Left ventricular geometry and endogenous ouabain in a Flemish population

The α2 isoform Na,K-ATPase modulates contraction of rat mesenteric small artery via cSrc-dependent Ca2+ sensitization

AIMS

The Na,K-ATPase is involved in a large number of regulatory activities including cSrc-dependent signalling. Upon inhibition of the Na,K-ATPase with ouabain, cSrc activation is shown to occur in many cell types. This study tests the hypothesis that acute potentiation of agonist-induced contraction by ouabain is mediated through Na,K-ATPase-cSrc signalling-dependent sensitization of vascular smooth muscle cells to Ca²⁺ .

METHODS

Agonist-induced rat mesenteric small artery contraction was examined in vitro under isometric conditions and in vivo in anaesthetized rats. Arterial wall tension and [Ca²⁺ ]_i in vascular smooth muscle cells were measured simultaneously. Changes in cSrc and myosin phosphatase targeting protein 1 (MYPT1) phosphorylation were analysed by Western blot. Protein expression was examined with immunohistochemistry. The α1 and α2 isoforms of the Na,K-ATPase were transiently downregulated by siRNA transfection in vivo.

RESULTS

Ten micromolar ouabain, but not digoxin, potentiated contraction to noradrenaline. This effect was not endothelium-dependent. Ouabain sensitized smooth muscle cells to Ca²⁺ , and this was associated with increased phosphorylation of cSrc and MYPT1. Inhibition of tyrosine kinase by genistein, PP2 or pNaKtide abolished the potentiating effect of ouabain on arterial contraction and Ca²⁺ sensitization. Downregulation of the Na,K-ATPase α2 isoform made arterial contraction insensitive to ouabain and tyrosine kinase inhibition.

CONCLUSION

Data suggest that micromolar ouabain potentiates agonist-induced contraction of rat mesenteric small artery via Na,K-ATPase-dependent cSrc activation, which increases Ca²⁺ sensitization of vascular smooth muscle cells by MYPT1 phosphorylation. This mechanism may be critical for acute control of vascular tone.

On the Many Actions of Ouabain: Pro-Cystogenic Effects in Autosomal Dominant Polycystic Kidney Disease

Ouabain and other cardenolides are steroidal compounds originally discovered in plants. Cardenolides were first used as poisons, but after finding their beneficial cardiotonic effects, they were rapidly included in the medical pharmacopeia. The use of cardenolides to treat congestive heart failure remained empirical for centuries and only relatively recently, their mechanisms of action became better understood. A breakthrough came with the discovery that ouabain and other cardenolides exist as endogenous compounds that circulate in the bloodstream of mammals. This elevated these compounds to the category of hormones and opened new lines of investigation directed to further study their biological role. Another important discovery was the finding that the effect of ouabain was mediated not only by inhibition of the activity of the Na,K-ATPase (NKA), but by the unexpected role of NKA as a receptor and a signal transducer, which activates a complex cascade of intracellular second messengers in the cell. This broadened the interest for ouabain and showed that it exerts actions that go beyond its cardiotonic effect. It is now clear that ouabain regulates multiple cell functions, including cell proliferation and hypertrophy, apoptosis, cell adhesion, cell migration, and cell metabolism in a cell and tissue type specific manner. This review article focuses on the cardenolide ouabain and discusses its various in vitro and in vivo effects, its role as an endogenous compound, its mechanisms of action, and its potential use as a therapeutic agent; placing especial emphasis on our findings of ouabain as a pro-cystogenic agent in autosomal dominant polycystic kidney disease (ADPKD).

Lanosterol Synthase Gene Polymorphisms and Changes in Endogenous Ouabain in the Response to Low Sodium Intake

Circulating levels of endogenous ouabain (EO), a vasopressor hormone of adrenocortical origin, are increased by sodium depletion. Furthermore, lanosterol synthase, an enzyme involved in cholesterol biosynthesis, has a missense polymorphism (rs2254524 V642L) that affects EO biosynthesis in adrenocortical cells. Here, we investigated the hypothesis that lanosterol synthase rs2254524 alleles in vivo impact the blood pressure (BP) and EO responses evoked by a low dietary Na intake (<100 mEq/d, 2 weeks) among patients with mild essential hypertension. During the low salt diet, the declines in both systolic BP (SBP: -8.7±1.7 versus -3.0±1.5; P=0.013) and diastolic BP (DBP: -5.1±0.98 versus -1.4±0.94 mm Hg; P<0.05), and the slope of the long-term pressure-natriuresis relationship affected significantly the presence of the lanosterol synthase rs2254524 A variant (AA: 0.71±0.22, AC 0.09±0.13, and CC 0.04±0.11 mEq/mm Hg/24 h; P=0.028). In addition, BP rose in ≈25% of the patients in response to the low salt diet and this was associated with increased circulating EO. Lanosterol synthase gene polymorphisms influence both the salt sensitivity of BP and changes in circulating EO in response to a low salt diet. The response of BP and EO to the low salt diet is markedly heterogeneous. Approximately 25% of patients experienced adverse effects, that is, increased BP and EO when salt intake was reduced and may be at increased long-term risk. The augmented response of EO to the low salt diet further supports the view that adrenocortical function is abnormal in some essential hypertensives.

Left ventricular structure and function in relation to steroid biosynthesis genes in a white population

BACKGROUND

Both endogenous ouabain (EO) and aldosterone are steroid hormones which might play a role in the pathogenesis of left ventricular (LV) hypertrophy and cardiac remodeling. Cholesterol side-chain cleavage enzyme (CYP11A1) and 3β-hydroxysteroid dehydrogenase (HSD3B1) are two key enzymes in the pathway of steroid biosynthesis.

METHODS

We investigated in 532 individuals (mean age, 50.3 years; 51.5% women) randomly recruited from a white European population whether LV structure and function were related to genetic variations in CYP11A1 and HSD3B1. We measured LV structure by conventional echocardiography and LV diastolic function by Doppler imaging of the transmitral blood flow and the mitral annular movement. We genotyped tag single nucleotide polymorphisms (SNPs) rs2279357, rs11638442 and rs2073475 in CYP11A1, and rs2236780, rs3765945, and rs6203 in HSD3B1.

RESULTS

While adjusting for covariables and accounting for family clusters, LV mass index decreased (P ≤ 0.049) across the CYP11A1 genotypes in rs2279357 (CC vs. CT vs. TT), rs11638442 (GG vs. GC vs. CC), and rs2073475 (GG vs. GA+AA). Carriers of the CYP11A1 TCG haplotype had lower (P ≤ 0.017) LV mass and LV mass index than noncarriers. Carriers of HSD3B1 GCC haplotype had lower peak early (Ea; P = 0.004) and higher peak late (Aa; P = 0.066) diastolic mitral annular velocities and therefore a lower Ea/Aa ratio (P = 0.041) as compared with noncarriers. Neither plasma endogenous ouabain nor 24-h urinary aldosterone were related to any of the SNPs or haplotypes (P ≥ 0.07).

CONCLUSIONS

Pending confirmation in other studies, LV mass and LV diastolic function seem to be related to genetic variation in the steroid biosynthesis.

Association of left ventricular mass with the AGTR1 A1166C polymorphism

BACKGROUND

The A1166C polymorphism is located within the microRNA-155 binding site of the human angiotensin II (Ang II) type-1 receptor (AGTR1) gene. The C allele interferes with the base-pairing complementariness between AGTR1 mRNA and microRNA-155 and thereby increases AGTR1 protein expression in vitro. We hypothesized that left ventricular (LV) mass is associated with the AGTR1 A1166C polymorphism.

METHODS

Among 708 individuals (mean age, 49.4 years; 51.8% women) randomly recruited in a white European population, we measured LV structure by two-dimensional guided M-mode echocardiography, the AGTR1 A1166C polymorphism and the 24-h urinary aldosterone. We applied a mixed model to assess phenotype-genotype associations while adjusting for covariables and accounting for relatedness.

RESULTS

The AA (49.1%), AC (42.8%), and CC (8.1%) genotypes were in Hardy-Weinberg equilibrium. Using a recessive model, CC homozygotes compared to A-allele carriers showed significant increases (P < 0.021) in LV mass index (+5.78 ± 2.25 g/m(2)), mean wall thickness (MWT) (+0.48 ± 0.15 mm), interventricular septum (IVS) (+0.60 ± 0.18 mm) and posterior wall thickness (PWT) (+0.34 ± 0.15 mm), but lower 24-h urinary aldosterone excretion (geometric mean, 22.4 vs. 19.0 nmol; P = 0.050). Sensitivity analyses in 552 participants untreated for hypertension were confirmatory.

CONCLUSIONS

LV mass index is associated with the AGTR1 A1166C polymorphism. Further research should clarify to what extent this association might be mediated via different expression of AGTR1 as modulated by microRNA-155.

Polymorphisms, hypertension and thiazide diuretics

It is 10 years since the discovery of the human genome; however, the study of the influence of genetic variants on drug effect - pharmacogenomics - has so far failed to create a major impact on day-to-day prescription practices. In the present article we analyze the main findings in candidate gene variants, gene combinations and whole-genome scans in relation to diuretic treatment. A critical analysis of the main reasons for some contrasting results will be discussed. The hypertension phases, in clinical trials dealing with genes and related pathophysiological mechanisms, may account for these inconsistent findings. The use of previously untreated versus treated patients is addressed. Finally, a positive study with a new genetic molecular strategy is described.

Heritability of left ventricular structure and function in Caucasian families

AIMS

The aim of this study was to investigate the heritability as well as genetic and environmental correlations of left ventricular (LV) structural and functional traits in complex pedigrees of a Caucasian population.

METHODS AND RESULTS

We randomly recruited 459 white European subjects from 52 families (50% women; mean age 45 years). LV structure was measured by M-mode and 2D echocardiography and LV function was measured by conventional Doppler and tissue Doppler imaging (TDI). Other measurements included blood pressure, anthropometric, and biochemical measurements. We estimated the heritability of LV traits while adjusting for covariables, including sex, age, body height and weight, systolic and diastolic blood pressures, and heart rate. With full adjustment, heritability of LV mass was 0.23 (P= 0.025). The TDI-derived mitral annular velocities Ea and Aa showed moderate heritability (h(2)= 0.36 and 0.53, respectively), whereas the mitral inflow A peak had weak heritability (h(2) = 0.25) and the E peak was not heritable (h(2) = 0.11). We partitioned the total phenotypic correlation when it reached significance, into a genetic and an environmental component. The genetic correlations were 0.61 between the E and Ea peaks and 0.90 between the A and Aa peaks.

CONCLUSION

Our study demonstrated moderate heritability for LV mass as well as the mitral annular Ea and Aa peaks. We also found significant genetic correlations between the E and Ea peaks and between the A and Aa peaks. Our current findings support the ongoing research to map and detect genetic variants that contribute to the variation in LV mass and other LV structural and functional phenotypes.

Main results of the ouabain and adducin for Specific Intervention on Sodium in Hypertension Trial (OASIS-HT): a randomized placebo-controlled phase-2 dose-finding study of rostafuroxin

BACKGROUND

The Ouabain and Adducin for Specific Intervention on Sodium in Hypertension (OASIS-HT) Trial was a phase-2 dose-finding study of rostafuroxin, a digitoxygenin derivative, which selectively antagonizes the effects of endogenous ouabain (EO) on Na+,K+-ATPase and mutated adducin. Rostafuroxin lowered blood pressure (BP) in some animal models and in humans.

METHODS

OASIS-HT consisted of 5 concurrently running double-blind cross-over studies. After 4 weeks without treatment, 435 patients with uncomplicated systolic hypertension (140-169 mm Hg) were randomized to rostafuroxin (0.05, 0.15, 0.5, 1.5 or 5.0 mg/d) or matching placebo, each treatment period lasting 5 weeks. The primary endpoint was the reduction in systolic office BP. Among the secondary endpoints were diastolic office BP, 24-h ambulatory BP, plasma EO concentration and renin activity, 24-h urinary sodium and aldosterone excretion, and safety. ANOVA considered treatment sequence (fixed effect), subjects nested within sequence (random), period (fixed), and treatment (fixed).

RESULTS

Among 410 analyzable patients (40.5% women; mean age, 48.4 years), the differences in the primary endpoint (rostafuroxin minus placebo) ranged from -0.18 mm Hg (P = 0.90) on 0.15 mg/d rostafuroxin to 2.72 mm Hg (P = 0.04) on 0.05 mg/d. In the 5 dosage arms combined, the treatment effects averaged 1.30 mm Hg (P = 0.03) for systolic office BP; 0.70 mm Hg (P = 0.08) for diastolic office BP; 0.36 mm Hg (P = 0.49) for 24-h systolic BP; and 0.05 mm Hg (P = 0.88) for 24-h diastolic BP. In the 2 treatment groups combined, systolic (-1.36 mm Hg) and diastolic (-0.97 mm Hg) office BPs decreased from week 5 to 10 (P for period effect ≤ 0.028), but carry-over effects were not significant (P ≥ 0.11). All other endpoints were not different on rostafuroxin and placebo. Minor side-effects occurred with similarly low frequency on rostafuroxin and placebo.

CONCLUSIONS

In 5 concurrently running double-blind cross-over studies rostafuroxin did not reduce BP at any dose.

TRIAL REGISTRATION

ClinicalTrials (NCT): NCT00415038.

Mice expressing ouabain-sensitive α1-Na,K-ATPase have increased susceptibility to pressure overload-induced cardiac hypertrophy

The Na,K-ATPase is a ubiquitous transmembrane pump and a specific receptor for cardiac glycosides such as ouabain and digoxin, which are used in the management of congestive heart failure (CHF). A potential role for these so-called endogenous cardiotonic steroids (CS) has been explored, and it has become apparent that such compounds are elevated and may play an important role in a variety of physiological and pathophysiological conditions such as hypertension and CHF. Recent evidence suggests that the Na,K-ATPase may act as a signal transducer upon CS binding and induce nonproliferative cardiac growth, implicating a role for endogenous CS in the development of cardiac hypertrophy and progressive failure of the heart. In the present study, we tested whether hypertrophic responses to pressure overload would be altered in mutant mice that specifically express ouabain-sensitive or ouabain-resistant α1- and α2-Na,K-ATPase subunits, as follows: α1-resistant, α2-resistant (α1(R/R)α2(R/R)); α1-sensitive, α2-resistant (α1(S/S)α2(R/R)); and α1-resistant, α2-sensitive (α1(R/R)α2(S/S), wild-type). In α1(S/S)α2(R/R) mice, pressure overload by transverse aortic coarctation induced severe left ventricular (LV) hypertrophy with extensive perivascular and replacement fibrosis at only 4 wk. Responses in α1(R/R)α2(S/S) and α1(R/R)α2(R/R) mice were comparatively mild. Mutant α1(S/S)α2(R/R) mice also had LV dilatation and depressed LV systolic contractile function by 4 wk of pressure overload. In separate experiments, chronic Digibind treatment prevented the rapid progression of cardiac hypertrophy and fibrosis in α1(S/S)α2(R/R) mice. These data demonstrate that mice with a ouabain-sensitive α1-Na,K-ATPase subunit have a dramatic susceptibility to the development of cardiac hypertrophy, and failure from LV pressure overload and provide evidence for the involvement of endogenous CS in this process.

Endogenous ouabain in renal Na(+) handling and related diseases

The Na(+) pump and its Endogenous modulator Ouabain (EO) can be considered as an ancestral enzymatic system, conserved among species ranging from Drosophila to humans, related to Na handling. In this review, we examine how EO is linked with vascular function in hypertension and if it impacts the pathogenesis of heart and renal failure. Moreover, the molecular mechanism of endogenous ouabain-linked hypertension involves the sodium pump/sodium-calcium exchanger duet. Biosynthesis of EO occurs in adrenal glands and is under the control of angiotensin II, ACTH and epinephrine. Elevated concentrations of EO and in the sub-nanomolar concentration range were found to stimulate proliferation and differentiation of cardiac and smooth muscle cells. They may have a primary role in the development of cardiac dysfunction and failure. Experimental data suggest that the Na/K-ATPase α(2)-catalytic subunit causes EO-induced vasoconstriction. Finally, maneuvers that promote Na depletion, as diuretic therapy or reduced Na intake, raise the EO levels. Taken together, these findings suggest a key role for EO in body Na homeostasis.