufagenin antibody to preeclamptic animals and humans restores the Na,K-ATPase activity in thoracic aorta and erythrocyte, respectively.22,27 Moreover, the elevated systolic blood pressure (SBP) is decreased to the normal pregnancy level when the antibody was administered.22,27 This suggests that the cardiotonic steroid–binding site of Na,K-ATPase might be involved in the regulation of blood pressure during pregnancy. The main goal of this study was to investigate the physiological role of the ouabain-binding site of the α2 Na,K-ATPase in gestational blood pressure regulation.