Development of an immunoassay for endogenous digitalislike factor

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.

Inhibition of endogenous ouabain by atrial natriuretic peptide is a guanylyl cyclase independent effect

Background

Endogenous ouabain (EO) and atrial natriuretic peptide (ANP) are important in regulation of sodium and fluid balance. There is indirect evidence that ANP may be involved in the regulation of endogenous cardenolides.

Methods

H295R are human adrenocortical cells known to release EO. Cells were treated with ANP at physiologic concentrations or vehicle (0.1% DMSO), with or without guanylyl cyclase inhibitor 1,2,4 oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). Cyclic guanosine monophosphate (cGMP), the intracellular second messenger of ANP, was measured by a chemiluminescent immunoassay and EO was measured by radioimmunoassay of C18 extracted samples.

Results

EO secretion is inhibited by ANP treatment, with the most prolonged inhibition (90 min vs ≤ 60 min) occurring at physiologic ANP concentrations (50 pg/mL). Inhibition of guanylyl cyclase with ODQ, also reduces EO secretion. The inhibitory effects on EO release in response to cotreatment with ANP and ODQ appeared to be additive.

Conclusions

ANP inhibits basal EO secretion, and it is unlikely that this is mediated through ANP-A or ANP-B receptors (the most common natriuretic peptide receptors) or their cGMP second messenger; the underlying mechanisms involved are not revealed in the current studies. The role of ANP in the control of EO synthesis and secretion in vivo requires further investigation.

Natural Products with Potential for Nonhormonal Male Contraception

Despite increased access to contraception over the last 60 years, unplanned pregnancies continue to contribute to economic disparities and overpopulation. Additionally, the burden of family planning falls primarily on women, as a reliable pharmaceutical male contraceptive has yet to be developed. The objective of this literature-based systematic review was to identify compounds for future study from natural sources with potential nonhormonal male contraceptive activity. After the exclusion of extracts and compounds with known hormonal mechanisms, 26 unique compounds were identified from natural species. The plant source, compound class, structure, target, mechanism of action, safety/toxicity profile, and in vitro, in vivo, and human studies for each compound were evaluated and discussed. β-Caryophyllene, embelin, oleanolic acid, triptonide, and N-butyldeoxynojirimycin (NB-DNJ) were selected as the five most promising compounds for future study using prespecified criteria such as number of studies, efficacy and safety profile, reversibility, and previous use in humans for any indication. In order to move forward with development of a male contraceptive from a natural source, additional studies are needed to determine the predicted safety and efficacy for in vivo and human clinical trials.

Endogenous cardiotonic steroids and cardiovascular disease, where to next?

Ever since British Physician William Withering first described the use of foxglove extract for treatment of patients with congestive heart failure in 1785, cardiotonic steroids have been used clinically to treat heart failure and more recently atrial fibrillation. Due to their ability to bind and inhibit the ubiquitous transport enzyme sodium potassium pump, thus regulating intracellular Na⁺ concentration in every living cell, they are also an essential tool for research into the sodium potassium pump structure and function. Exogenous CTS have been clearly demonstrated to affect cardiovascular system through modulation of vagal tone, cardiac contraction (via ionic changes) and altered natriuresis. Reports of a number of endogenous CTS, since the 1980s, have intensified research into their physiologic and pathophysiologic roles and opened up novel therapeutic targets. Substantive evidence pointing to the role of endogenous ouabain and marinobufagenin, the two most prominent CTS, in development of cardiovascular disease has accumulated. Nevertheless, their presence, structure, biosynthesis pathways and even mechanism of action remain unclear or controversial. In this review the current state-of-the-art, the controversies and the remaining questions surrounding the role of endogenous cardiotonic steroids in health and disease are discussed.

The sodium pump and digitalis drugs: Dogmas and fallacies

The sodium pump (Na/K-ATPase) is a plasma membrane enzyme that transports Na+ and K+ against their physiological gradients in most eukaryotic cells. Besides pumping ions, the enzyme may also interact with neighboring proteins to activate cell signaling pathways that regulate cell growth. Digitalis drugs, useful for the treatment of heart failure and atrial arrhythmias, inhibit the pumping function of Na/K-ATPase and stimulate its signaling function. In the current field of research on the sodium pump and digitalis drugs, some issues that are commonly accepted to be well established are not so, and this may impede progress. Here, several such issues are identified, their histories are discussed, and their open discussions are urged. The covered unsettled questions consist of (a) the suggested hormonal role of endogenous digitalis compounds; (b) the specificity of Na/K-ATPase as the receptor for digitalis compounds; (c) the relevance of the positive inotropic action of digitalis to its use for the treatment of heart failure; (d) the conflicting findings on digitalis-induced signaling function of Na/K-ATPase; and (e) the uncertainties about the structure of Na/K-ATPase in the native cell membrane.

Central and peripheral slow-pressor mechanisms contributing to Angiotensin II-salt hypertension in rats

Aims

High salt intake markedly enhances hypertension induced by angiotensin II (Ang II). We explored central and peripheral slow-pressor mechanisms which may be activated by Ang II and salt.

Methods and results

In protocol I, Wistar rats were infused subcutaneously with low-dose Ang II (150 ng/kg/min) and fed regular (0.4%) or high salt (2%) diet for 14 days. In protocol II, Ang II-high salt was combined with intracerebroventricular infusion of mineralocorticoid receptor (MR) blockers (eplerenone, spironolactone), epithelial sodium channel (ENaC) blocker (benzamil), angiotensin II type 1 receptor (AT1R) blocker (losartan) or vehicles. Ang II alone raised mean arterial pressure (MAP) ∼10 mmHg, but Ang II-high salt increased MAP ∼50 mmHg. Ang II-high salt elevated plasma corticosterone, aldosterone and endogenous ouabain but not Ang II alone. Both Ang II alone and Ang II-high salt increased mRNA and protein expression of CYP11B2 (aldosterone synthase gene) in the adrenal cortex but not of CYP11B1 (11-β-hydroxylase gene). In the aorta, Ang II-high salt increased sodium-calcium exchanger-1 (NCX1) protein. The Ang II-high salt induced increase in MAP was largely prevented by central infusion of MR blockers, benzamil or losartan. Central blockades significantly lowered plasma aldosterone and endogenous ouabain and markedly decreased Ang II-high salt induced CYP11B2 mRNA expression in the adrenal cortex and NCX1 protein in the aorta.

Conclusion

These results suggest that in Ang II-high salt hypertension, MR-ENaC-AT1R signalling in the brain increases circulating aldosterone and endogenous ouabain, and arterial NCX1. These factors can amplify blood pressure responses to centrally-induced sympatho-excitation and thereby contribute to severe hypertension.

Role of endogenous digitalis-like factors in the clinical manifestations of severe preeclampsia: a sytematic review

Endogenous digitalis-like factor(s), originally proposed as a vasoconstrictor natriuretic hormone, was discovered in fetal and neonatal blood accidentally because it cross-reacts with antidigoxin antibodies (ADAs). Early studies using immunoassays with ADA identified the digoxin-like immuno-reactive factor(s) (EDLF) in maternal blood as well, and suggested it originated in the feto-placental unit. Mammalian digoxin-like factors have recently been identified as at least two classes of steroid compounds, plant derived ouabain (O), and several toad derived bufodienolides, most prominent being marinobufagenin (MBG). A synthetic pathway for MBG has been identified in mammalian placental tissue. Elevated maternal and fetal EDLF, O and MBG have been demonstrated in preeclampsia (PE), and inhibition of red cell membrane sodium, potassium ATPase (Na, K ATPase (NKA)) by EDLF is reversed by ADA fragments (ADA-FAB). Accordingly, maternal administration of a commercial ADA-antibody fragment (FAB) was tested in several anecdotal cases of PE, and two, small randomized, prospective, double-blind clinical trials. In the first randomized trial, ADA-FAB was administered post-partum, in the second antepartum. In the post-partum trial, ADA-FAB reduced use of antihypertensive drugs. In the second trial, there was no effect of ADA-FAB on blood pressure, but the fall in maternal creatinine clearance (CrCl) was prevented. In a secondary analysis using the pre-treatment maternal level of circulating Na, K ATPase (NKA) inhibitory activity (NKAI), ADA-FAB reduced the incidence of pulmonary edema and, unexpectedly, that of severe neonatal intraventricular hemorrhage (IVH). The fall in CrCl in patients given placebo was proportional to the circulating level of NKAI. The implications of these findings on the pathophysiology of the clinical manifestations PE are discussed, and a new model of the respective roles of placenta derived anti-angiogenic (AAG) factors (AAGFs) and EDLF is proposed.

Endogenous cardiotonic steroids in kidney failure: a review and an hypothesis

In response to progressive nephron loss, volume and humoral signals in the circulation have increasing relevance. These signals, including plasma sodium, angiotensin II, and those related to volume status, activate a slow neuromodulatory pathway within the central nervous system (CNS). The slow CNS pathway includes specific receptors for angiotensin II, mineralocorticoids, and endogenous ouabain (EO). Stimulation of the pathway leads to elevated sympathetic nervous system activity (SNA) and increased circulating EO. The sustained elevation of circulating EO (or ouabain) stimulates central and peripheral mechanisms that amplify the impact of SNA on vascular tone. These include changes in synaptic plasticity in the brain and sympathetic ganglia that increase preganglionic tone and amplify ganglionic transmission, amplification of the impact of SNA on arterial tone in the vascular wall, and the reprogramming of calcium signaling proteins in arterial myocytes. These increase SNA, raise basal and evoked arterial tone, and elevate blood pressure (BP). In the setting of CKD, we suggest that sustained activation/elevation of the slow CNS pathway, plasma EO, and the cardiotonic steroid marinobufagenin, comprises a feed-forward system that raises BP and accelerates kidney and cardiac damage. Block of the slow CNS pathway and/or circulating EO and marinobufagenin may reduce BP and slow the progression to ESRD.

Neuroendocrine humoral and vascular components in the pressor pathway for brain angiotensin II: a new axis in long term blood pressure control

Central nervous system (CNS) administration of angiotensin II (Ang II) raises blood pressure (BP). The rise in BP reflects increased sympathetic outflow and a slower neuromodulatory pressor mechanism mediated by CNS mineralocorticoid receptors (MR). We investigated the hypothesis that the sustained phase of hypertension is associated also with elevated circulating levels of endogenous ouabain (EO), and chronic stimulation of arterial calcium transport proteins including the sodium-calcium exchanger (NCX1), the type 6 canonical transient receptor potential protein (TRPC6), and the sarcoplasmic reticulum calcium ATPase (SERCA2). Wistar rats received a chronic intra-cerebroventricular infusion of vehicle (C) or Ang II (A, 2.5 ng/min, for 14 days) alone or combined with the MR blocker, eplerenone (A+E, 5 µg/day), or the aldosterone synthase inhibitor, FAD286 (A+F, 25 µg/day). Conscious mean BP increased (P<0.05) in A (123±4 mm Hg) vs all other groups. Blood, pituitary and adrenal samples were taken for EO radioimmunoassay (RIA), and aortas for NCX1, TRPC6 and SERCA2 immunoblotting. Central infusion of Ang II raised plasma EO (0.58±0.08 vs C 0.34±0.07 nM (P<0.05), but not in A + E and A + F groups as confirmed by off-line liquid chromatography (LC)-RIA and LC-multistage mass spectrometry. Two novel isomers of EO were elevated by Ang II; the second less polar isomer increased >50-fold in the A+F group. Central Ang II increased arterial expression of NCX1, TRPC6 and SERCA2 (2.6, 1.75 and 3.7-fold, respectively; P<0.01)) but not when co-infused with E or F. Adrenal and pituitary EO were unchanged. We conclude that brain Ang II activates a CNS-humoral axis involving plasma EO. The elevated EO reprograms peripheral ion transport pathways known to control arterial Na⁺ and Ca²⁺ homeostasis; this increases contractility and augments sympathetic effects. The new axis likely contributes to the chronic pressor effect of brain Ang II.

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.

Relationship between ouabain and asthenozoospermia

A growing number of researches have shown that ouabain can regulate mammalian sperm function and male reproduction by modulating the sperm motility, capacitation and acrosome reaction in vitro. This study further examined the relationship between ouabain and asthenozoospermia. In this study, the rat was intraperitoneally injected with ouabain at different concentrations (low-dose ouabain group: 12.5 μg/kg body weight per day, and high-dose ouabain group: 25 μg/kg body weight per day) for 30 days to establish the asthenozoospermia model. The sperms from 60 males with normal fertility were incubated with ouabain of gradient concentrations (10(-7)-10(-2) mol/L) for 4 h. The sperm motility was evaluated under a microscope. Moreover, the endogenous ouabain (EO) level was determined in seminal plasma of mild or severe asthenozoospermia patients and males with normal fertility by competitive inhibition ELISA. The results showed that the sperm motility was significantly diminished in the rats treated with different concentrations of ouabain. The number of motile sperms (grades a and b) was decreased greatly in a time- and dose-dependent manner in 10(-5)-10(-2) mol/L ouabain groups (P<0.01), while no obvious change in sperm motility was observed in 10(-7)-10(-6)mol/L groups even for 4-h incubation (P>0.05). Furthermore, the EO level was significantly increased in asthenozoospermia patients as compared with that in males with normal fertility (25.27±1.71 μg/L in mild asthenozoospermia patients, 26.52±1.82 μg/L in severe asthenozoospermia patients, 19.31±1.45 μg/L in normal fertility men) (P<0.01). In conclusion, rat asthenozoospermia was successfully established by intraperitoneal injection of ouabain, and 10(-5) mol/L ouabain was sufficient enough to inhibit sperm motility in vitro. Moreover, EO, a normal constituent of seminal plasma, was highly expressed in asthenozoospermia males as compared with normal fertility ones.

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.

Physiological roles of endogenous ouabain in normal rats

Endogenous ouabain (EO)-like compounds are synthesized in and released from the adrenal gland. Although EO has been implicated in several pathological states such as hypertension and heart and kidney failure, its physiological roles in normal animal have not been elucidated. To address this issue, we studied the effects of reduction in plasma EO resulting from antiouabain antibody administration. Normal rats were treated for 28 days with antiouabain antibodies or rabbit IgG as control. Infusions were delivered through a jugular vein cannula by osmotic pumps, and blood pressure was monitored by tail-cuff plethysmography. The animals were housed in metabolic cages to measure water and food consumption and urine excretion. After 28 days, the thoracic aorta was isolated and used to study phenylephrine-induced contraction and atrial natriuretic peptide (ANP)-induced vasorelaxation. The adrenal gland cortex was enlarged in the antiouabain antibody-treated rats. Moreover, on the second day of treatment, there was a significant transient reduction in natriuresis in the antiouabain antibody-treated rats, suggesting that EO is a natriuretic hormone. Reduction in natriuresis was also observed when EO levels were reduced by active immunization resulting from sequential injection of ouabain-albumin. Furthermore, following 28 days of treatment, the response to phenylephrine was significantly lowered and that to ANP was significantly increased in aortic rings from antiouabain antibody-treated rats. These findings show for the first time that circulatory ouabain plausibly originating in the adrenal has physiological roles controlling vasculature tone and sodium homeostasis in normal rats.

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.

Analysis of ligand binding by bioaffinity mass spectrometry

BACKGROUND

Ligand binding is commonly analyzed using various immunoassays that are generally time-consuming and some may require secondary antibodies or gel electrophoresis which are also time-consuming and sometimes subjective. We introduced various examples for a more rapid approach using pre-activated surface chips which are analyzed by surface enhanced laser desorption/ionization-time of flight mass spectrometry (SELDI-TOF MS). Specific applications presented in this study include immobilization of antigen, antibody or oligo DNA on pre-activated chips with subsequent identification of the binding antibodies, antigens or DNA binding proteins to demonstrate the universal utility of this novel approach.

METHODS

BSA-digoxin conjugate (BSA-Dig), anti-digoxin antibody, anti-urinary trypsin inhibitor (uTi) antibody, or a double stranded oligo nucleotide based on the nucleotide sequence between -91 and -10 of the human CYP 450 2E1 promoter were immobilized on the Ciphergen pre-activated surface chips. Anti-digoxin antibody, BSA-digoxin conjugate, uTi, and CYP450 2E1 promoter binding protein were captured on the chip and identified by SELDI-TOF MS.

RESULTS

A protein with 141kDa was identified from anti-digoxin serum using BSA-Dig chips. This binding was competitively inhibited by addition of digoxin. Using anti-digoxin antibody, a peak at approximately 66kDa was detected in the preparation of BSA-Dig. This peak was also inhibited by free digoxin, suggesting BSA-Dig is detected. uTi fragments with approximately 3kDa to approximately 30kDa in the standard and urine samples were captured on the chip by anti-uTi antibody. Finally, we identified a 95-kDa CYP 450 2E1 promoter binding protein in HeLa cells nuclear extracts.

CONCLUSIONS

Bioaffinity SELDI-TOF MS is a powerful and versatile approach for analysis of ligands. It eliminates tracer-labeled secondary antibodies and allows for determination of molecular weights of binding proteins and their ligands directly. This approach may also be considered for the detection of enzymes, receptors, or any other specific ligands.

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.

Independent and incremental prognostic value of endogenous ouabain in idiopathic dilated cardiomyopathy

Increased circulating levels of endogenous ouabain (EO) have been observed in some heart failure patients, but their long term clinical significance is unknown. This study investigated the prognostic value of EO for worsening heart failure among 140 optimally treated patients (age 50+/-14 years; 104 male; NYHA class 1.9+/-0.7) with idiopathic dilated cardiomyopathy. Plasma EO was determined by RIA and by liquid chromatography mass spectrometry, values were linearly correlated (r = 0.89) in regression analysis. During follow-up (13+/-5 months), heart failure progression was defined as worsening clinical condition leading to one or more of the following: sustained increase in conventional therapies, hospitalization, cardiac transplant, or death. NYHA functional class, age, LVEF, peak VO2 and plasma levels of EO were predictive for heart failure progression. Heart failure worsened 1.5 fold (HR: 1.005; 95% CI: 1.001-1.007; p<0.01) for each 100 pmol/L increase in plasma EO. Moreover, those patients with higher plasma EO values had an odds ratio of 5.417 (95% CI: 2.044-14.355; p<0.001) for heart failure progression. Following multivariate analysis, LVEF, NYHA class and plasma EO remained significantly linked with clinical events. This study provides the first evidence that circulating EO is a novel, independent and incremental marker that predicts the progression of heart failure.

Ouabain-like compound changes rapidly on physical exercise in humans and dogs: effects of beta-blockade and angiotensin-converting enzyme inhibition

Ouabain, an inhibitor of the sodium pump, has been identified as a constituent of bovine adrenal glands. We were interested whether the release of this cardiotonic steroid is stimulated by physical exercise. Hence, athletes and healthy dogs were subjected to ergometry. Ouabain-like compound (OLC) was measured in venous blood by enzyme-linked immunosorbent assay as well as by (86)Rb+ uptake inhibition (as ouabain equivalents). OLC increased in venous blood of athletes after 15 minutes of ergometry from 2.5+/-0.5 to 86.0+/-27.2 nmol/L (n=51; P<0.001), as did the concentration of a circulating inhibitor of the sodium pump from 7.3+/-1.7 to 129.8+/-51 nmol/L (ouabain equivalents, P<0.05). Half-maximal increase in heart rate and systolic blood pressure occurred at 5.1+/-1.2 nmol/L and at 30+/-1 nmol/L OLC, respectively. On rest, OLC decreased in humans and dogs with a half-life of 3 to 5 minutes. In beagles exposed to moderate exercise on a treadmill for 13 minutes, levels of OLC increased 46-fold (from 3.7+/-0.8 to 166.9+/-91.8 nmol/L; n=6; P<0.005). This effect was suppressed when the dogs had been treated for 3 weeks with the beta1-adrenergic receptor blocker atenolol or the angiotensin-converting enzyme inhibitor benazepril. We conclude that OLC changes rapidly during exercise and is under the control of norepinephrine and angiotensin II.

Production of ouabain-like factor in normal and ischemic rat heart

Endogenous ouabain-like factor (OLF) has been detected in mammalian plasma, adrenal gland, and hypothalamus. We investigate whether cardiac tissue may also produce OLF. HPLC chromatographic separation of cardiac extracts showed that RIA-determined OLF activity coincided with the elution profile of exogenous ouabain and with the ability to inhibit 86Rb uptake in human erythrocytes. OLF activity was remarkably higher in excised hearts (3.94 +/- 0.84 pmol/g wet weight by RIA) than in rat blood (0.05 +/- 0.02 pmol/ml). Similar values were obtained in perfused working hearts, without significant changes over time from 5 to 30 minutes of aerobic perfusion. Significant OLF release in the perfusion buffer was also observed (0.54 +/- 0.05 pmoles over 30 minutes). In hearts subjected to 15 minutes of aerobic perfusion followed by 15 minutes of global myocardial ischemia OLF concentration was remarkably increased (8.59 +/- 1.13 versus 4.58 +/- 0.57 pmol/g wet weight by RIA, P < 0.01; an increase after ischemia was confirmed by the assay of 86Rb uptake). Our findings suggest that the rat heart is able to produce OLF, and that its concentration increases during ischemia. Myocardial OLF might modulate the Na/K-ATPase, producing relevant effects on ionic homeostasis and/or gene transcription.

Endogenous glycosides in critically ill patients

OBJECTIVE

To determine the incidence of critically ill patients displaying endogenous digitalis-like-immunoreactive substances (DLIS) and to examine the relationship of these hormones to routine laboratory variables, the underlying disease, myocardial function, hemodynamic status, severity of illness, systemic inflammation, and mortality rate.

DESIGN

Sera of 401 consecutive critically ill patients, not treated with cardiac glycosides, were analyzed for DLIS (digitoxin and digoxin, TDx; Abbott Diagnostics, North Chicago, IL) and endogenous ouabain. Normal values of endogenous ouabain were determined in 62 healthy volunteers. We measured pro- and anti-inflammatory mediators (L-selectin, tumor necrosis factor-alpha, interleukin-1beta, interleukin-2, interleukin-6, interleukin-10), C-reactive protein, and serum amyloid A protein as well as patients' Acute Physiology and Chronic Health Evaluation II and Goris scores. In a subgroup of patients with a pulmonary artery catheter (n = 95), we determined cardiac output, pulmonary artery occlusion pressure, systemic and pulmonary vascular resistance, left ventricular stroke volume, and right and left stroke work.

SETTING

Two surgical intensive care units of an university hospital.

SUBJECTS

Sera of 401 consecutive critically ill patients.

INTERVENTIONS

Blood sampling.

MEASUREMENTS AND MAIN RESULTS

Of the 401 patients tested, 343 had nonmeasurable DLIS concentrations (DLIS-negative), and 58 (14.5%) had positive digoxin (n = 18) or digitoxin (n = 34) concentrations (DLIS-positive) or were positive in both tests (n = 6). Mean endogenous ouabain concentrations were nine-fold increased in DLIS-positive (3.59 +/- 1.43 nmol/L) and three-fold increased in DLIS-negative (1.34 +/-.81 nmol/L) patients compared with controls (0.38 +/- 0.31 nmol/L). DLIS and ouabain concentrations closely correlated with the Acute Physiology and Chronic Health Evaluation II and Goris score and were associated with increased concentrations of transaminases, bilirubin, aldosterone, cortisol, serum creatinine, fractional sodium excretion, proinflammatory mediators, C-reactive protein, and serum amyloid A (p <or=.009). The hospital mortality rates of DLIS-positive and DLIS-negative patients were 12% and 3.2%, respectively, and for patients with ouabain concentrations above and below 2 nmol/L 38.6% and 0.6%, respectively. In DLIS-positive patients with pulmonary artery catheter (n = 23), cardiac output, stroke volume, and left ventricular stroke work were decreased, and pulmonary artery occlusion pressure and central venous pressure were increased (p <or=.009).

CONCLUSIONS

Different types of endogenous glycosides including endogenous ouabain are elevated in a significant proportion of critically ill patients. The occurrence of these substances is associated with increased morbidity and hospital mortality rates, possibly due to systemic inflammatory processes. DLIS but not endogenous ouabain concentrations were found to be related to left ventricular function.

11-hydroxylation in the biosynthesis of endogenous ouabain: multiple implications

Accumulating evidence indicates that mammals use steroidal glycosides with "digitalis-like" activity. An endogenous ouabain (EO) has been described and is linked with long-term changes in sodium balance and cardiovascular structure and function. In the adrenal gland, the biosynthesis of EO and similar compounds appears to involve cholesterol side-chain cleavage with sequential metabolism of pregnenolone and progesterone. The more distal events in the biosynthesis have not been elucidated. Preliminary work using primary cell cultures from the bovine adrenal cortex suggests that the biosynthesis of EO is affected by inhibitors of 11beta-hydroxylase. Direct participation of 11-hydoxylase in EO synthesis would lead to an 11beta isomer of ouabain in mammals and, in vivo, an 11beta-oriented hydroxyl group would spontaneously form a mixture of two 11-19 hemiketal isomers. The latter isomers would likely be converted back to a single 11beta isomer of ouabain during isolation. The existence of an additional ring in the hemiketals, along with reduced flexion of the steroidal A, B, and C rings, raises the possibility that their in vivo physiological targets and actions differ from the isolated form of EO.

Structure-activity relationships for the hypertensinogenic activity of ouabain: role of the sugar and lactone ring

Elevated levels of an endogenous ouabain circulate in many patients with essential hypertension. However, in contrast to ouabain, digoxin does not induce hypertension. This study investigated the hypothesis that within a single cardiac glycoside, the structural elements that induce hypertension differ from those responsible for high potency as a sodium pump inhibitor. Normal male Sprague-Dawley rats received infusions of vehicle (VEH), rhamnose (RHA), ouabain (OUA), ouabagenin (OGN), dihydro-ouabain (DHO), iso-ouabain (ISO), and a lactone ring opened analog (ORO) at 30 microgram. kg(-1). 24 h(-1) for 5 weeks via subcutaneous osmotic pumps. Cuff pressures were taken weekly. At the end of the study, trunk blood was harvested, extracted by C18 column, and subjected to high-performance liquid chromatography. Fractions were analyzed for OUA, OGN, and DHO by immunoassay. In OUA-, OGN-, and DHO-infused rats, 1 main peak of immunoreactivity corresponding to the infused agent was found. No evidence of in vivo conversion to OUA or DHO was found for any analog except ORO. At 5 weeks, systolic blood pressures in VEH, RHA, OUA, OGN, DHO, ISO, and ORO were 132+/-2.5, 133+/-1.5, 159+/-2.6,* 154+/-4,* 167+/-4,* 171+/-2.2,* and 169+/-2.4* mm Hg, respectively (*P<0.01 versus VEH and RHA, P<0.05 versus OUA). The hypertensinogenic activity was greater than OUA in 3 analogs (DHO, ISO, and ORO) in which the lactone was saturated, conformationally restrained by linkage with the oxygen at C14, or opened, respectively. These compounds were weak inhibitors of dog kidney Na,K-ATPase. Thus, RHA and the unsaturated lactone ring are crucial to the high potency of OUA as an inhibitor of the sodium pump but appear to be unrelated to its ability to induce hypertension. The conclusion that this form of hypertension is mediated primarily by the steroid nucleus suggests also that OUA may have a mechanism of action independent of the sodium pump.

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.

Secretion of a lactone-hydrogenated ouabain-like effector of sodium, potassium-adenosine triphosphatase activity by adrenal cells

Ouabain-like factor (OLF), a mammalian cardenolide, is a counterpart to plant-derived ouabain and is found in the adrenal, hypothalamus, and blood of several mammalian species. We now report the existence of a mammalian lactone-hydrogenated ouabain-like factor (dihydro-OLF) in secretions from cultured mouse adrenal Y-1 cells. Dihydro-OLF structurally and functionally mimics plant-derived dihydroouabain. We measured both OLF and the newly discovered dihydro-OLF using five independent techniques: immunoreactivity with two specific antisera, one against ouabain and one against dihydroouabain; chromatographic mobility; spectral absorbance characteristics; and concentration-dependent inhibition and phosphorylation of Na,K-adenosine triphosphatase. All measured physical attributes of dihydro-OLF mimic those of plant-derived dihydroouabain, including a spectral shift maxima, 220 nm (OLF) to 196 nm (dihydro-OLF), with appropriately decreased molar absorptivity. Dihydro-OLF (IC50 = 590 nM) is a 10-fold less potent Na+,K+-adenosine triphosphatase inhibitor than its oxidized mammalian counterpart OLF (IC50 = 60 nM), just as dihydroouabain is less potent than ouabain. Dihydro-OLF is also 3-fold more potent than a recently identified isomer of plant-derived dihydroouabain (IC50 = 1,700 nM). Using antiouabain and antidihydroouabain antisera we estimate that 3 x 10(7) mouse adrenal Y-1 cells secreted 1.3 ng OLF and 8.9 ng dihydro-OLF. The relative abundance of dihydro-OLF is consistently greater than that of its oxidized form, OLF, in bovine adrenals (22-fold), human serum (13-fold), and secretions from cultured mouse Y-1 cells (5-fold). The discoveries of OLF, OLF-genin, and now dihydro-OLF constitute an intriguing structural polymorphism probably involved in the synthesis, regulation, and metabolic control of these new hormone-like compounds.

Monoclonal Antibodies That Distinguish Between Two Related Digitalis Glycosides, Ouabain and Digoxin

The exogenous digitalis glycosides, ouabain and digoxin, have been widely used in humans to treat congestive heart failure and cardiac arrhythmias. Several reports have also pointed to the existence of endogenous ouabain- and digoxin-like compounds, but their precise roles in mammalian physiology and various disorders of the circulation are not clear. In an attempt to produce specific Abs for the purification and identification of endogenous ouabain-like compounds, somatic cell fusion was used to produce mAbs specific for ouabain. Our attempts to produce ouabain-specific mAbs were unsuccessful when ouabain was coupled to exogenous proteins such as bovine γ-globulins, BSA, and human serum albumin. However, when ouabain was coupled to an Ab of A/J mice origin and the same strain of mouse was used for immunization with ouabain-Ab conjugate, three Abs (1-10, 5A12, and 7-1) specific for ouabain were obtained. In assays of fluorescence quenching and saturation equilibrium with tritiated ouabain, Ab 1-10 exhibited 200 nM affinity for ouabain. These three mAbs are distinguished from existing Abs to ouabain and digoxin by their specificity for ouabain and lack of cross-reactivity with digoxin. Specificity studies showed that the loss of cross-reactivity was correlated with the presence of a hydroxyl group at either position 12β (digoxin) or 16β (gitoxin) of the steroid ring. These Abs can be used to develop assays for detection and characterization of ouabain-like molecules in vivo.

Natriuresis and inhibition of Na+/K(+)-ATPase: modulation of response by physiological manipulation

1. There is considerable evidence for the existence of an endogenous inhibitor of Na+/K(+)-ATPase. The exact physiological nature and role of this postulated agent remains unclear, although it would be predicted that one of its actions would be stimulation of renal sodium excretion. 2. The natriuretic effect of renal arterial infusion of ouabain is relatively slow in onset and is sustained. 3. The natriuresis is not modified by changes in sodium status, unlike the natriuretic effect of atrial natriuretic peptide. 4. The natriuretic action of ouabain is enhanced dramatically by acute volume expansion or chronic mineralocorticoid treatment, which both result in hypokalaemia, hypertension and hypervolaemia. 5. The natriuretic response to small increments in blood pressure is markedly enhanced by treatment with ouabain. 6. We hypothesize that the interaction between the inhibition of Na+/K(+)-ATPase and elevated blood pressure could result in the shedding of sodium in conditions where there are increased levels of circulating endogenous digitalis-like factors.

Observations on the nature, biosynthesis, secretion and significance of endogenous ouabain

The human circulation contains four readily distinguishable biologically active inhibitors of the sodium pump that appear to be endogenous to mammals. Of these, one has been purified to homogeneity and by numerous chromatographic, mass spectral, biochemical, and physiological analyses has been shown to be a novel steroidal isomer of ouabain in which the location and orientation of two or more steroidal hydroxyl groups differ. The human endogenous "ouabain" (EO) is a high affinity reversible inhibitor of the pump with inotropic and vasopressor activity. Circulating levels of EO depend upon the adrenal cortex and metabolic events preceding and following pregnenolone formation are involved in EO biosynthesis. Within the adrenal gland, the stimulus-secretion mechanisms for EO secretion are distinct from those for aldosterone highlighting different regulation. Among Caucasians with essential hypertension, 30-45% have elevated circulating levels of EO. Sustained elevation of plasma ouabain in rats induces chronic hypertension with characteristics similar to those in patients and whose severity is determined by inherited factors and renal function. In conclusion, at least one of the mammalian counterparts to the cardiac glycosides is a novel steroidal isomer of ouabain. The isomer is secreted by the adrenal cortex, and augments cardiovascular function. The observation of this entity in the human circulation, the demonstration of its biosynthesis, and the existence of specific receptors suggest to us that EO is a novel adrenocortical hormone and may be part of a broader family of novel mammalian steroids that regulate the sodium pump and other processes.

Role of the ouabain-like factor and Na-K pump in rat and human genetic hypertension

Endogenous ouabain-like factor (OLF) is present in mammal tissues and after standardized extraction procedure can be similarly quantified by two independent assays: RIA and Na-KATPase inhibition. OLF was quantified both from plasma and tissues obtained from MHS hypertensive and MNS normotensive rats, maintained under the same environmental and dietary conditions, and from plasma of healthy volunteers and essential hypertensive patients. OLF biochemical characterization shows that it behaves like ouabain except for a 1000-fold higher affinity for the ouabain low-affinity Na-KATPase isoforms than ouabain. Tissue and plasma levels of OLF are higher in MHS than in MNS rats and are not influenced by exogenous OLF sources. Plasma OLF is also increased in a subgroup of hypertensive patients. Both in rats and humans a primary cell membrane alteration affecting ion transports seems to be linked to the increased levels of OLF. An antihypertensive compound which selectively antagonizes the pressor effect of OLF and corrects the ion transport defect is under development and can represent a new pharmacological approach to the treatment of hypertension.

Biosynthesis of digitalis-like compounds in rat adrenal cells: hydroxycholesterol as possible precursor

The biosynthesis of digitalis-like compounds (DLC) was determined in bovine and rat adrenal homogenates, as well as in primary rat adrenal cells, by following changes in the concentration of DLC using three independent sensitive bioassays: inhibition of [3H]-ouabain binding to red blood cells and competitive ouabain and bufalin ELISA. The amounts of DLC in bovine and rat adrenal homogenates, as measured by the two first bioassays, increased with time when the mixtures were incubated under tissue culture conditions. Rat primary adrenal cells were incubated in the presence of [1,2-(3)H]-25-hydroxycholesterol, [26,27-(3)H]-25-hydroxycholesterol or [7-(3)H]-pregnenolone. The radioactive products, as well as the digitalis-like activity, were fractionated by three sequential chromatography systems. When [1,2-(3)H]-25-hydroxycholesterol or [7-(3)H]-pregnenolone was added to the culture medium, the radioactivity was co-eluted with digitalis-like activity, suggesting that at least one of the DLC might originate in hydroxycholesterol. In contrast, when the culture medium was supplemented with [26,27-(3)H]-25-hydroxycholesterol, the radioactivity was not co-eluted with the digitalis-like activity, indicating that side chain cleavage is the first step in the synthesis of digitalis-like compounds by rat adrenal.

Bovine adrenals and hypothalamus are a major source of proscillaridin A- and ouabain-immunoreactivities

Besides an isomer of the cardenolide ouabain, a material with a similar HPLC retention time as ouabain but cross-reactivating with antibodies against the bufadienolide proscillaridin A and inhibiting the sodium pump is known to circulate in human blood plasma (B. SICH et al., Hypertension 27, 1073-1078 (1996).). The concentrations of both substances are known to correlate with the blood pressure. It was the intention of this work to localize tissues that contain the highest concentrations of the proscillaridin A immunoreactive material, to correlate its concentration with that of ouabain and to get information whether the concentration of this material simply reflects the number of sodium pumps of the tissue extracted. Specific antibodies for each cardiotonic steroid were used to test the tissue concentration. This report shows that in bovine tissues the distribution pattern of proscillaridin A and ouabain immunoreactivities are similar and that hypothalamus and adrenals show the highest concentrations. The cross-reactive material did not reflect the number of sodium pumps per g of wet weight tissue as measured by [3H]ouabain binding. Therefore, it is unlikely that the tissue concentrations in both immunoreactivities reflects the tissue capacity of sodium pumps labeled with cardiotonic steroids via the blood plasma. The study rather favors the concept that two different types of inhibitors of the sodium pump exist within both tissues.

Digoxin-like immunoreactivity in early infantile death

The aim of this study was to determine if the level of digoxin-like immunoreactivity in post-mortem sera obtained from infants differs according to the cause of death and if the level is related to age, post-mortem interval, cardiac pathology or adrenal weight. Twelve infants whose deaths were attributed to sudden infant death syndrome (SIDS), and 11 infants who died from other causes, had blood sampled between 3 to 53 hours post-mortem from their right atrial cavity. Digoxin-like immunoreactivity was measured, using a specific and sensitive digoxin radioimmunoassay, and was detected in 7 of the infants who died of SIDS and 7 of those who died from other causes. The highest levels were seen in two patients who died from meningococcal sepsis and haemorrhage, hyperpyrexia and encephalopathy syndrome, respectively. No correlation was detected between the digoxin-like immunoreactivity level, gender, age at death, post-mortem interval or cardiac pathology. Digoxin-like immunoreactivity levels correlated with adrenal weight. It is concluded that digoxin-like immunoreactivity is frequently found in infant sera, but levels are not specific to and are no higher in SIDS infants than infants dying of other conditions.

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.

Effect of carbidopa on the excretion of sodium, dopamine, and ouabain-like substance in the rat

The effect of carbidopa, a competitive inhibitor of dopamine synthesis on the excretion of sodium, dopamine (DA), and endogenous sodium transport inhibitor (ESTI), was examined in rats on normal and high salt diets for 7 days. ESTI activity was measured (1) as ouabain-like substance (OLS) by radioimmunoassay using an antibody raised against ouabain, (2) by inhibition of purified Na+,K+-ATPase activity (ATPI), and (3) as digoxin-like substance (DLS) using a commercial digoxin assay. The OLS immunoassay was validated against rubidium transport studies. High salt diet caused an increase in OLS and ATPI but not DLS. Sodium excretion in rats on normal sodium intake was not affected by carbidopa treatment but was significantly lower in the high sodium diet group during the first 5 days of the study. In the low salt group, carbidopa treatment caused significant increases in OLS. We conclude that ESTI (measured as OLS) and DA are important in the natriuresis of salt loading.

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.

Further evidence for an endogenous digitalis-like compound in newborn and adult plasma detected by anti-ouabain antiserum

It is widely but not unanimously accepted that one or more endogenous digitalis-like factors (EDLF) circulate in human plasma. In this paper we provide confirmatory evidence that newborn plasma contains a factor with immunological and biological properties similar to ouabain and demonstrate that this factor may be present also in the adult. In fact, we obtained in newborn and adult plasma extracts, identical HPLC elution profiles of ouabain-like immunoreactivity and 86Rb erythrocyte uptake inhibitory activity with a major peak corresponding to the retention time of ouabain. The fact that immunoreactivity and biological digitalis-like activity in the peak are due to an identical substance is strongly supported by the correlation between RIA and 86Rb uptake inhibitory values observed in the purified fractions. Finally, the strong correlation between immunoreactivity observed in plasma samples after simple SepPak C18 extraction and after additional HPLC suggests that less purified samples may be assayed for screening purposes. However, for a more quantitative assessment, this simple extraction method needs a subsequent HPLC purification for eliminating an overestimation of values due to cross-reacting impurities.

Angiotensin II stimulates secretion of endogenous ouabain from bovine adrenocortical cells via angiotensin type 2 receptors

Angiotensin II stimulates secretion of corticosteroids and ouabain-like activity from adrenocortical cells. Distinct adrenocortical angiotensin II receptor subtypes (AT1, AT2) have been described, and the present studies investigated their roles in steroid secretion. Using primary bovine adrenocortical cell cultures under serum free conditions, angiotensin II stimulated the secretions of aldosterone, cortisol, and endogenous ouabain as verified by high-performance chromatography. The dose-response curves for stimulated steroid secretion were parallel with unitary slopes while the half-maximally effective concentrations of angiotensin II were 0.31 to 0.38 nmol/L for secretions of aldosterone and cortisol and 2.3 nmol/L for endogenous ouabain. The nonselective mammalian antagonist (Sar1-Ile8) angiotensin II blocked stimulated secretion of all three steroids without affecting basal output. In the presence of the AT1 antagonist DuP753, angiotensin II-stimulated secretions of aldosterone and cortisol were blocked while secretion of endogenous ouabain was unaffected. In the presence of the AT2 antagonist PD123319, both basal and angiotensin II-stimulated secretions of aldosterone and cortisol were normal while stimulated secretion of endogenous ouabain was inhibited. The secretion of endogenous ouabain was activated maximally by the AT2 agonist CGP42112 under conditions in which aldosterone secretion was unaffected. These results demonstrate that AT2 receptors stimulate secretion of endogenous ouabain from bovine adrenocortical cells. The specificity of AT1 and AT2 receptor stimulation indicates that separate signaling mechanisms having minimal cross talk control the adrenocortical secretions of corticosteroids and cardiac-active steroids. Adrenocortical AT2 receptors may be important in the adaptation to low salt diets and other conditions in which angiotensin II is increased.

Purification and characterization of endogenous digoxin-like immunoreactive factors in chicken blood

Studies have been performed to determine whether an endogenous material capable of binding to digoxin antibodies is present in the chicken plasma. In the blood of 12 chickens without feed control, endogenous digoxin-like immunoreactive factors (DLIF) binding of digoxin antibodies in enzyme immunoassays amounted to 866 / 302 pg digoxin equivalents/mL of plasma (mean +/- SEM). Immunoreactivity of DLIF increased to 1848***331 pg/mL with a double value of control after boiling and acid pretreating the plasma. The major purification steps employed in this report were gel filtration column chromatography, high performance liquid chromatography (HPLC) and isoelectric focusing (IEF). Using HPLC for the separation, at least 10 chicken DLIFs with different molecular weight (MW) have been found. The MW of the smallest is 300 daltons (Da) while the largest is 100 kDa. The value of the isoelectric point of the most abundant type of DLIF from untreated chicken plasma is 6.3 as determined by IEF. The partially purified DLIF inhibits Na+, K(+)-ATPase from a porcine cerebral cortex as well as three human red blood cell membrane preparations in a dose-response fashion.

Different methods for measuring endogenous digitalis-like factor(s)

It is well recognized that one of the difficulties in the search for endogenous ligand(s) with digitalis-like properties (endogenous digitalis-like factor(s), EDLF) in mammals has been the lack of a unique, specific method for the accurate measurement of EDLF. Using C18 solid-phase extracts of plasma from normal adults and various patient groups, and purified extracts from umbilical cord plasma by affinity resin chromatography and HPLC, different methods to measure EDLF were evaluated. These were: (a) a human placenta radioreceptor assay (RRA) developed on the premise that competition for cardiac glycoside receptors was an absolute requirement for EDLF; (b) the inhibition of 86Rb uptake in human erythrocytes to estimate the potassium transport by the sodium pump; (c) an enzyme immunoassay specific for ouabain recently introduced in the market (DuPont Ouabain EIA Reagent Pack). The human placenta RRA was found to have the same ease of application as immunoassay, but could have major advantages in detecting active molecules, being "biologically more meaningful". Ouabain immunoreactivity correlated with EDLF values obtained by RRA, but in some instances the two assays were completely unrelated. Moreover, the high specificity of the DuPont antibody for ouabain (< 3% cross reactivity with digoxin) could be disadvantageous to detect EDLF not strictly resembling ouabain. The 86Rb uptake inhibition method correlated with RRA for EDLF purified by HPLC. It tested the complete enzymatic cycle and could therefore better reflect the in-vivo inhibitory activity of EDLF. However, it appeared not suitable for the routine EDLF evaluation in clinical studies since it was susceptible to sample osmolarity and required daily isolation of human erythrocytes possibly from the same donor. Results of the present study demonstrate that every assay has its limitations, and would suggest the use of multiple assays for EDLF detection.

Pulse pressure correlates in humans with a proscillaridin A immunoreactive compound

Endogenous digitalis-like factors in humans are presumably cardenolides and bufadienolides. To test whether bufadienolide-like substances may circulate in human blood, we used antibodies from rabbits against the bufadienolide proscillaridin A to measure the concentration of cross-reacting material in human plasma with an indirect enzyme-linked immunosorbent assay. IgG had an apparent affinity of 2 x 10(-9) mol/L for proscillaridin A. It was specific for bufadienolides and did not cross-react with cardenolides or several steroid hormones. Extraction of human plasma with ethanol and fractionation of this extract over a high-performance liquid chromatographic reverse-phase C18 column with a propanol/isopropanol gradient resulted in the separation of three peaks of increasing hydrophobicity (ED1, ED2, ED3) that inhibited the sodium pump of human red blood cells and cross-reacted with proscillaridin A antibodies. The concentration of the proscillaridin A immunoreactivity ED1 in normotensive subjects had a geometric mean of 0.1 nmol/L, with a dispersion factor of 8.77. ED1 correlated positively in a group of 60 normotensive subjects, 22 patients with hypertension, and 19 patients with chronic renal failure with mean arterial blood pressure (log ED1 [nmol/L] = 0.013 x mm Hg-2.17, r = .25, P < .05), systolic pressure (log ED1 [nmol/L] = 0.010 x mm Hg-2.23, r = .32, P < .01), and pulse pressure (log ED1 [nmol/L] = 0.019 x mm Hg-1.80, r = .38, P < .0001). There was no correlation with other parameters of the donors. We conclude that several substances cross-reacting with proscillaridin A antibodies and inhibiting the sodium pump of human red blood cells circulate in human blood. The level of one of these substances (ED1) correlates with mean arterial and pulse pressures.

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.

Na,K-ATPase inhibitors from bovine hypothalamus and human plasma are different from ouabain: nanogram scale CD structural analysis

The specific, high affinity binding of plant-derived digitalis glycosides by the mammalian sodium and potassium transporting adenosine triphosphatase (Na,K-ATPase, or sodium pump), a plasma membrane enzyme with critical physiological importance in mammalian tissues, has raised the possibility that a mammalian analog of digitalis might exist. We previously isolated and structurally characterized from bovine hypothalamus a novel isomer of the plant glycoside, ouabain, which differs structurally only in the attachment site and/or the stereochemistry of the steroid moiety [Tymiak et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 8189-8193]. Hamlyn and co-workers reported a molecule purified from human plasma which by mass spectrometry could not be distinguished from plant ouabain [Hamlyn et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 6259-6263]. Since rhamnoside cardiotonic steroids are not known as natural products from mammalian sources, it became important to compare these two pure isolates to determine if the same or structurally distinct compounds has been found. Our results indicate that the human and bovine Na,K-ATPase-inhibitors are identical, but different from plant ouabain. This supports the notion that the human sodium pump may be under specific physiological regulation by a mammalian analog of the digitalis glycosides.