Does plasma immunoreactive ouabain originate from the adrenal gland?

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.

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 of cardiac glycosides: Structure, toxicokinetics, clinical signs, diagnosis and antineoplastic potential

Cardiac glycosides (CGs) are secondary compounds found in plants and amphibians and are widely distributed in nature with potential cardiovascular action. Their mechanism is based on the blockage of the heart's sodium potassium ATPase, with a positive inotropic effect. Some of the most well-known CGs are digoxin, ouabain, oleandrin, and bufalin. They have similar chemical structures: a lactone ring, steroid ring, and sugar moiety. Digoxin, ouabain, and oleandrin are classified as cardenolides, consisting of a lactone ring with five carbons, while bufalin is classified as bufodienolides, with a six-carbon ring. Small structural differences determine variations in the toxicokinetics and toxicodynamics of such substances. Most case reports of poisoning caused by CGs are associated with cardiovascular toxicity, causing a variety of arrhythmias and lesions in the heart tissue. Experimental studies also describe important similarities among different CGs, especially regarding species sensitivity. Recent studies furthermore focus on their antineoplastic potential, with controversial results. Data from research studies and case reports were reviewed to identify the main characteristics of the CGs, including toxicokinetics, toxicodynamics, clinical signs, electrocardiographic, pathological findings, antineoplastic potential and the main techniques used for diagnostic purposes.

The Expanding Spectrum of Primary Aldosteronism: Implications for Diagnosis, Pathogenesis, and Treatment

Primary aldosteronism is characterized by aldosterone secretion that is independent of renin and angiotensin II and sodium status. The deleterious effects of primary aldosteronism are mediated by excessive activation of the mineralocorticoid receptor that results in the well-known consequences of volume expansion, hypertension, hypokalemia, and metabolic alkalosis, but it also increases the risk for cardiovascular and kidney disease, as well as death. For decades, the approaches to defining, diagnosing, and treating primary aldosteronism have been relatively constant and generally focused on detecting and treating the more severe presentations of the disease. However, emerging evidence suggests that the prevalence of primary aldosteronism is much greater than previously recognized, and that milder and nonclassical forms of renin-independent aldosterone secretion that impart heightened cardiovascular risk may be common. Public health efforts to prevent aldosterone-mediated end-organ disease will require improved capabilities to diagnose all forms of primary aldosteronism while optimizing the treatment approaches such that the excess risk for cardiovascular and kidney disease is adequately mitigated. In this review, we present a physiologic approach to considering the diagnosis, pathogenesis, and treatment of primary aldosteronism. We review evidence suggesting that primary aldosteronism manifests across a wide spectrum of severity, ranging from mild to overt, that correlates with cardiovascular risk. Furthermore, we review emerging evidence from genetic studies that begin to provide a theoretical explanation for the pathogenesis of primary aldosteronism and a link to its phenotypic severity spectrum and prevalence. Finally, we review human studies that provide insights into the optimal approach toward the treatment of primary aldosteronism.

Endogenous cardiotonic steroids in chronic renal failure

BACKGROUND

Previous reports demonstrated that digitalis-like cardiotonic steroids (CTS) contribute to the pathogenesis of end-stage renal disease. The goal of the present study was to define the nature of CTS in patients with chronic kidney disease (CKD) and in partially nephrectomized (PNx) rats.

METHODS

In patients with CKD and in healthy controls, we determined plasma levels of marinobufagenin (MBG) and endogenous ouabain (EO) and erythrocyte Na/K-ATPase activity in the absence and in the presence of 3E9 anti-MBG monoclonal antibody (mAb) and Digibind. Levels of MBG and EO were also determined in sham-operated Sprague-Dawley rats and in rats following 4 weeks of PNx.

RESULTS

In 25 patients with CKD plasma, MBG but not EO was increased (0.86 ± 0.07 versus 0.28 ± 0.02 nmol/L, P < 0.01) and erythrocyte Na/K-ATPase was inhibited (1.24 ± 0.10 versus 2.80 ± 0.09 μmol Pi/mL/h, P < 0.01) as compared to that in 19 healthy subjects. Ex vivo, 3E9 mAb restored Na/K-ATPase in erythrocytes from patients with CKD but did not affect Na/K-ATPase from control subjects. Following chromatographic fractionation of uremic versus normal plasma, a competitive immunoassay based on anti-MBG mAb detected a 3-fold increase in the level of endogenous material having retention time similar to that seen with MBG. A similar pattern of CTS changes was observed in uremic rats. As compared to sham-operated animals, PNx rats exhibited 3-fold elevated levels of MBG but not that of EO.

CONCLUSIONS

In chronic renal failure, elevated levels of a bufadienolide CTS, MBG, contribute to Na/K-ATPase inhibition and may represent a potential target for therapy.

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.

The digitalis-like steroid hormones: new mechanisms of action and biological significance

Digitalis-like compounds (DLC) are a family of steroid hormones synthesized in and released from the adrenal gland. DLC, the structure of which resembles that of plant cardiac glycosides, bind to and inhibit the activity of the ubiquitous cell surface enzyme Na(+), K(+)-ATPase. However, there is a large body of evidence suggesting that the regulation of ion transport by Na(+), K(+)-ATPase is not the only physiological role of DLC. The binding of DLC to Na(+), K(+)-ATPase induces the activation of various signal transduction cascades that activate changes in intracellular Ca(++) homeostasis, and in specific gene expression. These, in turn, stimulate endocytosis and affect cell growth and proliferation. At the systemic level, DLC were shown to be involved in the regulation of major physiological parameters including water and salt homeostasis, cardiac contractility and rhythm, systemic blood pressure and behavior. Furthermore, the DLC system has been implicated in several pathological conditions, including cardiac arrhythmias, hypertension, cancer and depressive disorders. This review evaluates the evidence for the different aspects of DLC action and delineates open questions in the field.

Circulating endogenous digitalis-like factor(s) (EDLF) in man is derived from the adrenals and its secretion is ACTH-dependent

Adrenal origin and ACTH-dependent secretion of endogenous digitalis-like factor(s) (EDLF) was investigated. Twelve normal weight normotensive subjects (normal group) and 10 patients with Addison's disease (Addison group) were subjected to prolonged ACTH stimulation with 1 mg tetracosactin-depot im. Blood sampling was at 0 and 240 min. Digitalis-like reactivity was monitored in plasma extracts (combined organic solvent solid phase method) by digoxin and ouabain radioimmunoassay (RIAD and RIAO, respectively). 3H-ouabain concentration on erythrocytes (OBS) was also determined. Na+, K+-ATPase inhibition by normal plasma extract was tested by measuring Vmax and Km of 86Rb+-transport into human erythrocytes. In the normal group basal median plasma concentrations RIAD (0.07 nmol/l) and RIAO (0.89 nmol/l) increased significantly after ACTH administration (median 0.31 and 1.83, respectively; Wilcoxon, p<0.01). In contrast, in the Addison group no plasma RIAD and RIAO reactivity was detected before or after ACTH administration with minor exceptions. The OBS remained unchanged in the Addison group at 0 and 240 min; in the normal group there was a significant decline at 240 min (Wilcoxon, p<0.05) implying increase in circulating EDLF after ACTH stimulation. In the 86Rb+-transport experiments, 2 nmol/l ouabain or 2 nmol/l plasma-extracted ouabain reactivity both significantly impaired substrate affinity equally increasing Km without affecting Vmax. In men, the adrenals may produce and secrete EDLF, whose secretion appears to be ACTH-dependent.

Calpain is a mediator of myocardial injury in experimental uremia: is it activated by endogenous ouabain?

BACKGROUND

Incidence of cardiovascular disease is more than 20-fold higher in patients with chronic renal failure than in aged-matched individuals with normal renal function. Little is understood about the causes or the mechanism of uremia-induced cardiovascular injury, but the involvement of calpain as a possible mediator has recently been under investigation. Mean calpain activity was found to be 3.4-fold higher in the hearts of uremic rats than in control or spontaneously hypertensive (SHR) rats. In addition, calpain activity was found to be stimulated in myoblasts (Girardi) treated with media enriched with uremic serum compared with cells treated with serum from healthy volunteers. In this study, we assessed the impact of calpain activation in uremia and explored the possibility that calpain might be activated in uremia by endogenous ouabain. Ouabain is known to be elevated in uremia and is strongly associated with left ventricular hypertrophy in essential hypertension.

METHODS

Calpain activity was measured in situ in human-derived myoblasts treated with low doses of ouabain similar to those concentrations found in uremic patients.

RESULTS

Low concentrations of ouabain (10 nmol/L) caused a highly significant increase in calpain activity, which could be completely inhibited by the simultaneous chelation of intracellular and extracellular Ca2+, and by the chelation of extracellular Ca2+ alone.

CONCLUSIONS

Calpain activity can be stimulated by nanomolar concentrations of ouabain due to an influx of extracellular Ca2+. As circulating ouabain is known to be elevated in uremia and strongly associated with LVH remodeling, we hypothesize that endogenous ouabain might be one of the factors that facilitates the remodeling of the left ventricle in patients with renal failure.

Increased circulating levels of ouabain-like factor in patients with asymptomatic left ventricular dysfunction

BACKGROUND

Much evidence has been accumulated that human plasma contains digitalis-like factor(s) with Na/K ATPase inhibitor properties. Increased concentrations of ouabain-like factor (OLF) have been reported in patients with moderate to severe hypertension and in patients with overt congestive heart failure due to dilated cardiomyopathy.

AIM

The presence of circulating OLF has not been investigated in borderline to mild hypertension or in the early stage of dilated cardiomyopathy.

METHODS AND RESULTS

The study population consisted of 18 normal volunteers, 24 patients with borderline to mild hypertension, 47 patients with asymptomatic left ventricular dysfunction (ALVD) due to dilated cardiomyopathy and 26 patients with cardiac arrhythmias but normal left ventricular function. OLF values (pM ouabain equivalent) were assayed in extracted plasma, using a radioimmunoassay for ouabain. OLF was, respectively, 29.4+/-20.6 pM in normal controls, 39.1+/-23.8 pM in hypertensives, 35+/-18 pM in patients with cardiac arrhythmias, 52.3+/-25.8 pM in ALVD patients not treated with digoxin and 64.6+/-29.6 pM in ALVD patients treated with digoxin. Patients with ALVD, both treated and not treated with digoxin, had OLF significantly higher (P<0.05) than all the other groups. In patients with ALVD no correlation between OLF and left ventricular ejection fraction was observed. In the hypertensive group no correlation between OLF and both diastolic and systolic pressure was found.

CONCLUSION

Increased concentrations of OLF were observed in patients with left ventricular dysfunction due to dilated cardiomyopathy, before the occurrence of overt heart failure, suggesting that OLF may be an early marker of the disease.

Radioiodinated Tyrosyl-Ouabain and Measurement of a Circulating Ouabain-like Compound

Background: Assays for endogenous ouabain, a cardiac glycoside believed to be involved in blood pressure and volume regulation, are characterized by laboratory-specific plasma values that are measured by different assays. Because of this variability, our study focused on the development of a new 125I-labeled ouabain derivative for RIA of high sensitivity.

Methods: We generated rabbit antisera against a ouabain-thyroglobulin conjugate. A tyrosylated ouabain derivative for radioiodination was synthesized using periodate and sodium cyanoborohydride reagents.

Results: Mass spectrometric analyses showed that the main product of the tyrosylating reaction was tyrosyl-ouabain (molecular mass, 702 Da). This was radioiodinated with Chloramine-T and used as a tracer in a RIA, which gave an assay detection limit of 5 pmol/L (4 ng/L), 2–100 times lower than that in the corresponding 3H-RIAs and 2–20 times lower than ouabain ELISAs, making it possible to measure low plasma concentrations of immunoreactive ouabain. Different amounts of SepPak C18-extracted plasma samples displaced the 125I-labeled tyrosyl-ouabain tracer at the same rate at which authentic ouabain was displaced. Plasma immunoreactive ouabain coeluted with authentic ouabain in two different HPLC conditions. Using the new RIA, we found plasma ouabain concentrations, assayed as immunoreactive equivalents, of 10.0 ± 1.3 pmol/L in healthy women and 12.0 ± 0.9 pmol/L in healthy men (mean ± SE; n = 10), as well as 41.2 ± 9.6 pmol/L in rats. The concentrations were 2–90 times lower than those previously reported using different assay methods.

Conclusions: Our ouabain 125I-RIA enables reliable measurements of low endogenous concentrations of a ouabain-like compound for both physiological and clinical purposes.

Detection of digitalis compounds using a surface plasmon resonance-based biosensor

An automated surface plasmon resonance-based biosensor system has been used to detect endogenous and exogenous digitalis-like factors (EDLF) in the pmolar range in real time. EDLF was purified from umbilical cord blood. EDLF has been suggested to play a role in hypertension and in perinatal adaptation. Highly specific polyclonal anti-ouabain antibodies showed a high affinity binding capacity for ouabain, ouabagenin and strophantidin with an IC50 value of 5 x 10(-10) M, 7.0 x 10(-10) M and 2 x 10(-8) M, respectively. EDLF cross-reacted with antibodies and its concentration in plasma at IC50 was around 50 pmol ouabain equivalent. This study shows the potential usefulness of the biosensor technology for biomolecular interaction analysis. The features of this technology (fully automated, measured in real time, sharpened response) offer several advantages compared with a traditional immunoassay like radioimmunoassay (RIA) in the detection of digitalis compounds in human fluids.

Endogenous ouabain secretion in man is not regulated by ACTH

It has been suggested that endogenous ouabain-like substance (OLS) is of adrenal origin and the secretion of OLS may be ACTH dependent. To determine if OLS is influenced by the pituitary-adrenal axis, we studied the effect of adrenal stimulation (0.25 mg Synacthen) and suppression (1 mg Dexamethasone) on two separate groups of nine subjects. Serum OLS was measured by a radioimmunoassay (RIA) developed in our lab, and cortisol and ACTH were measured by commercial assay kits. Dexamethasone significantly (P< 0.001) suppressed serum cortisol and ACTH concentrations, without effecting endogenous OLS concentration (0.64+/-0.17 vs 0.85+/-0.18nmol/l). Synacthen increased the concentration of cortisol in serum (p < 0.001) over the test period; OLS concentration, again, remained unchanged (0.45+/-0.04 vs 0.43+/-0.05 nmol/l). In further studies, serum concentrations of cortisol and OLS were compared between left (LAV) and right (RAV) adrenal veins with that from the inferior vena cava (IVC). Concentration of cortisol in the LAV and RAV was five-fold greater than that in IVC. However, there was no difference in OLS concentration at the corresponding sites. In addition, serum OLS concentrations in patients having undergone bilateral adrenalectomy or diagnosed with Addison's disease (0.62+/-0.19 nmol/l) were similar to concentrations in healthy subjects (0.67+/-0.21 nmol/l). Examination of bovine adrenal, liver, kidney, heart and human placenta demonstrated that OLS content of bovine adrenal was comparable with other tissues analysed. HPLC studies of human serum and bovine adrenal gland produced identical elution profiles, resolving a single peak which coincided with the retention time observed for standard ouabain. We conclude that the adrenal is unlikely to be the source of endogenous OLS, the secretion of which appears to be independent of ACTH.

Ouabain-like factors in human urine: identification of a Na-K-ATPase inhibitor as vanadium-diascorbate adduct

We investigated endogenous Na-K-ATPase inhibitors, i.e. ouabain-like factors(OLFs), in the urine of salt-loaded healthy subjects. During an intake of > 30g NaCl/day 24h-urines were collected, lyophilized, redissolved and acidified to pH 3.5. With gelchromatography the inhibitory activity eluted in a post-salt fraction FIV from Sephadex G-25. When this fraction was again passed through Sephadex G-10, one of three OLFs eluted in the early subfractions FIV/1-2 close to H-ouabain and cross-reacted strongly with a ouabain antibody (NEN). Two additional OLFs with Mr around 400 eluted in a late subfraction FIV/8 which resolved after reverse-phase HPLC into a more polar OLF- (water phase) and a more apolar OLF-2 (20% acetonitrile). Only the more apolar OLF-2 cross-reacted with digoxin and ouabain antibodies. OLF-1 and OLF-2 purified to single compounds by preparative thin layer chromatography inhibited Na-K-ATPase with IC50 of around 1.5 x 10(-5) M and 1.5 x 10(-4) M, respectively. Identification of OLF-2 was first attempted because most material was available for further processing. Data from mass-spectroscopy, nuclear magnetic resonance (1H-NMR) and infrared spectroscopy characterized OLF-2 as structurally unrelated to ouabain but resembling ascorbic acid derivatives, i.e. vanadium (V) diascorbates (Mr 403) with similar elution times from RP-HPLC as OLF-2. They inhibited the enzyme in its E2-configuration with IC50 of 9 x 10(-5) M and 2 x 10(-6) M for V(IV)- and V(V)-diascorbate, respectively. OLF-1, OLF-2 and V-diascorbate raise intracellular free calcium in inner medullary collecting duct and vascular smooth muscle cells which also contract in vitro. V-diascorbate was also natriuretic in a bioassay. We suggest that V-diascorbates represent one of several OLFs excreted in human urine.

Effect of high salt intake on plasma and tissue concentration of endogenous ouabain-like substance in the rat

The effect of high salt intake on serum concentration and tissue distribution of ouabain-like substance (OLS) was examined in rats. Sprague-Dawley rats (n=8) were placed on a high salt diet by the inclusion of 1.8% sodium chloride in drinking water for 7 days and a 'control' group (n=8) was maintained on normal drinking water during the study period. Serum and tissue OLS was measured by radioimmunoassay after solid phase extraction. High salt intake significantly increased serum OLS concentration (1.43 +/- 0.06 vs 1.14 +/- 0.05 nmol/L; mean +/- SEM, P=0.002). In both groups, the adrenal showed significantly (p < 0.001) higher OLS content compared to liver, kidney, heart and brain. HPLC of rat serum extract resolved a major peak with a retention time identical to that of standard ouabain, further confirming the nature of OLS. We conclude that high salt intake increases endogenous production of OLS, which appears to originate from the adrenal gland in the rat.

Ouabain-like factor quantification in mammalian tissues and plasma: comparison of two independent assays

The resolution of controversies that concern the detectability of an endogenous ouabain-like factor (OLF) in mammalian tissues and plasma was approached by the application of a standardized method for its extraction and quantification. Two independent assays were used to quantify the OLF: (1) a radioimmunoassay, which used a polyclonal anti-ouabain antiserum, and (2) a radioenzymatic assay based on the inhibition of dog kidney Na+,K+-ATPase. Plasma and tissues were obtained from the Milan hypertensive strain (MHS) and the Milan normotensive strain (MNS) of rats and from healthy human volunteers. Results indicate that (1) a single high-performance liquid chromatography (HPLC) fraction identical to that of ouabain was identified by both assay methods in the rat hypothalamus and hypophysis and in both rat and human plasma; (2) dilution curves of OLF and standard ouabain were parallel and with a similar Kd, both in radioimmunoassay (3 nmol/L) and ATPase assay (14 nmol/L); (3) after HPLC, OLF was similarly quantified by the two methods in the hypothalamus, hypophysis, adrenals, and plasma of rats and in human plasma; (4) OLF was present in larger amounts in the hypothalamus, hypophysis, and plasma of MHS rats than that of MNS rats; (5) the HPLC fraction of human plasma was quantified similarly by both assays (range, 60 to 150 pmol/L); (6) recovery of standard ouabain in pre-HPLC plasma extracts was approximately 90%; and (7) pre-HPLC OLF concentrations in human plasma ranged between 0.05 and 0.75 nmol/L. Rat cerebral tissues and both rat and human plasma contained measurable amounts of OLF, which were quantified similarly by radioimmunoassay and ATPase assay, both before and after HPLC fractionation. The increased MHS tissue and plasma levels of OLF are in keeping with the pathogenetic role of this factor in MHS hypertension.

Development of enzyme immunoassay for endogenous ouabain-like compound in human plasma

Widespread evidence supports the existence of an endogenous digitalis-like compound in mammals. We report here the development of a novel enzyme immunoassay for ouabain that, in conjunction with a detailed HPLC study, identifies a ouabain-like compound (OLC) in extracted human plasma. The assay is sensitive—minimum detection limit for OLC 37 pmol/L (11 pmol/L in plasma)—and has a working range (between-assay CV &lt;10%) of 180–10 000 pmol/L (54–3000 pmol/L in plasma). Mean recoveries of ouabain added to plasma ranged from 90% to 100%, and plasma extracts diluted in parallel to the standard curve. Plasma OLC concentrations in 10 healthy volunteers averaged 92 pmol/L (range 55–168), assuming 100% cross-reactivity of OLC in the ouabain assay. HPLC analysis with two distinct chromatographic conditions demonstrated that endogenous human plasma OLC co-eluted with authentic ouabain. The enzyme immunoassay is rapid and easy to perform and will support further investigation of the nature of this controversial endogenous steroid.

Bovine adrenocortical cells in culture synthesize an ouabain-like compound

Ouabain or a closely related isomer, and 'ouabain-like compound' (OLC), has been identified in plasma, by Hamlyn et al., using several physico-chemical and biological methods. Using a radioimmunoassay, the same authors later characterized an identical compound in adrenal cortex tissue and culture medium from adrenocortical cells. Nevertheless, other groups, using different immunosera, were not able to detect OLC in adrenal cortex and adrenocortical cells medium. In this report, we confirm the presence of OLC in bovine adrenal cortex and in fasciculata cells culture medium. The compound that we obtained has the same chromatographic properties as ouabain on HPLC using two types of elution systems. It presents the same mass spectrum and is able to bind to erythrocytes membranes Na(+)-K(+)-ATPase. In primary cultures of adrenocortical cells, its biosynthesis is increased after addition of pregnenolone or progesterone suggesting that these compounds may represent intermediate substrates in the biosynthetic pathway. Rhamnose readily enters the adrenocortical cell and increases slightly the biosynthesis of OLC. The present studies confirm that bovine adrenocortical cells in primary culture release an OLC with no differences with authentic ouabain using, HPLC, mass spectrometry and radioreceptor assay and suggest that OLC may be a product related to the adrenocortical steroidogenic pathway.

Ouabainlike compound in hypertension associated with ectopic corticotropin syndrome

Molecular mechanisms related to sodium retention have been implicated in the pathogenesis of hypertension. It is unclear how sodium retention leads to a rise in blood pressure, but ouabainlike compound may act as a final common pathway in sodium-induced hypertension. In ectopic corticotropin syndrome, hypertension has been attributed to cortisol inactivation overload, giving rise to mineralocorticoid-type hypertension. We sequentially measured plasma and urinary levels of ouabainlike compound over 2 months to evaluate its role in the hypertensive mechanisms in a 64-year-old man with this syndrome caused by lung cancer. His data included hypokalemia and increased cortisol concentrations, corticotropin levels, and urinary 17-hydroxycorticosteroid excretion. Plasma renin activity was suppressed. Plasma and urinary levels of ouabainlike compound were markedly increased concomitantly with high blood pressure. The maximum plasma level was 40-fold the normal range of the subject. After chemotherapy, ouabainlike compound levels gradually decreased in parallel with the decline in blood pressure and rise in potassium concentration. A correlation was observed between plasma and urinary levels of ouabainlike compound (P < .05). Plasma and urinary levels of ouabainlike compound correlated with systolic (P < .01) and diastolic (P < .05) pressures, respectively. The peak of ouabainlike compound in plasma and urine coincided with that of authentic ouabain on high-performance liquid chromatography. Ouabainlike compound derived from urine inhibited [3H]ouabain binding to human erythrocytes. These findings suggest that ouabainlike compound with biological activity could partly account for hypertension in ectopic corticotropin syndrome.

Endogenous sodium pump inhibition: current status and therapeutic opportunities

One might ask, given the number of false trails that have been pursued, why we, and so many others, have continued to pursue the elusive digitalis-like factor? The answer can be found in the many review articles cited above [4-13]. In animal models of volume-dependent hypertension, evidence favoring sodium pump inhibition as at least a contributing factor, is essentially overwhelming. These observations are supported by multiple lines of less direct evidence in humans which are also compatible with a contribution of a circulating sodium pump inhibitor. Indeed, if multiple premature claims announcing the isolation of the digitalis-like factor had not appeared, this would be one of a large number of interesting scientific areas in which identification of a responsible vector was expected momentarily. The disenchantment so often expressed, we believe, reflects a response to those premature claims. We echo a recent review on the digoxin-like sodium pump inhibitor story from one of the productive groups in this area. "Now that there is little doubt that endogenous digoxin-like inhibitors of sodium transport exist..., the link between these substances, salt intake and vascular tone must be pursued with increasing vigor" [12]. That pursuit, of course, will be easier if the criteria concerning the responsible mediator are employed systematically. Because the current situation resembles so strikingly the situation late in the nineteenth century--when efforts focused on the attempt to identify a specific microorganism as the agent responsible for specific disease--we employed Koch's Postulates as the organizing principle. The challenge faced by Robert Koch over a century ago is identical to the challenge that those of us who are interested in digitalis-like factors face today. Passionate advocacy and equally impassioned denial can be seen as a stage in the scientific process when the problem is important and has proven to be more intractable than anticipated. Substantial, but still circumstantial evidence supports strongly a role for a circulating digitalis-like factor not only in normal sodium homeostasis and in the pathogenesis of salt-sensitive hypertension, but also in the pathogenesis of a wide array of processes that have an uncertain etiology. Although supported by many lines of evidence, this intriguing concept remains controversial, in large part because the responsible factor has proven to be very elusive. Informed opinion today ranges from arguments that the agent does not exist to contrary arguments that the agent has been identified. A very large number of candidates from a wide range of chemical classes have been proposed. Indeed, the large number of candidates, none supported by absolutely definitive evidence, has contributed to the controversy. In this essay, we have attempted to define and illustrate the information that will be required before a candidate becomes widely accepted.

Ouabain increases aldosterone release from bovine adrenal glomerulosa cells: role of renin-angiotensin system

To evaluate the potential physiological significance of ouabain or a ouabainlike substance, we investigated the effect of nanomolar concentrations of ouabain on aldosterone release by cultured bovine adrenal glomerulosa cells. Ouabain (10 nM) increased aldosterone release from 0.35 to 0.89 ng.mg-1.4 h-1 in the serum-containing medium. Losartan prevented this increase. When angiotensinogen was added to the nonserum medium, 10 nM ouabain enhanced the aldosterone release. Losartan again blocked the increase. These findings together with a stimulation of renin release by ouabain indicate that angiotensin II generated by the adrenal cell renin-angiotensin system in the presence of exogenous serum or exogenous angiotensinogen is necessary for the ouabain-induced stimulation of aldosterone release. Ouabain (10 nM) enhanced the intracellular calcium concentration increase elicited by 0.1 nM angiotensin II severalfold. Addition of 1 nM ouabain enhanced the aldosterone secretion resulting from the addition of 1 nM angiotensin II. Nanomolar levels of ouabain, therefore, interact with both locally formed and exogenous angiotensin II to stimulate aldosterone production. A suggested mechanism is that ouabain increases calcium stores in the endoplasmic reticulum, thereby increasing the agonist response.

Evidence for an endogenous ouabain-like immunoreactive factor in human newborn plasma coeluted with ouabain on HPLC

The identification in human plasma of ouabain as an endogenous digitalis-like factor (EDLF) claimed by Hamlyn et al. has recently been contradicted by two studies which failed to demonstrate endogenous ouabain-like immunoreactivity in HPLC fractions in which exogenous ouabain was eluted. In this paper we report the results obtained on the cross-reactivity with antiouabain antibodies of an EDLF purified by us from human newborn cord plasma. We found that this EDLF coeluted with ouabain on HPLC and cross-reacted both with rabbit anti-ouabain antiserum and with the purified antibodies, which excluded possible interferences due to antibodies directed against non-ouabain portions of the immunogen. Similar but not identical slopes of the ouabain and EDLF displacements curves were observed in all competition ELISA experiments. The inhibitory effect of EDLF on erythrocyte 86Rb uptake was reversed by antiouabain antiserum and antibodies. The concentration of EDLF in newborn plasma, in the four different purifications studied ranged from 30 to 380 pM ouabain equivalents (o.e.) by ELISA and from 100 to 300 pM o.e. by 86Rb uptake. Our data thus support the existence, in human newborn plasma, of a factor with both biological and immunological ouabain-like properties, although not necessarily identical to ouabain.

Insulin resistance is associated with high plasma ouabain-like immunoreactivity concentration in NIDDM

The aim of the present study was to elucidate the pathophysiologic significance of circulating ouabain as a link between insulin resistance (IR) and hypertension (HT) in NIDDM. Euglycaemic (4.5 mmol/l) hyperinsulinaemic (360-580 pmol/l) clamping was performed using an artificial endocrine pancreas. Plasma ouabain-like immunoreactivity (OLI) was determined by radioimmunoassay using a highly specific antibody to ouabain. HT was defined as systolic blood pressure > 140 mm Hg and/or diastolic > 90 mm Hg or being treated with antihypertensive agents. The values (mean +/- SEM) of glucose infusion rate (GIR) and plasma OLI were compared among the four groups classified using IR and HT as factors. Group I (IR-/HT-, n = 15): GIR 7.20 +/- 0.36 mg.kg-1.min-1, OLI 130.8 +/- 20.9 pmol/l, which was not different from that in eight normal control subjects (7.69 +/- 0.40 mg.kg-1.min-1 and 142.6 +/- 32.3 pmol/l, respectively); Group II (IR-/HT+, n = 13): 5.89 +/- 0.36 mg.kg-1.min-1, 172.5 +/- 35.0 pmol/l; Group III (IR+/HT-, n = 14) 1.91 +/- 0.28 mg.kg-1.min-1, 576.6 +/- 161.5 pmol/l (p < 0.01 vs Group I and II); Group IV (IR+/HT+, n = 15) 1.79 +/- 0.22 mg.kg-1.min-1, 703.1 +/- 170.1 pmol/l (p < 0.01 vs Group I and II), respectively. Six of 57 NIDDM patients studied exhibited very high (> 1500 pmol/l) plasma OLI concentrations, showed marked insulin resistance and were all hypertensive. When analysed as a whole, plasma OLI was negatively correlated with GIR (p < 0.001), but was not correlated with arterial blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Ouabain is not detectable in human plasma

An enzyme-linked immunosorbent assay is described for the measurement of ouabain in human plasma. This assay is specific for ouabain, strophanthidin, and ouabagenin, with other steroids, including digoxin and vasopressor hormones, exhibiting negligible cross-reactivity. Assay sensitivity was 0.06 nmol/L if 1 mL plasma was extracted and less than 0.005 nmol/L when 20 mL plasma was analyzed. Extracted plasma samples showed ouabainlike immunoreactivity that diluted in parallel with the ouabain standard curve. Repeated extraction and assay of single plasma samples, however, did not produce consistent results in the assay. Increased specificity was obtained by high-performance liquid chromatography of sample extracts before assay. When high-performance liquid chromatographic profiles of plasma spiked with ouabain standard or following bolus intravenous injections of ouabain into normal human volunteers were compared with profiles of unspiked plasma, there was no support for the immunoreactive material in the latter samples being ouabain. We propose that if ouabain is present in the human circulation, its concentration is less than 0.005 nmol/L.