DOCA-Salt Hypertension Does Not Require the Ouabain-Sensitive Binding Site of the α2 Na,K-ATPase

The development of hypertension is firmly associated with high salt intake, and reducing daily salt intake results in a decrease in both systolic blood pressure (SBP) and diastolic blood pressure of mildly hypertensive patients.1–3 It is, therefore, vital to understand the detailed mechanisms relating salt intake and blood pressure regulation. In this regard, isolation of endogenous cardiotonic steroids such as ouabain and marinobufagenin from human and experimental animals, and findings of their involvement in cardiovascular and renal diseases, have a significant impact on the understanding of complex mechanisms of hypertension.4 Although there are inconsistent findings about the levels of plasma endogenous ouabain after salt administration, acute NaCI loading, deoxycorticosterone acetate (DOCA)-salt treatment, and high-salt diet have been shown to be associated with an increase in plasma ouabain in rats5,6 and humans.7 Augmented levels of plasma marinobufagenin have been detected in acute plasma volume expansion,8 high-salt diet,9 pregnancy-related hypertension,10 essential hypertension, primary aldosteronism, renal failure,11 and adrenocorticotropic hormone (ACTH)-induced hypertension.12 Furthermore, Huang et al. and Krep et al. reported that central and peripheral administration of Digibind, an antibody Fab fragment to digoxin, significantly lowered the blood pressure of hypertensive high-salt and DOCA-salt treated rats.13,14 Na,K-ATPase consists of three subunits, α, β and FXYD protein family.15 Four α isoforms of Na,K-ATPase, α1–α4 have been identified and show variable tissue distribution: the αl isoform is expressed abundantly in most tissues; the α2 isoform is detected in brain, heart, skeletal and vascular smooth muscle, and adipocytes; the α3 isoform is predominant in neurons and ovaries; the α4 isoform is exclusively expressed in sperm. In most mammals, including humans, all four α isoforms are sensitive to ouabain, but in mice and rats, the αl Na,K-ATPase is remarkably resistant to ouabain,16,17 leading investigators to postulate that the sensitive α2 isoform plays an important regulatory role in the cardiovascular system, despite its more restricted expression pattern. In an attempt to explore the specific role of these various isoforms, we have generated genetically modified mice with specifically altered sensitivities to ouabain: a ouabain-resistant α2 Na,K-ATPase (α1R/Rα2R/R) and a ouabain-sensitive (“humanized”) α1 Na,K-ATPase (αS/Sα2R/R).18,19 We have shown that mice expressing either a ouabain-sensitive α1 or α2 isoform (α1R/Rα2S/S or α1S/Sα2R/R) can develop ACTH-dependent hypertension, whereas the mice with two resistant isoforms (α1R/Rα2R/R) remain normotensive during ACTH treatment. α1R/Rα2R/R mice also show lower blood pressure during pregnancy.20 By contrast, we found that 2-kidney, 1-clip renovascular hypertension is equivalent in all three genotypes, and therefore does not depend on a ouabain-sensitive α1 or α2 subunit.21 Since previous studies have specifically reported a link between DOCA-salt hypertension and endogenous cardiotonic steroids, the present study was performed to further investigate the role of the cardiotonic steroid-binding site of Na,K-ATPase in DOCA-salt hypertension, as another model of high blood pressure. Our findings indicate that DOCA salt hypertension does not require a sensitive α2 subunit.