Identification of a ouabain-like compound in toad skin and plasma as a bufodienolide derivative

Bufadienolide-Fatty Acid Conjugates from the Fertilized Eggs of Toad Bufo gargarizans: Isolation, Characterization, Toxicity, and Antiproliferative Evaluation

Bufadienolides (BDs) are a class of naturally occurring toxins present in amphibian toads. Serving as the chemical weapons, they exist not only in the adult toads but also in toad eggs. Guided by mass spectrometry (MS)-based component analysis and feature-based molecular networking (FBMN), 30 bufadienolide-fatty acid conjugates (BDFs) were isolated from the fertilized eggs of toad Bufo gargrizans, including 25 previously undescribed compounds (1-25). Their chemical structures were elucidated by extensive spectroscopic analysis, chemical methods, and GC-MS. The toxicities of all BDFs and their corresponding free BDs were assessed using the zebrafish model. The structure-toxicity relationship analysis showed that the modification of BDs by hydroxy fatty acids can cause a significant increase of the toxicity. Furthermore, all the isolated compounds were evaluated for their antiproliferative activities in pancreatic cancer cell lines ASPC-1 and PANC10.05. The structure-activity relationship (SAR) analysis revealed that BDFs with hellebrigenin as the bufogenin moiety (6 and 7) exhibited the most potent antiproliferative effect. Further investigation into their functional mechanism demonstrated that 6 and 7 induced apoptosis in pancreatic cancer cells PANC10.05 and significantly suppressed the expression of the apoptosis-related gene c-MYC. In addition, 6 and 7 effectively inhibited the expression of the PI3K/Akt/mTOR pathway in PANC10.05. Moreover, we assessed the efficacy of 6 and 7 on cancer cells from various tissues and observed their broad-spectrum antiproliferative activity.

3α-Hydroxybufadienolides in Bufo gallbladders: structural insights and biotransformation

Bufadienolides, naturally occurring steroids primarily found in toads, have garnered attention for their pharmacological properties and ecological significance. In this study, we isolated and identified 21 bufadienolides from the gallbladders of Bufo gargarizans, comprising four new compounds and 17 known ones. Notably, the predominance of 15 bufadienolides with a 3α-OH configuration in toad bile differs significantly from the 3β-OH bufadienolides found in venom secreted by toad glands. Moreover, our investigation into the biotransformation of 3β-OH and 3α-OH bufadienolides in the liver and kidney tissues of toads revealed an irreversible conversion from 3β-OH to 3α-OH bufadienolides, suggesting a crucial role in toad self-detoxification. These findings provide valuable insights into the structural diversity of bufadienolides and advance our understanding of their medical and ecological significance.

Transcriptomic analysis reveals potential candidate pathways and genes involved in toxin biosynthesis in true toads

Synthesized chemical defenses have broadly evolved across countless taxa and are important in shaping evolutionary and ecological interactions within ecosystems. However, the underlying genomic mechanisms by which these organisms synthesize and utilize their toxins are relatively unknown. Herein, we use comparative transcriptomics to uncover potential toxin synthesizing genes and pathways, as well as interspecific patterns of toxin synthesizing genes across ten species of North American true toads (Bufonidae). Upon assembly and annotation of the ten transcriptomes, we explored patterns of relative gene expression and possible protein-protein interactions across the species to determine what genes and/or pathways may be responsible for toxin synthesis. We also tested our transcriptome dataset for signatures of positive selection to reveal how selection may be acting upon potential toxin producing genes. We assembled high quality transcriptomes of the bufonid parotoid gland, a tissue not often investigated in other bufonid related RNAseq studies. We found several genes involved in metabolic and biosynthetic pathways (e.g. steroid biosynthesis, terpenoid backbone biosynthesis, isoquinoline biosynthesis, glucosinolate biosynthesis) that were functionally enriched and/or relatively expressed across the ten focal species that may be involved in the synthesis of alkaloid and steroid toxins, as well as other small metabolic compounds that cause distastefulness in bufonids. We hope that our study lays a foundation for future studies to explore the genomic underpinnings and specific pathways of toxin synthesis in toads, as well as at the macroevolutionary scale across numerous taxa that produce their own defensive toxins.

Bufadienolides from the Skin Secretions of the Neotropical Toad Rhinella alata (Anura: Bufonidae): Antiprotozoal Activity against Trypanosoma cruzi

Toads in the family Bufonidae contain bufadienolides in their venom, which are characterized by their chemical diversity and high pharmacological potential. American trypanosomiasis is a neglected disease that affects an estimated 8 million people in tropical and subtropical countries. In this research, we investigated the chemical composition and antitrypanosomal activity of toad venom from Rhinella alata collected in Panama. Structural determination using mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy led to the identification of 10 bufadienolides. Compounds identified include the following: 16β-hydroxy-desacetyl-bufotalin-3-adipoyl-arginine ester (1), bufotalin (2), 16β-hydroxy-desacetyl-bufotalin-3-pimeloyl-arginine ester (3), bufotalin-3-pimeloyl-arginine ester (4), 16β-hydroxy-desacetyl-bufotalin-3-suberoyl-arginine ester (5), bufotalin-3-suberoyl-arginine ester (6), cinobufagin-3-adipoyl-arginine ester (7), cinobufagin-3-pimeloyl-arginine ester (8), cinobufagin-3-suberoyl-arginine ester (9), and cinobufagin (10). Among these, three new natural products, 1, 3, and 5, are described, and compounds 1-10 are reported for the first time in R. alata. The antitrypanosomal activity assessed in this study revealed that the presence of an arginyl-diacid attached to C-3, and a hydroxyl group at C-14 in the structure of bufadienolides that is important for their biological activity. Bufadienolides showed cytotoxic activity against epithelial kidney Vero cells; however, bufagins (2 and 10) displayed low mammalian cytotoxicity. Compounds 2 and 10 showed activity against the cancer cell lines MCF-7, NCI-H460, and SF-268.

Cardiac Glycosides in Human Physiology and Disease: Update for Entomologists

Cardiac glycosides, cardenolides and bufadienolides, are elaborated by several plant or animal species to prevent grazing or predation. Entomologists have characterized several insect species that have evolved the ability to sequester these glycosides in their tissues to reduce their palatability and, thus, reduce predation. Cardiac glycosides are known to interact with the sodium- and potassium-activated adenosine triphosphatase, or sodium pump, through a specific receptor-binding site. Over the last couple of decades, and since entomologic studies, it has become clear that mammals synthesize endogenous cardenolides that closely resemble or are identical to compounds of plant origin and those sequestered by insects. The most important of these are ouabain-like compounds. These compounds are essential for the regulation of normal ionic physiology in mammals. Importantly, at physiologic picomolar or nanomolar concentrations, endogenous ouabain, a cardenolide, stimulates the sodium pump, activates second messengers, and may even function as a growth factor. This is in contrast to the pharmacologic or toxic micromolar or milimolar concentrations achieved after consumption of exogenous cardenolides (by consuming medications, plants, or insects), which inhibit the pump and result in either a desired medical outcome, or the toxic consequence of sodium pump inhibition.

Toxins and pharmacologically active compounds from species of the family Bufonidae (Amphibia, Anura)

ETHNOPHARMACOLOGICAL RELEVANCE

Among amphibians, 15 of the 47 species reported to be used in traditional medicines belong to the family Bufonidae, which demonstrates their potential in pharmacological and natural products research. For example, Asian and American tribes use the skin and the parotoid gland secretions of some common toads in the treatment of hemorrhages, bites and stings from venomous animals, skin and stomach disorders, as well as several types of cancers.

OVERARCHING OBJECTIVE

In addition to reviewing the occurrence of chemical constituents present in the family Bufonidae, the cytotoxic and biomedical potential of the active compounds produced by different taxa are presented.

METHODOLOGY

Available information on bioactive compounds isolated from species of the family Bufonidae was obtained from ACS Publications, Google, Google Scholar, Pubmed, Sciendirect and Springer. Papers written in Chinese, English, German and Spanish were considered.

RESULTS

Recent reports show more than 30% of amphibians are in decline and some of bufonid species are considered to be extinct. For centuries, bufonids have been used as traditional folk remedies to treat allergies, inflammation, cancer, infections and other ailments, highlighting their importance as a prolific source for novel drugs and therapies. Toxins and bioactive chemical constituents from skin and parotid gland secretions of bufonid species can be grouped in five families, the guanidine alkaloids isolated and characterized from Atelopus, the lipophilic alkaloids isolated from Melanophryniscus, the indole alkaloids and bufadienolides known to be synthesized by species of bufonids, and peptides and proteins isolated from the skin and gastrointestinal extracts of some common toads. Overall, the bioactive secretions of this family of anurans may have antimicrobial, protease inhibitor and anticancer properties, as well as being active at the neuromuscular level.

CONCLUSION

In this article, the traditional uses, toxicity and pharmacological potential of chemical compounds from bufonids have been summarized. In spite of being reported to be used to treat several diseases, neither extracts nor metabolites from bufonids have been tested in such illness like acne, osteoporosis, arthritis and other illnesses. However, the cytotoxicity of these metabolites needs to be evaluated on adequate animal models due to the limited conditions of in vitro assays. Novel qualitative and quantitative tools based on MS spectrometry and Nuclear Magnetic Resonance spectroscopy is now available to study the complex secretions of bufonids.

Structures and inhibitory activity against breast cancer cells of new bufadienolides from the eggs of toad Bufo bufo gargarizans

New bufadienolides with inhibitory activity against breast cancer cells were isolated from the eggs of toad Bufo bufo gargarizans.

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.

Changes in plasma concentrations of immunoreactive ouabain in the tilapia in response to changing salinity: is ouabain a hormone in fish?

Ouabain, a cardiac glycoside and inhibitor of Na(+), K(+)-ATPase, is now believed to be a steroid hormone in mammals, involved in blood pressure and volume regulation and possibly acting as a natriuretic hormone. We have identified ouabain-like immunoreactivity in the plasma and tissues of a euryhaline teleost, the tilapia (Oreochromis mossambicus), by means of solid-phase extraction followed by a specific radioimmunoassay. Plasma concentrations of immunoreactive ouabain were 5-20pg/ml. Ouabain immunoreactivity was detected in all the tissues examined, with highest concentrations in the head kidney followed by intestine and body kidney. When the fish in fresh water were transferred to seawater, plasma osmolality increased significantly after 2, 4, 8, and 24h. Significant increases were observed in plasma ouabain immunoreactivity after 4 and 24h, and a significant correlation was seen between ouabain immunoreactivity and plasma osmolality. There was also a significant correlation between the plasma osmolality and cortisol concentrations. Upon transfer from seawater to fresh water, significant increases were seen in plasma cortisol after 4 and 8h and in immunoreactive ouabain after 4h. When the correlation was analyzed using all the data obtained during the two transfer experiments, plasma ouabain immunoreactivity and cortisol were significantly correlated with plasma osmolality, whereas there was a significant negative correlation between plasma prolactin and osmolality. A significant positive correlation was also seen between plasma cortisol and ouabain immunoreactivity. These results suggest that immunoreactive ouabain may be involved, together with cortisol, in the maintenance of hydromineral balance in the tilapia.

Effects of resibufogenin from toad venom on isolated Purkinje fibers

Chan su (dried toad venom) is a traditional Chinese medicine obtained from the skin venom gland of the toad. Chan su has long been used as a therapeutic agent in China and other Asian countries. Recent reports indicate that Chan su toxicity carries a high mortality rate in the United States. This study focused upon the cardiac electrophysiological and electro-toxicity effects of resibufogenin (RBG), one of the major components in Chan su. Action potentials of isolated sheep and canine heart Purkinje fibers were studied using glass microelectrode recording techniques. The results indicate that RBG significantly affected all parameters of transmembrane action potential, induced delayed after depolarization, and triggered arrhythmias in sheep and canine Purkinje fibers.

Isolation and characterization of novel endogenous digitalis-like factors in the ovary of the giant toad, Bufo marinus

We have previously described the structures of four novel unconjugated bufadienolides in the ovary of the toad, Bufo marinus. In this study, we report the separation and characterization of three novel bufadienolide conjugates. These compounds were purified by HPLC, and their structures were determined to be 11alpha, 19-dihydroxytelocinobufagin-3-(12-hydroxydodecanoic acid) ester, 11alpha,19-dihydroxytelocinobufagin-3-(14-hydroxy-7-tetra decenoic acid) ester, and 11alpha, 19-dihydroxytelocinobufagin-3-(14-hydroxytetradecanoic acid) ester on the basis of NMR and MS data. Numerous dicarboxylic acid esters of bufadienolides have previously been described, but the three bufadienolide conjugates described in this report differ from previously described esters in that they contain hydroxylated monocarboxylic acids. The function of these three conjugates is not known but they are, like bufotoxins, potent inhibitors of Na+, K+-ATPase and may play a developmental role in the differentiation of toad oocytes.

Effects of Bufo marinus skin toxins on human fetal extracorporeal blood vessels

The effects of extracts of Bufo marinus toad skin toxin on human isolated umbilical arterial rings and the fetal vessels of perfused placentae were examined and compared with those of ouabain, an inhibitor of Na+/K(+)-ATPase. Umbilical artery rings and fetal vessels of the perfused placenta responded to extracts, or ouabain, with constriction which persisted after the removal of each agent. Extraction of the skin, using various solvents, revealed that the umbilical artery constriction was due mainly to the effects of water-soluble, polar compounds. Fractionation of a water extract and bioassay on the rat isolated aorta revealed maximum vasoconstrictor activity in a low mol. wt fraction. During Na+/K(+)-ATPase inactivation in the fetal circulation of the human placenta, by perfusion with K(+)-free Kreb's solution, reactivation of the enzyme by K+ infusion caused vasodilatation. This effect was inhibited both by water extracts of load skin and by ouabain. Thus, properties of some of the endogenous compounds in B. marinus skin resemble those of ouabain, by causing persistent constriction of human fetal blood vessels. A component of the vasoconstrictor response probably results from inhibition of vascular smooth muscle Na+/K(+)-ATPase, but it is likely that a contribution is also made by additional vasoconstrictor substances contained in B. marinus toxin.

Bufo toads and bufotenine: fact and fiction surrounding an alleged psychedelic

This paper investigates the supposedly psychedelic Bufo toad and the allegedly psychedelic drug bufotenine, which is contained in the skin and glands of this toad. The bufo toad has held a place in human mythologies and medicines worldwide since archaic times. Used by ancient peoples for a variety of purposes, its most spectacular effects, according to lore, involve magical and shamanic or occult uses for casting spells and for divination. In the Middle Ages, the Bufo toad was celebrated as a panacea and persecuted as a powerful poison. More recently, in the 1960s the Bufo toad was resurrected as a countercultural icon, with people purportedly licking or smoking the secretions to get high. Bufotenine has been at the center of a scientific debate since its discovery in 1893. This paper examines the extensive literature surrounding the Bufo toad and bufotenine, and untangles many of the myths and the misinformation that continue to vex both science and popular reporting. Finally, to promote further investigation, a comprehensive bibliography is provided that charts the history of the Bufo toad and bufotenine.

A cardiotonic steroid bufalin-like factor in human plasma induces leukemia cell differentiation

A Na+,K(+)-ATPase inhibitor, bufalin, has been shown previously to induce leukemia cell differentiation. The presence of a circulating Na+,K(+)-ATPase inhibitor has been proposed in mammals. The aim of this study was to explore an endogenous bufalin-like factor that induces leukemia cell differentiation. We found a fraction, designated as fraction A, obtained from human plasma extract that inhibits the growth of several human-derived leukemia cell lines. The effect of the fraction was retained after protease digestion or heat treatment. Murine leukemia cells and ouabain-resistant cells, which are insensitive to bufalin, appeared to be refractory to fraction A in terms of growth inhibition. Fraction A also induced functional and morphological maturation in THP-1 cells. Fraction A was recognized by anti-bufalin anti-serum and inhibited 3H-bufalin binding to K562 cells. These findings suggest that fraction A shows a similar behavior to that of bufalin on leukemia cells by inhibiting Na+,K(+)-ATPase. We propose that an endogenous Na+,K(+)-ATPase inhibitor in human plasma may play a role in cell differentiation.

Endogenous natriuretic factors 3: isolation and characterization of human natriuretic factors LLU-alpha, LLU-beta 1, and LLU-gamma

A low molecular weight endogenous substance believed to be responsible for extracellular fluid homeostasis in mammals has been sought for many years. Our goal is to isolate and structurally characterize this putative "natriuretic hormone." We have developed an assay using the conscious rat to measure prolonged natriuresis (Benaksas et al (1993) Life Sciences, 52, 1045-1054), the activity originally described for this putative substance. Using this assay we have identified a number of natriuretic compounds isolated from human uremic urine. The collected urine is processed by ultrafiltration (< or = 3 kDa), gel filtration chromatography (G-25) and extraction with isopropanol and diethyl ether. The organic soluble material is then subjected to sequential high-performance liquid chromatography. We report here the initial characterization of two pure isolates (LLU-alpha and LLU-gamma) obtained by this method, and the structural elucidation of a third pure compound, LLU-beta 1, a natriuretic and previously unreported metabolite of the drug diltiazem.

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

Previously, we reported that the venom of Bufo marinus toad contains a Na+,K(+)-ATPase inhibitor with potent vasoconstrictor activity. In the present study, using thin-layer chromatography in Silicagel 60 F254 + 366, we separated a vasoactive substance from a mixture of steroids from Bufo marinus venom. Based on chromatographic mobility of this substance and typical color reaction after its vizualization with SbCl3, we identified it as a previously described steroid, marinobufagenin. Vasoconstrictor and Na+,K+ pump inhibitory properties of marinobufagenin were studied in isolated rat aortic rings and compared with those of ouabain. Ouabain (10-100 mumol.1-1) produced weak vasoconstriction, which was blocked by 2 mumol.1-1 phentolamine. 10 mumol.1-1 ouabain stimulated, and at higher concentrations inhibited, the Na+,K+ pump. 2 mumol.1-1 phentolamine abolished the activating effect of 10 mumol.1-1 ouabain on the Na+,K+ pump, but did not alter the inhibitory action of higher concentrations of ouabain. By contrast, marunibufagenin elicited rapid and strong vasoconstriction and inhibited ouabain-sensitive 86Rb uptake. Antidigoxin antibody antagonized the vasoconstrictor responses to marinobufagenin, but not to ouabain. 2 mumol.1-1 phentolamine did not alter the constrictor effect of marinobufagenin. In solid-phase digoxin immunoassay, marinobufagenin demonstrated higher digoxin-like immunoreactivity than ouabain.

Chronic blood pressure effects of bufalin, a sodium-potassium ATPase inhibitor, in rats

Endogenous Na+,K(+)-ATPase inhibitors may have a role in the mechanism of low-renin hypertension. Two such compounds have been characterized: ouabain from human plasma and resibufogenin from toad plasma. Previously, we examined the acute effects of ouabain and bufalin (which has the same structure as resibufogenin except for one H+) in normal rats. Bufalin raised blood pressure, but ouabain had little effect. In contrast, given chronically, ouabain substantially increased blood pressure in normal rats and 70% reduced renal mass rats on a salt-free diet. We have now examined the chronic effects of bufalin in rats. Normal rats received 14.8 micrograms/kg per day bufalin or an equimolar dose of ouabain intraperitoneally for 6 weeks; 70% reduced renal mass rats also received 14.8 micrograms/kg per day bufalin. Another group of normal rats received 29.6 micrograms/kg per day bufalin intraperitoneally for 6 weeks. Respective control animals received vehicle. In contrast to ouabain, blood pressure did not increase in normal rats receiving the 14.8 micrograms dose of bufalin. However, normal rats receiving 29.6 micrograms bufalin and 70% reduced renal mass rats receiving 14.8 micrograms bufalin developed significant increases in blood pressure. Increases in blood pressure were associated with decreases in myocardial Na+,K(+)-ATPase activity and correlated with increased plasma Na+,K(+)-ATPase inhibitory activity. Thus, although bufalin is a more potent pressor agent than ouabain when both agents are given acutely, ouabain is at least as potent a vasopressor agent as bufalin when given chronically. Thus, both are pressor agents, more so in the presence of reduced renal mass, when given chronically in the rat.

Identification of digitalis-like compounds in human cataractous lenses

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

Long-term ouabain administration produces hypertension in rats

Ouabain has recently been identified as an endogenous Na(+)-K+ pump inhibitor. We administered ouabain chronically to normotensive rats with varying degrees of reduced renal mass (RRM) and to normal two-kidney rats to see whether hypertension could be produced. Normal male Wistar rats and rats with 25%, 60%, and 70% RRM received ouabain (13.9 micrograms/kg per day IP) in normal saline for 4 weeks followed by ouabain (27.8 micrograms/kg per day IP) for 3 to 4 more weeks. Respective control animals received vehicle only. Blood pressure was recorded weekly by tail plethysmography. Animals received tap water and standard rat chow, except for 70% RRM rats, which received distilled water and sodium-free chow. After 6 to 8 weeks of treatment, with rats under thiobutabarbital anesthesia, direct blood pressure was determined. Plasma, tissue, and urinary ouabain levels were measured with a specific radioimmunoassay. Animals receiving ouabain developed significant increases in mean blood pressure compared with control animals (70% RRM, 147 +/- 4 vs 116 +/- 4 mm Hg; 60% RRM, 140 +/- 4 vs 107 +/- 3 mm Hg; 25% RRM, 131 +/- 5 vs 100 +/- 2 mm Hg; no RRM, 116 +/- 4 vs 98 +/- 5 mm Hg). Plasma ouabain levels measured 24 hours after the last ouabain dose were not different in animals receiving ouabain vs those receiving vehicle. However, kidney tissue ouabain levels were significantly greater (6.39 +/- 1.17 vs 2.36 +/- 0.52 micrograms/kg, P < .05) in animals receiving ouabain. In conclusion, ouabain, given chronically, is associated with the development of hypertension in RRM rats as well as in normal rats. Blood pressure was greater in animals with greater degrees of RRM for a given ouabain dose.

Digitalis-like and vasoconstrictor effects of endogenous digoxin-like factor(s) from the venom of Bufo marinus toad

Digitalis glycoside-like properties of the Bufo marinus toad crude venom and one of its constituents, bufalin, were studied in various assay systems. In concentrations 0.3-30 micrograms/ml crude venom increased the contractility of isolated electrically driven rat atria, constricted rat aortic rings, inhibited ouabain-sensitive Na+,K(+)-ATPase in rat erythrocytes and the Na+,K(+)-pump in rat aorta, and cross-reacted with antidigoxin antibody from the dissociation enhanced lanthanide fluoroimmunoassay (DELFIA). These effects were unaffected by adrenoceptor blockers and the 5-HT antagonist, deseril, but were blocked by antidigoxin antibody. Bufalin (10-30 microM) increased myocardial contractility and inhibited Na+,K(+)-ATPase in rat erythrocytes similarly to crude Bufo marinus venom. In rat aorta bufalin showed weak and delayed vasoconstrictor activity which was antagonized by 2 microM phentolamine, and had a biphasic effect on the Na+,K(+)-pump; 0.5-1.0 microM bufalin stimulated the pump, while higher concentrations inhibited its activity. Although the effects of bufalin were blocked by antidigoxin antibody, bufalin showed very low digoxin-like immunoreactivity in the DELFIA. These observations suggest that, in addition to bufalin, Bufo marinus venom contains at least one more digitalis-like steroid with significant intrinsic vasoconstrictor activity which, unlike bufalin, constricts the blood vessels acting directly via inhibition of the sodium pump in the vascular smooth muscle membrane.

Effects of direct renal arterial infusion of bufalin and ouabain in conscious sheep

1. The presence of an endogenous digitalis-like factor (EDLF) in the plasma of both normal and volume expanded animals is well documented. In this study we have used ouabain and bufalin as pharmacological analogues to mimic the renal effects of EDLF and to investigate whether any interaction occurs between atrial natriuretic factor (ANF) and EDLF. 2. Conscious Na replete sheep with chronically indwelling catheters in the renal artery received renal arterial infusion of ouabain (1000 micrograms h-1) or bufalin (500 micrograms h-1) for 60 min. Renal arterial infusion of bufalin increased sodium excretion (UNaV) from 120 +/- 13 to 596 +/- 161 mumol min-1 after 45 min. Bufalin infusion did not alter glomerular filtration rate (GFR), effective renal plasma flow (ERPF), or lithium clearance. Ouabain infusion increased UNaV from 124 +/- 57 to 764 +/- 123 mumol min-1 in the first hour after infusion. 3. ANF infusion increased UNaV from 159 +/- 34 to 583 +/- 134 mumol min-1. When renal arterial bufalin infusion was followed by renal arterial ANF infusion (50 micrograms h-1) UNaV was increased from 155 +/- 31 to 795 +/- 96 mumol min-1. This increase in UNaV is approximately equal to the sum of the separate effects of bufalin and ANF. 4. The natriuretic effects of ouabain at pharmacological doses in sheep are confirmed by this study. The data presented here do not support the hypothesis that EDLF sensitizes the kidney to the natriuretic effects of ANF.

Digitalis-like compounds in animal tissues

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

Digitalis toxicity caused by toad venom

A case of toad venom-induced digitalis toxicity is presented. A pause of 13.5 s was noted in the patient taking a Chinese medication which contained toad venom. This is the first case report of clinical digitalis toxicity related to toad venom in Western society.

Digitalis-like compounds in the toad Bufo viridis: tissue and plasma levels and significance in osmotic stress

Digitalis-like compounds (DLC), constituents of animal tissues, are possible regulators of the Na+, K(+)-ATPase implicated in water and salt homeostasis. The distribution of DLC in the toad (Bufo viridis) was determined following methanol extraction and partial purification. DLC highest levels were found in the skin but it was also detected in the plasma and many internal organs. Short term (hours) exposure of the toad to hypertonic shock (1.5% NaCl) induced an increase in plasma osmolarity due to an increase in Na+ and Cl- levels. This treatment induced a transient, three fold, increase of DLC levels in the brain and transient reduction of its levels in the ventral skin. Acclimation of the toads to burrowing conditions for six weeks resulted in an increase in plasma osmolarity due to a large increase in plasma urea with a small increase in ion concentrations. Under these conditions DLC levels in the dorsal skin increased by 100% without alteration of its levels in the plasma, brain and ventral skin. DLC levels in the toad brain of control animals, showed a significant dependence on season, being highest in the summer and lowest in the winter. DLC levels in the skin peaked in May while the levels in the plasma were season independent. The changes in DLC levels induced by the short- as well as long-term perturbations in the animal environmental salinity together with the seasonal differences suggest that DLC in the toad is involved in water and salt homeostasis of these animals, but may also participate in other unknown functions.

Endogenous digitalis-like factors

The postulate of a natriuretic factor inhibiting the sodium pump in the kidney led to the detection of increased concentrations of endogenous digitalis-like factors in blood after salt loading, in essential hypertension, in pregnancy-induced hypertension and in chronic hypervolaemia. The recent isolation of ouabain or a close isomer thereof from human plasma and the demonstration of a compound similar if not identical to digoxin in adrenals and human urine shows that mammals like non-vertebrates and toads may synthesize cardiac glycosides in their adrenals and possibly in hypothalamus. The hypothalamus also forms other compounds of unknown structure which bind to the cardiac glycoside receptor site. The differential functions of endogenously formed ouabain and of a digoxin-like substance are unclear. The detailed knowledge of the physiological role of both endogenously formed cardiac glycosides in the regulation of blood pressure has still to be worked out.

Effects of bufalin on norepinephrine turnover in canine saphenous vein

Abundant experimental data suggest that an endogenous digitalislike factor is responsible for some essential hypertension. Some forms of hypertension have also been associated with increased levels of catecholamines. We therefore designed experiments to investigate the role of digitalislike factors in the regulation of norepinephrine turnover in the neurovascular junction. We chose bufalin, an amphibian-derived compound that shares many of the physiological properties postulated as characteristic of digitalislike compounds, as a model of the mammalian compound. In vitro experiments in canine saphenous veins showed that, in addition to inhibiting norepinephrine uptake, bufalin increased norepinephrine overflow by an amount larger than could be explained solely by uptake inhibition. The effect of bufalin on norepinephrine overflow is inhibited by tetrodotoxin, which suggests a dependence of this response on Na+ influx through the neuronal membranes. We propose that Na+,K(+)-ATPase inhibition resulting in neuronal depolarization is responsible for the augmented norepinephrine turnover caused by bufalin and that these indirect effects of norepinephrine on the cardiovascular system may play a role in the etiology of hypertension.

Effects of three sodium-potassium adenosine triphosphatase inhibitors

Reports from several laboratories suggest the presence of an ouabainlike compound in plasma and various animal tissues, particularly during acute volume expansion and in low-renin hypertension. It has been hypothesized that this compound, through inhibition of the Na(+)-K+ pump, can constrict blood vessels, enhance vasoconstriction in response to agonists, increase cardiac contractility, raise blood pressure, and cause natriuresis/diuresis and therefore is implicated in the pathophysiology of the low-renin, volume-expanded type of hypertension. However, so far, only two steroid Na(+)-K+ pump inhibitors (namely, a bufodienolide derivative [resibufogenin], obtained from toad skin and plasma and a factor with the same carbon, oxygen, and hydrogen content as ouabain obtained from the plasma of volume-expanded humans) have been purified and structurally characterized. To determine whether such endogenous Na(+)-K+ pump inhibitors can in fact produce the above effects on the cardiovascular and renal systems, we infused commercially available bufalin (aglycone, identical to resibufogenin except for one H+), ouabain, and ouabagenin (aglycone) at equimolar doses in normotensive rats. Relative to ouabain, bufalin produced significantly greater dose-dependent increases in blood pressure, left ventricular rate of pressure change, heart rate, and excretion of urinary volume and sodium. Ouabagenin was without effect on any of these parameters. These data indicate that a Na(+)-K+ pump inhibitor can cause an increase in blood pressure despite potent diuretic and natriuretic effects and that, in rats, bufalin is much more potent in this respect than ouabain or ouabagenin.

Comparison of the effects of a bufodienolide and ouabain on neuronal and smooth muscle preparations

The effect of a bufodienolide (monohydroxy-14,15-epoxy-20,22-dienolide glycoside) purified from toad skin was compared with that of ouabain on 3H-noradrenaline release and on the tension of rabbit pulmonary arterial strips. This compound exerted an ouabain-like activity. The neuronal effects of this bufodienolide derivative on squid axon were also studied and compared with those of ouabain. Both compounds enhanced the resting and stimulation-evoked (2 Hz, 360 shocks) release of 3H-noradrenaline. Moreover, in the presence of either this bufodienolide or ouabain, the tension of the rabbit artery increased gradually, and the contraction evoked by electrical stimulation was potentiated. Both compounds enhanced, in a prazosin-sensitive way, smooth muscle responses to noradrenaline and to electrical stimulation. In higher concentrations, they contracted smooth muscle cells of pulmonary artery, an action which was insensitive to prazosin. The bufodienolide was about 8 times more active in inhibition of 22Na efflux than was ouabain, but did not affect Ca efflux, which is not sensitive to ouabain. It is therefore concluded that compounds with an inhibitory effect on Na+,K(+)-ATPase are able to affect chemical neurotransmission of blood vessels in such a way that in lower concentrations they potentiate the release of noradrenaline, and in higher concentrations they contract directly the smooth muscle. These findings indicate that such compounds if they are present in the circulation might be involved in the physiological regulation of blood pressure or in the genesis of hypertension.

Effect of salt acclimation on digitalis-like compounds in the toad

Digitalis-like compounds (DLC) were shown to be a normal constituent of the skin and plasma of toads. In order to assess the possible physiological role of these compounds in the toad, their levels were determined in the brain, plasma and skin following acclimation in different NaCl solutions. We demonstrate that an increase in salt concentrations in the animal medium from 0 to 1.2% decreased the levels of DLC in the brain by 50% without altering significantly its levels in the plasma and skin. An increase in medium salt concentration to 1.5% resulted in a 50% increase of DLC levels in the skin without changing its levels in the plasma or brain. These results suggest that skin and brain DLC may participate in the long-term salt and water homeostasis in the toad, while the plasma compound either participates in the short-term regulations of salt and water homeostasis or have some other, unknown, function.

[Endogenous natriuretic and ouabain-like factors. Their potential role in volume and blood pressure regulation]

The existence of an endogenous natriuretic hormone and ouabain-like factors (OLF) has been postulated for many years. This postulate was based on our original observation that a small M.W. fraction in the serum after acute expansion of the extracellular fluid volume (ECFV) not only exhibited natriuretic activity but also inhibited the Na-K-ATPase enzyme in vitro similar to ouabain. Since then, numerous studies confirmed the presence of OLFs in serum, urine, cerebrospinal fluid, and various organs including the heart and hypothalamus. Some of these OLFs are well-known endogenous compounds, such as free unsaturated fatty acids, which inhibit in vitro transmembranous sodium transport, Na-K-ATPase and 3H-ouabain binding to its membrane receptor or cross-react with digoxin antibodies. Chemically yet undefined OLFs of potentially hypothalamic origin were detected in various models of experimental and clinical hypertension and are suggested to play a pathophysiological role especially in salt- and volume-dependent forms of hypertension. Our results show that OLFs isolated from the urine of salt-loaded healthy subjects strongly enhance basal and vasopressin-stimulated release of calcium in vascular smooth muscle cells and platelets similar to the effects we had observed with endothelin. This urine fraction also exhibits natriuretic activity which increases in parallel with sodium intake. Further chromatographic separation and amino acid analysis confirmed the peptidic nature (M.W. less than 1000) of the natriuretic factor(s). However, the two biological activities, namely natriuretic and ouabain-like activities, reside in distinct and chemically different compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Digitalislike circulating factor in hypertension: potential messenger between salt balance and intracellular sodium

Sodium chloride has no clearly established local direct action on blood vessels to produce constriction; on the contrary, it has an immediate local indirect action via osmolality, which produces vasodilation. Thus in order to explain salt-induced hypertension, a delayed remote indirect vasoconstrictor action must be postulated. This indirect vasoconstrictor action is apparently the result of volume expansion. Acute volume expansion imparts three physiologic properties to the plasma; these are the ability to inhibit Na,K-ATPase and the Na-K pump, to cause natriuresis, and to sensitize blood vessels to vasoconstrictor agents. Similarly, low-renin, volume-expanded hypertension endows the plasma with the capacity to inhibit the Na,K-ATPase pump, to sensitize blood vessels to vasoconstrictor agents, and to raise blood pressure. These properties apparently result from a circulating digitalislike substance(s), perhaps derived from the hypothalamus and/or adrenals. We here review the considerable effort expended in identifying the agent or agents, and conclude that both steroidal and peptidic structure must be considered. Regardless of its structure, we hypothesize that when sodium excretion does not keep pace with sodium intake, its release leads to increased contractile activity of cardiac and vascular smooth muscle and hence hypertension. Inhibition of the Na-K pump increases the intracellular sodium concentration, particularly when superimposed on genetic- or aldosterone-induced increased sodium permeability, resulting in depolarization and increased calcium influx (vascular smooth muscle) or altered Na(+)-Ca2+ exchange and decreased calcium efflux (heart muscle). The increased intracellular calcium concentration then accounts for the increased contractile activity. Depolarization may also increase the sensitivity of vascular smooth muscle to vasoconstrictor agents such as norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic effects of bufalin in the anesthetized dog

Studies in Lichstein's laboratory suggest that the endogenous digitalislike substance implicated in low renin hypertension might be a steroidal dienolide derivative. If this is true, the bufadienolides should block potassium vasodilation and enhance norepinephrine vasoconstriction, constrict blood vessels, raise blood pressure, and produce natriuresis and diuresis. We have therefore examined these parameters while infusing bufalin (aglycone), a bufadienolide, intrabrachially and intravenously in the anesthetized dog. Intrabrachial infusion of 5-25 micrograms/min with brachial arterial blood flow held constant at 100 ml/min produced a dose-dependent increase in perfusion pressure with rapid onset and offset, a progressive decrease in the vasodilator response to intrabrachial injection of 1 ml iso-osmotic potassium chloride solution (but not to acetylcholine), and an increase in the vasoconstrictor response to intrabrachial injection of 0.1 microgram norepinephrine. Intravenous infusion at 5-50 micrograms/min produced a dose-dependent increase in systemic arterial blood pressure, rate of change of ventricular pressure (dP/dt), and after the highest dose, cardiac irregularities. Natriuresis and diuresis were not observed. Thus, bufalin does in fact have some of the physiological properties required to be considered a candidate for the digitalislike substance found in low renin hypertension.

Circulating digitalis-like factors

The ability of extracts of mammalian plasma and tissue to mimic the biologic activities of the digitalis glycosides has suggested the existence of endogenous regulators for Na, K ATPase. Purification of plasma extracts has identified several classes of circulating lipids with digitalis-like activity including free fatty acids, lysophospholipids, and arachidonic acid metabolites of the lipoxygenase pathway. Circulating steroids with digitalis-like activity include dehydroepiandrosterone sulfate and hydrocortisone. Evidence for other, more unique compounds has also been published although their structure has not yet been determined. Analysis of tissue suggests that hypothalamus contains a unique, low molecular digitalis-like factor (DLF) which also circulates in plasma. Some studies suggest that the hypothalamic factor is also present in other parts of the brain and in the adrenal. Some of these endogenous DLF may function as modulators of cardiovascular function by regulating renal sodium excretion and peripheral vascular resistance in both physiological and pathophysiological situations.

Endogenous digitalislike compounds. A tentative update of chemical and biological studies

Endogenous digitalislike compound (or compounds) has been described as involved in some diseases. Questions remain concerning its chemical nature, origin, and biological properties. The methods of measuring the compound are based on biological properties of digitalis, mainly Na+, K+-adenosine triphosphatase (ATPase) inhibition and related properties. Chemically, digitalislike compound has been described as a peptide, as fatty acids, and as a steroid. Its origin could be the brain, particularly the hypothalamus and pituitary gland. The adrenal glands were also proposed as its origin. The reported biological properties of digitalislike compound are mainly dependent on Na+, K+-ATPase inhibition. No definitive conclusions can be drawn from the available data.

Noradrenaline releasing effect of an ouabain-like compound on pulmonary artery

The effect of ouabain and an endogenous ouabain-like compound (OLC) was studied on (3H)noradrenaline [3H)NA) release from rabbit pulmonary artery. Similarly to ouabain, OLC enhanced the release of (3H)NA in response to electrical stimulation. These findings are the first evidence that OLC is able to act presynaptically and enhance the amount of (3H)NA release per impulse. It is suggested that this mechanism might be involved in the physiological regulation of blood pressure or in the genesis of hypertension.