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

Alterations of Na+/K+-ATPase function in caveolin-1 knockout cardiac fibroblasts

Recent studies have demonstrated that the Na(+)/K(+)-ATPase is not only an ion pump, but also a membrane receptor that confers the ligand-like effects of cardiotonic steroids (CTS) such as ouabain on protein kinases and cell growth. Because CTS have been implicated in cardiac fibrosis, this study examined the role of caveolae in the regulation of Na(+)/K(+)-ATPase function and CTS signaling in cardiac fibroblasts. In cardiac fibroblasts prepared from wild-type and caveolin-1 knockout [Cav-1(-/-)] mice, we found that the absence of caveolin-1 did not affect total cellular amount or surface expression of Na(+)/K(+)-ATPase alpha1 subunit. However, it did increase ouabain-sensitive (86)Rb(+) uptake. While knockout of caveolin-1 increased basal activities of Src and ERK1/2, it abolished the activation of these kinases induced by ouabain but not angiotensin II. Finally, ouabain stimulated collagen synthesis and cell proliferation in wild type but not Cav-1(-/-) cardiac fibroblasts. Thus, we conclude that caveolae are important for regulating both pumping and signal transducing functions of Na(+)/K(+)-ATPase. While depletion of caveolae increases the pumping function of Na(+)/K(+)-ATPase, it suppresses CTS-induced signal transduction, growth, and collagen production in cardiac fibroblasts.

Endogenous ouabain-like compounds in locus coeruleus modulate rapid eye movement sleep in rats

Although the detailed mechanism of spontaneous generation and regulation of rapid eye movement sleep (REMS) is yet unknown, it has been reported that noradrenergic REM-OFF neurons in the locus coeruleus (LC) cease firing during REMS and, if they are kept active, REMS is significantly reduced. On the other hand, the activity as well as expression of Na-K ATPase has been shown to increase in the LC following REMS deprivation. Ouabain is a specific inhibitor of Na-K ATPase, and endogenous ouabain-like compounds are present in the brain. These findings led us to propose that a decrease in the level of ouabain-like compounds spontaneously available in and around the LC would stimulate and increase the REM-OFF neuronal activities in this region and thus would reduce REMS. To test this hypothesis, we generated anti-ouabain antibodies and then microinjected it bilaterally into the LC in freely moving chronically prepared rats and recorded electrophysiological signals for evaluation of sleep-wakefulness states; suitable control experiments were also conducted. Injection of anti-ouabain antibodies into the LC, but not into adjacent brain areas, significantly reduced percent REMS (mean +/- SEM) from 7.12 (+/-0.74) to 3.63 (+/-0.65). The decrease in REMS was due to reduction in the mean frequency of REMS episode, which is likely due to increased excitation of the LC REM-OFF neurons. Control microinjections of normal IgG did not elicit this effect. These results support our hypothesis that interactions of naturally available endogenous ouabain-like compounds with the Na-K ATPase in the LC modulate spontaneous REMS.

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.

Oleandrin-mediated inhibition of human tumor cell proliferation: importance of Na,K-ATPase alpha subunits as drug targets

Cardiac glycosides such as oleandrin are known to inhibit the Na,K-ATPase pump, resulting in a consequent increase in calcium influx in heart muscle. Here, we investigated the effect of oleandrin on the growth of human and mouse cancer cells in relation to Na,K-ATPase subunits. Oleandrin treatment resulted in selective inhibition of human cancer cell growth but not rodent cell proliferation, which corresponded to the relative level of Na,K-ATPase alpha3 subunit protein expression. Human pancreatic cancer cell lines were found to differentially express varying levels of alpha3 protein, but rodent cancer cells lacked discernable expression of this Na,K-ATPase isoform. A correlation was observed between the ratio of alpha3 to alpha1 isoforms and the level of oleandrin uptake during inhibition of cell growth and initiation of cell death; the higher the alpha3 expression relative to alpha1 expression, the more sensitive the cell was to treatment with oleandrin. Inhibition of proliferation of Panc-1 cells by oleandrin was significantly reduced when the relative expression of alpha3 was decreased by knocking down the expression of alpha3 isoform with alpha3 siRNA or increasing expression of the alpha1 isoform through transient transfection of alpha1 cDNA to the cells. Our data suggest that the relative lack of alpha3 (relative to alpha1) in rodent and some human tumor cells may explain their unresponsiveness to cardiac glycosides. In conclusion, the relatively higher expression of alpha3 with the limited expression of alpha1 may help predict which human tumors are likely to be responsive to treatment with potent lipid-soluble cardiac glycosides such as oleandrin.

Association between sodium- and potassium-activated adenosine triphosphatase alpha isoforms and bipolar disorders

BACKGROUND

The sodium- and potassium-activated adenosine triphosphatase (Na+, K+-ATPase) is a major plasma membrane transporter for sodium and potassium. We recently suggested that bipolar disorders (BD) may be associated with alterations in brain Na+, K+-ATPase. We further conjectured that the differences in Na+, K+-ATPase in BD patients could result partially from genetic variations in Na+, K+-ATPase alpha isoforms.

METHODS

To test our hypothesis, we undertook a comprehensive study of 13 tagged single nucleotide polymorphisms (SNPs) across the three genes of the brain alpha isoforms of Na+, K+- ATPase (ATP1A1, ATP1A2, and ATP1A3, which encode the three alpha isoforms, alpha1, alpha2, and alpha3, respectively) identified using HapMap data and the Haploview algorithm. Altogether, 126 subjects diagnosed with BD from 118 families were genotyped (parents and affected siblings). Both individual SNPs and haplotypes were tested for association using family-based association tests as provided in the UNPHASED and PBAT set of programs.

RESULTS

Significant nominal association with BD was observed for six single SNPs (alpha1: rs11805078; alpha2: rs2070704, rs1016732, rs2854248, and rs2295623; alpha3: rs919390) in the three genes of Na+, K+-ATPase alpha isoforms. Haplotype analysis of the alpha2 isoform (ATP1A2 gene) showed a significant association with two loci haplotypes with BD (rs2295623: rs2070704; global p value = .0198, following a permutation test).

CONCLUSIONS

This study demonstrates for the first time that genetic variations in Na+, K+-ATPase are associated with BD, suggesting a role of this enzyme in the etiology of this disease.

Signal pathways in ouabain-induced proliferation of leukemia cells

BACKGROUND

Cardiotonic steroids (CTSs) can bind to Na(+)/K(+)-ATPase and activate protein kinase cascades, resulting in changes in cell proliferation, differentiation or apoptosis in a cell-specific manner. We explored the participation of ouabain-activated signaling pathways in growth regulation of leukemia cells.

METHODS

Lymphocytic leukemia Jhhan cells and megakaryocytic leukemia M07e cells were incubated at different concentrations of ouabain (0, 1 and 10 nmol) for 24 hours. Cell proliferation was measured by methyl thiazolyl tetrazolium (MTT) assay. To probe the role of ouabain-induced signaling in control of cell growth, we employed Src kinase inhibitor PP2 and the MEK inhibitor PD98059, respectively. The expression of Na(+)/K(+)-ATPase alpha1 subunit of leukemia cells was evaluated by RT-PCR and Western blotting.

RESULTS

One nmol and 10 nmol ouabain promoted proliferation of both Jhhan and M07e cells. Ouabain also up-regulated the expression of Na(+)/K(+)-ATPase alpha1 subunit. Addition of either PP2 or PD98059 blocked the effects of ouabain on cell proliferation.

CONCLUSION

Ouabain activates Src and ERK1/2 pathways and regulates the proliferation of leukemia cells.

The hemodynamic effects of intravenous digoxin-binding fab immunoglobulin in severe preeclampsia: a double-blind, randomized, clinical trial

OBJECTIVE

An endogenous digitalis-like factor (EDLF) has been implicated in the pathophysiology of preeclampsia (PE). This hypothesis is supported by two cases of preeclampsia in which administration of digoxin immune Fab (DIF) reduced mean arterial pressure (MAP).

STUDY DESIGN

To study this observation further, we performed a double-blind, placebo-controlled, randomized clinical trial to examine the effects on MAP of intravenous DIF given after delivery in 26 subjects with severe preeclampsia. Treating obstetricians were blinded to subject assignment and were allowed to use standard antihypertensive drugs during the trial.

RESULTS

The primary outcome, a significant difference in blood pressure between the two groups over the 24-h period of observation after the intervention, was not supported. However, mean MAP was significantly lower in the DIF-treated subjects for the first 4 h after therapy as compared with controls (P=0.05). Six subjects (46.2%) in the placebo arm were given conventional antihypertensive medications by their obstetrician for blood pressure >160 mm Hg systolic or >110 mm Hg diastolic, compared with zero subjects in the treatment arm (P=0.01). A trend towards increased creatinine clearance was observed in DIF-treated subjects (137.6+/-42.6 versus 104.1+/-43.4, P=0.07).

CONCLUSION

These results support the hypothesis that EDLF contributes to the elevated blood pressure in preeclampsia and suggests a possible role for DIF as a treatment for this condition.

Erythrocyte sodium/potassium ATPase activity in severe preeclampsia

OBJECTIVE

Elevated blood levels of endogenous digitalis-like factors (EDLF) may decrease erythrocyte sodium pump activity in preeclampsia. As the highest EDLF levels might be expected in severe preeclampsia, we investigated sodium pump activity in that group of patients.

STUDY DESIGN

Erythrocyte sodium pump activity was determined by (86)Rubidium uptake (in nM per hour per 10(6) cells) in women with severe preeclampsia and those with normal pregnancies, matched for gestational age, and in healthy nonpregnant women (n=12 in each group). Differences between groups were analyzed by a two-sided Student t-test.

RESULT

Sodium pump activity was significantly increased in normotensive pregnancies as compared with normotensive non-pregnant women (81.4+/-8.4 vs 61.1+/-7.4, mean+/-s.d., p<0.05), and was decreased 43% in severe preeclamptic pregnancies as compared with normotensive pregnancies (46.4+/-14.1 vs 81.4+/-8.4, p<0.05).

CONCLUSION

Severe preeclampsia is associated with significantly lower erythrocyte sodium pump activity than normotensive pregnancy. These data suggest that plasma levels of a biologically active EDLF are elevated in patients with severe preeclampsia.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

Na+/K+-ATPase alpha subunits as new targets in anticancer therapy

BACKGROUND

The sodium pump (Na(+)/K(+)-ATPase) could be a target for the development of anticancer drugs as it serves as a signal transducer, it is a player in cell adhesion and its aberrant expression and activity are implicated in the development and progression of different cancers. Cardiotonic steroids (CS) are the natural ligands and inhibitors of the sodium pump and this supports the possibility of their development as anticancer agents targeting overexpressed Na(+)/K(+)-ATPase alpha subunits.

OBJECTIVES

To highlight and further develop the concept of using Na(+)/K(+)-ATPase alpha1 and alpha3 subunits as targets in anticancer therapy and to address the question of the actual usefulness of further developing CS as anticancer agents.

CONCLUSIONS

Targeting overexpressed Na(+)/K(+)-ATPase alpha subunits using novel CS might open a new era in anticancer therapy and bring the concept of personalized medicine from aspiration to reality. Clinical data are now needed to further support this proposal. Furthermore, future medicinal chemistry should optimize new anticancer CS to target Na(+)/K(+)-ATPase alpha subunits with the aim of rendering them more potent and less toxic.

Distribution of the Na,K-ATPase alpha subunit in the rat spiral ganglion and organ of corti

Processing of sound in the cochlea involves both afferent and efferent innervation. The Na,K-ATPase (NKA) is essential for cells that maintain hyperpolarized membrane potentials and sodium and potassium concentration gradients. Heterogeneity of NKA subunit expression is one mechanism that tailors physiology to particular cellular demands. Therefore, to provide insight into molecular differences that distinguish the various innervation pathways in the cochlea, we performed a variety of double labeling experiments with antibodies against three of the alpha isoforms of the NKA (NKA alpha 1-3) and markers identifying particular subsets of neurons or supporting cells in whole mount preparations of the organ of Corti and spiral ganglion. We found that the NKA alpha 3 is abundantly expressed within the membranes of the spiral ganglion somata, the type I afferent terminals contacting the inner hair cells, and the medial efferent terminals contacting the outer hair cells. We also found expression of the NKA alpha 1 in the supporting cells that neighbor the inner hair cells and express the glutamate transporter GLAST. These findings suggest that both the NKA alpha 1 and NKA alpha 3 are poised to play an essential role in the regulation of the type I afferent synapses, the medial efferent synapses, and also glutamate transport from the afferent-inner hair cell synapse.

Digoxin treatment is associated with an increased incidence of breast cancer: a population-based case-control study

INTRODUCTION

Laboratory and epidemiologic studies have suggested a modifying effect of cardiac glycosides (for example, digoxin and digitoxin) on cancer risk. We explored the association between digoxin treatment and invasive breast cancer incidence among postmenopausal Danish women.

METHODS

We used Danish registries to identify 5,565 postmenopausal women diagnosed with incident invasive breast carcinoma between 1 January 1991 and 31 December 2007, and 55,650 matched population controls. Cardiac glycoside prescriptions were ascertained from county prescription registries. All subjects had at least 2 years of recorded prescription drug and medical history data. We estimated the odds ratio associating digoxin use with breast cancer in conditional logistic regression models adjusted for age, county of residence, and use of anticoagulants, non-steroidal anti-inflammatory drugs (NSAIDs), aspirin, and hormone replacement therapy. We also explored the impact of confounding by indication and detection bias.

RESULTS

Digoxin was the sole cardiac glycoside prescribed to subjects during the study period. There were 324 breast cancer cases (5.8%) and 2,546 controls (4.6%) with a history of digoxin use at least 1 year before their index date (adjusted odds ratio (OR): 1.30; 95% confidence interval: 1.14 to 1.48). The breast cancer OR increased modestly with increasing duration of digoxin exposure (adjusted OR for 7 to 18 years of digoxin use: 1.39; 95% confidence interval: 1.10 to 1.74). The association was robust to adjustment for age, receipt of hormone replacement therapy, coprescribed drugs, and confounding by indication. A comparison of screening mammography rates between cases and controls showed no evidence of detection bias.

CONCLUSIONS

Our results suggest that digoxin treatment increases the risk of invasive breast cancer among postmenopausal women.

Novel therapeutic applications of cardiac glycosides

Cardiac glycosides are a diverse family of naturally derived compounds that bind to and inhibit Na+/K+-ATPase. Members of this family have been in clinical use for many years for the treatment of heart failure and atrial arrhythmia, and the mechanism of their positive inotropic effect is well characterized. Exciting recent findings have suggested additional signalling modes of action of Na+/K+-ATPase, implicating cardiac glycosides in the regulation of several important cellular processes and highlighting potential new therapeutic roles for these compounds in various diseases. Perhaps most notably, the increased susceptibility of cancer cells to these compounds supports their potential use as cancer therapies, and the first generation of glycoside-based anticancer drugs are currently in clinical trials.

The prince and the pauper. A tale of anticancer targeted agents

Cancer rates are set to increase at an alarming rate, from 10 million new cases globally in 2000 to 15 million in 2020. Regarding the pharmacological treatment of cancer, we currently are in the interphase of two treatment eras. The so-called pregenomic therapy which names the traditional cancer drugs, mainly cytotoxic drug types, and post-genomic era-type drugs referring to rationally-based designed. Although there are successful examples of this newer drug discovery approach, most target-specific agents only provide small gains in symptom control and/or survival, whereas others have consistently failed in the clinical testing. There is however, a characteristic shared by these agents: -their high cost-. This is expected as drug discovery and development is generally carried out within the commercial rather than the academic realm. Given the extraordinarily high therapeutic drug discovery-associated costs and risks, it is highly unlikely that any single public-sector research group will see a novel chemical "probe" become a "drug". An alternative drug development strategy is the exploitation of established drugs that have already been approved for treatment of non-cancerous diseases and whose cancer target has already been discovered. This strategy is also denominated drug repositioning, drug repurposing, or indication switch. Although traditionally development of these drugs was unlikely to be pursued by Big Pharma due to their limited commercial value, biopharmaceutical companies attempting to increase productivity at present are pursuing drug repositioning. More and more companies are scanning the existing pharmacopoeia for repositioning candidates, and the number of repositioning success stories is increasing. Here we provide noteworthy examples of known drugs whose potential anticancer activities have been highlighted, to encourage further research on these known drugs as a means to foster their translation into clinical trials utilizing the more limited public-sector resources. If these drug types eventually result in being effective, it follows that they could be much more affordable for patients with cancer; therefore, their contribution in terms of reducing cancer mortality at the global level would be greater.

Dose-dependent effect of intracerebroventricular injection of ouabain on the phosphorylation of the MEK1/2-ERK1/2-p90RSK pathway in the rat brain related to locomotor activity

Intracerebroventricular (ICV) injection of ouabain, a specific Na-K ATPase inhibitor, induced behavioral changes in rats, a putative animal model for bipolar disorder. The binding of ouabain to Na-K ATPase is known to affect signaling molecules in vitro such as extracellular signal-regulated kinase1/2 (ERK1/2). Although ERK has been suggested to be related to the behavioral alterations induced by various psychotomimetics, the effect of ouabain on ERK in the brain related to behavioral changes has not been examined. After ICV injection of ouabain in rats, we investigated changes in the phosphorylation of mitogen-activated protein kinase kinase1/2 (MEK1/2), ERK1/2, and p90 ribosomal s6 kinase (p90RSK) in rat striatum, frontal cortex, and hippocampus along with changes in locomotor activity. Ouabain induced the following biphasic dose-dependent changes in locomotor activity: no change with 10(-6) M, a statistically significant decrease with 10(-5) M, no change with 10(-4) M, and a statistically significant increase with 0.5x10(-3) and 10(-3) M. The phosphorylation level of MEK1/2, ERK1/2, and p90RSK in rat striatum showed dose-dependent changes similar to those observed in locomotor activity with relatively high correlation. The phosphorylation of these molecules in rat frontal cortex and hippocampus also changed in a similar dose-dependent pattern. Taken together, ouabain induced biphasic dose-dependent changes in locomotor activity and the phosphorylation of the ERK1/2 pathway. These findings suggest a possible relationship between ouabain-induced behavioral changes and ERK activity in the brain and suggest an important role of ERK in regulating locomotor activity and mood state.

Cardenolides from Antiaris toxicaria as potent selective Nur77 modulators

Toxicarioside D (1), a new cardenolide, along with 10 other known ones, was isolated from the stem of Antiaris toxicaria LESCH. by bioassay-guided fractionation. Their structures were determined on the basis of spectroscopic analysis. All the reported compounds effectively inhibited the growth of various cancer cell lines at nanomolar concentrations. Inhibition of cancer cell growth was accompanied with induction of the expression of Nur77, a potent apoptotic member of the steroid/thyroid hormone receptor superfamily.

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.

Endogenous digitalis: pathophysiologic roles and therapeutic applications

Endogenous digitalis-like factors, also called cardiotonic steroids, have been thought for nearly half a century to have important roles in health and disease. The endogenous cardiotonic steroids ouabain and marinobufagenin have been identified in humans, and an effector mechanism has been delineated by which these hormones signal through the sodium/potassium-transporting ATPase. These findings have increased interest in this field substantially. Although cardiotonic steroids were first considered important in the regulation of renal sodium transport and arterial pressure, subsequent work has implicated these hormones in the control of cell growth, apoptosis and fibrosis, among other processes. This Review focuses on the role of endogenous cardiotonic steroids in the pathophysiology of essential hypertension, congestive heart failure, end-stage renal disease and pre-eclampsia. We also discuss potential therapeutic strategies that have emerged as a result of the increased understanding of the regulation and actions of cardiotonic steroids.

Chronic exposure to dietary sterol glucosides is neurotoxic to motor neurons and induces an ALS-PDC phenotype

Epidemiological studies of the Guamanian variants of amyotrophic lateral sclerosis (ALS) and parkinsonism, amyotrophic lateral sclerosis-parkinsonism dementia complex (ALS-PDC), have shown a positive correlation between consumption of washed cycad seed flour and disease occurrence. Previous in vivo studies by our group have shown that the same seed flour induces ALS and PDC phenotypes in out bred adult male mice. In vitro studies using isolated cycad compounds have also demonstrated that several of these are neurotoxic, specifically, a number of water insoluble phytosterol glucosides of which beta-sitosterol beta-D: -glucoside (BSSG) forms the largest fraction. BSSG is neurotoxic to motor neurons and other neuronal populations in culture. The present study shows that an in vitro hybrid motor neuron (NSC-34) culture treated with BSSG undergoes a dose-dependent cell loss. Surviving cells show increased expression of HSP70, decreased cytosolic heavy neurofilament expression, and have various morphological abnormalities. CD-1 mice fed mouse chow pellets containing BSSG for 15 weeks showed motor deficits and motor neuron loss in the lumbar and thoracic spinal cord, along with decreased glutamate transporter labelling, and increased glial fibrillary acid protein reactivity. Other pathological outcomes included increased caspase-3 labelling in the striatum and decreased tyrosine-hydroxylase labelling in the striatum and substantia nigra. C57BL/6 mice fed BSSG-treated pellets for 10 weeks exhibited progressive loss of motor neurons in the lumbar spinal cord that continued to worsen even after the BSSG exposure ended. These results provide further support implicating sterol glucosides as one potential causal factor in the motor neuron pathology previously associated with cycad consumption and ALS-PDC.

Diverse biological responses to different cardiotonic steroids

Cardiotonic steroids (CS) such as ouabain, digoxin and bufalin, are steroidal drugs prepared from the seeds and dried leaves of the genus Digitalis, and the skin and parotid gland of amphibians, are used as a cardiac stimulant. Steroids similar or identical to the cardiotonic steroids were identified in human tissues. The available literature unequivocally supports the notion that these endogenous CS function as hormones in mammals. Recent studies show that although similar in structure, the different CS exhibit diverse biological responses. This was shown at the molecular, cellular, tissue and whole animal levels. This review summarizes these diversities, raises a possible explanation for their presence and discusses their implication on the physiological role of the different steroids.

Synergistic interaction between ouabain and 8-methoxy-3,9-dihydroxy coumestan, a non-steroidal synthetic inhibitor of Na+,K+-ATPase

The use of combination drugs is very common in therapeutics as in the treatment of infectious diseases, cancer and heart failure but controversies about analysis of these interactions are frequent. The aim of the present work was to characterize the interaction between ouabain and 8-methoxy-3,9-dihydroxy coumestan (LQB93), a non-steroidal synthetic inhibitor of Na+,K+-ATPase, as well as the interaction between ouabain and ouabagenin, two cardiac glycosides sharing the same binding site. Inhibition of rat kidney Na+,K+-ATPase with increasing concentrations of the drugs alone or of mixtures of ouabain:ouabagenin and LQB93:ouabain in a fixed 1:4 ratio was performed. In other experiments, increasing concentrations of LQB93 (or ouabain) in the presence of a fixed concentration of ouabain (or ouabagenin) were used for determining the concentration pairs eliciting 50% inhibition in order to construct isobolograms. The mixture (experimental) curve for the ouabain:ouabagenin combination was superimposed on the additive (theoretical) curve indicating additivity, in accordance with the isobolographic analysis. On the other hand, the empirical curve for LQB93:ouabain (IC50=10.6 microM) was significantly shifted to the left in relation to the theoretical curve (IC50=30.7 microM) indicating synergism, further confirmed by the isobolographic analysis. As a conclusion, we show that the combination of a newly synthesized non-steroidal inhibitor and ouabain have a synergistic effect on Na+,K+-ATPase, further supporting a mechanism of inhibition different from ouabain. Present data also support the use of both the isobolograms and combination curves for the assessment of drug interactions occurring at the same molecular target, a situation poorly investigated.