Endogenous Ligand of α ₁ Sodium Pump, Marinobufagenin, Is a Novel Mediator of Sodium Chloride–Dependent Hypertension

Abstract

Background-Digitalis-like sodium pump ligands (SPLs) effect natriuresis via inhibition of renal tubular Na ϩ ,K ϩ -ATPase but may induce vasoconstriction. The present study investigated the potential roles of 2 putative endogenous SPLs, an ouabain-like compound (OLC) and an ␣ 1 Na ϩ ,K ϩ -ATPase inhibitor, marinobufagenin (MBG), in regulating natriuresis and blood pressure (BP) responses to sustained and acute NaCl loading in Dahl salt-sensitive rats (DS). Methods and Results-During 4 weeks of an 8% NaCl diet, DS exhibited a progressive increase in MBG renal excretion (66Ϯ13 pmol/24 hours at week 4 versus 11Ϯ1 pmol/24 hours at baseline, nϭ48), which paralleled an increase in systolic BP (174Ϯ10 mm Hg at week 4 versus 110Ϯ2 mm Hg at baseline). By contrast, OLC excretion peaked at week 1 and returned to baseline levels. Administration of an anti-MBG, but not anti-ouabain antibody, to DS after 3 weeks of a high NaCl diet lowered BP (139Ϯ7 versus 175Ϯ5 mm Hg, PϽ0.001, nϭ5). Acute NaCl loading (2 hours) of DS (nϭ5) increased MBG and OLC excretion and natriuresis. Pretreatment of acutely NaCl-loaded DS with an anti-MBG antibody (nϭ5) reduced the excretion of sodium and MBG but not that of OLC. An anti-ouabain antibody (nϭ5) reduced sodium excretion and both OLC and MBG. Key Words: hypertension Ⅲ Na ϩ -K ϩ -exchanging ATPase Ⅲ sodium Ⅲ bufadienolides Ⅲ ouabain T he role of endogenous digitalis-like sodium pump ligands (SPLs) in the pathogenesis of hypertension has been disputed for almost 3 decades. 1 Endogenous SPLs are believed to promote natriuresis via renal Na ϩ pump inhibition and may link the Na ϩ retention that occurs during NaCl loading to an increase in arterial pressure via inhibition of the Na ϩ ,K ϩ -ATPase in cardiovascular tissues. [2][3][4] An endogenous ouabain-like compound (OLC) was the first mammalian SPL to be purified. 5 Subsequent studies showed that mammals also produce another class of SPLs, ie, bufadienolides. 6 -8 Bufadienolides, which were discovered in Amphibia, inhibit Na ϩ ,K ϩ -ATPase and cross-react with digitalis antibodies. 9 In Amphibia, skin bufadienolides are responsible for Na ϩ excretion, 9 and bufadienolide levels in toads vary with changes in environmental salinity. 10 Several bufadienolides have been suggested as candidate SPLs in mammals, including marinobufagenin (MBG), 7 which acts in vitro as a vasoconstrictor 11,12 and exhibits greater affinity to rodent ouabain-resistant ␣ 1 isoform of the Na ϩ ,K ϩ -ATPase than to an ␣ 3 isoform. 11,13,14 MBG immunoreactive material purified from human and rat urine was found to be identical to MBG from the toad, Bufo marinus, by mass spectral analysis 15 and by its ability to inhibit rat kidney Na ϩ ,K ϩ -ATPase. 14 Enhanced MBG production occurs in volume-expanded dogs 16 and rats 17 and in pathological states associated with fluid retention, including preeclampsia, 12 hypertension in endstage renal disease, 18 and salt-sensitive hypertension in Dahl rats. 14 In NaCl-induced hypertension in Dahl salt-sensitive rats (DS), which exhibit...

Na/K-ATPase Tethers Phospholipase C and IP3 Receptor into a Calcium-regulatory Complex

“…Because there is sufficient evidence that ouabain and marinobufagenin are likely the endogenous cardiotonic steroids, our findings brings about an interesting question as to whether this mode of regulation is relevant to in vivo physiology. To this end, several laboratories have reported that endogenous ouabain and marinobufagenin are circulated at sub-nM to nM concentrations in normal individuals and that volume expansion can significantly increase their concentrations (Hamlyn et al, 1991;Fedorova et al, 2002;Bauer et al, 2005). Thus, it is conceivable that the described regulation may be relevant to in vivo physiopathology if future in vivo studies confirm our in vitro experiments.…”

Endogenous digitalis: pathophysiologic roles and therapeutic applications

“…After acute and chronic salt-loading in these animals, a transient increase in circulating endogenous ouabain precedes a sustained increase in circulating marinobufagenin. 82,83 This observation has led us to postulate that endogenous ouabain, acting as a neurohormone, triggers release of marinobufagenin, which in turn causes increases in cardiac contractility, peripheral vasoconstriction and natriuresis by inhibiting the Na + /K + -ATPase (Figure 3). 102,103 We have subsequently demonstrated that the greatest transient increases in brain endogenous ouabain level occur in the amygdala, hippocampus and supraoptic nucleus of the hypothalamus.…”

Endogenous sodium pump inhibitors, diabetes mellitus and preeclampsia

“…The placentas were removed and placental and fetal weights determined. Blood was collected for Na,K-ATPase measurements in erythrocytes [14], and for plasma MBG measurements [13].…”

“…Activity of NKA in erythrocytes was determined, as reported previously in detail [14]. Erythrocytes were preincubated with Tween-20 (0.5%) in sucrose (250 mmol/L) and Tris buffer (20 mmol/L; pH 7.4, 37°C) for 30 minutes, and were incubated for 30 minutes in the medium (mmol/L): Na 100, K 10, MgCl 2 3, EDTA 0.5, Tris 50, ATP 2 (pH 7.4, 37°C) in the final dilution 1:40.…”

“…One of the factors contributing to the pathogenesis of PE are CTS [3,10,11]. Pregnancy is associated with renal sodium retention [12,13], a major stimulus for MBG production [14]. In normal pregnancy, moderatly elevated levels of MBG do not alter vascular tone [10,13].…”