Effects of two endogenous Na+,K(+)-ATPase inhibitors, marinobufagenin and ouabain, on isolated rat aorta

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.

Salt and Aldosterone - Reciprocal and Combined Effects in Preclinical Models and Humans

Primary aldosteronism is an endocrine disorder caused by excessive production of aldosterone by the adrenal glands, and is recognized as the most important cause of endocrine hypertension. With specific therapy, this type of hypertension is potentially curable. In the general population, high salt intake increases the risk for cardiovascular diseases like stroke. In populations with aldosterone excess, observational and experimental data suggest that aldosterone-induced organ damage requires a combination of high dietary salt intake and high plasma aldosterone, i.e., plasma aldosterone levels inappropriately high for salt status. Therefore, understanding the relationship between plasma aldosterone levels and dietary salt intake and the nature of their combined effects is crucial for developing effective prevention and treatment strategies. In this review, we present an update on findings about primary aldosteronism and salt intake and the underlying mechanisms governing their interaction.

Toad venom bufadienolides and bufotoxins: An updated review

Bufadienolides, naturally found in toad venoms having steroid-like structures, reveal antiproliferative effects at low doses. However, their application as anticancer drugs is strongly prevented by their Na+ /K+ -ATPase binding activities. Although several kinds of research were dedicated to moderating their Na+ /K+ -ATPase binding activity, still deeper fundamental knowledge is required to bring these findings into medical practice. In this work, we reviewed data related to anticancer activity of bufadienolides such as bufalin, arenobufagin, bufotalin, gamabufotalin, cinobufotalin, and cinobufagin and their derivatives. Bufotoxins, derivatives of bufadienolides containing polar molecules mainly belonging to argininyl residues, are reviewed as well. The established structures of bufotoxins have been compiled into a one-page figure to review their structures. We also highlighted advances in the structure-modification of the structure of compounds in this class. Drug delivery approaches to target these compounds to tumor cells were discussed in one section. The issues related to extraction, identification, and quantification are separated into another section.

New Insights on the Role of Marinobufagenin from Bench to Bedside in Cardiovascular and Kidney Diseases

Marinobufagenin (MBG) is a member of the bufadienolide family of compounds, which are natural cardiac glycosides found in a variety of animal species, including man, which have different physiological and biochemical functions but have a common action on the inhibition of the adenosine triphosphatase sodium-potassium pump (Na+/K+-ATPase). MBG acts as an endogenous cardiotonic steroid, and in the last decade, its role as a pathogenic factor in various human diseases has emerged. In this paper, we have collated major evidence regarding the biological characteristics and functions of MBG and its implications in human pathology. This review focused on MBG involvement in chronic kidney disease, including end-stage renal disease, cardiovascular diseases, sex and gender medicine, and its actions on the nervous and immune systems. The role of MBG in pathogenesis and the development of a wide range of pathological conditions indicate that this endogenous peptide could be used in the future as a diagnostic biomarker and/or therapeutic target, opening important avenues of scientific research.

Toxic profile of marinobufagin from poisonous Amazon toads and antitumoral effects on human colorectal carcinomas

ETHNOPHARMACOLOGICAL RELEVANCE

South Americans natives have extensively used the toad "kururu" to reduce/treat skin infections, cutaneous lesions and sores. They release secretions rich in bufadienolides, polyhydroxy steroids with well-documented cardiotonic and antiproliferative actions, but in vivo antitumoral evaluations in mammals are rare, and toxicological safety has been left in second place.

AIMS OF THE STUDY

This investigation used in silico, in vitro and in vivo tools to evaluate acute and subacute toxic effects of marinobufagin and the anticancer action in tumor-bearing mice models.

MATERIALS AND METHODS

Initially, in silico toxic predictions were performed, followed by in vitro assays using human and murine normal and tumor lines. Next, acute and subacute studies on mice investigated the behavior, hematological and intestinal transit profile and antitumoral activity of marinobufagin in sarcoma 180- and HCT-116 colorectal carcinoma-transplanted mice for 7 and 15 days, respectively. Ex vivo and in vivo cytogenetic assays in Sarcoma 180 and bone marrow cells and histopathological examinations were also executed.

RESULTS

In silico studies revealed ecotoxicological effects on crustaceans (Daphnia sp.), fishes (Pimephales promelas and Oryzias latipes), and algae. A 24-h marinobufagin-induced acute toxicity included signals of central activity, mainly (vocal frenzy, absence of body tonus, increased ventilation, ataxia, and equilibrium loss), and convulsions and death at 10 mg/kg. The bufadienolide presented effective in vitro cytotoxic action on human lines of colorectal carcinomas in a similar way to ouabain and tumor reduction in marinobufagin-treated SCID-bearing HCT-116 heterotopic xenografts. Animals under subacute nonlethal doses exhibited a decrease in creatinine clearance with normal levels of blood urea, probably as a result of a marinobufagin-induced renal perfusion fall. Nevertheless, only minor morphological side effects were identified in kidneys, livers, hearts and lungs.

CONCLUSIONS

Marinobufagin has in vitro and in vivo anticancer action on colorectal carcinoma and mild and reversible alterations in key metabolic organs without direct chemotherapy-induced gastrointestinal effects at subacute exposure, but it causes acute ataxia, equilibrium loss, convulsions and death at higher acute exposure.

Vasoconstrictor and hemodynamic effects of a methanolic extract from Rhinella marina toad poison

Rhinella marina toad is abundant in Brazil. Its poison contains cardiac glycosides called bufadienolides, which are extensively investigated for their bioactivity. Our aim was to characterize the vasoactivity of Rhinella marina poison (RmP) on the aorta of male Wistar rats. For this, the RmP was first collected and processed to obtain an alcoholic extract. To determine cardiovascular effects of RmP, we performed in vivo tests by administering RmP intravenously in doses of 0.1-0.8 mg/kg. Vascular reactivity was also performed through concentration-response curves to RmP (10 ng/mL to 200 μg/mL) in aortic segments with and without endothelium. RmP induced a concentration-dependent contraction in rat aorta which was partly endothelium-mediated. Nitric oxide contributes with this response in view that incubation with L-NAME increased the contractile response. Additionally, treatment with indomethacin [cyclooxygenase, (COX) inhibitor], nifedipine (L-type voltage-gated calcium channels blocker), and BQ-123 (ET_A receptors antagonist) decreased maximum response, and ketanserin (5-HT₂ receptors antagonist) decreased pEC₅₀, suggesting active participation of these pathways in the contractile response. On the other hand, apocynin (NADPH oxidase inhibitor) did not alter contractility. Incubation with prazosin (α₁-adrenergic receptor antagonist) abolished the contractile response, suggesting that the RmP-induced contraction is dependent on the adrenergic pathway. In the Na⁺/K⁺ ATPase protocol, a higher Emax was observed in the RmP experimental group, suggesting that RmP potentiated Na⁺/K⁺ATPase hyperpolarizing response. When this extract was injected (i.v.) in vivo, increase in blood pressure and decrease in heart rate were observed. The results were immediate and transitory, and occurred in a dose-dependent manner. Overall, these data suggest that the poison extract of R. marina toad has an important vasoconstrictor action and subsequent vasopressor effects, and its use can be investigated to some cardiovascular disorders.

Chemistry and the Potential Antiviral, Anticancer, and Anti-Inflammatory Activities of Cardiotonic Steroids Derived from Toads

Cardiotonic steroids (CTS) were first documented by ancient Egyptians more than 3000 years ago. Cardiotonic steroids are a group of steroid hormones that circulate in the blood of amphibians and toads and can also be extracted from natural products such as plants, herbs, and marines. It is well known that cardiotonic steroids reveal effects against congestive heart failure and atrial fibrillation; therefore, the term "cardiotonic" has been coined. Cardiotonic steroids are divided into two distinct groups: cardenolides (plant-derived) and bufadienolides (mainly of animal origin). Cardenolides have an unsaturated five-membered lactone ring attached to the steroid nucleus at position 17; bufadienolides have a doubly unsaturated six-membered lactone ring. Cancer is a leading cause of mortality in humans all over the world. In 2040, the global cancer load is expected to be 28.4 million cases, which would be a 47% increase from 2020. Moreover, viruses and inflammations also have a very nebative impact on human health and lead to mortality. In the current review, we focus on the chemistry, antiviral and anti-cancer activities of cardiotonic steroids from the naturally derived (toads) venom to combat these chronic devastating health problems. The databases of different research engines (Google Scholar, PubMed, Science Direct, and Sci-Finder) were screened using different combinations of the following terms: “cardiotonic steroids”, “anti-inflammatory”, “antiviral”, “anticancer”, “toad venom”, “bufadienolides”, and “poison chemical composition”. Various cardiotonic steroids were isolated from diverse toad species and exhibited superior anti-inflammatory, anticancer, and antiviral activities in in vivo and in vitro models such as marinobufagenin, gammabufotalin, resibufogenin, and bufalin. These steroids are especially difficult to identify. However, several compounds and their bioactivities were identified by using different molecular and biotechnological techniques. Biotechnology is a new tool to fully or partially generate upscaled quantities of natural products, which are otherwise only available at trace amounts in organisms.

Nutraceutical, Dietary, and Lifestyle Options for Prevention and Treatment of Ventricular Hypertrophy and Heart Failure

Although well documented drug therapies are available for the management of ventricular hypertrophy (VH) and heart failure (HF), most patients nonetheless experience a downhill course, and further therapeutic measures are needed. Nutraceutical, dietary, and lifestyle measures may have particular merit in this regard, as they are currently available, relatively safe and inexpensive, and can lend themselves to primary prevention as well. A consideration of the pathogenic mechanisms underlying the VH/HF syndrome suggests that measures which control oxidative and endoplasmic reticulum (ER) stress, that support effective nitric oxide and hydrogen sulfide bioactivity, that prevent a reduction in cardiomyocyte pH, and that boost the production of protective hormones, such as fibroblast growth factor 21 (FGF21), while suppressing fibroblast growth factor 23 (FGF23) and marinobufagenin, may have utility for preventing and controlling this syndrome. Agents considered in this essay include phycocyanobilin, N-acetylcysteine, lipoic acid, ferulic acid, zinc, selenium, ubiquinol, astaxanthin, melatonin, tauroursodeoxycholic acid, berberine, citrulline, high-dose folate, cocoa flavanols, hawthorn extract, dietary nitrate, high-dose biotin, soy isoflavones, taurine, carnitine, magnesium orotate, EPA-rich fish oil, glycine, and copper. The potential advantages of whole-food plant-based diets, moderation in salt intake, avoidance of phosphate additives, and regular exercise training and sauna sessions are also discussed. There should be considerable scope for the development of functional foods and supplements which make it more convenient and affordable for patients to consume complementary combinations of the agents discussed here. Research Strategy: Key word searching of PubMed was employed to locate the research papers whose findings are cited in this essay.

Chronic α1-Na/K-ATPase inhibition reverses the elongation of the axon initial segment of the hippocampal CA1 pyramidal neurons in Angelman syndrome model mice

Angelman syndrome (AS) is a neurodevelopmental disorder caused by the loss of function of the maternal UBE3A gene. The hippocampus is one of the most prominently affected brain regions in AS model mice, manifesting in severe hippocampal-dependent memory and plasticity deficits. Previous studies in AS mice reported an elongated axon initial segment (AIS) in pyramidal neurons (PNs) of the hippocampal CA1 region. These were the first reports in mammals to show AIS elongation in vivo. Correspondingly, this AIS elongation was linked to enhanced expression of the α1 subunit of Na⁺/K⁺-ATPase (α1-NaKA). Recently, it was shown that selective pharmacological inhibition of α1-NaKA by marinobufagenin (MBG) in adult AS mice rescued the hippocampal-dependent deficits via normalizing their compromised activity-dependent calcium (Ca⁺²) dynamics. In the herein study, we showed that a chronic selective α1-NaKA inhibition reversed the AIS elongation in hippocampal CA1 PNs of adult AS mice, and differentially altered their excitability and intrinsic properties. Taken together, our study is the first to demonstrate in vivo structural plasticity of the AIS in a mammalian model, and further elaborates on the modulatory effects of elevated α1-NaKA levels in the hippocampus of AS mice.

The Insecticidal Activity of Rhinella schneideri (Werner, 1894) Paratoid Secretion in Nauphoeta cinerea Cocroaches

Rhinella schneideri is a common toad found in South America, whose paratoid toxic secretion has never been explored as an insecticide. In order to evaluate its insecticidal potential, Nauphoeta cinerea cockroaches were used as an experimental model in biochemical, physiological and behavioral procedures. Lethality assays with Rhinella schneideri paratoid secretion (RSPS) determined the LD₅₀ value after 24 h (58.07µg/g) and 48 h exposure (44.07 µg/g) (R² = 0.882 and 0.954, respectively). Acetylcholinesterase activity (AChE) after RSPS at its highest dose promoted an enzyme inhibition of 40%, a similar effect observed with neostigmine administration (p < 0.001, n= 5). Insect locomotion recordings revealed that RSPS decreased the distance traveled by up to 37% with a concomitant 85% increase in immobile episodes (p < 0.001, n = 36). RSPS added to in vivo cockroach semi-isolated heart preparation promoted an irreversible and dose dependent decrease in heart rate, showing a complete failure after 30 min recording (p < 0.001, n ≥ 6). In addition, RSPS into nerve-muscle preparations induced a dose-dependent neuromuscular blockade, reaching a total blockage at 70 min at the highest dose applied (p < 0.001, n ≥ 6). The effect of RSPS on spontaneous sensorial action potentials was characterized by an increase in the number of spikes 61% (p < 0.01). Meanwhile, there was 42% decrease in the mean area of those potentials (p < 0.05, n ≥ 6). The results obtained here highlight the potential insecticidal relevance of RSPS and its potential biotechnological application.

Potent nonopioid antinociceptive activity of telocinobufagin in models of acute pain in mice

Introduction:

In recent decades, several researches have been conducted in search of new analgesics that do not present the side effects of opioids. In this context, animal venoms contain natural painkillers that have been used for the development of new analgesics.

Objective:

The aims of this study were to evaluate the antinociceptive effects of telocinobufagin (TCB), a bufadienolide isolated from Rhinella jimi venom, in murine acute pain models, and to verify the participation of the opioid system in these effects.

Methods:

TCB was purified from R. jimi venom by high-performance liquid chromatography, and its structure was confirmed by spectrometric techniques. TCB was administered intraperitoneally (i.p.) (0.062, 0.125, 0.25, 0.5, and 1 mg·kg⁻¹) and orally (p.o.) (0.625, 1.125, 2.5, 5, and 10 mg·kg⁻¹) in mice, which were then subjected to pain tests: acetic acid–induced writhing, formalin, tail-flick, and hot-plate. Involvement of the opioid system in TCB action was evaluated by naloxone i.p. injected (2.5 mg·kg⁻¹) 20 minutes before TCB administration. In addition, the TCB action on the μ, δ, and κ opioid receptors was performed by radioligand binding assays.

Results:

In all the tests used, TCB showed dose-dependent antinociceptive activity with more than 90% inhibition of the nociceptive responses at the doses of 1 mg·kg⁻¹ (i.p.) and 10 mg·kg⁻¹ (p.o.). Naloxone did not alter the effect of TCB. In addition, TCB did not act on the μ, δ, and κ opioid receptors.

Conclusion:

The results suggest that TCB may represent a novel potential nonopioid therapeutic analgesic for treatment of acute pains.

Biochemical and Biological Profile of Parotoid Secretion of the Amazonian Rhinella marina (Anura: Bufonidae)

Skin secretions of frogs have a high chemical complexity. They have diverse types of biomolecules, such as proteins, peptides, biogenic amines, and alkaloids. These compounds protect amphibians' skin against growth of bacteria, fungi, and protozoa and participate in defense system against attack from predators. Therewith, this work performed biochemical and biological profile of macroglands parotoid secretion from cane toad. For poison analysis, we performed molecular exclusion and reverse phase chromatography, electrophoresis, and mass spectrometry. Antimicrobial, antiplasmodial, leishmanicidal, cytotoxicity, genotoxicity, and inflammatory activity of crude and/or fractions of R. marina secretion were also evaluated. Fractionation prior to filtration from poison showed separation of low mass content (steroids and alkaloids) and high molecular mass (protein). Material below 10 kDa two steroids, marinobufagin and desacetylcinobufagin, was detected. Crude extract and fractions were active against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Plasmodium falciparum, Leishmania guyanensis, and Leishmania braziliensis. Crude extract was also active against cancer cells although it was not cytotoxic for normal cells. This extract did not show significant DNA damage but it showed an important inflammatory effect in vivo. The information obtained in this work contributes to the understanding of the constituents of R. marina secretion as well as the bioactive potential of these molecules.

Marinobufagenin extraction from Rhinella marina toad glands: Alternative approaches for a systematized strategy

Marinobufagenin is a bufadienolide compound detected mainly in skin and parotoid gland secretions of Rhinella marina (L.) toad. Bufadienolides regulate the Na⁺ /K⁺ -ATPase pump by inhibiting the cardiotonic steroid dependent-site and act as cardiac inotropes with vasoconstrictive properties. Marinobufagenin and other bufadienolides, such as telocinobufagin and bufalin, are thought to be found endogenously in mammals in salt-sensitive hypertensive states such as essential hypertension, congestive heart-failure, and preeclampsia. The role of marinobufagenin as antimicrobial agent and its cytotoxic potential have also been recognized. The particular interest around marinobufagenin prompts us to consider the Rhinella marina toad venom as a possible source for molecules with pharmacological and/or diagnostic potential. In this article, two different approaches of extraction and purification of marinobufagenin from Rhinella marina (L.) venom are studied: (i) Preparative thin-layer chromatography combined to mass spectrometry and/or ultraviolet detection and (ii) solid-phase extraction coupled with fractionation on high-performance liquid chromatography. Different chromatographic conditions are tested for each approach. The solid-phase extraction combined with high-performance liquid chromatography fractionation approach was preferred as it offered a greater yield, was less time-consuming and allowed us to selectively isolate marinobufagenin. Both protocols aim to provide efficient and convenient methods for toad venom extraction, based on an easily automatable and systematized strategy.

Revealing of endogenous Marinobufagin by an ultra-specific and sensitive UHPLC-MS/MS assay in pregnant women

Marinobufagenin (MBG) is a bufadienolide cardiac inotrope implicated in volume expansion-mediated hypertensive states including essential hypertension and preeclampsia (PE). Endogenous MBG is an inhibitor of the α1-isoform of Na⁺,K⁺-ATPase with vasoconstrictive and cardiotonic properties, causing hypertension and natriuresis. Elevated endogenous MBG-like material levels have been described by immunoassays in salt-sensitive pregnant and preeclamptic rats as well as in preeclamptic human patients. The rise of endogenous MBG-like material appears prior the development of the main symptoms of PE, leading us to consider MBG as one of the potential biomarkers for PE. The weak specificity and the high variability of the published immunoassays gives no certification about endogenous MBG existence. This led us to set-up a highly specific and sensitive analytical method to detect MBG in plasma at low levels relying on liquid chromatography combined to mass spectrometry (UHPLC-MS/MS) with recording of 7 highly specific MRM transitions for MBG. Pure MBG standard used in the method development was obtained by purification from the Bufo marinus toad venom. d₃-25-hydroxyvitamin D3 was used as internal standard. An increasing organic gradient with mobile phase A and B composed of 97:3 (v/v) H₂O: MeOH and 50:45:5 (v/v/v) MeOH:IPA:H₂O at pH 4.5 respectively was used on a Pursuit 3 PFP column (100 mm × 3 mm; 3 µm) to allow elution and separation of the plasmatic compounds. Chromatographic analyses of plasma samples were preceded by a precipitation of proteins pretreatment. The developed UHPLC-MS/MS assay has been applied to early-pregnant women plasma samples allowing us to investigate MBG plasma levels. Thanks to the high specificity of the assay we were able to authenticate and certify the presence of endogenous MBG in early-pregnant women plasma with the use of the 7 selected specific mass transitions. These pioneering preliminary results are giving a promising perspective for early preeclampsia risk assessment in pregnant women.

Marinobufagenin inhibits glioma growth through sodium pump α1 subunit and ERK signaling-mediated mitochondrial apoptotic pathway

Malignant glioma is one of the most challenging central nervous system diseases to treat and has high rates of recurrence and mortality. Current therapies often fail to control tumor progression or improve patient survival. Marinobufagenin (MBG) is an endogenous mammalian cardiotonic steroid involved in sodium pump inhibition. Currently, various studies have indicated the potential of MBG in cancer treatments; however, the precise mechanisms are poorly understood. The functions of MBG were examined using colony formation, migration, cell cycle, and apoptosis assays in glioma cells. A mitochondrial membrane potential assay was performed to determine the mitochondrial transmembrane potential change, and cytochrome c release from mitochondria was assayed by fluorescence microscopy. An immunofluorescence assay was performed, and the nuclear translocation of NF‐κB in glioma cells was confirmed by confocal microscopy. Western blotting and RT‐qPCR were used to detect the protein and gene expression levels, respectively. In addition, transfection experiment of ATP1A1‐siRNA was further carried out to confirm the role of sodium pump α1 subunit in the anticancer effect of MBG in human glioma. The apoptosis‐promoting and anti‐inflammatory effects of MBG were further investigated, and the sodium pump α1 subunit and the ERK signaling pathway were found to be involved in the anticancer effect of MBG. The in vivo anticancer efficacy of MBG was also tested in xenografts in nude mice. Thus, therapies targeting the ERK signaling‐mediated mitochondrial apoptotic pathways regulated by MBG might represent potential treatments for human glioma, and this study could accelerate the finding of newer therapeutic approaches for malignant glioma treatment.

Genetic Control of Serum Marinobufagenin in the Spontaneously Hypertensive Rat and the Relationship to Blood Pressure

BACKGROUND

We have investigated serum levels of immunoreactive marinobufagenin (MBG) in 16- to 20-week-old spontaneously hypertensive rats (SHRs)-A3 and in the normotensive Wistar-Kyoto (WKY) rat strain in the absence of salt loading, and we have investigated the genetic control of serum MBG.

METHODS AND RESULTS

We genotyped the F2 progeny of an SHR-A3×WKY intercross using a genome-wide panel of 253 single-nucleotide polymorphism markers that were dimorphic between SHR-A3 and WKY and measured serum MBG by ELISA. Serum MBG levels were lower in SHR-A3 than WKY rats (0.39±0.07 and 1.27±0.40 nmol/L, respectively), suggesting that MBG may not play a role in the markedly divergent blood pressure measured by telemetry in rats of these 2 strains (SHR-A3 and WKY, 198.3±4.43 and 116.8±1.51 mm Hg, respectively). The strain difference in serum MBG was investigated to determine whether genomic regions influencing MBG might be identified by genetic mapping. Quantitative trait locus mapping indicated a single locus influencing serum MBG in the region of chromosome 6q12. Homozygosity of WKY alleles at this locus was associated with increased serum MBG levels. We surveyed whole genome sequences from our SHR-A3 and WKY lines, seeking coding sequence variation between SHR-A3 and WKY within the mapped locus that might explain the inherited strain difference in serum MBG.

CONCLUSIONS

We identified amino acid substitution in the sterol transport protein Abcg5, present in SHR-A3, but absent in WKY, that is a potential mechanism influencing MBG levels.

Ouabain attenuates ovalbumin-induced airway inflammation

PURPOSE

Ouabain, an Na⁺/K⁺-ATPase inhibitor hormone, presents immunomodulatory actions, including anti-inflammatory effect on acute inflammation models.

METHODS

In the present study, the effect of ouabain in a model of allergic airway inflammation induced by ovalbumin (OVA) was assessed.

RESULTS

Initially, it was observed that ouabain treatment inhibited cellular migration induced by OVA on bronchoalveolar lavage fluid (BALF), mostly granulocytes, without modulating macrophage migration. In addition, it was observed, by flow cytometry, that ouabain reduces CD3^high lymphocytes cells on BALF. Furthermore, treatment with ouabain decreased IL-4 and IL-13 levels on BALF. Ouabain also promoted pulmonary histological alterations, including decreased cell migration into peribronchiolar and perivascular areas, and reduced mucus production in bronchioles regions observed through hematoxylin-eosin (HE) and by periodic acid-Schiff stain, respectively. Allergic airway inflammation is characterized by high OVA-specific IgE serum titer. This parameter was also reduced by the treatment with ouabain.

CONCLUSIONS

Therefore, our data demonstrate that ouabain negatively modulates allergic airway inflammation induced by OVA.

Comparison of Digitalis Sensitivities of Na+/K+-ATPases from Human and Pig Kidneys

Digitalis drugs are selective inhibitors of the plasma membrane Na⁺/K⁺-ATPase. There are many studies on molecular mechanisms of digitalis interaction with purified pig kidney enzyme, with the tacit assumption that it is a good model of human kidney enzyme. However, previous studies on crude or recombinant human kidney enzymes are limited, and have not resulted in consistent findings on their digitalis sensitivities. Hence, we prepared comparably purified enzymes from human and pig kidneys and determined inhibitory constants of digoxin, ouabain, ouabagenin, bufalin, and marinobufagenin (MBG) on enzyme activity under optimal turnover conditions. We found that each compound had the same potency against the two enzymes, indicating that (i) the pig enzyme is an appropriate model of the human enzyme, and (ii) prior discrepant findings on human kidney enzymes were either due to structural differences between the natural and recombinant enzymes or because potencies were determined using binding constants of digitalis for enzymes under nonphysiological conditions. In conjunction with previous findings, our newly determined inhibitory constants of digitalis compounds for human kidney enzymes indicate that (i) of the compounds that have long been advocated to be endogenous hormones, only bufalin and MBG may act as such at kidney tubules, and (ii) beneficial effects of digoxin, the only digitalis with extensive clinical use, does not involve its inhibitory effect on renal tubular Na⁺/K⁺-ATPase.

Effects of cardiotonic steroids on isolated perfused kidney and NHE3 activity in renal proximal tubules

Cardiotonic steroids (CS) are known as modulators of sodium and water homeostasis. These compounds contribute to the excretion of sodium under overload conditions due to its natriuretic property related to the inhibition of the renal Na⁺/K⁺-ATPase (NKA) pump α1 isoform. NHE3, the main route for Na⁺ reabsorption in the proximal tubule, depends on the Na⁺ gradient generated by the NKA pump. In the present study we aimed to investigate the effects of marinobufagin (MBG) and telocinobufagin (TBG) on the renal function of isolated perfused rat kidney and on the inhibition of NKA activity. Furthermore, we investigated the mechanisms for the cardiotonic steroid-mediated natriuretic effect, by evaluating and comparing the effects of bufalin (BUF), ouabain (OUA), MBG and TBG on NHE3 activity in the renal proximal tubule in vivo. TBG significantly increased GFR, UF, natriuresis and kaliuresis in isolated perfused rat kidney, and inhibits the activity of NKA at a much higher rate than MBG. By stationary microperfusion technique, the perfusion with BUF, OUA, TBG or MBG promoted an inhibitory effect on NHE3 activity, whereas BUF was the most effective agent, and demonstrated a dose-dependent response, with maximal inhibition at 50nM. Furthermore, our data showed the role of NKA-Src kinase pathway in the inhibition of NHE3 by CS. Finally, a downstream step, MEK1/2-ERK1/2 was also investigated, and, similar to Src inhibition, the MEK1/2 inhibitor (U0126) suppressed the BUF effect. Our findings indicate the involvement of NKA-SRc-Kinase-Ras-Raf-ERK1/2 pathway in the downregulation of NHE3 by cardiotonic steroids in the renal proximal tubule, promoting a reduction of proximal sodium reabsorption and natriuresis.

Are endogenous cardenolides controlled by atrial natriuretic peptide

Endogenous cardenolides are digoxin-like substances and ouabain-like substances that have been implicated in the pathogenesis of hypertension and mood disorders in clinical and pre-clinical studies. Regulatory signals for endogenous cardenolides are still unknown. These endogenous compounds are believed to be produced by the adrenal gland in the periphery and the hypothalamus in the central nervous system, and constitute part of an hormonal axis that may regulate the catalytic activity of the α subunit of Na(+)/K(+)-ATPase. A review of literature suggests that there is great overlap in physiological environments that are associated with either elevations or reductions in the levels of atrial natriuretic peptide (ANP) and endogenous cardenolides. This suggests that these two factors may share a common regulatory signal or perhaps that ANP may be involved in the regulation of endogenous cardenolides.

Endogenous digitalis-like factors: an overview of the history

The sodium pump is a ubiquitous cell surface enzyme, a Na, K ATPase, which maintains ion gradients between cells and the extracellular fluid (ECF). The extracellular domain of this enzyme contains a highly conserved binding site, a receptor for a plant derived family of compounds, the digitalis glycosides. These compounds inhibit the enzyme and are used in the treatment of congestive heart failure and certain cardiac arrhythmias. The highly conserved nature of this enzyme and its digitalis receptor led to early suggestions that endogenous regulators might exist. Recent examination of this hypothesis emerged from research in two separate areas: the regulation of ECF volume by a natriuretic hormone (NH), and the regulation of peripheral vascular resistance by a circulating inhibitor of vascular Na, K ATPase. These two areas merged with the hypothesis that NH and the vascular Na, K ATPase inhibitor were in fact the same entity, and that it played a causative role in the pathophysiology of certain types of hypertension. The possibility that multiple endogenous digitalis-like factors (EDLFs) exist emerged from efforts to characterize the circulating enzyme inhibitory activity. In this review, the development of this field from its beginnings is traced, the current status of the structure of EDLFs is briefly discussed, and areas for future development are suggested.

Natriuretic effect of bufalin in isolated rat kidneys involves activation of the Na+-K+-ATPase-Src kinase pathway

Bufadienolides are structurally related to the clinically relevant cardenolides (e.g., digoxin) and are now considered as endogenous steroid hormones. Binding of ouabain to Na(+)-K(+)-ATPase has been associated, in kidney cells, to the activation of the Src kinase pathway and Na(+)-K(+)-ATPase internalization. Nevertheless, whether the activation of this cascade also occurs with other cardiotonic steroids and leads to diuresis and natriuresis in the isolated intact kidney is still unknown. In the present work, we perfused rat kidneys for 120 min with bufalin (1, 3, or 10 μM) and measured its vascular and tubular effects. Thereafter, we probed the effect of 10 μM 3-(4-chlorophenyl)1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4amine (PP2), a Src family kinase inhibitor, and 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio] butadiene (UO126), a highly selective inhibitor of both MEK1 and MEK2, on bufalin-induced renal alterations. Bufalin at 3 and 10 μM profoundly increased several parameters of renal function in a time- and/or concentration-dependent fashion. At a concentration that produced similar inhibition of the rat kidney Na(+)-K(+)-ATPase, ouabain had a much smaller diuretic and natriuretic effect. Although bufalin fully inhibited the rat kidney Na(+)-K(+)-ATPase in vitro, its IC(50) (33 ± 1 μM) was threefold higher than the concentration used ex vivo and all its renal effects were blunted by PP2 and UO126. Furthermore, the phosphorylated (activated) ERK1/2 expression was increased after bufalin perfusion and this effect was totally prevented after PP2 pretreatment. The present study shows for the first time the direct diuretic, natriuretic, and kaliuretic effects of bufalin in isolated rat kidney and the relevance of Na(+)-K(+)-ATPase-mediated signal transduction.

The central mechanism underlying hypertension: a review of the roles of sodium ions, epithelial sodium channels, the renin-angiotensin-aldosterone system, oxidative stress and endogenous digitalis in the brain

The central nervous system has a key role in regulating the circulatory system by modulating the sympathetic and parasympathetic nervous systems, pituitary hormone release, and the baroreceptor reflex. Digoxin- and ouabain-like immunoreactive materials were found >20 years ago in the hypothalamic nuclei. These factors appeared to localize to the paraventricular and supraoptic nuclei and the nerve fibers at the circumventricular organs and supposed to affect electrolyte balance and blood pressure. The turnover rate of these materials increases with increasing sodium intake. As intracerebroventricular injection of ouabain increases blood pressure via sympathetic activation, an endogenous digitalis-like factor (EDLF) was thought to regulate cardiovascular system-related functions in the brain, particularly after sodium loading. Experiments conducted mainly in rats revealed that the mechanism of action of ouabain in the brain involves sodium ions, epithelial sodium channels (ENaCs) and the renin-angiotensin-aldosterone system (RAAS), all of which are affected by sodium loading. Rats fed a high-sodium diet develop elevated sodium levels in their cerebrospinal fluid, which activates ENaCs. Activated ENaCs and/or increased intracellular sodium in neurons activate the RAAS; this releases EDLF in the brain, activating the sympathetic nervous system. The RAAS promotes oxidative stress in the brain, further activating the RAAS and augmenting sympathetic outflow. Angiotensin II and aldosterone of peripheral origin act in the brain to activate this cascade, increasing sympathetic outflow and leading to hypertension. Thus, the brain Na(+)-ENaC-RAAS-EDLF axis activates sympathetic outflow and has a crucial role in essential and secondary hypertension. This report provides an overview of the central mechanism underlying hypertension and discusses the use of antihypertensive agents.

Renal ischemia regulates marinobufagenin release in humans

Cardiotonic steroids, including marinobufagenin, are a group of new steroid hormones found in plasma and urine of patients with congestive heart failure, myocardial infarction, and chronic renal failure. In animal studies, partial nephrectomy induces marinobufagenin elevation, cardiac hypertrophy, and fibrosis. The objective of this study is to test the effect of renal ischemia on marinobufagenin levels in humans with renal artery stenosis (RAS). To test this, plasma marinobufagenin levels were measured in patients with RAS of the Prospective Randomized Study Comparing Renal Artery Stenting With or Without Distal Protection, non-RAS patient controls who were scheduled for coronary angiography, and normal healthy individuals. Marinobufagenin levels were significantly higher in patients with RAS compared with those of the other 2 groups. Multivariate analysis shows that occurrence of RAS is independently related to marinobufagenin levels. In addition, renal artery revascularization by stenting partially reversed marinobufagenin levels in the patients with RAS (0.77±0.06 nmol/L at baseline; 0.66±0.06 nmol/L at 24 hours; and 0.61±0.05 nmol/L at 1 month). In conclusion, we have found that marinobufagenin levels are increased in patients with RAS, whereas reversal of renal ischemia by stenting treatment reduces marinobufagenin levels. These results suggest that RAS-induced renal ischemia may be a major cause of marinobufagenin release.

Ouabain stimulates Na-K-ATPase through a sodium/hydrogen exchanger-1 (NHE-1)-dependent mechanism in human kidney proximal tubule cells

Recent investigations demonstrate increased Na/H exchanger-1 (NHE-1) activity and plasma levels of ouabain-like factor in spontaneously hypertensive rats. At nanomolar concentrations, ouabain increases Na-K-ATPase activity, induces cell proliferation, and activates complex signaling cascades. We hypothesize that the activity of NHE-1 and Na-K-ATPase are interdependent. To test whether treatment with picomolar ouabain regulates Na-K-ATPase through an NHE-1-dependent mechanism, we examined the role of NHE-1 in ouabain-mediated stimulation of Na-K-ATPase in kidney proximal tubule cell lines [opossum kidney (OK), HK-2, HKC-5, and HKC-11] and rat kidney basolateral membranes. Ouabain stimulated Na-K-ATPase activity and tyrosine phosphorylation in cells that express NHE-1 (OK, HKC-5, and HKC-11) but not in HK-2 cells that express very low levels of NHE-1. Inhibition of NHE-1 with 5 microM EIPA, a NHE-1-specific inhibitor, prevented ouabain-mediated stimulation of (86)Rb uptake and Na-K-ATPase phosphorylation in OK, HKC-5, and HKC-11 cells. Expression of wild-type NHE-1 in HK2 cells restored regulation of Na-K-ATPase by picomolar ouabain. Treatment with picomolar ouabain increased membrane expression of Na-K-ATPase and enhanced NHE-1-Na-K-ATPase alpha1-subunit association. Treatment with ouabain (1 microg x kg body wt(-1) x day(-1)) increased Na-K-ATPase activity, expression, phosphorylation, and association with NHE-1 increased in rat kidney cortical basolateral membranes. Eight days' treatment with ouabain (1 microg x kg body wt(-1) x day(-1)) resulted in increased blood pressure in these rats. These results suggest that the association of NHE-1 with Na-K-ATPase is critical for ouabain-mediated regulation of Na-K-ATPase and that these effects may play a role in cardioglycoside-stimulated hypertension.

Marinobufagenin enhances cardiac contractility in mice with ouabain-sensitive alpha1 Na+-K+-ATPase

Endogenous Na(+) pump inhibitors are thought to play important (patho)physiological roles and occur in two different chemical forms in the mammalian circulation: cardenolides, such as ouabain, and bufadienolides, such as marinobufagenin (MBG). Although all alpha Na(+)-K(+)-ATPase isoforms (alpha(1-4)) are sensitive to ouabain in most species, in rats and mice the ubiquitously expressed alpha(1) Na(+)-K(+)-ATPase is resistant to ouabain. We have previously shown that selective modification of the putative ouabain binding site of either the alpha(1) or alpha(2) Na(+)-K(+)-ATPase subunit in mice substantially alters the cardiotonic influence of exogenously applied cardenolides. To determine whether the ouabain binding site also interacts with MBG and if this interaction plays a functional role, we evaluated cardiovascular function in alpha(1)-resistant/alpha(2)-resistant (alpha(1)(R/R)alpha(2)(R/R)), alpha(1)-sensitive/alpha(2)-resistant (alpha(1)(S/S)alpha(2)(R/R)), and alpha(1)-resistant/alpha(2)-sensitive mice (alpha(1)(R/R)alpha(2)(S/S), wild type). Cardiovascular indexes were evaluated in vivo by cardiac catheterization at baseline and during graded infusions of MBG. There were no differences in baseline measurements of targeted mice, indicating normal hemodynamics and cardiac function. MBG at 0.025, 0.05, and 0.1 nmol*min(-1)*g body wt(-1) significantly increased cardiac performance to a greater extent in alpha(1)(S/S)alpha(2)(R/R) compared with alpha(1)(R/R)alpha(2)(R/R) and wild-type mice. The increase in LVdP/dt(max) in alpha(1)(S/S)alpha(2)(R/R) mice was greater at higher concentrations of MBG compared with both alpha(1)(R/R)alpha(2)(R/R) and alpha(1)(R/R)alpha(2)(S/S) mice (P < 0.05). These results suggest that MBG interacts with the ouabain binding site of the alpha(1) Na(+)-K(+)-ATPase subunit and can thereby influence cardiac inotropy.

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

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

The cardiotonic steroid hormone marinobufagenin induces renal fibrosis: implication of epithelial-to-mesenchymal transition

We recently demonstrated that the cardiotonic steroid marinobufagenin (MBG) induced fibrosis in rat hearts through direct stimulation of collagen I secretion by cardiac fibroblasts. This stimulation was also responsible for the cardiac fibrosis seen in experimental renal failure. In this study, the effect of MBG on the development of renal fibrosis in rats was investigated. Four weeks of MBG infusion triggered mild periglomerular and peritubular fibrosis in the cortex and the appearance of fibrotic scars in the corticomedullary junction of the kidney. MBG also significantly increased the protein levels and nuclear localization of the transcription factor Snail in the tubular epithelia. It is known that activation of Snail is associated with epithelial-to-mesenchymal transition (EMT) during renal fibrosis. To examine whether MBG alone can trigger EMT, we used the porcine proximal tubular cell line LLC-PK1. MBG (100 nM) caused LLC-PK1 cells grown to confluence to acquire a fibroblast-like shape and have an invasive motility. The expressions of the mesenchymal proteins collagen I, fibronectin, and vimentin were increased twofold. However, the total level of E-cadherin remained unchanged. These alterations in LLC-PK1 cells in the presence of MBG were accompanied by elevated expression and nuclear translocation of Snail. During the time course of EMT, MBG did not have measurable inhibitory effects on the ion pumping activity of its natural ligand, Na(+)-K(+)-ATPase. Our data suggest that the MBG may be an important factor in inducing EMT and, through this mechanism, elevated levels of MBG in chronic renal failure may play a role in the progressive fibrosis.

Ouabain-Sensitive alpha1 Na,K-ATPase enhances natriuretic response to saline load

Na,K-ATPase is ubiquitously expressed and is essential for maintaining electrochemical and osmotic gradients. The alpha subunit of Na,K-ATPase is the receptor for cardiotonic steroids, which act through the ouabain-binding site and are important in cardiovascular regulation. Interestingly, the presence of endogenous Na,K-ATPase ligands has been implicated in the natriuretic response to perturbations such as hypertension and salt loading; therefore, it is important to characterize the role of the ouabain-binding sites in this context. Because the alpha1 isoform of mice and rats is relatively ouabain resistant, gene-targeting strategies were used to produce mice with reversed responses of the alpha1 and/or alpha2 isoforms to ouabain to assess for altered natriuretic responses to acute salt loading. Regardless of the sensitivity of the alpha2 isoform to ouabain, conferring ouabain sensitivity to alpha1 augmented the natriuretic response to an acute salt load. In addition, when endogenous Na,K-ATPase inhibitors were sequestered with an anti-digoxin antibody fragment, the sodium excretion rates in the ouabain-sensitive alpha1 isoform mice were equivalent to the ouabain-resistant alpha1 isoform mice. These data suggest that the ouabain-binding site of the alpha1 Na,K-ATPase can participate in the natriuretic response to a salt load by responding to endogenous Na,K-ATPase ligands.

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.

Examination of the cellular mechanisms by which marinobufagenin inhibits cytotrophoblast function

Marinobufagenin (MBG) is an endogenous mammalian cardiotonic steroid involved in the inhibition of Na(+)/K(+)-ATPase. Increased plasma levels have been reported in patients with volume expansion-related hypertension. We have recently demonstrated that MBG impairs first trimester cytotrophoblast (CTB) cell proliferation, migration, and invasion, which may play a role in the development of preeclampsia. However, whether apoptosis contributes to altered CTB cell function by MBG remains unknown. Using the human extravillous CTB cell line SGHPL-4, we examined the effect of MBG and a similar Na(+)/K(+)-ATPase inhibitor, ouabain, on the phosphorylation status of Jnk, p38, and Src. Additionally, we measured apoptosis by caspase 9 and 3/7 activity and by annexin-V staining. We also investigated interleukin-6 (IL-6) secretion with or without p38 and Jnk inhibition. MBG significantly increased the phosphorylation of Jnk, p38, and Src and increased the expression of caspase 9 and 3/7 indicating the activation of apoptosis. MBG treatment also stimulated the expression of the early apoptosis marker, annexin-V, which was prevented by Jnk and p38 inhibition. MBG also stimulated the secretion of IL-6, which was attenuated by p38 inhibition. Ouabain had similar effects to those of MBG, suggesting that the apoptotic effects on CTB cells may be mediated by inhibition of Na(+)/K(+)-ATPase. In conclusion, the MBG-induced impairment of CTB function occurs via activation of Jnk, p38, and Src leading to increased apoptosis and IL-6 secretion. These observations may have clinical applicability with respect to the therapy of preeclampsia.

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.

Marinobufagenin interferes with the function of the mineralocorticoid receptor

Marinobufagenin (MBG) is a cardiotonic steroid of the bufadienolide class of compounds which has the ability to inhibit the ubiquitous enzyme, Na+/K+-ATPase, resulting in natriuresis. The involvement of MBG in the pathogenesis of volume expansion-mediated forms of hypertension has been suggested for some time, and we have proposed that MBG participates in the hypertension noted in preeclampsia. We examined the hypothesis that MBG might contribute to these forms of hypertension by promoting the activity of the mineralocorticoid receptor (MR). However, our data demonstrate that instead, MBG interferes with the functioning of the MR by inhibiting the transcriptional activity of the receptor, and this is reflected in a reduced interaction between the SRC-3 coactivator and the MR. Thus, the ability of MBG to cause a natriuresis may be due, not only to inhibition of Na+/K+-ATPase activity, but also to its ability to interfere with MR-dependent expression of the Na/K/H exchanger in the late distal nephron.

ANP differentially modulates marinobufagenin-induced sodium pump inhibition in kidney and aorta

NaCl loading and plasma volume expansion stimulate 2 natriuretic systems, vasoconstrictor, digitalis-like Na/K-ATPase inhibitors and vasorelaxant ANP peptides. Several hormones, including ANP, regulate activity of the Na/K-ATPase by modulation of its phosphorylation state. We studied effects of ANP on Na/K-ATPase phosphorylation and inhibition by an endogenous sodium pump ligand, marinobufagenin, in the aorta and renal medulla from male Sprague-Dawley rats. Marinobufagenin dose-dependently inhibited the Na/K-ATPase in renal and vascular membranes at the level of higher (nanomolar) and lower affinity (micromolar) binding sites. Marinobufagenin (1 nmol/L) inhibited Na/K-ATPase in aortic sarcolemma (18%) and in renal medulla (19%). prepro-ANP 104 to 123 (ppANP) and alpha-human ANP ([alpha-hANP] both 1 nmol/L) potentiated marinobufagenin-induced Na/K-ATPase inhibition in the kidney, but reversed the effect of marinobufagenin in the aorta. Similarly, ppANP and alpha-hANP modulated the sodium pump (ouabain-sensitive (86)Rb uptake) inhibitory effects of marinobufagenin in the aorta and renal medulla. In renal medulla, ppANP and alpha-hANP induced alpha-1 Na/K-ATPase phosphorylation, whereas in aorta, both peptides dephosphorylated Na/K-ATPase. The effect of ppANP on Na/K-ATPase phosphorylation and inhibition was mimicked by a protein kinase G activator, 8-Br-PET-cGMP (10 micromol/L), and prevented by a protein kinase G inhibitor, KT5823 (60 nmol/L). Our results suggest that alpha-1 Na/K-ATPase inhibition by marinobufagenin in the kidney is enhanced via Na/K-ATPase phosphorylation by ANP, whereas in the aorta, ANP exerts an opposite effect. The concurrent production of a vasorelaxant, ANP, and a vasoconstrictor, marinobufagenin, potentiate each other's natriuretic effects, but ANP peptides may offset the deleterious vasoconstrictor effect of marinobufagenin.

Central role for the cardiotonic steroid marinobufagenin in the pathogenesis of experimental uremic cardiomyopathy

Patients with chronic renal failure develop a "uremic" cardiomyopathy characterized by diastolic dysfunction, cardiac hypertrophy, and systemic oxidant stress. Patients with chronic renal failure are also known to have increases in the circulating concentrations of the cardiotonic steroid marinobufagenin (MBG). On this background, we hypothesized that elevations in circulating MBG may be involved in the cardiomyopathy. First, we observed that administration of MBG (10 microg/kg per day) for 4 weeks caused comparable increases in plasma MBG as partial nephrectomy at 4 weeks. MBG infusion caused increases in conscious blood pressure, cardiac weight, and the time constant for left ventricular relaxation similar to partial nephrectomy. Decreases in the expression of the cardiac sarcoplasmic reticulum ATPase, cardiac fibrosis, and systemic oxidant stress were observed with both MBG infusion and partial nephrectomy. Next, rats were actively immunized against a MBG-BSA conjugate or BSA control, and partial nephrectomy was subsequently performed. Immunization against MBG attenuated the cardiac hypertrophy, impairment of diastolic function, cardiac fibrosis, and systemic oxidant stress seen with partial nephrectomy without a significant effect on conscious blood pressure. These data suggest that the increased concentrations of MBG are important in the cardiac disease and oxidant stress state seen with renal failure.

Sodium pump alpha2 subunits control myogenic tone and blood pressure in mice

A key question in hypertension is: How is long-term blood pressure controlled? A clue is that chronic salt retention elevates an endogenous ouabain-like compound (EOLC) and induces salt-dependent hypertension mediated by Na(+)/Ca(2)(+) exchange (NCX). The precise mechanism, however, is unresolved. Here we study blood pressure and isolated small arteries of mice with reduced expression of Na(+) pump alpha1 (alpha1(+/-)) or alpha2 (alpha2(+/-)) catalytic subunits. Both low-dose ouabain (1-100 nm; inhibits only alpha2) and high-dose ouabain (> or =1 microm; inhibits alpha1) elevate myocyte Ca(2)(+) and constrict arteries from alpha1(+/-), as well as alpha2(+/-) and wild-type mice. Nevertheless, only mice with reduced alpha2 Na(+) pump activity (alpha2(+/-)), and not alpha1 (alpha1(+/-)), have elevated blood pressure. Also, isolated, pressurized arteries from alpha2(+/-), but not alpha1(+/-), have increased myogenic tone. Ouabain antagonists (PST 2238 and canrenone) and NCX blockers (SEA0400 and KB-R7943) normalize myogenic tone in ouabain-treated arteries. Only the NCX blockers normalize the elevated myogenic tone in alpha2(+/-) arteries because this tone is ouabain independent. All four agents are known to lower blood pressure in salt-dependent and ouabain-induced hypertension. Thus, chronically reduced alpha2 activity (alpha2(+/-) or chronic ouabain) apparently regulates myogenic tone and long-term blood pressure whereas reduced alpha1 activity (alpha1(+/-)) plays no persistent role: the in vivo changes in blood pressure reflect the in vitro changes in myogenic tone. Accordingly, in salt-dependent hypertension, EOLC probably increases vascular resistance and blood pressure by reducing alpha2 Na(+) pump activity and promoting Ca(2)(+) entry via NCX in myocytes.

A novel endogenous digitalis, telocinobufagin, exhibits elevated plasma levels in patients with terminal renal failure

BACKGROUND AND OBJECTIVES

There are several potential endogenous digitalis-like factors (EDLF) in mammalian body fluids, and marinobufagenin (MBG) may be the most potent EDLF. Improved assays are needed to confirm the potency of these metabolites. In the present study, we have identified MBG and telocinobufagin (TCB) in human plasma by high-resolution mass spectrometry (MS) and nuclear magnetic resonance (NMR).

METHODS AND RESULTS

The high-resolution MS analysis revealed the molecular masses of TCB and MBG to be the same as their respective theoretical values. Using a tandem mass spectrometer, the mass-charge ratio for TCB was determined to be 403.2 for the parent ion and 349.2 for the daughter ion. The mass-charge ratio for MBG was m/z 383.2 and m/z 401.2. The NMR study revealed that the signals for MBG and TCB were the same as those obtained by MS analysis. In human blood, MBG and TCB were also identified by liquid chromatography (LC) as well as MS. In the LC/MS assay, proscillaridin A was used as an internal standard. The plasma was pretreated with Sep-Pak C18, and then 50 microL was applied to the C8 high-performance liquid chromatography (HPLC) column. The mean plasma concentration of MBG in healthy volunteers (0.94 +/- 0.28 ng/mL) was significantly lower than that in patients undergoing regular hemodialysis (3.81 +/- 1.92 ng/mL). The concentration of TCB in the healthy volunteers (1.80 +/- 0.55 ng/mL) was also significantly lower than that in patients with terminal renal failure (6.86 +/- 4.30 ng/mL).

CONCLUSION

These results indicate that the major EDLF is TCB because its plasma concentration is the highest among the reported endogenous digitalis candidates.

Marinobufagenin may mediate the adverse impact of salty diets on renal calcium retention by impairing the efficiency of renal tubular sodium–calcium exchange

For reasons yet to be clarified, salt loading and plasma volume expansion decrease renal calcium retention; consequently, high-salt diets are thought to increase risk for osteoporosis and renal stones. These measures also can evoke increased adrenal production of the natriuretic factor marinobufagenin (MBG), recently implicated in the genesis of essential hypertension. MBG achieves natriuresis via potent selective inhibition of the alpha-1-type sodium pump, expressed throughout the nephron. In as much as renal calcium retention is largely dependent on efficient activity of calcium-sodium exchangers situated in the basolateral membranes of tubular epithelium, it is evident that an increased intracellular sodium concentration consequent to sodium pump inhibition could blunt the activity of these exchangers. Thus, it is postulated that MBG mediates the impact of salt loading on renal calcium retention.

Marinobufagenin may mediate the impact of salty diets on left ventricular hypertrophy by disrupting the protective function of coronary microvascular endothelium

Individuals who eat salty diets and who are "salt-sensitive" tend to have increased left ventricular mass, independent of blood pressure; this phenomenon awaits an explanation. It is clear that local up-regulation of angiotensin II (AngII) production and activity play a key role in the induction of left ventricular hypertrophy (LVH). Recent evidence suggests that a healthy coronary microvascular endothelium opposes this effect by serving as a paracrine source of nitric oxide (NO), a natural antagonist of AngII activity, and that up-regulation of this mechanism can account for the protective role of bradykinin with respect to LVH. The coronary microvasculature also possesses NAD(P)H oxidase activity that can generate superoxide, inimical to the bioactivity of endothelial NO. There is now good reason to believe that the triterpenoid marinobufagenin (MBG), a selective inhibitor of the alpha-1 isoform of the sodium pump, mediates the impact of salty diets on blood pressure; production of MBG by the adrenal cortex is boosted when salt-sensitive animals are fed salty diets. It is hypothesized that coronary microvascular endothelium expresses the alpha-1 isoform of the sodium pump, and that MBG thus can target this endothelium. If that is the case, MBG would be expected to decrease membrane potential in these cells; as a consequence, superoxide production would be up-regulated, NO synthase activity would be down-regulated, and myocardial NO bioactivity would thus be suppressed. This would offer a satisfying explanation for the impact of salt and salt-sensitivity on risk for LVH. If expression of the alpha-1 isoform of the sodium pump is a more general property of vascular endothelium, MBG may suppress NO bioactivity in other regions of the vascular tree, thereby contributing to other adverse effects elicited by salty diets: reduced arterial compliance, medial hypertrophy, impaired endothelium-dependent vasodilation, hypertensive/diabetic glomerulopathy, increased risk for stroke, and hypertension.

Cardiotonic steroids differentially affect intracellular Na+ and [Na+]i/[K+]i-independent signaling in C7-MDCK cells

Recently, we reported that ouabain kills renal epithelial and vascular endothelial cells independently of elevation of the [Na(+)](i)/[K(+)](i) ratio. These observations raised the possibility of finding cardiotonic steroids (CTS) that inhibit the Na(+),K(+) pump without attenuating cell survival and vice versa. To test this hypothesis, we compared CTS action on Na(+),K(+) pump, [Na(+)](i) content, and survival of Madin-Darby canine kidney cells. At a concentration of 1 microM, ouabain and other tested cardenolides, as well as bufadienolides such as bufalin, cinobufagin, cinobufotalin, and telobufotoxin, led to approximately 10-fold inhibition of the Na(+),K(+) pump, a 2-3-fold decrease in staining with dimethylthiazol-diphenyltetrazolium (MTT), and massive death indicated by detachment of approximately 80% of cells and caspase-3 activation. In contrast, Na(+),K(+) pump inhibition and elevation of [Na(+)](i) seen in the presence of 3 microM marinobufagenin (MBG) and marinobufotoxin did not affect MTT staining and cell survival. Inhibition of the Na(+),Rb(+) pump in K(+)-free medium was not accompanied by a decline of MTT staining and cell detachment but increased sensitivity to CTS. In K(+)-free medium, half-maximal inhibition of (86)Rb influx was observed in the presence of 0.04 microM ouabain and 0.1 microM MBG, whereas half-maximal detachment and decline of MTT staining were detected at 0.03 and 0.004 microM of ouabain versus 10 and 3 microM of MBG, respectively. Both ouabain binding and ouabain-induced [Na(+)](i),[K(+)](i)-independent signaling were suppressed in the presence of MBG. Thus, our results show that CTS exhibit distinctly different potency in Na(+),K(+) pump inhibition and triggering of [Na(+)](i)/[K(+)](i)-independent signaling, including cell death.

Marinobufagenin may mediate the impact of salty diets on left ventricular hypertrophy by disrupting the protective function of coronary microvascular endothelium

Individuals who eat salty diets and who are "salt-sensitive" tend to have increased left ventricular mass, independent of blood pressure; this phenomenon awaits an explanation. It is clear that local up-regulation of angiotensin II (AngII) production and activity play a key role in the induction of left ventricular hypertrophy (LVH). Recent evidence suggests that a healthy coronary microvascular endothelium opposes this effect by serving as a paracrine source of nitric oxide (NO), a natural antagonist of AngII activity, and that up-regulation of this mechanism can account for the protective role of bradykinin with respect to LVH. The coronary microvasculature also possesses NAD(P)H oxidase activity that can generate superoxide, inimical to the bioactivity of endothelial NO. There is now good reason to believe that the triterpenoid marinobufagenin (MBG), a selective inhibitor of the alpha-1 isoform of the sodium pump, mediates the impact of salty diets on blood pressure;production of MBG by the adrenal cortex is boosted when salt-sensitive animals are fed salty diets. It is hypothesized that coronary microvascular endothelium expresses the alpha-1 isoform of the sodium pump, and that MBG thus can target this endothelium. If that is the case, MBG would be expected to decrease membrane potential in these cells;as a consequence, superoxide production would be up-regulated, NO synthase activity would be down-regulated, and myocardial NO bioactivity would thus be suppressed. This would offer a satisfying explanation for the impact of salt and salt-sensitivity on risk for LVH. If expression of the alpha-1 isoform of the sodium pump is a more general property of vascular endothelium, MBG may suppress NO bioactivity in other regions of the vascular tree, thereby contributing to other adverse effects elicited by salty diets: reduced arterial compliance, medial hypertrophy, impaired endothelium-dependent vasodilation, hypertensive/diabetic glomerulopathy, increased risk for stroke, and hypertension.

Ouabain- and Marinobufagenin-Induced Proliferation of Human Umbilical Vein Smooth Muscle Cells and a Rat Vascular Smooth Muscle Cell Line, A7r5

Background— We studied the growth-promoting effects of 2 sodium pump–selective cardiotonic steroids, ouabain and marinobufagenin, on cultured cells from vascular smooth muscle (VSMCs) from human umbilical vein and a rat VSMC line, A7r5.

Methods and Results— Both ouabain and marinobufagenin activated proliferation of these cells in a concentration-dependent manner, reflecting the cardiotonic steroid sensitivity of the specific α 1 subunit contained within each cell source. The observed effective concentration ranges of both compounds was below that necessary to induce cytoplasmic ion alterations by sodium pump inhibition.

Conclusions— These data indicate that the ouabain-activated proliferative effect previously observed in canine VSMCs occurs in other VSMC sources. This growth effect seems to be initiated by drug interaction with the sodium pump, reflected by the affinity of the steroid for the pump, and is independent of altered transmembrane ionic gradients.

Phorbol diacetate potentiates na(+)-k(+) ATPase inhibition by a putative endogenous ligand, marinobufagenin

Several vasoconstrictor agents can regulate the phosphorylation status of the Na(+)-K(+) ATPase (NKA). We have recently demonstrated that mammalian tissues contain an endogenous bufadienolide, digitalis-like alpha(1)-NKA-selective ligand, marinobufagenin (MBG). Protein kinase C induces phosphorylation of the alpha(1)-NKA isoform, the major isoform in vascular smooth muscle, kidney, and heart cells. We hypothesized that protein kinase C-induced phosphorylation of NKA can potentiate the effect of endogenous digitalis-like ligands, and that such potentiation can occur in an NKA isoform-specific fashion. A protein kinase C activator, phorbol 12,13-diacetate (PDA, 50 nmol/L), induced phosphorylation of the alpha1-NKA from human mesenteric artery (HMA) sarcolemma and rat kidney but not that of the alpha(3)-NKA from rat fetal brain. In HMA sarcolemma, which predominantly contains alpha(1)-NKA, PDA (50 nmol/L) potentiated the NKA-inhibitory effect of MBG at the level of high-affinity binding sites (0.05 +/- 0.03 nmol/L versus 4.0 +/- 1.7 nmol/L, P<0.05). In contrast, PDA did not affect the NKA inhibition by ouabain, an alpha(3)-NKA ligand. In isolated endothelium-denuded HMA artery rings, 50 nmol/L PDA potentiated the MBG-induced vasoconstriction (EC(50), 17 +/- 6 nmol/L versus 150 +/- 40 nmol/L; P<0.01). Our results suggest that alpha(1)-isoform-specific NKA inhibition by the endogenous digitalis-like ligand, MBG, is substantially enhanced via NKA phosphorylation by protein kinase C. Thus, an interaction of protein kinase C-dependent phosphorylation and MBG on NKA activity may underlie the synergistic vasoactive effects of MBG and other endogenous vasoconstrictors in hypertension.

Interaction of NaCl and behavioral stress on endogenous sodium pump ligands in rats

Our study investigated the hypothesis that the combination of a high NaCl diet and social isolation stress would increase systolic blood pressure (SBP) and endogenous sodium pump ligands (SPL), ouabainlike compound (OLC), and marinobufagenin (MBG). Excretion of MBG and OLC, SBP, and organ weights were studied in four groups (n = 8) of male Fisher 344 x Norwegian brown rats: controls, socially isolated (Iso), 4% NaCl diet (Salt), and the combination of Salt and Iso (Iso+Salt). In Salt, MBG excretion increased by 78% (P < 0.01), whereas SBP and OLC remained unchanged. In Iso, SBP and MBG did not change, but OLC peaked on day 1. In the Iso+Salt, SBP increased by 9 mmHg, MBG excretion increased (42.0 +/- 7.6 vs. 10.0 +/- 1.5 pmol/24 h, P < 0.01), whereas OLC peaked at day 1 (25.0 +/- 2.5 vs. 10.0 +/- 2.0 pmol/24 h, P < 0.01) and remained elevated. Heart and kidney weights were increased in Salt and Iso+Salt. Aortic weights were increased in Iso and Iso+Salt. Thus a high NaCl intake stimulates MBG excretion, whereas isolation stress stimulates OLC. The combination of Salt and Iso is accompanied by marked stimulation of both SPL.

Acute pressor actions of ouabain do not enhance the actions of phenylephrine or norepinephrine in anesthetized rats

The inhibition of high-affinity isoforms of the Na+,K+-ATPase by nanomolar levels of ouabain has been proposed to enhance the actions of vasoconstrictor agents that act via a Ca+2-dependent mechanism. The present study tested this hypothesis by evaluating the effects of ouabain (6 and 18 microg/kg, i.v.) on the vasopressor actions of phenylephrine and norepinephrine in anesthetized, reflex-blocked rats. In separate groups of animals, dose-response curves for increases in diastolic pressure produced by phenylephrine were generated after the administration of saline (control), ouabain (18 microg/kg), L-omega-N-nitro arginine methyl ester (L-NAME, 3 micromol/kg) and angiotensin II (15 ng/kg per min). Treatment with ouabain (18 microg/kg) produced an increase in diastolic pressure of 19+/-3 mm Hg but did not significantly alter the potency or maximal response produced by phenylephrine. In contrast, treatment with angiotensin II and L-NAME, agents known to enhance the actions of alpha-adrenoceptor agonists, increased the potency of phenylephrine. In animals in which the pressor actions of norepinephrine were evaluated before and after the administration of ouabain (6 microg/kg), ouabain did not alter the pressor response to norepinephrine. Blockade of alpha-adrenoceptors with phentolamine was found to attenuate as well as partially reverse the increase in diastolic pressure produced by ouabain. These observations suggest that ouabain produces a pressor response by actions on sympathetic nerve endings as well as on vascular smooth muscle and that these actions do not alter the sensitivity to phenylephrine or norepinephrine.