Advances in understanding the role of cardiac glycosides in control of sodium transport in renal tubules

Cardiotonic steroids have been used for the past 200 years in the treatment of congestive heart failure. As specific inhibitors of membrane-bound Na(+)/K(+) ATPase, they enhance cardiac contractility through increasing myocardial cell calcium concentration in response to the resulting increase in intracellular Na concentration. The half-minimal concentrations of cardiotonic steroids required to inhibit Na(+)/K(+) ATPase range from nanomolar to micromolar concentrations. In contrast, the circulating levels of cardiotonic steroids under physiological conditions are in the low picomolar concentration range in healthy subjects, increasing to high picomolar levels under pathophysiological conditions including chronic kidney disease and heart failure. Little is known about the physiological function of low picomolar concentrations of cardiotonic steroids. Recent studies have indicated that physiological concentrations of cardiotonic steroids acutely stimulate the activity of Na(+)/K(+) ATPase and activate an intracellular signaling pathway that regulates a variety of intracellular functions including cell growth and hypertrophy. The effects of circulating cardiotonic steroids on renal salt handling and total body sodium homeostasis are unknown. This review will focus on the role of low picomolar concentrations of cardiotonic steroids in renal Na(+)/K(+) ATPase activity, cell signaling, and blood pressure regulation.

Mechanistic insight into digoxin inactivation by Eggerthella lenta augments our understanding of its pharmacokinetics

The human gut microbiota plays a key role in pharmacology, yet the mechanisms responsible remain unclear, impeding efforts toward personalized medicine. We recently identified a cytochrome-encoding operon in the common gut Actinobacterium Eggerthella lenta that is transcriptionally activated by the cardiac drug digoxin. These genes represent a predictive microbial biomarker for the inactivation of digoxin. Gnotobiotic mouse experiments revealed that increased protein intake can limit microbial drug inactivation. Here, we present a biochemical rationale for how the proteins encoded by this operon might inactivate digoxin through substrate promiscuity. We discuss digoxin signaling in eukaryotic systems, and consider the possibility that endogenous digoxin-like molecules may have selected for microbial digoxin inactivation. Finally, we highlight the diverse contributions of gut microbes to drug metabolism, present a generalized approach to studying microbe-drug interactions, and argue that mechanistic studies will pave the way for the clinical application of this work.

Aberrant regulation of endogenous ouabain-like factor in bipolar subjects

Ill phases of bipolar illness are associated with abnormalities in ion regulation and intracellular ion concentrations. Previously, it has been reported that mania is characterised by lower circulating levels of ion regulating endogenous cardenolides, and that bipolar subjects lack the normal seasonal variation of these factors. Since endogenous cardenolides are elaborated in settings of extensive physical activity, euthymic bipolar and psychiatrically normal control subjects were asked to exercise to exhaustion. Plasma concentrations of endogenous cardenolides were measured at baseline, 60 min, peak exercise and post-recovery. Ouabain-like immunoreactive factor (OLF) was lower at baseline (0.005+/-S.D. 0.01 ng/mL in bipolar vs. 0.072+/-0.06 ng/mL in normal control subjects, P=0.019), lower at 60 min (0.007+/-S.D. 0.02 ng/mL in bipolar vs. 0.075+/-0.06 ng/mL in normal control subjects, P=0.029), and tended to be lower at peak exercise (0.009+/-S.D. 0.02 ng/mL in bipolar vs. 0.131+/-0.21 ng/mL in normal control subjects, P=0.15) in bipolar subjects compared to non-psychiatric controls. Other endogenous cardenolides did not vary significantly. The endogenous cardenolide, OLF, may be aberrantly controlled in bipolar illness.

Mammalian cardenolides in cancer prevention and therapeutics

Digoxin-like immunoreactive factor (DLIF) and ouabain-like factor (OLF) are the mammalian counterparts to the plant-derived cardiotonic steroids digoxin and ouabain. Compelling evidence indicates that the cardiotonic steroids may have anticancer properties. Recent evidence indicates that low (nanomolar) concentrations of DLIF selectively induce cell death in transformed cells, while sparing normal cells, and is even more potent than the plant-derived compounds. The discovery that these endogenous molecules may play a role in the regulation of cancer cell proliferation provides a potentially new paradigm for the physiologic role of DLIF and OLF. In addition, the possible use of digoxin itself as a therapeutic agent in cancer has been explored, and evidence suggests that its conversion to dihydrodigoxin may be involved in regulating anticancer activity. The mechanism(s) for the pro-apoptotic property of these compounds is not known. In this brief review, we will discuss the proposed mechanism of action of digoxin, ouabain, DLIF, and OLF as anticancer compounds and discuss the effects that metabolic conversion to their dihydro-derivatives may have on this activity. From the perspective of therapeutic drug monitoring, these findings suggest some potential new challenges in the need to measure concentrations of digoxin and dihydrodigoxin as well as their endogenous counterparts DLIF and OLF in serum.

Digoxin-like immunoreactive factors induce apoptosis in human acute T-cell lymphoblastic leukemia

BACKGROUND

Plant-derived cardenolides reportedly possess anticancer properties in human leukemic cells via selective induction of apoptosis, cell cycle arrest, and differentiation. Selective induction of apoptosis with mammalian-derived digoxin-like immunoreactive factor (DLIF) could provide new strategies for anticancer drug development or the identification of biomarkers for cancer. We investigated whether DLIFs selectively induce apoptosis in human lymphoblastic leukemic cells.

METHODS

We compared the relative potencies of digoxin, ouabain, and DLIF on induction of programmed cell death in Jurkat cells (an acute T-leukemic cell line), K-562 (a myelogenous leukemia cell line), and nonpathologic human peripheral blood mononuclear cells (PBMCs). Apoptosis was measured by flow cytometry with the annexin V/propidium iodide method.

RESULTS

Digoxin and ouabain induced apoptosis in Jurkat cells [digoxin 50% inhibitory concentration (IC(50)), 24 nmol/L; ouabain IC(50), 26 nmol/L]. Neither digoxin nor ouabain induced apoptosis in K-562 cells or PBMCs. DLIF was more potent (IC(50), 1.9 nmol/L) and >2-fold more effective than digoxin or ouabain at inducing maximum apoptosis in Jurkat cells. The IC(50) values in the apoptosis assays were >100-fold lower (DLIF) and 20-fold lower (digoxin and ouabain) than the IC(50) required for Na(+)- and K(+)-dependent ATPase (DLIF, 200 nmol/L; digoxin, 910 nmol/L; ouabain, 600 nmol/L).

CONCLUSION

DLIF selectively induces apoptosis in a human acute T-cell lymphoblastic leukemia cell line but not in K-562 cells or PBMCs. These data suggest a new physiological role for these endogenous hormone-like factors.

Strategies for developing biomarkers of heart failure

BACKGROUND

Heart failure (HF) is a devastating disease with increasing prevalence in elderly populations. One-half of all patients die within 5 years of diagnosis. The annual cost of treating patients with HF in the US is more than $20 billion, which is estimated to be greater than that of myocardial infarction and all cancers combined. Given the complex pathophysiology and varied manifestations of HF, interest has intensified in developing biological markers to predict susceptibility and aid in the early diagnosis and management of this disease.

METHODS

We searched Medline via Ovid for studies published during the period 1966-2003 regarding various biomarkers suggested for HF. Our review focused on developing strategies for discovering and using new biomarkers, particularly those potentially linked to pathophysiologic mechanisms. We also point out strategic advantages, limitations, and methods available for measuring each of the currently proposed markers.

RESULTS

Biomarkers reviewed include those released from the heart during normal homeostasis (natriuretic peptides), those produced elsewhere that act on the heart (endogenous cardiotonic steroids and other hormones), and those released in response to tissue damage (inflammatory cytokines). The concept of using a combination of multiple markers based on diagnosis, prognosis, and acute vs chronic disease is also discussed. In view of recent advances in our understanding of molecular biochemical derangements observed during cardiac failure, we consider the concept of myocardial remodeling and the heart as part of an endocrine system as strategies.

CONCLUSION

Strategically, biomarkers linked to mechanisms involved in the etiology of HF, such as dysregulation of ion transport, seem best suited for serving as early biological markers to predict and diagnose disease, select therapy, or assess progression.

Heterogeneous post-column immunoreaction detection using magnetized beads and a laboratory-constructed electromagnetic separator

The nature of immune reactors allows development of quantitative analytical methods that are highly selective and can often be used directly with complex biological matrixes such as blood, plasma or urine. A major limitation of immunoassay is that antibodies are sometimes unable to discriminate structurally similar species such as drug metabolites and synthetic analogs. The problem associated with the lack of discrimination can be circumvented by coupling immunoassay with liquid chromatography post-column. The most commonly used separation method in post-column immunoreaction detection is the affinity column. Affinity columns may create undesired effects such as a compromise of the chromatographic separation efficiency, the requirement for an antibody with fast reaction kinetics and the need for flushing the column. This paper reports a post-column immunoreaction detection system coupled with a laboratory-constructed on-line magnetic separation flow chamber that is designed to overcome these problems. The system uses disposable magnetic beads as a solid-phase support for separation that can be easily removed from the system. The model analytes chosen for this study were digoxin and its metabolites due to the commercial availability of monoclonal antibodies for these compounds. Digoxin was separated using a chromatographic method prior to being interfaced through a liquid handler system to the immunoreactor. Compatibility of the HPLC mobile phase was determined to be acceptable with a mixing ratio of 1:3 between the LC fraction and immunoreagent solution. The dynamic range of the calibration curve in digoxin-spiked phosphate buffer was found to be 0.25-12 ng/ml and a quadratic fit was found to provide the best fit to the data with a correlation coefficient of 0.9974. The residual error for all standards was less than 15%. The percentage RSDs for the two controls, 2 and 10 ng/ml, were 6.88 and 4.82% (n = 6) and the percentage errors were 7.07 and -6.89% (n = 6), respectively.

Digoxin and its related endogenous factors

The digitalis drugs are plant-derived cardenolide compounds used medicinally for several hundred years. These drugs elicit inotropic and chronotropic effects on the heart, but they also affect many other tissues. The mechanism of action involves inhibition of the ion-transport activity of a membrane-associated protein called Na, K-ATPase (sodium pump). Present theory holds that the sodium pump is the principal molecular receptor for the digitalis drugs. Recent evidence indicates the presence of naturally occurring digitalis-like compounds in mammals. It is believed these compounds, collectively known as either digitalis-like (DLF) or ouabain-like (OLF) factors, may be endogenous hormones regulating the biological activity of the sodium pump and its isoforms. The presence of deglycosylated and other congeners of one specific DLF, the digoxin-like immunoreactive factor (DLIF), has very recently been described in humans. Digoxin as a drug is the most widely prescribed digitalis in the U.S., and its measurement in serum has established a model for present-day therapeutic drug monitoring (TDM). Historically, the accurate measurement of digoxin in blood has been difficult. This article focuses on the present understanding of the clinical use of digoxin, factors that affect the accuracy of measuring digoxin, the principle of measuring metabolically active species of digoxin, and the effects of DLIF and other interfering substances in digoxin immunoassay.

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.

High-performance liquid chromatography with on-line post-column immunoreaction detection of digoxin and its metabolites based on fluorescence energy transfer in the far-red spectral region

The combination of immunoassays with separation techniques such as chromatography can result in enhanced selectivity and sensitivity. This paper describes an on-line chromatography with immunochemical post-column fluorescence energy transfer detection for digoxin and its metabolites. R-phycoerythrin (PE) was used as the donor and an indodicarbocyanine dye (Cy5) as the acceptor label. These labels allow the detection in the far-red spectral region, which is more selective for biological samples. Hence, digoxin was labeled with PE using the activated digoxigenin-NHS-ester and monoclonal anti-digoxin antibody was labeled with Cy5. Digoxin and its metabolites was injected into the HPLC system followed by post-column injection of R-phycoerythrin labeled digoxin and by Cy5 labeled anti-digoxin antibody. Incubation time was provided using an open tubular reactor coil at room temperature. The detection was performed by measurement of the sensitized emission of Cy5 at 670 nm due to fluorescence energy transfer from PE labeled with digoxin. The system was optimized with regard to the concentrations of the used post-column reagents as well as incubation time and temperature. The dynamic range of digoxin spiked in 0.01 M phosphate buffer (pH 7.4) was 0.05 to 10 ng/ml with a correlation coefficient of 0.989. The limit of detection was 33 pg/ml. The precision of two controls, 0.4 and 4 ng/ml, was found to be 2.2 and 8.7% RSD, respectively, accuracy was 10.7 and 20.3% (n=6 in each case).

Two biologically active isomers of dihydroouabain isolated from a commercial preparation

Ouabain is a plant-derived cardiac glycoside that inhibits the catalytic activity of Na(+),K(+)-ATPase (sodium pump; NKA). Dihydroouabain, a derivative of ouabain with a reduced lactone ring, is commonly used as a sodium pump antagonist. It has been assumed that commercially available dihydroouabain is homogeneous. We now report that preparations of dihydroouabain contain two components each with a different potency for inhibition of sodium pump activity. We used reverse-phase HPLC chromatography, UV spectrophotometry, electrospray ionization-mass spectrometry (ESI-MS), nuclear magnetic resonance (NMR) spectroscopy and two independent bioassays to characterize these compounds. The two dihydroouabain fractions (Dho-A and Dho-B) resolved by 3 min chromatographically, had UV absorbance maxima at 196 nm, and comprised 37% and 63% of the stock dihydroouabain, respectively. The molar potency of each component for inhibition of NKA from porcine cerebral cortex differed by 4. 4-fold (Dho-A, IC(50) = 7.13 +/- 0.8 microM; Dho-B, IC(50) = 1.63 +/- 0.12 microM). The relative potencies were 9% and 40% of those of ouabain, respectively. A similar pattern for phosphorylation of NKA was observed. Mass spectrometry (ESI-MS) and fragmentation patterns are consistent with Dho-A and Dho-B being isomers of identical molecular mass (587 Da) and each with six hydroxyl groups, a deoxyhexose sugar moiety and a lactone ring. Furthermore, NMR spectroscopy revealed structural differences between Dho-A and Dho-B by displaying noticeably different chemical shifts at only two groups of proton resonances assigned to H-21 and H-22. The ESI-MS and NMR results confirm the presence of the isomerism at C20 of the lactone ring. Our results demonstrate the existence of two molecular forms of dihydroouabain, each with a different biological potency. These findings underscore the importance of characterizing the purity of dihydroouabain commercial preparations. It also provides possible molecular models for investigating the metabolism of endogenous ouabain-like factors recently reported in mammals.

Interference from digitoxin-like immunoreactive factors reduced in a new monoclonal chemiluminescent digitoxin assay

Endogenous digoxin-like immunoreactive factors (DLIF) can interfere with some digoxin immunoassays. We looked for similar interference, called digitoxin-like immunoreactive factors (DTLIF) in two digitoxin immunoassays: A new chemiluminescent assay (CLIA), processed on the automated random access immunoassay system ACS:180, and a fluorescent polarization assay (FPIA), processed on the semiautomated TDx batch analyzer. One hundred thirty-seven samples of sera were tested from nondigitalized pregnant women, patients with liver or kidney diseases, and cord blood. The CLIA digitoxin assay uses a murine monoclonal antibody and requires no sample pretreatment; the FPIA digitoxin assay uses a polyclonal rabbit antibody and requires sample precipitation. Both assays have a similar dynamic range and sensitivity and give comparable results with commercial controls and external quality control survey samples. Although the CLIA detected no digitoxin in any sample tested, the FPIA showed apparent digitoxin concentrations of more than 2.0 ng/ml for 100% and 44% among cord blood and liver disease specimens, respectively. The highest DTLIF concentration was found in serum from a patient with liver disease (18.1 ng/ml). When spiked with 32 ng/ml digitoxin, six of the samples containing DTLIF generated FPIA digitoxin values of 6% to 27.5% more than the expected digitoxin levels. Two specimens with no detectable DTLIF activity were run as controls, and when spiked with digitoxin, showed target digitoxin concentrations in the FPIA. The CLIA recovered near the target digitoxin values (32 ng/ml) in all spiked samples. It was concluded that the polyclonal FPIA digitoxin assay may give discordant digitoxin concentrations in some patient groups because of interference from digitoxin-like immunoreactive factors. The CLIA digitoxin assay is not affected by DTLIF interference.

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.

Bidirectional (positive/negative) interference in a digoxin immunoassay: importance of antibody specificity

The importance of high specificity in immunoassays used in therapeutic monitoring is highlighted by a case study in which therapeutic-to-toxic borderline digoxin levels were measured by a digoxin immunoassay in the serum sample from a patient administered digitoxin rather than digoxin. The sample, mistakenly sent to the laboratory for digoxin analysis, gave discordant results in three digoxin immunoassays: 1.99 and 0.79 ng/ml in assays using polyclonal antibodies (fluorescence-polarization immunoassay and microparticle enzyme immunoassay, respectively), and <0.1 ng/ml in a chemiluminescent immunoassay using more specific monoclonal antibody. The presence of digitoxin (approximately 40 ng/ml) in the sample was confirmed by three different digitoxin immunoassays. Based on these results, the interference of different levels of digitoxin was studied in the presence of 0, 0.85, 1.9, and 4.7 ng/ml digoxin in all three digoxin assays. The chemiluminescent assay showed no significant interference. The fluorescence-polarization immunoassay showed positive interference in all cases; however, the microparticle enzyme immunoassay showed a bidirectional interference: a positive interference observed at digoxin level <1.8 ng/ml, changing to a negative interference at higher digoxin concentrations. The authors conclude that in countries such as Germany, where both digoxin and digitoxin may be prescribed, caution should be used to interpret digoxin immunoassay results. Digoxin assays, with cross-reactivity to digitoxin <0.1% should be used.

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.

Digoxinlike immunoreactive factor isolated from human pleural fluid is structurally similar to digoxin

To further define the chemical structure of human endogenous digoxinlike immunoreactive factors (DLIF) we used human pleural effusions as a source of the substance. Digoxinlike immunoreactive factor activity was detected by radioimmunoassay in the pleural fluid of each of four patients; average concentration was 0.35 ng/mL. The chemical profile of DLIF was determined by initial extraction and concentration of DLIF by ion exchange chromatography followed by reverse phase-high-pressure liquid chromatography (RP-HPLC) separation and purification. Using high-pressure liquid chromatography cochromatography of DLIF, together with several radioactively marked glycosides, we observed a single peak of DLIF activity that was chromatographically identical to digoxin. The present study further supports the recent finding that DLIF is related structurally to the cardiac glycosides, and for the first time it has been proven that DLIF is present in pleural fluids.