Role of renal Na+,K(+)-ATPase in the regulation of sodium excretion under normal conditions and in acute congestive heart failure

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 Signaling and Salt Sensitivity: The Role of Oxidative Stress

Other than genetic regulation of salt sensitivity of blood pressure, many factors have been shown to regulate renal sodium handling which contributes to long-term blood pressure regulation and have been extensively reviewed. Here we present our progress on the Na/K-ATPase signaling mediated sodium reabsorption in renal proximal tubules, from cardiotonic steroids-mediated to reactive oxygen species (ROS)-mediated Na/K-ATPase signaling that contributes to experimental salt sensitivity.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Ouabain-stimulated trafficking regulation of the Na/K-ATPase and NHE3 in renal proximal tubule cells

We have demonstrated that ouabain regulates protein trafficking of the Na/K-ATPase α1 subunit and NHE3 (Na/H exchanger, isoform 3) via ouabain-activated Na/K-ATPase signaling in porcine LLC-PK1 cells. To investigate whether this mechanism is species-specific, ouabain-induced regulation of the α1 subunit and NHE3 as well as transcellular (22)Na(+) transport were compared in three renal proximal tubular cell lines (human HK-2, porcine LLC-PK1, and AAC-19 originated from LLC-PK1 in which the pig α1 was replaced by ouabain-resistant rat α1). Ouabain-induced inhibition of transcellular (22)Na(+) transport is due to an ouabain-induced redistribution of the α1 subunit and NHE3. In LLC-PK1 cells, ouabain also inhibited the endocytic recycling of internalized NHE3, but has no significant effect on recycling of endocytosed α1 subunit. These data indicated that the ouabain-induced redistribution of the α1 subunit and NHE3 is not a species-specific phenomenon, and ouabain-activated Na/K-ATPase signaling influences NHE3 regulation.

Risk Factors for Acute Renal Insufficiency Induced by Diuretics in Patients With Congestive Heart Failure

BACKGROUND

In patients with congestive heart failure (CHF), continuous diuretic therapy may result in acute renal insufficiency (ARI). This study examines factors contributing to this complication.

METHODS

We analyzed clinical data from 318 consecutive patients who were hospitalized for CHF. All were treated with diuretics and had echocardiography performed within 4 days of hospitalization. Systolic left ventricular (LV) dysfunction is defined as an ejection fraction less than 50%, and diastolic LV dysfunction, as an ejection fraction of 50% or greater in the presence of LV hypertrophy and a reversed E/A ratio.

RESULTS

ARI, defined as a 25% increase in serum creatinine level, occurred in 110 patients (35%) after diuretic therapy. Risk factors for ARI on univariate analyses were older age, higher baseline serum creatinine level, lower baseline serum sodium level, lower mean arterial pressure (MAP) during diuretic therapy, and greater doses and longer duration of diuretic therapy. In multivariate analyses, ARI occurred more frequently in patients with systolic (40%) than diastolic dysfunction (28%). The use of digoxin in patients with systolic LV dysfunction was observed to decrease the risk for ARI by 61%, independent of other agents used for the treatment of patients with CHF.

CONCLUSION

Age, baseline renal function and serum sodium concentration, MAP, and intensity of diuretic therapy can identify individuals at risk for ARI while receiving diuretic therapy for CHF. This complication is observed more often in individuals with systolic dysfunction, and its risk may be decreased with the use of digoxin.

Digitalis-like immunoreactive substances in maternal and umbilical cord plasma: a comparative sensitivity study of fluorescence polarization immunoassay and microparticle enzyme immunoassay

Digitalis-like immunoreactive substances (DLIS) obtained from maternal and umbilical cord plasma at delivery were measured by fluorescence polarization immunoassay (FPIA; TDX, Abbott) and microparticle enzyme immunoassay (MEIA; IMX, Abbott). In each sample, concentrations of dehydroepiandrosterone, dehydroepiandrosterone sulfate, estradiol, estriol, hydrocortisone, progesterone, and testosterone were measured by radioimmunoassay, and cross-reaction tests of DLIS with these substances were conducted. By FPIA, the concentration of DLIS in umbilical cord plasma (0.55 +/- 0.22 ng/mL) was significantly higher than that in maternal plasma (0.23 +/- 0.11 ng/mL). In the cross-reaction tests, when the concentration of dehydroepiandrosterone sulfate was higher than 1.0 microg/mL or that of progesterone was higher than 0.5 microg/mL, DLIS were detected by FPIA. However, DLIS were not found either in the samples or in the cross-reaction tests by MEIA. By radioimmunoassay, there was no significant difference in the dehydroepiandrosterone sulfate concentration between the maternal plasma (2,917 +/- 1,001 ng/mL) and the umbilical cord plasma (1,957 +/- 376 ng/mL). The progesterone concentration in the umbilical cord plasma (310.0 +/- 85.7 ng/mL) was significantly higher than that in the maternal plasma (126.4 +/- 38.5 ng/mL). These results suggest that dehydroepiandrosterone sulfate in maternal plasma and progesterone in maternal and umbilical cord plasma may be measured as digoxin by FPIA.

Role of endogenous brain "ouabain" in the sympathoexcitatory and pressor effects of sodium

Endogenous cardiac glycoside inhibitors of the Na, K-ATPase (called "ouabain" here) with structures similar to plant ouabain have been isolated in several tissues, including the adrenal cortex and the brain. Recent studies have demonstrated that "ouabain" in the anteroventral third ventricle (AV3V) region of the hypothalamus mediates the sympathoexcitatory and pressor responses to a high sodium diet in Dahl salt-sensitive (Dahl-S) and spontaneously hypertensive (SHR) rats. Although the mechanisms regulating the biosynthesis, release and deactivation of CNS "ouabain" remain unknown, the discovery of the importance of brain "ouabain" in cardiovascular regulation creates a novel path for the development of antihypertensive pharmacopeia.

Comparison of the effects of ouabain and brain natriuretic peptide in saline-loaded sheep

1. It has been claimed that ouabain is an endogenous hormone that may be pivotal in the pathogenesis of some forms of hypertension and may exaggerate natriuresis in situations characterized by volume overload. We compared the haemodynamic, renal and endocrine effects of ouabain (at approximately 187 ng/kg per min for 2 h) with those of brain natriuretic peptide (BNP; at 5 pmol/kg per min for 2 h) in nine saline-loaded sheep in a balanced, randomized, single-blind, placebo-controlled crossover study. 2. Brain natriuretic peptide infusion reduced mean arterial pressure whereas ouabain infusion caused no change. Haematocrit rose steadily during BNP infusion but fell during ouabain infusion. Neither ouabain nor BNP affected urine volume, sodium, potassium or creatinine excretion. Mean heart rate declined during the ouabain and placebo infusions, but was not altered during BNP infusion. Endogenous ouabain concentrations were not detectable at baseline or during BNP or placebo infusions, but rose to concentrations of 11 +/- 1.3 nmol/L during the ouabain infusion. 3. These results suggest that ouabain is not an endogenous hormone present at physiologically relevant concentrations. Furthermore, ouabain does not cause natriuresis during saline-loading in sheep and is therefore unlikely to be responsible for the exaggerated natriuresis seen in some forms of hypertension.

Circulating Na+/K+-ATPase inhibitors: effects of neuropeptides, volume expansion and salt loading in conscious rats

1. In mammalian plasma, many different inhibitors of Na+/K(+)-ATPase are present, but it is not clear whether their net effect on NA+/K(+)-ATPase activity changes during the regulation of electrolyte and fluid balance. We studied Na+/K(+)-ATPase inhibition by plasma extracts in conscious rats during short- and long-term body fluid regulation. 2. Male, adult, conscious, freely moving Wistar rats were subjected to one of the following protocols: (i) intracerebro-ventricular (i.c.v.) injections of angiotension II (AngII; 1, 10 and 100 ng), the AngII receptor antagonist losartan (1 microgram), atrial natriuretic peptide (ANP-III; 1 microgram) or isotonic saline (IS); (ii) intra-arterial (i.a.) injections of IS (6 or 10 mL), hypertonic saline (HS; 1.2% NaCl, 5 mL) or hypertonic plasma expander (HPS; 3.5% hetastarch in HS, 5 mL); or (iii) a low salt-high salt-low salt diet sequence (0.18/1.8/0.18% NaCl chow for 5 days each with controls receiving 0.18% NaCl on all days). Bodyweight, the intake of food and water, urine volume and Na+ concentration and weight of faeces were determined daily. Plasma samples were withdrawn repeatedly throughout the respective protocols, extracted on C18-reversed phase columns and assayed for their effect on the activity of different Na+/K(+)-ATPase preparations. 3. The inhibition of rat brain Na+/K(+)-ATPase by plasma extracts was not significantly changed by i.c.v. injection of AngII, losartan, ANP-III and IS within the observation period (30 min from respective stimuli). Similarly, no significant changes occurred after acute volume expansion by i.a. injection of IS or HS within 120 min; upon HPS, however, Na+/K(+)-ATPase inhibition was decreased by approximately 20% (P < 0.05), probably due to passive dilution. During the high-salt diet, fluid retention was effectively counteracted by an adaptive increase of urinary sodium excretion. Throughout the protocol, inhibition of pig brain Na+/K(+)-ATPase by plasma extracts did not differ significantly between groups. 4. It is concluded from these results that the short- or long-term control of body fluids in conscious rats is not associated with systematic changes in Na+/K(+)-ATPase inhibition by plasma factors.

Value of digoxin in heart failure and sinus rhythm: new features of an old drug?

Digoxin has been a controversial drug since its introduction >200 years ago. Although its efficacy in patients with heart failure and atrial fibrillation is clear, its value in patients with heart failure and sinus rhythm has often been questioned. In the 1980s, reports of some large-scale trials indicated that digoxin, with or without vasodilators or angiotensin-converting enzyme inhibitors, reduced signs and symptoms of congestive heart failure and improved exercise tolerance. This beneficial influence was mainly found in patients with more advanced heart failure and dilated ventricles, whereas the effect in those with mild disease appeared to be less pronounced. In the last few years, new data have shown that digoxin may also have clinical value in mild heart failure, either when used in combination with other drugs or when administered alone. As neurohumoral activation has increasingly been recognized to be a contributing factor in the disease progression of chronic heart failure, the modulating effects of digoxin on neurohumoral and autonomic status have received more attention. Also, there is evidence that relatively low doses of digoxin may be at least as effective as higher doses and have a lower incidence of side effects. Further, the recognition that the use of digoxin too early after myocardial infarction may be harmful and the development of other drugs, in particular angiotensin-converting enzyme inhibitors, have obviously changed the place of digoxin in the treatment of chronic heart failure. The large-scale survival trial by the Digitalis Investigators Group (DIG), whose preliminary results have recently been presented, has shown that although digoxin has a neutral effect on total mortality during long-term treatment, it reduces the number of hospital admissions and deaths due to worsening heart failure. The potentially new features of the old drug digoxin are discussed in this review.

Chronic ouabain infusion does not cause hypertension in sheep

Ouabain is claimed to be a hormone of adrenal origin, capable of raising arterial pressure in rats. We infused ouabain in conscious sheep under carefully controlled circumstances to determine its effects on blood pressure, urine electrolytes, and vasoactive hormones. Eight healthy ewes were studied while taking a constant intake of dietary sodium and potassium. Ouabain infusion at 0.25 mg daily over 22 days reduced heart rate and arterial pressure and had no effect on pressor responsiveness to incremental intravenous infusions of angiotensin II. Ouabain induced minor, but statistically significant, decrements in urine volume, urinary sodium excretion, plasma renin and angiotensin II concentrations, and a rise in plasma aldosterone and cortisol. Plasma ouabain levels averaged 1.37 +/- 0.28 nmol/l during ouabain infusion. In conclusion, high-dose chronic ouabain infusion in sheep did not elevate arterial pressure or alter pressor responsiveness to angiotensin II, was antidiuretic and antinatriuretic, and induced minor perturbations in circulating renin, angiotensin II, aldosterone, and cortisol.

Ouabain-inhibiting activity of aldosterone antagonists

It has been suggested that endogenous substances (known as ouabain-like factors, OLF), secreted from the central nervous system in response to salt and water retention, inhibit the cell membrane Na+/K+ pump in the renal tubules and reduce sodium reabsorption. However, by also acting upon vascular smooth muscle cells, they may induce cell Na+ and Ca++ accumulation, vasoconstriction and systemic hypertension. Recently, an endogenous Na+/K+ pump inhibitor was isolated from human plasma; this inhibitor is indistinguishable from the cardiac glycoside ouabain based on biochemical and immunological criteria. Its plasma concentration is close to the therapeutic range for ouabain (around 0.4 nmol/L). Since plant ouabain promotes natriuresis, vasoconstriction, and hypertension; endogenous ouabain may therefore control extracellular fluid volume and blood pressure. The highest plasma concentrations of endogenous ouabain and OLF were found in congestive heart failure, aldosterone producing adenoma, human and animal models of volume expanded hypertension (reduced renal mass and DOCA-salt hypertension), and in Milan hypertensive rats (MHS). Aldosterone antagonists (canrenone and canrenoate) exert both agonist and antagonist effects on the digitalis receptor site of the Na+/K+ pump. They are effective antihypertensive agents in animal models of hypertension sustained by OLF (reduced renal mass-Na+ and DOCA-salt hypertension in rats). Moreover, in a subgroup of essential hypertensives, 4 weeks of canrenoate administration reduced blood pressure, heightened red blood cell Na+/K+ pump activity, and antagonized ouabain-induced vasoconstriction. None of these effects was seen in the other hypertensives. These data suggest that aldosterone antagonists stimulate the Na+/K+ pump inhibited by endogenous ouabain and exert their antihypertensive action at least in part through this mechanism.

Double-blind placebo-controlled study of ibopamine and digoxin in patients with mild to moderate heart failure: results of the Dutch Ibopamine Multicenter Trial (DIMT)

OBJECTIVES

This study was conducted to determine the efficacy and safety of long-term treatment with the orally active dopamine agonist ibopamine in patients with mild to moderate chronic congestive heart failure and to compare the results with those of treatment with digoxin and placebo.

BACKGROUND

Ibopamine and digoxin are drugs that exert hemodynamic and neurohumoral effects. Because there is accumulating evidence that progression of disease in chronic heart failure is related not only to hemodynamic but also to neurohumoral factors, both drugs might be expected to have a favorable long-term effect.

METHODS

We studied 161 patients with mild to moderate chronic heart failure (80% in New York Heart Association functional class II and 20% in class III), who were treated with ibopamine (n = 53), digoxin (n = 55) or placebo (n = 53) for 6 months. Background therapy consisted of furosemide (0 to 80 mg); all other drugs for heart failure were excluded. Clinical assessments were made at baseline and after 1, 3 and 6 months.

RESULTS

Of the 161 patients, 128 (80%) completed the study. Compared with placebo, digoxin but not ibopamine significantly increased exercise time after 6 months (p = 0.008 by intention to treat analysis). Ibopamine was only effective in patients with relatively preserved left ventricular function, as it significantly increased exercise time in this subgroup (for patients with a left ventricular ejection fraction > 0.30; p = 0.018 vs. placebo). No patient receiving digoxin withdrew from the study because of progression of heart failure, compared with six patients receiving ibopamine and two receiving placebo. At 6 months, plasma norepinephrine was decreased with digoxin and ibopamine therapy (-106 and -13 pg/ml, respectively) but increased with placebo administration (+62 pg/ml) (both p < 0.05 vs. placebo). Plasma aldosterone was unaffected, but renin was decreased by both agents after 6 months (p < 0.05 vs. placebo). Total mortality and ambulatory arrhythmias were not significantly affected by the two drugs.

CONCLUSIONS

Ibopamine and digoxin both inhibit neurohumoral activation in patients with mild to moderate chronic heart failure. However, the clinical effects of these drugs are different and appear to be related to the degree of left ventricular dysfunction.