Variation in apparent serum digitalis-like factor levels with different digoxin antibodies. The "immunochemical fingerprint"

Depth of the Steroid Core Location Determines the Mode of Na,K-ATPase Inhibition by Cardiotonic Steroids

Cardiotonic steroids (CTSs) are specific inhibitors of Na,K-ATPase (NKA). They induce diverse physiological effects and were investigated as potential drugs in heart diseases, hypertension, neuroinflammation, antiviral and cancer therapy. Here, we compared the inhibition mode and binding of CTSs, such as ouabain, digoxin and marinobufagenin to NKA from pig and rat kidneys, containing CTSs-sensitive (α1S) and -resistant (α1R) α1-subunit, respectively. Marinobufagenin in contrast to ouabain and digoxin interacted with α1S-NKA reversibly, and its binding constant was reduced due to the decrease in the deepening in the CTSs-binding site and a lower number of contacts between the site and the inhibitor. The formation of a hydrogen bond between Arg111 and Asp122 in α1R-NKA induced the reduction in CTSs’ steroid core deepening that led to the reversible inhibition of α1R-NKA by ouabain and digoxin and the absence of marinobufagenin’s effect on α1R-NKA activity. Our results elucidate that the difference in signaling, and cytotoxic effects of CTSs may be due to the distinction in the deepening of CTSs into the binding side that, in turn, is a result of a bent-in inhibitor steroid core (marinobufagenin in α1S-NKA) or the change of the width of CTSs-binding cavity (all CTSs in α1R-NKA).

Bufalin Inhibits Proliferation and Induces Apoptosis in Osteosarcoma Cells by Downregulating MicroRNA-221

Bufalin, a major component of the Chinese medicine ChanSu, which is prepared from the skin and parotid venom glands of toads, has shown cytotoxicity in several malignant tumors. Here, we reported that bufalin inhibited proliferation and induced mitochondria-dependent apoptosis in U-2OS and Saos-2 osteosarcoma cells with intracellular reactive oxygen species (ROS) production. By microRNA (miR) array analysis and quantitative reverse transcription polymerase chain reaction, we found that miR-221 was downregulated after treatment with bufalin. In accordance with TargetScan prediction and luciferase reporter assay, Bcl2 binding component 3 (BBC3) was the direct target of miR-221. Furthermore, upregulating miR-221 by its MIMIC and suppressing BBC3 by small interfering RNA (siRNA) reversed the effects of bufalin on osteosarcoma cells. Collectively, our data indicate that bufalin inhibits cell proliferation and induces mitochondria-dependent apoptosis in osteosarcoma cells through downregulating miR-221 and triggering BBC3 expression.

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.

Characterization of the neutralizing activity of digoxin-specific Fab toward ouabain-like steroids

Digoxin-specific Fab (Digibind) is a mixture of antidigoxin Fab fragments prepared from sheep sera and is used as a treatment for digoxin poisoning. Digoxin-specific Fab has been shown to neutralize an endogenous Na+/K+ ATPase inhibitor (endogenous digoxin-like Na+/K+ ATPase regulatory factor; EDLF) in rats and humans and to lower blood pressure. Although the exact structure of EDLF is unknown, compounds identical to or structurally related to ouabain, bufalin, and marinobufagenin have been detected in mammalian plasma. In this study, some structural characteristics of EDLF were inferred from the ability of digoxin-specific Fab to neutralize the Na+/K+ ATPase inhibitory activity of several known cardenolides and bufodienolides. Additional structural information was obtained from [3H]ouabain binding and enzyme-linked immunosorbent assay experiments. Digoxin-specific Fab had the ability to interact to some extent with all of the cardenolides and bufodienolides tested. However, digoxin-specific Fab was more than 20-fold more potent in neutralizing ouabain and bufalin than marinobufagenin. The antihypertensive effect of digoxin-specific Fab seen in preeclampsia and animal models of hypertension may therefore be due to a molecule identical to or structurally similar to ouabain or bufalin.

Circulating bufodienolide and cardenolide sodium pump inhibitors in preeclampsia

OBJECTIVE

To determine plasma levels of the endogenous bufodienolide Na+/K+ ATPase inhibitor, marinobufagenin-like factor (MBG), in normotensive pregnancy and in preeclampsia, to compare changes of MBG with that of ouabain-like compound (OLC), and to characterize the purified MBG immunoreactive factor from preeclamptic plasma.

DESIGN AND METHODS

Consecutive sample study. The levels of MBG and OLC compounds were measured in extracted plasma by solid phase fluoroimmunoassays. MBG and ouabain immunoreactive materials were partially purified from preeclamptic plasma via reverse-phase high-performance liquid chromatography (HPLC) and studied for their ability to cross react with MBG and ouabain antibodies, and to inhibit the Na+/K+ ATPase from human mesenteric arteries. Vasoconstrictor effect of authentic MBG was studied in isolated rings of human umbilical arteries.

RESULTS

In 11 nonpregnant control individuals, plasma concentrations of MBG and OLC were 0.190+/-0.04 nmol/l and 0.297+/-0.037 nmol/l, respectively. In the third trimester of noncomplicated pregnancy (n = 6), plasma MBG increased (0.625+/-0.067 nmol/l, P<0.05), and OLC did not (0.32+/-0.07 nmol/l). In 15 patients with preeclampsia, plasma levels of both MBG and OLC increased dramatically (2.63+/-0.10 nmol/l and 0.697+/-0.16 nmol/l, respectively, P<0.01 versus both control groups). When fractionated by reverse phase HPLC, OLC was eluted by 18% acetonitrile, and MBG by 48% acetonitrile. Serially diluted samples of MBG and OLC immunoreactive materials from HPLC fractions reacted with MBG and ouabain antibody in solid phase immunoassay in a concentration dependent fashion. Authentic MBG caused contractile responses of isolated rings of human mesenteric arteries in a concentration-dependent manner. Similarly to the authentic MBG, HPLC purified MBG immunoreactive material from preeclamptic plasma inhibited Na+/K+ ATPase purified from human mesenteric artery.

CONCLUSIONS

Our observations demonstrate the coexistence of two endogenous cardiotonic steroids in preeclamptic plasma, a more polar OLC and a less polar MBG-like compound. Substantial increases in plasma OLC and MBG immunoreactivity in preeclampsia, along with the vasoconstrictor properties of authentic MBG and Na+,K+ ATPase inhibitory activity of human MBG immunoreactive factor, suggest, that in preeclampsia, plasma concentrations of MBG are enough to substantially inhibit the sodium pump in cardiovascular tissues, and are in accordance with the views attributing endogenous digitalis-like factors a pathogenic role in the preeclamptic hypertension.

A new interference in some digoxin assays: anti-murine heterophilic antibodies

BACKGROUND

We describe a patient with cirrhotic liver disease and atrial fibrillation who was treated with spironolactone and digoxin. He was hospitalized because of an incidental finding of a high serum digoxin level (4.2 micrograms/L), but he remained asymptomatic without emerging arrhythmias. Despite discontinuation of both drugs, his serum digoxin level persisted at or above 3.0 micrograms/L for approximately 5 weeks, drawing into question the accuracy of the digoxin assay.

METHODS

Additional digoxin methods gave lower, discrepant results, providing evidence of an assay interference, and several possible sources of digoxin false positivity were evaluated. This included assessment of the contribution of digoxin-like immunoreactive factor (DLIF), digoxin metabolites, and spironolactone. Because the routine digoxin assay used a monoclonal antibody, we also tested for another hypothetical interference: human heterophilic ("anti-mouse") antibodies.

RESULTS

We found no contribution from DLIF, digoxin antibodies, or spironolactone to the apparent digoxin results. However, the use of protein A to complex and selectively remove immunoglobulin G molecules markedly lowered the apparent digoxin value, as did the less specific process of ultrafiltration.

CONCLUSIONS

These results suggest a previously unreported cause of digoxin false positivity: heterophilic antibodies, which have been reported to bind murine monoclonal antibodies in other assays. Because newer digoxin assays now use murine monoclonal antibodies, the possible presence of heterophilic, anti-mouse antibodies should now be considered in the interpretation of a high digoxin level.

Analytical performance of a monoclonal digoxin assay with increased specificity on the ACS:180

Digoxin metabolites cross-react in the Ciba Corning ACS digoxin assay in proportion to their bioactivity, but have greater (near 100%) cross-reactivity in the Abbott TDx, Baxter Stratus, and Ciba Corning Magic RIA digoxin assays. We studied the analytical performance of the ACS digoxin assay and compared it with these other assays. Coefficients of variation ranged from 5.5% at 3.11 ng/ml to 8.8% at 0.57 ng/ml. Mean analytical recovery was 96.4%. Results on dilutions were linear in the range of 0.6-5.0 ng/ml. We observed no interference by hemoglobin, bilirubin, or triglycerides. Dihydrodigoxin and digitoxin had lower cross-reactivity in the ACS and Stratus assays than in the TDx and Magic assays. Digoxin-like immunoreactive factor (DLIF) in patients' sera was not detected in the ACS assay but was in the TDx, Stratus, and Magic assays. Digibind therapy seemingly did not affect digoxin results by ACS or Stratus, but did for up to 10 days after therapy for TDx and Magic. We compared digoxin results for 121 sera from 49 patients. Deming regression analysis was performed on the first specimen from each patient: ACS = 1.08(TDx)-0.17 ng/ml (r = 0.961, Sy,x = 0.164); ACS = 1.16(Stratus)-0.46 ng/ml (r = 0.973, Sy,x = 0.123); ACS = 1.00(Magic)-0.20 ng/ml (r = 0.982, Sy,x = 0.110). Discrepant results (> 2Sy,x from the regression line) were usually lower by the ACS assay (87%). Nine of 11 patients with discrepant results had renal insufficiency or hepatic disease, conditions commonly associated with increased DLIF. These observations may be explained by the improved specificity of the ACS digoxin assay.

Specificity of the volume-sensitive sodium pump inhibitor isolated from human peritoneal dialysate in chronic renal failure

Compromised renal function predisposes to volume-dependent hypertension. Increased plasma levels of a sodium pump inhibitor as a possible pathogenetic factor have been demonstrated by many investigators in such patients, but efforts to identify the responsible agent have led to many, diverse candidates. Our premise in this study is that candidacy must depend on the satisfaction of rigorous criteria, including a specific action of the agent on the sodium pump. These criteria included reversibility, concentration dependence, receptor mediation, and an action at the appropriate step in the enzyme cycle. These criteria were applied to a potent [Na,K]ATPase inhibitor we have identified in the peritoneal dialysate of patients with chronic renal failure, present only during extracellular fluid volume expansion, the levels of which are correlated with the blood pressure rise that results from excessive NaCl and water intake. In microsomes that contained both [Na,K]ATPase and other ATPases, this candidate inhibited only the Na and K dependent, ouabain-sensitive ATPase. It displaced ouabain from the cardioglycoside binding site and its binding was linked to inhibition. Inhibition was produced by slowing the pump's dephosphorylation step, the exact action of all cardioglycosides. Finally, the candidate cross reacted with a digoxin Fab fragment and this Fab reversed its inhibition of [Na,K]ATPase. Together, these experiments demonstrate that the PD candidate specifically, and reversibly, inhibits the sodium pump via the cardioglycoside binding site, and hence, meets this crucial criterion for candidacy.

Endogenous sodium pump inhibition: current status and therapeutic opportunities

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

Endogenous marinobufagenin-like immunoreactive factor and Na+, K+ ATPase inhibition during voluntary hypoventilation

In previous studies investigators found that conditioned hypoventilatory breathing potentiated a sodium-sensitive form of hypertension in dogs that was not mediated by sympathetic nervous system arousal. Our study investigated effects of 30 minutes of voluntary hypoventilation, maintained by a respiratory gas monitor and feedback procedure, in 16 normotensive humans of both sexes on (1) plasma concentrations of endogenous digitalis-like factors (ouabain-like and marinobufagenin-like immunoreactivity), (2) activity of erythrocyte Na+, K+ -ATPase, (3) inhibitory activity of plasma Na+, K+ -ATPase, and (4) blood pressure. Increased end tidal PCO2 (41 +/- 0.78 mm Hg versus 37.6 +/- 1.03 mm Hg) was associated with (1) an increase in plasma marinobufagenin-like immunoreactivity (1.23 +/- 0.47 versus 4.96 +/- 1.19 nmol/L), (2) an inhibition of Na+, K+ -ATPase in red blood cells (3.68 +/- 0.22 versus 2.15 +/- 0.25 mmol Pi/mL-1/h-1; P < .01), (3) increase in plasma Na+, K+ -ATPase inhibitory activity (34.9 +/- 4.0% versus 48.8 +/- 2.1%, P < .02), and (4) increases in systolic (112.4 +/- 2.6 versus 107.6 +/- 1.8 mm Hg) and diastolic (73.5 +/- 2.1 versus 68.8 +/- 2.1 mm Hg) blood pressures. Plasma levels of ouabain-like immunoreactivity did not increase significantly. Incubation of erythrocytes obtained during hypoventilation with antidigoxin antibody restored the Na+, K+ -ATPase activity (3.99 +/- 0.34 mmol Pi/mL-1/h-1). Cessation of hypoventilation was associated with decreases in diastolic blood pressure (70.5 +/- 2.2 mm Hg) and restoration of Na+, K+ -ATPase activity in erythrocytes (2.99 +/- 0.43 mmol Pi/mL-1/h1).(ABSTRACT TRUNCATED AT 250 WORDS)

Ouabain is not detectable in human plasma

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

Identification of digitalis-like compounds in human cataractous lenses

Human cataractous lens nuclei extract inhibited, in a dose-dependent fashion, [3H]ouabain binding to rat brain synaptosomes and microsomal Na(+)- and K(+)-dependent adenosine triphosphate (Na+, K(+)-ATPase) activity and interacted with anti-digoxin antibodies. The compounds responsible for these activities, termed digitalis-like compounds (DLC), were also detected in bovine, rat, cat and rabbit, normal, transparent lenses, but the levels were only 0.7-5.4% of the average levels in the cataractous human lenses. DLC from the human cataractous lenses were purified by a procedure consisting of organic extractions and batch chromatography followed by filtration through a 3000 Da cut-off filter and subsequent separations using reverse-phase high-performance liquid chromatography. The presence of DLC in the different fractions obtained in the chromatograms was monitored by their ability to inhibit [3H]ouabain binding and Na+, K(+)-ATPase activity. Based on chemical ionization mass spectrometry together with ultraviolet spectrometry and biological characterization, it is suggested that new bufodienolides, 19-norbufalin and 19-norbufalin peptide derivatives are responsible for the endogenous DLC activity. It is proposed that these compounds may regulate Na+, K(+)-ATPase activity in the lens under some physiological and pathological conditions.

Digitalis-like compounds in animal tissues

The Na+, K+ activated adenosine triphosphatase is present in the membrane of eukaryotic cells and represents a major pathway for Na+ and K+ transport across the plasma membrane. Cardiac glycosides such as ouabain or digoxin suppress this enzyme activity by binding to a specific receptor on the membrane. Studies conducted in this and other laboratories have proven the existence of digitalis-like compounds in animal tissues which may serve as in vivo regulators of the Na+, K(+)-pump activity. This review summarizes the attempts to identify these compounds from animal tissues and examines the potential physiological role of some of the identified compounds.