Salt, endogenous ouabain and blood pressure interactions in the general population

Sensational site: the sodium pump ouabain-binding site and its ligands

Cardiotonic steroids (CTS), used by certain insects, toads, and rats for protection from predators, became, thanks to Withering's trailblazing 1785 monograph, the mainstay of heart failure (HF) therapy. In the 1950s and 1960s, we learned that the CTS receptor was part of the sodium pump (NKA) and that the Na+/Ca2+ exchanger was critical for the acute cardiotonic effect of digoxin- and ouabain-related CTS. This "settled" view was upended by seven revolutionary observations. First, subnanomolar ouabain sometimes stimulates NKA while higher concentrations are invariably inhibitory. Second, endogenous ouabain (EO) was discovered in the human circulation. Third, in the DIG clinical trial, digoxin only marginally improved outcomes in patients with HF. Fourth, cloning of NKA in 1985 revealed multiple NKA α and β subunit isoforms that, in the rodent, differ in their sensitivities to CTS. Fifth, the NKA is a cation pump and a hormone receptor/signal transducer. EO binding to NKA activates, in a ligand- and cell-specific manner, several protein kinase and Ca2+-dependent signaling cascades that have widespread physiological effects and can contribute to hypertension and HF pathogenesis. Sixth, all CTS are not equivalent, e.g., ouabain induces hypertension in rodents while digoxin is antihypertensinogenic ("biased signaling"). Seventh, most common rodent hypertension models require a highly ouabain-sensitive α2 NKA and the elevated blood pressure is alleviated by EO immunoneutralization. These numerous phenomena are enabled by NKA's intricate structure. We have just begun to understand the endocrine role of the endogenous ligands and the broad impact of the ouabain-binding site on physiology and pathophysiology.

Na+, K+-ATPase α Isoforms and Endogenous Cardiac Steroids in Prefrontal Cortex of Bipolar Patients and Controls

Bipolar disorder is a chronic multifactorial psychiatric illness that affects the mood, cognition, and functioning of about 1–2% of the world’s population. Its biological basis is unknown, and its treatment is unsatisfactory. The α1, α2, and α3 isoforms of the Na⁺, K⁺-ATPase, an essential membrane transporter, are vital for neuronal and glial function. The enzyme and its regulators, endogenous cardiac steroids like ouabain and marinobufagenin, are implicated in neuropsychiatric disorders, bipolar disorder in particular. Here, we address the hypothesis that the α isoforms of the Na⁺, K⁺-ATPase and its regulators are altered in the prefrontal cortex of bipolar disease patients. The α isoforms were determined by Western blot and ouabain and marinobufagenin by specific and sensitive immunoassays. We found that the α2 and α3 isoforms were significantly higher and marinobufagenin levels were significantly lower in the prefrontal cortex of the bipolar disease patients compared with those in the control. A positive correlation was found between the levels of the three α isoforms in all samples and between the α1 isoform and ouabain levels in the controls. These results are in accordance with the notion that the Na⁺, K⁺-ATPase-endogenous cardiac steroids system is involved in bipolar disease and suggest that it may be used as a target for drug development.

Na,K-ATPase as a target for endogenous cardiotonic steroids: What's the evidence?

With an exception of few reports, the plasma concentration of ouabain and marinobufagenin, mostly studied cardiotonic steroids (CTS) assessed by immunoassay techniques, is less than 1 nM. During the last 3 decades, the implication of these endogenous CTS in the pathogenesis of hypertension and other volume-expanded disorders is widely disputed. The threshold for inhibition by CTS of human and rodent α1-Na,K-ATPase is ∼1 and 1000 nM, respectively, that rules out the functioning of endogenous CTS (ECTS) as natriuretic hormones and regulators of cell adhesion, cell-to-cell communication, gene transcription and translation, which are mediated by dissipation of the transmembrane gradients of monovalent cations. In several types of cells ouabain and marinobufagenin at concentrations corresponding to its plasma level activate Na,K-ATPase, decrease the [Na⁺]_i/[K⁺]_i-ratio and increase cell proliferation. Possible physiological significance and mechanism of non-canonical Na⁺_i/K⁺_i-dependent and Na⁺_i/K⁺_i-independent cell responses to CTS are discussed.

Ouabain, endogenous ouabain and ouabain-like factors: The Na+ pump/ouabain receptor, its linkage to NCX, and its myriad functions

In this brief review we discuss some aspects of the Na⁺ pump and its roles in mediating the effects of ouabain and endogenous ouabain (EO): i) in regulating the cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) via Na/Ca exchange (NCX), and ii) in activating a number of protein kinase (PK) signaling cascades that control a myriad of cell functions. Importantly, [Ca²⁺]_CYT and the other signaling pathways intersect at numerous points because of the influence of Ca²⁺ and calmodulin in modulating some steps in those other pathways. While both mechanisms operate in virtually all cells and tissues, this article focuses primarily on their functions in the cardiovascular system, the central nervous system (CNS) and the kidneys.

A Review on Adducin from Functional to Pathological Mechanisms: Future Direction in Cancer

Adducin (ADD) is a family of membrane skeleton proteins including ADD1, ADD2, and ADD3 that are encoded by distinct genes on different chromosomes. Adducin is primarily responsible for the assembly of spectrin-actin network that provides physical support to the plasma membrane and mediates signal transduction in various cellular physiological processes upon regulation by protein kinase C-dependent and calcium/calmodulin-dependent pathways. Abnormal phosphorylation, genetic variations, and alternative splicing of adducin may contribute to alterations in cellular functions involved in pathogenic processes. These alterations are associated with a wide range of diseases including cancer. This paper begins with a discussion on how adducin partakes in the structural formation of membrane skeleton, its regulation, and related functional characteristics, followed by a review on the pathogenesis of hypertension, biliary atresia, and cancer with respect to increased disease susceptibility mediated by adducin polymorphism and/or dysregulation. Given the functional diversity of adducin in different cellular compartments, we aim to provide a knowledge base whereby its pathophysiological roles can be better understood. More importantly, we aim to provide novel insights that may be of significance in turning the adducin model to clinical application.

Protein Interaction and Na/K-ATPase-Mediated Signal Transduction

The Na/K-ATPase (NKA), or Na pump, is a member of the P-type ATPase superfamily. In addition to pumping ions across cell membrane, it is engaged in assembly of multiple protein complexes in the plasma membrane. This assembly allows NKA to perform many non-pumping functions including signal transduction that are important for animal physiology and disease progression. This article will focus on the role of protein interaction in NKA-mediated signal transduction, and its potential utility as target for developing new therapeutics.

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.

Endogenous digitalis-like factors: an overview of the history

The sodium pump is a ubiquitous cell surface enzyme, a Na, K ATPase, which maintains ion gradients between cells and the extracellular fluid (ECF). The extracellular domain of this enzyme contains a highly conserved binding site, a receptor for a plant derived family of compounds, the digitalis glycosides. These compounds inhibit the enzyme and are used in the treatment of congestive heart failure and certain cardiac arrhythmias. The highly conserved nature of this enzyme and its digitalis receptor led to early suggestions that endogenous regulators might exist. Recent examination of this hypothesis emerged from research in two separate areas: the regulation of ECF volume by a natriuretic hormone (NH), and the regulation of peripheral vascular resistance by a circulating inhibitor of vascular Na, K ATPase. These two areas merged with the hypothesis that NH and the vascular Na, K ATPase inhibitor were in fact the same entity, and that it played a causative role in the pathophysiology of certain types of hypertension. The possibility that multiple endogenous digitalis-like factors (EDLFs) exist emerged from efforts to characterize the circulating enzyme inhibitory activity. In this review, the development of this field from its beginnings is traced, the current status of the structure of EDLFs is briefly discussed, and areas for future development are suggested.

No endogenous ouabain is detectable in human plasma by ultra-sensitive UPLC-MS/MS

BACKGROUND

The presence of a binding site for cardiac glycosides, such as digitoxin and digoxin, in the sodium-potassium-ATPase, stimulated attempts to isolate endogenous cardiotonic steroids. Using immunoassays, clinical studies found the cardenolide ouabain to be secreted endogenously in response to exercise and untreated hypertension and to be correlated with severity of clinical conditions such as kidney failure and dilated cardiomyopathy. The assays used were not standardized and the mean concentrations of endogenous ouabain reported for healthy controls ranged from 60 to 530 pmol/l. None of these immunoassays is available any more. Therefore, the aim of this study was to develop a highly specific and reliable method for measurement of ouabain in human plasma based on isotope dilution liquid chromatography tandem-mass spectrometry (ID-LC-MS/MS).

METHOD

An ultra-sensitive and specific ultra performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) method was developed which applied solid phase extraction of plasma for sample preparation.

RESULTS

The method was comprehensively validated and had a lower limit of quantification of 1.7 pmol/l. However, despite this very low detection limit ouabain was not observed in plasma samples from patients with and without heart failure.

CONCLUSION

Our results suggest that immunoassays previously used to quantify assumed endogenous ouabain detected compounds which are not structurally identical with ouabain. Cross reactivity of structurally related compounds of endogenous origin may cause these discrepancies between immunological and mass spectrometric analyses. Conclusive characterization of assumed endogenous counterparts of digoxin in a biomarker discovery approach seems to require distinct analytical techniques.

Natriuretic hormones, endogenous ouabain, and related sodium transport inhibitors

The work of deWardener and colleagues stimulated longstanding interest in natriuretic hormones (NHs). In addition to the atrial peptides (APs), the circulation contains unidentified physiologically relevant NHs. One NH is controlled by the central nervous system (CNS) and likely secreted by the pituitary. Its circulating activity is modulated by salt intake and the prevailing sodium concentration of the blood and intracerebroventricular fluid, and contributes to postprandial and dehydration natriuresis. The other NH, mobilized by atrial stretch, promotes natriuresis by increasing the production of intrarenal dopamine and/or nitric oxide (NO). Both NHs have short (<35 min) circulating half lives, depress renotubular sodium transport, and neither requires the renal nerves. The search for NHs led to endogenous cardiotonic steroids (CTS) including ouabain-, digoxin-, and bufadienolide-like materials. These CTS, given acutely in high nanomole to micromole amounts into the general or renal circulations, inhibit sodium pumps and are natriuretic. Among these CTS, only bufalin is cleared sufficiently rapidly to qualify for an NH-like role. Ouabain-like CTS are cleared slowly, and when given chronically in low daily nanomole amounts, promote sodium retention, augment arterial myogenic tone, reduce renal blood flow and glomerular filtration, suppress NO in the renal vasa recta, and increase sympathetic nerve activity and blood pressure. Moreover, lowering total body sodium raises circulating endogenous ouabain. Thus, ouabain-like CTS have physiological actions that, like aldosterone, support renal sodium retention and blood pressure. In conclusion, the mammalian circulation contains two non-AP NHs. Identification of the CNS NH should be a priority.

A functional common polymorphism of the ABCB1 gene is associated with chronic kidney disease and hypertension in Chinese

BACKGROUND

Permeability glycoprotein is encoded by the ATP-binding cassette B1 gene (ABCB1) and is an extruder of toxic metabolites in the kidney. A functional common polymorphism (C3435T, rs1045642) in the human ABCB1 gene has been found to be associated with allograft outcome in kidney transplant patients. In this study, we investigated the association of the C3435T polymorphism with renal function and blood pressure (BP) in 2 Chinese populations.

METHODS

The discovery and replication populations were recruited from a mountainous area (Zhejiang Province) and a newly urbanized suburban area (Shanghai), respectively. We genotyped all subjects using the ABI SNapShot method. Chronic kidney disease (CKD) was defined as an estimated glomerular filtration rate <60 ml/min × 1.73 m(2) or 24-hour urinary albumin excretion ≥30 mg.

RESULTS

In the discovery population of 1,987 subjects, after adjustment for covariables, TT homozygosity (n = 217) was associated with a higher risk of CKD (n = 369; odds ratio (OR) = 1.73; P = 0.003) and with higher systolic BP (+3.1 mm Hg; P = 0.03) and pulse pressure (+3.4 mm Hg; P = 0.001). These associations were dependent on age (Pint ≤ 0.05). In subjects aged ≥60 years (n = 374), the corresponding OR or difference was 2.40 for CKD, 15.1 mm Hg for systolic BP, and 12.4 mm Hg for pulse pressure (P < 0.001). In similar adjusted analyses in the replication population of 2,427 elderly (≥60 years) subjects, TT homozygosity was also associated with a higher risk of CKD (OR = 1.39; P = 0.02) and an enhanced association of hypertension with CKD (OR = 1.50; P = 0.04).

CONCLUSIONS

The ABCB1 C3435T polymorphism might predict CKD, especially in the elderly.

Salt sensitivity, endogenous ouabain and hypertension

PURPOSE OF REVIEW

Endogenous cardiotonic steroids (CTS) exert long-term effects on salt and blood pressure homeostasis. Here we discuss recent observations on mechanisms of salt sensitivity that involve endogenous ouabain and novel pathways in the brain and discuss their possible relationship to arterial and renal function in hypertension.

RECENT FINDINGS

Chronic elevation of brain sodium promotes sustained hypertension mediated by central endogenous ouabain and the Na(+) pump α-2 catalytic subunit. The intermediary pressor mechanism in the brain involves aldosterone biosynthesis, activation of mineralocorticoid receptors and increased epithelial sodium channel activity. In the periphery, elevated plasma CTS raise contractility and blood pressure by augmentation of sympathetic nerve responses, increasing arterial Ca(2+) signaling and blunting nitric oxide production in the renal medulla and collecting ducts.

SUMMARY

Endogenous ouabain in the brain appears to play a critical role in salt sensitivity and hypertension. In the periphery, the J-shaped relationship of plasma endogenous ouabain in response to short-term changes in salt balance in humans raises the possibility that endogenous ouabain contributes to the increased risk of adverse cardiovascular events associated with both low and high salt intakes.

Activation of c-SRC underlies the differential effects of ouabain and digoxin on Ca(2+) signaling in arterial smooth muscle cells

Cardiotonic steroids (CTS) of the strophanthus and digitalis families have opposing effects on long-term blood pressure (BP). This implies hitherto unrecognized divergent signaling pathways for these CTS. Prolonged ouabain treatment upregulates Ca(2+) entry via Na(+)/Ca(2+) exchanger-1 (NCX1) and TRPC6 gene-encoded receptor-operated channels in mesenteric artery smooth muscle cells (ASMCs) in vivo and in vitro. Here, we test the effects of digoxin on Ca(2+) entry and signaling in ASMC. In contrast to ouabain treatment, the in vivo administration of digoxin (30 μg·kg(-1)·day(-1) for 3 wk) did not raise BP and had no effect on resting cytolic free Ca(2+) concentration ([Ca(2+)](cyt)) or phenylephrine-induced Ca(2+) signals in isolated ASMCs. Expression of transporters in the α2 Na(+) pump-NCX1-TRPC6 Ca(2+) signaling pathway was not altered in arteries from digoxin-treated rats. Upregulated α2 Na(+) pumps and a phosphorylated form of the c-SRC protein kinase (pY419-Src, ~4.5-fold) were observed in ASMCs from rats treated with ouabain but not digoxin. Moreover, in primary cultured ASMCs from normal rats, treatment with digoxin (100 nM, 72 h) did not upregulate NCX1 and TRPC6 but blocked the ouabain-induced upregulation of these transporters. Pretreatment of ASMCs with the c-Src inhibitor PP2 (1 μM; 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine) but not its inactive analog eliminated the effect of ouabain on NCX1 and TRPC6 expression and ATP-induced Ca(2+) entry. Thus, in contrast to ouabain, the interaction of digoxin with α2 Na(+) pumps is unable to activate c-Src phosphorylation and upregulate the downstream NCX1-TRPC6 Ca(2+) signaling pathway in ASMCs. The inability of digoxin to upregulate c-Src may underlie its inability to raise long-term BP.

Arterial stiffness, central hemodynamics, and cardiovascular risk in hypertension

This review summarizes several scientific contributions at the recent Satellite Symposium of the European Society of Hypertension, held in Milan, Italy. Arterial stiffening and its hemodynamic consequences can be easily and reliably measured using a range of noninvasive techniques. However, like blood pressure (BP) measurements, arterial stiffness should be measured carefully under standardized patient conditions. Carotid-femoral pulse wave velocity has been proposed as the gold standard for arterial stiffness measurement and is a well recognized predictor of adverse cardiovascular outcome. Systolic BP and pulse pressure in the ascending aorta may be lower than pressures measured in the upper limb, especially in young individuals. A number of studies suggest closer correlation of end-organ damage with central BP than with peripheral BP, and central BP may provide additional prognostic information regarding cardiovascular risk. Moreover, BP-lowering drugs can have differential effects on central aortic pressures and hemodynamics compared with brachial BP. This may explain the greater beneficial effect provided by newer antihypertensive drugs beyond peripheral BP reduction. Although many methodological problems still hinder the wide clinical application of parameters of arterial stiffness, these will likely contribute to cardiovascular assessment and management in future clinical practice. Each of the abovementioned parameters reflects a different characteristic of the atherosclerotic process, involving functional and/or morphological changes in the vessel wall. Therefore, acquiring simultaneous measurements of different parameters of vascular function and structure could theoretically enhance the power to improve risk stratification. Continuous technological effort is necessary to refine our methods of investigation in order to detect early arterial abnormalities. Arterial stiffness and its consequences represent the great challenge of the twenty-first century for affluent countries, and "de-stiffening" will be the goal of the next decades.

Endogenous ouabain: a link between sodium intake and hypertension

The sodium pump, an ancestral enzyme with conserved ability to bind ouabain, plays a key role in salt conservation and is regulated by aldosterone and endogenous ouabain (EO). Plasma EO is elevated in about 45% of patients with essential hypertension and correlates with blood pressure. The relationship of EO with Na(+) balance is complex. Na(+) depletion raises circulating EO, whereas acute saline loads have no effect on EO in essential hypertension, and ambient levels of EO are unrelated to the saline sensitivity of blood pressure. Short-term periods of high dietary salt elevate EO and the relationship with salt balance in normal individuals is V-shaped, whereas the long-term relationship is likely to be L-shaped. Normal individuals suppress the high EO transient triggered by high-salt diets and avoid hypertension. In contrast, patients with elevated EO on normal Na(+) intakes have hypertension related to poor modulation of EO biosynthesis, clearance, or both.

Endogenous ouabain and the renin-angiotensin-aldosterone system: distinct effects on Na handling and blood pressure in human hypertension

OBJECTIVE

To evaluate whether the renin-angiotensin-aldosterone system (RAAS) and endogenous ouabain system differently affect renal Na handling and blood pressure.

METHODS

Three hundred and one patients in whom we compared blood pressure, and renal Na tubular reabsorption in the basal condition and 2 h (T120) after saline infusion.

RESULTS

Following multivariate-adjusted linear and quartiles analysis, baseline mean blood pressure (MBP) was significantly higher (113.7 ± 1.33 mmHg) in the fourth versus the first endogenous ouabain quartile (103.8 ± 1.04 mmHg) and the trend across the quartiles was highly significant (β = 0.23, P = 3.53e-04). In contrast, an inverse relationship was present in the renin activity (PRA) quartiles with MBP highest in the first (112.5 ± 1.26) and lowest in the fourth PRA quartile (107.6 ± 1.48, P = 0.039). Following an acute saline load, changes in MBP and the slope of the pressure-natriuresis relationship were inversely related across the PRA quartiles. The fractional excretion of sodium (FENa) showed a negative linear trend going from the first to the third endogenous ouabain quartiles (2.35 ± 0.17 and 1.90 ± 0.14%, P = 0.05). Patients in the fourth endogenous ouabain quartile (>323 pmol/l) showed increased FENa T120 (2.78 ± 0.18%, P < 0.01) and increased Na tubular rejection fraction (P = 0.007) after Na load. After the saline load, there was a biphasic relationship between plasma endogenous ouabain and FENa favoring Na retention at low endogenous ouabain and Na excretion at high endogenous ouabain levels.

CONCLUSION

The RAAS and endogenous ouabain system are two independent and complementary systems having an inverse (RAAS) or a direct (endogenous ouabain system) relationship with hemodynamic parameters.

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.

Sympathetic nerves and the endothelium influence the vasoconstrictor effect of low concentrations of ouabain in pressurized small arteries

We hypothesized that in salt-dependent forms of hypertension, endogenous ouabain acts on arterial smooth muscle to cause enhanced vasoconstriction. Here, we tested for the involvement of the arterial endothelium and perivascular sympathetic nerve terminals in ouabain-induced vasoconstriction. Segments of rat mesenteric or renal interlobar arteries were pressurized to 70 mmHg at 37 degrees C and exposed to ouabain (10(-11)-10(-7) M). Removal of the endothelium enhanced ouabain-induced vasoconstriction by as much as twofold (at an ouabain concentration of 10(-9) M). A component of the ouabain-induced vasoconstriction is due to the enhanced spontaneous release of norepinephrine (NE) from nerve terminals in the arterial wall. The alpha(1)-adrenoceptor blocker prazosin (10(-6) M) decreased ouabain-induced vasoconstrictions by as much as 50%. However, neither the contraction induced by sympathetic nerve activity (SNA) nor the NE release evoked by SNA (measured directly by carbon fiber amperometry) was increased by ouabain (<10(-7) M). Nevertheless, the converse case was true: after brief bursts of SNA, vasoconstrictor responses to ouabain were transiently increased (1.75-fold). This effect may be mediated by neuropeptide Y and Y(1) receptors on smooth muscle. In arteries lacking the endothelium and exposed to prazosin, ouabain (10(-11) M and greater) caused vasoconstriction, indicating a direct effect of very "low" concentrations of ouabain on arterial smooth muscle. In conclusion, in intact arteries, the endothelium opposes ouabain (10(-11)-10(-7)M)-induced vasoconstriction, which is caused by both enhanced spontaneous NE release and direct effects on smooth muscle. Ouabain (<10(-7)M) does not enhance SNA-mediated contractions, but SNA enhances ouabain-induced contractions. The effects of endogenous ouabain may be accentuated in forms of hypertension that involve sympathetic nerve hyperactivity and/or endothelial dysfunction.

The Renin-Angiotensin System in the Development of Salt-Sensitive Hypertension in Animal Models and Humans

Hypertension is still one of the major causes of death from cardiovascular failure. Increased salt intake may aggravate the rise in blood pressure and the development of consequential damage of the heart, the vessels and other organs. The general necessity of restricted salt intake regardless of blood pressure or salt sensitivity has been a matter of debate over the past decades. This review summarizes the main pathogenic mechanisms of hypertension and salt sensitivity in rat models, particularly in the spontaneously hypertensive rat (SHR), and in patients with essential hypertension (EH). Although SHRs are commonly considered to be salt-resistant, there is much evidence that salt loading may deteriorate blood pressure and cardiovascular function even in these animals. Similarly, EH is not a homogenous disorder - some patients, but not all, exhibit pronounced salt sensitivity. The renin-angiotensin system (RAS) plays a key role in the regulation of blood pressure and salt and fluid homeostasis and thus is one of the main targets of antihypertensive therapy. This review focuses on the contribution of the RAS to the pathogenesis of salt-sensitive hypertension in SHRs and patients with EH.

Endogenous cardiotonic steroids and salt-sensitive hypertension

Endogenous cardiotonic steroids (CTS), also called digitalis like factors, have been postulated to play important roles in pathogenesis of hypertension for nearly half of a century. For the past 50 years biomedical scientists have been in quest of an unidentified factor or hormone that both increases blood pressure and renal sodium excretion; this "natriuretic hormone" was, in fact, postulated to interact with the Na/K-ATPase. Recent discoveries have led to the identification of steroid molecules which are present in humans, rodents and amphibians, and which, in a complex manner, interact with each other and with the other systems that regulate renal salt handling and contribute to the salt-sensitivity of blood pressure. Recent findings include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of mechanisms by which CTS can signal through the Na/K-ATPase. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the central regulation of blood pressure and regulation of cell growth, and development of cardiovascular and renal fibrosis in particular.

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.

Endogenous ouabain in Ménière's disease

OBJECTIVE

Endogenous Ouabain (EO) has been demonstrated to modulate the activity of Na+, K+ -ATPase. Our purpose was to measure plasma levels of EO in Ménière's Disease (MD) subjects as a possible predisposing factor to developing and maintaining hydrops.

STUDY DESIGN

Case-control study.

SETTINGS

University hospital.

PATIENTS

Thirty-nine MD subjects and 29 controls with a lifetime negative history for vertigo and dizziness.

MAIN OUTCOME MEASURES

Plasma levels of EO.

RESULTS

Plasma EO in MD subjects was in the range between 33 and 504 pmol/L (median, 135.5 pmol/L), whereas in the control group, plasma EO varied between 70 and 724 pmol/L (median, 205 pmol/L). The Mann-Whitney U test detected a statistically significant difference (p = 0.0001).

CONCLUSION

Low plasma levels of EO have been proposed to augment Na-K pump activity, whereas high EO levels show an inhibitory effect on the pump activity. A proper pump activity may be necessary to keep the right ionic amount and osmolarity in endolymph. Although other possibilities may be considered, we suggest that altered control mechanisms of pump activity may be related to the pathogenesis and maintenance of MD.

Left ventricular geometry and endogenous ouabain in a Flemish population

BACKGROUND

Endogenous ouabain, an endogenous digitalis-like steroid, is synthesized in the adrenal glands and possibly in the hypothalamus. In vitro and in vivo, endogenous ouabain triggers growth and proliferation of cardiomyocytes.

OBJECTIVE

We therefore investigated the association between left ventricular structure and function and plasma endogenous ouabain in a general population.

METHODS

We randomly recruited 536 individuals from a general population (50.7% women, mean age 53.1 years). Measurements included echocardiographic left ventricular structure and function, blood pressure, plasma endogenous ouabain, and the 24-h urinary excretion of sodium.

RESULTS

The geometric mean plasma endogenous ouabain was 95.5 pmol/l (interquartile range 79.4-120.2 pmol/l). We expressed effect sizes for a 1-SD increase in plasma endogenous ouabain (0.21 on the logarithmic scale), while accounting for important covariables. For a 1-SD increment in plasma endogenous ouabain, SBP, left ventricular posterior wall, the interventricular septum, and relative wall thickness increased by 1.59 mmHg (P = 0.009), 0.138 mm (P = 0.003), 0.152 mm (P = 0.013), and 0.71 x 10 (P = 0.008), respectively. In a sensitivity analysis, involving 431 individuals aged 29-71 years (10-90th percentile interval), a 1-SD increase in plasma endogenous ouabain was associated with opposite trends in ejection fraction (+0.90%, P = 0.005) and left ventricular systolic longitudinal strain (-0.48%, P = 0.011). Moreover, in individuals whose sodium excretion was above median (160 mmol/24 h), the aforementioned associations reached a higher level of statistical significance.

CONCLUSION

Our population-based study suggested that endogenous ouabain might have a trophic effect on the myocardium, independent of blood pressure and other covariables. The clinical implication of these findings remains to be elucidated.

Upregulation of Na+ and Ca2+ transporters in arterial smooth muscle from ouabain-induced hypertensive rats

Prolonged ouabain administration (25 microg kg(-1) day(-1) for 5 wk) induces "ouabain hypertension" (OH) in rats, but the molecular mechanisms by which ouabain elevates blood pressure are unknown. Here, we compared Ca(2+) signaling in mesenteric artery smooth muscle cells (ASMCs) from normotensive (NT) and OH rats. Resting cytosolic free Ca(2+) concentration ([Ca(2+)](cyt); measured with fura-2) and phenylephrine-induced Ca(2+) transients were augmented in freshly dissociated OH ASMCs. Immunoblots revealed that the expression of the ouabain-sensitive alpha(2)-subunit of Na(+) pumps, but not the predominant, ouabain-resistant alpha(1)-subunit, was increased (2.5-fold vs. NT ASMCs) as was Na(+)/Ca(2+) exchanger-1 (NCX1; 6-fold vs. NT) in OH arteries. Ca(2+) entry, activated by sarcoplasmic reticulum (SR) Ca(2+) store depletion with cyclopiazonic acid (SR Ca(2+)-ATPase inhibitor) or caffeine, was augmented in OH ASMCs. This reflected an augmented expression of 2.5-fold in OH ASMCs of C-type transient receptor potential TRPC1, an essential component of store-operated channels (SOCs); two other components of some SOCs were not expressed (TRPC4) or were not upregulated (TRPC5). Ba(2+) entry activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol [a measure of receptor-operated channel (ROC) activity] was much greater in OH than NT ASMCs. This correlated with a sixfold upregulation of TRPC6 protein, a ROC family member. Importantly, in primary cultured mesenteric ASMCs from normal rats, 72-h treatment with 100 nM ouabain significantly augmented NCX1 and TRPC6 protein expression and increased resting [Ca(2+)](cyt) and ROC activity. SOC activity was also increased. Silencer RNA knockdown of NCX1 markedly downregulated TRPC6 and eliminated the ouabain-induced augmentation; silencer RNA knockdown of TRPC6 did not affect NCX1 expression but greatly attenuated its upregulation by ouabain. Clearly, NCX1 and TRPC6 expression are interrelated. Thus, prolonged ouabain treatment upregulates the Na(+) pump alpha(2)-subunit-NCX1-TRPC6 (ROC) Ca(2+) signaling pathway in arterial myocytes in vitro as well as in vivo. This may explain the augmented myogenic responses and enhanced phenylephrine-induced vasoconstriction in OH arteries (83) as well as the high blood pressure in OH rats.

Diverse actions of ouabain and its aglycone ouabagenin in renal cells

The cellular actions of ouabain are complex and involve different pathways, depending on the cell type and experimental conditions. Several studies have reported that Madin-Darby canine kidney (MDCK) cellular sensitivity to ouabain is not related to Na-K-ATPase inhibition, and others showed that some cell types, such as Ma104, are resistant to ouabain toxicity albeit their Na-K-ATPase isoforms possess high affinity for this glycoside. We describe here that the effects of ouabain and ouabagenin also diverge in MDCK and Ma104 cells, being MDCK cells more resistant to ouabagenin, while Ma104 cells are resistant to both molecules. This feature seems to correlate with induction of cell signaling, since ouabain, but not ouabagenin, induced an intense and sustained increase in tyrosine phosphorylation levels in MDCK cells. Moreover, ouabain-induced phosphorylation in Ma104 cells was approximately half than that observed in MDCK cells. The proportion between alpha and beta subunits of Na-K-ATPase was similar in MDCK cells, though Ma104 cells presented more alpha subunits, located mainly at the cytoplasm. Furthermore, a fluorescent ouabain-analog labeled mainly the cytoplasm of Ma104 cells, the opposite of that seen in MDCK cells, corroborating the results using anti-Na-K-ATPase antibodies. Hence, the results suggest that ouabain and ouabagenin differ in terms of Na-K-ATPase inhibition and cell signaling activation in MDCK cells. Additionally, MDCK and Ma104 cell lines respond differently to ouabain, perhaps due to an intrinsic ability of this glycoside to selectively reach the cytoplasm of Ma104 cells.

Steroid biosynthesis and renal excretion in human essential hypertension: association with blood pressure and endogenous ouabain

BACKGROUND

Endogenous ouabain (EO) has been linked with long-term changes in sodium balance and cardiovascular structure and function. The biosynthesis of EO involves, cholesterol side-chain cleavage (CYP11A1), 3-beta-hydroxysteroid dehydrogenase (HSD3B) with sequential metabolism of pregnenolone and progesterone. Furthermore, the renal excretion of cardiac glycosides is mediated by the organic anion transporter (SLCO4C1) at the basolateral membrane and the P-glycoprotein (PGP) (encoded by MDR1) at the apical membrane of the nephron.

METHODS

Average 24-h ambulatory blood pressures were recorded in 729 untreated essential hypertensives. Aldosterone (Aldo), EO, urinary Na+, and K+ excretions were determined. Single-nucleotide polymorphism (SNP) and haplotype-based association study was performed with a total of 26 informative SNPs.

RESULTS

Plasma EO was significantly directly related to both day (r = 0.131, P < 0.01) and nighttime diastolic blood pressure (DBP) (r = 0.143, P < 0.01), and remained significantly related after correction for confounders (sex, body mass index, age). Genotype analysis for EO levels and daytime DBP gave significant results for CYP11A1 rs11638442 and MDR1 rs1045642 (T/C Ile1145) in which the minor allele tracked with higher EO levels (T/T 210.3 (147-272) vs. C/C 270.7 (193-366) pmol/l, P < 0.001). Association was found between HSD3B1 polymorphisms and/or haplotypes with blood pressure (systolic blood pressure (SBP) 140.3 (11.7) vs. 143.8 (11.2) mm Hg, P < 0.01) and plasma Aldo (P < 0.05). Haplotype-based analyses support the data of SNP analysis.

CONCLUSIONS

Among patients with essential hypertension, cholesterol side-chain cleavage and MDR1 loci are related to circulating EO and DBP, most likely by influencing EO synthesis and transmembrane transport, respectively. In contrast, variants in HSD3B1 are related with SBP probably via Aldo.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

Chronic ouabain treatment induces vasa recta endothelial dysfunction in the rat

Descending vasa recta (DVR) are 15-microm vessels that perfuse the renal medulla. Ouabain has been shown to augment DVR endothelial cytoplasmic Ca(2+) ([Ca(2+)](CYT)) signaling. In this study, we examined the expression of the ouabain-sensitive Na-K-ATPase alpha2 subunit in the rat renal vasculature and tested effects of acute ouabain exposure and chronic ouabain treatment on DVR. Immunostaining with antibodies directed against the alpha2 subunit verified its expression in both DVR pericytes and endothelium. Acute application of ouabain (100 or 500 nM) augmented the DVR nitric oxide generation stimulated by acetylcholine (ACh; 10 microM). At a concentration of 1 mM, ouabain constricted microperfused DVR, whereas at 100 nM, it was without effect. Acute ouabain (100 nM) did not augment constriction by angiotensin II (0.5 or 10 nM), whereas l-nitroarginine methyl ester-induced contraction of DVR was slightly enhanced. Ouabain-hypertensive (OH) rats were generated by chronic ouabain treatment (30 microg.kg(-1).day(-1), 5 wk). The acute endothelial [Ca(2+)](CYT) elevation by ouabain (100 nM) was absent in DVR endothelia of OH rats. The [Ca(2+)](CYT) response to 10 nM ACh was also eliminated, whereas the response to 10 microM ACh was not. The endothelial [Ca(2+)](CYT) response to bradykinin (100 nM) was significantly attenuated. We conclude that endothelial responses may offset the ability of acute ouabain exposure to enhance DVR vasoconstriction. Chronic exposure to ouabain, in vivo, leads to hypertension and DVR endothelial dysfunction, manifested as reduced [Ca(2+)](CYT) responses to both ouabain- and endothelium-dependent vasodilators.

Endogenous digitalis: pathophysiologic roles and therapeutic applications

Endogenous digitalis-like factors, also called cardiotonic steroids, have been thought for nearly half a century to have important roles in health and disease. The endogenous cardiotonic steroids ouabain and marinobufagenin have been identified in humans, and an effector mechanism has been delineated by which these hormones signal through the sodium/potassium-transporting ATPase. These findings have increased interest in this field substantially. Although cardiotonic steroids were first considered important in the regulation of renal sodium transport and arterial pressure, subsequent work has implicated these hormones in the control of cell growth, apoptosis and fibrosis, among other processes. This Review focuses on the role of endogenous cardiotonic steroids in the pathophysiology of essential hypertension, congestive heart failure, end-stage renal disease and pre-eclampsia. We also discuss potential therapeutic strategies that have emerged as a result of the increased understanding of the regulation and actions of cardiotonic steroids.

Comparison of non-digitalis binding properties of digoxin-specific Fabs using direct binding methods

Digibind and DigiFab are commercial formulations of polyclonal, ovine, digoxin-specific Fabs in clinical use for treatment of digoxin intoxication. Of interest for extending its use to other clinical indications, Digibind has also been reported to neutralize the effect of endogenous digoxin-like molecules, including ouabain, that are linked to clinical disorders ranging from preeclampsia to congestive heart failure. Although Digibind and DigiFab are equivalent in their digoxin-binding activity, the antigens used to produce these Fabs are different. We therefore explored, using native (3)H-digoxin and (3)H-ouabain in four different types of solution-phase binding methods, whether they might exhibit different profiles with respect to ouabain and other digoxin-like factors. Consistent with previous results, both Fab preparations bound digoxin with the same affinities and capacities. However, (3)H-ouabain was found to bind with high affinity only to Fab sub-populations present in both products. Interestingly, this sub-population was twice as large for Digibind compared to DigiFab. Competition experiments also showed differences in specificity within Fab sub-populations. Therefore, the equivalence in digoxin-binding activity of the two Fab preparations does not extend to ouabain-binding capacity and Fab specificity, with implications for clinical differentiation between the preparations in treatment of disorders related to control of non-digoxin cardenolides. The existence of a small but perhaps clinically relevant sub-population of antibodies was detected using specific radioligands. This sub-population could not have been detected nor quantified using standard cross-reactivity in an ELISA assay.

Cardiac glycosides regulate endothelial tissue factor expression in culture

BACKGROUND

Tissue factor (TF) plays an important role in acute coronary syndromes and stent thrombosis. This study investigates whether Na(+)/K(+)-ATPase regulates TF expression in human endothelial cells.

METHODS AND RESULTS

Ouabain inhibited tumor necrosis factor (TNF)-alpha-induced endothelial TF protein expression; maximal inhibition occurred at 10(-5) mol/L, reached more than 70%, and was observed throughout the 5 hours stimulation period. The decrease in protein expression was paralleled by a reduced TF surface activity. Similarly, lowering of extracellular potassium concentration inhibited TNF-alpha-induced TF protein expression. In contrast, ouabain did not affect TNF-alpha-induced expression of full-length TF mRNA for up to 5 hours of stimulation; instead, expression of alternatively-spliced TF mRNA was upregulated after 3 and 5 hours of stimulation. Ouabain did not affect TNF-alpha-induced activation of the MAP kinases p38, extracellular signal-regulated kinase (ERK), and c-Jun terminal NH(2) kinase; activation of Akt and p70S6 kinase remained unaltered as well. Similar to the MAP kinases, ouabain did not affect TNF-alpha-induced degradation of IkappaB-alpha. Ouabain had no effect on TF protein degradation.

CONCLUSIONS

Na(+)/K(+)-ATPase is required for protein translation of endothelial TF in culture. This observation provides novel insights into posttranscriptional regulation of TF expression.

Ouabain modulation of cellular calcium stores and signaling

Ouabain-like factors modulate intracellular Ca2+ concentrations and Ca2+ stores. Recently, a role for Na+-K+-ATPase Na+ transport inhibition as a pivotal event in ouabain signaling was questioned (Kaunitz JD. Am J Physiol Renal Physiol 290: F995–F996, 2006). In the present study, we used a mathematical model of Ca2+ trafficking in cytoplasm and subplasmalemmal microdomains to simulate the pathways through which ouabain can affect Ca2+ signaling: inhibition of active transport by Na+-K+-ATPase α1- and α2-isoforms, activation of inositol trisphosphate (IP3) production, and increased IP3 receptor (IP3R) conductance. A fundamental prediction is that Na+-K+-ATPase inhibition favors sarcoplasmic reticulum Ca2+ store loading, whereas Src-mediated increases in IP3 production and IP3R sensitization favor store depletion. The model predicts that α2-isoform inhibition generates a peak-and-plateau pattern of cytosolic Ca2+ concentration ([Ca2+]cyt) elevation, whereas α1-isoform inhibition yields a monophasic rise. The effects of ouabain-mediated increases in IP3 production or IP3R conductance on [Ca2+]cyt depend on their relative distributions between cellular microdomains and the bulk cytoplasm. Simulations suggest that the intracellular localization of IP3 production is a pivotal determinant of the changes in compartmental Ca2+ concentrations that can be induced by ouabain. As a consequence of sequestration of the ouabain-sensitive α2-isoform into microdomains, inhibition of the α2-isoform in rodents is not predicted to significantly affect cytosolic Na+ concentration. Model simulations support the hypothesis that ouabain can enhance agonist-evoked [Ca2+]cyt transients when its predominant effect is to inhibit α2-isoform Na+ transport and, thereby, increase Ca2+ loading into sarcoplasmic reticulum stores.

Endogenous and exogenous cardiac glycosides: their roles in hypertension, salt metabolism, and cell growth

Cardiotonic steroids (CTS), long used to treat heart failure, are endogenously produced in mammals. Among them are the hydrophilic cardenolide ouabain and the more hydrophobic cardenolide digoxin, as well as the bufadienolides marinobufagenin and telecinobufagin. The physiological effects of endogenous ouabain on blood pressure and cardiac activity are consistent with the "Na(+)-lag" hypothesis. This hypothesis assumes that, in cardiac and arterial myocytes, a CTS-induced local increase of Na(+) concentration due to inhibition of Na(+)/K(+)-ATPase leads to an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) via a backward-running Na(+)/Ca(2+) exchanger. The increase in [Ca(2+)](i) then activates muscle contraction. The Na(+)-lag hypothesis may best explain short-term and inotropic actions of CTS. Yet all data on the CTS-induced alteration of gene expression are consistent with another hypothesis, based on the Na(+)/K(+)-ATPase "signalosome," that describes the interaction of cardiac glycosides with the Na(+) pump as machinery activating various signaling pathways via intramembrane and cytosolic protein-protein interactions. These pathways, which may be activated simultaneously or selectively, elevate [Ca(2+)](i), activate Src and the ERK1/2 kinase pathways, and activate phosphoinositide 3-kinase and protein kinase B (Akt), NF-kappaB, and reactive oxygen species. A recent development indicates that new pharmaceuticals with antihypertensive and anticancer activities may be found among CTS and their derivatives: the antihypertensive rostafuroxin suppresses Na(+) resorption and the Src-epidermal growth factor receptor-ERK pathway in kidney tubule cells. It may be the parent compound of a new principle of antihypertensive therapy. Bufalin and oleandrin or the cardenolide analog UNBS-1450 block tumor cell proliferation and induce apoptosis at low concentrations in tumors with constitutive activation of NF-kappaB.

Modulation of multidrug resistance protein (MRP1/ABCC1) expression: a novel physiological role for ouabain

Besides being a (Na(+),K(+))-ATPase inhibitor, high doses of the hormone ouabain have also been reported to modulate both the expression and activity of proteins belonging to the ATP binding cassette family of transporters, such as ABCC7 (CFTR), ABCB1 (P-glycoprotein), and ABCC1 (MRP1). Although these proteins are present in the kidney, only ABCB1 has a putative physiological role in this organ, secreting endobiotics and xenobiotics. In the present work, we studied the relationship between ouabain and ABCC1 expression and function, aiming to establish a physiological role for ouabain. It was observed that prolonged (24 h) but not short (30 min) incubation with 1 nmol/L or higher ouabain concentrations decreased the expression of ABCC1 protein and induced its mRNA expression. This decrease was rapidly reversible, reaching control levels after incubation of cells in ouabain-free medium for 3 h, denoting a hormonal action. Moreover, concentrations equal or higher than 100 nmol/L ouabain also induced impairment of ABCC1 activity, increasing the accumulation of carboxyfluorescein diacetate, an ABCC1 fluorescent substrate. Because ouabain is now accepted as an endogenous hormone, our results suggest that ABCC1 is regulated by hormones related to body volume control, which may have implications for the treatment of hypertensive cancer patients. Moreover, providing ABCC1 is expressed in several other tissues, such as brain, testis, and the immune system, and is related to the transport of glutathione, it is possible that ouabain release may control a number of functions within these organs and tissues by modulating both the expression and the activity of ABCC1.

Endogenous and exogenous cardiac glycosides and their mechanisms of action

Cardiac glycosides have been used for decades to treat congestive heart failure. The recent identification of cardiotonic steroids such as ouabain, digoxin, marinobufagenin, and telocinobufagin in blood plasma, adrenal glands, and hypothalamus of mammals led to exciting new perspectives in the pathology of heart failure and arterial hypertension. Biosynthesis of ouabain and digoxin occurs in adrenal glands and is under the control of angiotensin II, endothelin, and epinephrine released from cells of the midbrain upon stimulation of brain areas sensing cerebrospinal Na(+) concentration and, apparently, the body's K(+) content. Rapid changes of endogenous ouabain upon physical exercise may favor the economy of the heart by a rise of intracellular Ca(2)(+) levels in cardiac and atrial muscle cells. According to the sodium pump lag hypothesis, this may be accomplished by partial inhibition of the sodium pump and Ca(2+) influx via the Na(+)/Ca(2+) exchanger working in reverse mode or via activation of the Na(+)/K(+)-ATPase signalosome complex, generating intracellular calcium oscillations, reactive oxygen species, and gene activation via nuclear factor-kappaB or extracellular signal-regulated kinases 1 and 2. Elevated concentrations of endogenous ouabain and marinobufagenin in the subnanomolar 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 because (i) offspring of hypertensive patients evidently inherit elevated plasma concentrations of endogenous ouabain; (ii) such elevated concentrations correlate positively with cardiac dysfunction, hypertrophy, and arterial hypertension; (iii) about 40% of Europeans with uncomplicated essential hypertension show increased concentrations of endogenous ouabain associated with reduced heart rate and cardiac hypertrophy; (iv) in patients with advanced arterial hypertension, circulating levels of endogenous ouabain correlate with BP and total peripheral resistance; (v) among patients with idiopathic dilated cardiomyopathy, high circulating levels of endogenous ouabain and marinobufagenin identify those individuals who are predisposed to progressing more rapidly to heart failure, suggesting that endogenous ouabain (and marinobufagenin) may contribute to toxicity upon digoxin therapy. In contrast to endogenous ouabain, endogenous marinobufagenin may act as a natriuretic substance as well. It shows a higher affinity for the ouabain-insensitive alpha(1) isoform of Na(+)/K(+)-ATPase of rat kidney tubular cells and its levels are increased in volume expansion and pre-eclampsia. Digoxin, which is synthesized in adrenal glands, seems to counteract the hypertensinogenic action of ouabain in rats, as do antibodies against ouabain, for example, (Digibind) and rostafuroxin (PST 2238), a selective ouabain antagonist. It lowers BP in ouabain- and adducin-dependent hypertension in rats and is a promising new class of antihypertensive medication in humans.

Adducin and hypertension

Adducin is a heterodimeric cytoskeleton protein consisting of an alpha-subunit and either a beta- or gamma-subunit. In rats and humans, mutation of the alpha-adducin subunit leads to the stimulation of the sodium (Na(+)), potassium (K(+))-adenosine triphosphate (ATP)-ase activity in renal tubular cells, increased renal Na(+) reabsorption, and, subsequently, hypertension. Ouabain is a hormone that is released by the hypothalamus and, possibly, the adrenal glands. In renal tubular cells it modulates Na(+)/K(+)-ATPase activity and regulates natriuresis. Plasma ouabain levels increase with the number of copies of the mutated alpha-adducin allele. Rostafuroxin is a digitoxygenin derivative that selectively displaces ouabain from the Na(+)/K(+)-ATPase receptor and lowers blood pressure in rats and humans. In this short editorial review, we summarize the recent experimental, clinical and epidemiological evidence that contributed to our understanding of the pathogenetic mechanisms that lead to hypertension associated with the alpha-adducin Gly460Trp polymorphism and its interaction with ouabain. We propose that a pharmacogenomic approach, as applied in an ongoing Phase II dosage study of rostafuroxin, will be a critical step in moving the adducin hypothesis from experimental and observational studies to clinical application.

Salt intake and depletion increase circulating levels of endogenous ouabain in normal men

High-salt diets elevate circulating Na+ pump inhibitors, vascular resistance, and blood pressure. Ouabain induces a form of hypertension mediated via the alpha2-Na+ pump isoform and the calcium influx mode of the vascular sodium calcium exchanger (NCX). Whereas elevated levels of an endogenous ouabain (EO) and NCX have been implicated in salt-sensitive hypertension, acute changes in sodium balance do not affect plasma EO. This study investigated the impact of longer-term alterations in sodium balance on the circulating levels and renal clearance of EO in normal humans. Thirteen normal men consumed a normal diet, high-salt diet, and hydrochlorothiazide (HCTZ), each for 5-day periods to alter sodium balance. EO and other humoral and urinary variables were determined daily. On a normal diet, urinary sodium excretion (140 +/- 16 meq/day), plasma EO (0.43 +/- 0.08 nmol/l) and urinary EO excretion (1.04 +/- 0.13 nmol/day) were at steady state. On the 3rd day of a high-salt diet, urine sodium excretion (315 +/- 28 meq/day), plasma EO (5.8 +/- 2.2 nmol/l), and the urinary EO excretion (1.69 +/- 0.27 nmol/day) were significantly increased, while plasma renin activity and aldosterone levels were suppressed. The salt-evoked increase in plasma EO was greater in older individuals, in subjects whose baseline circulating EO was higher, and in those with low renal clearance. During HCTZ, body weight decreased and plasma renin activity, aldosterone, and EO (1.71 +/- 0.77 nmol/l) rose, while urinary EO excretion remained within the normal range (1.44 +/- 0.31 nmol/day). Blood pressure fell in one subject during HCTZ. HPLC of the plasma extracts showed one primary peak of EO immunoreactivity with a retention time equivalent to ouabain. High-salt diets and HCTZ raise plasma EO by stimulating EO secretion, and a J-shaped curve relates sodium balance and EO in healthy men. Under normal dietary conditions, approximately 98% of the filtered load of EO is reabsorbed by the kidney, and differences in the circulating levels of EO are strongly influenced by secretion and urinary excretion of EO. The dramatic impact of high-salt diets on plasma EO is consistent with its proposed role as a humoral vasoconstrictor that links salt intake with vascular function in hypertension.

Screening for the alpha-adducin Gly460Trp variant in hypertensive patients: a cost-effectiveness analysis

BACKGROUND

Studies have shown that approximately 80% of hypertensive patients do not take diuretics despite their recommendation as a first-line therapy. A recent study reported that hypertensive patients with the Gly460Trp variant in the alpha-adducin gene are more likely to benefit from diuretic therapy. The objective of this study was to evaluate the potential cost effectiveness of screening for the alpha-adducin Gly460Trp variant among hypertensive patients.

METHODS

A decision analytic Markov model was developed to estimate the clinical and economic outcomes comparing screening for the Gly460Trp variant to identify patients for addition of a diuretic compared to no screening and no addition of diuretic (usual care) in a hypothetical cohort of treated hypertensive patients not receiving diuretic therapy. We used a lifetime horizon and payer perspective. Cost, utility and epidemiological data were obtained from the literature. One-way, probabilistic, and scenario sensitivity analyses were conducted to evaluate the uncertainty in the results.

RESULTS

The screening strategy increased quality adjusted life years (QALYs) by 0.14 (95% confidence range [CR]: 0.05, 0.36) and saved dollar 1834 (dollar 505, dollar 5174) compared to usual care. The most influential input was the strength of the interaction between the alpha-adducin gene variant and diuretic effect.

CONCLUSIONS

Our results suggest that screening for the alpha-adducin gene variant may be a useful mechanism to identify patients most likely to benefit from diuretic therapy and improve compliance with current treatment guidelines.

Sodium excretion as a modulator of genetic associations with cardiovascular phenotypes in the European Project on Genes in Hypertension

Hypertension is a chronic age-related disorder, affecting nearly 20% of all adult Europeans. This disease entails debilitating cardiovascular complications and is the leading cause for drug prescriptions in Europeans older than 50 years. Intensive research over the past two decades has so far failed to identify common genetic polymorphisms with a major impact on blood pressure or associated cardiovascular phenotypes, suggesting that multiple genes each with a minor impact, along with gene-gene and gene-environment interactions, play a role. The European Project on Genes in Hypertension (EPOGH) is a large-scale, family-based study in which participants from seven different populations were phenotyped and genotyped according to standardized procedures. This review article summarizes the initial 5-year findings and puts these observations into perspective against other published studies. The EPOGH demonstrated that phenotype-genotype relations strongly depend on host factors such as gender and lifestyle, in particular salt intake as reflected by the 24-h urinary excretion of sodium. The EPOGH therefore highlights the concept that phenotype-genotype relations can only be studied within a defined ecogenetic context.

Genetic variations of tubular sodium reabsorption leading to “primary” hypertension: from gene polymorphism to clinical symptoms

The definition of the most appropriate strategy to demonstrate causation of a given genetic-molecular mechanism in a complex multifactorial polygenic disease like hypertension is hampered by the underestimation of the complexity arising from the genetic and environmental interactions. To disentangle this complexity, we developed a strategy based on six steps: 1) isolation of a rodent model of hypertension (Milan hypertensive strain and Milan normotensive strain) that shares some pathophysiological abnormalities with human primary hypertension; 2) definition in the model of the sequence of events linking these abnormalities to a genetic molecular mechanism; 3) determination of the polymorphism of the three adducin genes discovered in the model both in rats and in humans; 4) comparison at biochemical and physiological levels between the rodent models and the hypertensive carriers of the “mutated” gene variants; 5) evaluation of the impact of the adducin genes in hypertension and its organ complications with association and linkage studies in humans, also considering the genetic and environmental interactions; and 6) development of a pharmacogenomic approach aimed at establishing the therapeutic benefit of a drug interfering with the sequence of events triggered by adducin and their effect's size. The bulk of data obtained demonstrates the importance of a multidisciplinary approach considering a variety of genetic and environmental interactions. Adducin functions within the cells as a heterodimer composed of a combination of three subunits. Each of these subunits is coded by genes mapping to different chromosomes. Therefore, the interaction among these genes, taken together with the interactions with other modulatory genes or with the environment, is indispensable to establish the adducin clinical impact. The hypothesis that adducin polymorphism favors the development of hypertension via an increased tubular sodium reabsorption is well supported by a series of consistent experimental and clinical data. Many mechanistic aspects, underlying the link between these genes and clinical symptoms, need to be clarified. The clinical effect size of adducin must be established also with the contribution of pharmacogenomics with a drug that selectively interferes with the sequence of events triggered by the mutated adducin.