Effects of resibufogenin in experimental hypertension

Analysis of Resibufogenin on Cardiac conduction reveals a species difference in the cardiac electrophysiology: Rats versus guinea pigs

Resibufogenin (RBG) is a chemical ingredient of Chan Su. In our research, we found RBG affected cardiac rhythm in a negative chronotropic way in vivo. The cardiac Mapping system ex vivo and the patch clamp in vitro were used to explore how RBG influenced the cardiac electrophysiological properties. The negative chronotropic action of RBG at 100 μM might be attribute to prolongation in the atrioventricular conduction time and reduction in the ventricular conduction velocity. Using whole-cell patch clamp in ventricular myocytes of adult rats, we found that RBG prolonged the action potential duration (APD) in APD₂₀, APD₅₀, and APD₉₀ at 100 μM and inhibited calcium currents (I_Ca), total outward potassium currents (I_K), and transient outward potassium current (I_to) in a concentration-dependent manner, but not on the inward rectifying potassium current (I_K1). Notably, RBG had a potent proarrhythmic action ex vivo in the isolated perfused guinea pig hearts at 10 μM, but not in rats. To avoid the potential cardiotoxicity derived from the distributional differences of ion channels among species, the effect of RGB on I_Kr in hERG-HEK293 cells was detected. The IC₅₀ of RGB on I_Kr was more than 100 μM. In summary, all these results indicated that the negative chronotropic action of RBG relied on the blocking activities on multiple ion channels, and the species-difference of proarrhythmic effects might result from lack of the I_to on the myocardial membrane of guinea pigs. Anyhow, the cardiotoxicity observed in guinea pigs required further detailed studies to mitigate the potential risks in the clinical application of Chan Su.

Leptin-induced increase in blood pressure and markers of endothelial activation during pregnancy in Sprague Dawley rats is prevented by resibufogenin, a marinobufagenin antagonist

Levels of leptin and marinobufagenin (MBG), a cardiotonic steroid, are elevated in the serum of women with pre-eclampsia. Besides this, leptin administration to pregnant rats increases systolic blood pressure (SBP), urinary protein excretion and serum markers of endothelial activation. The link between leptin and MBG is unknown and it is also unclear if leptin-induced increases in blood pressure and proteinuria in the pregnant rat could be prevented by an MBG antagonist. To ascertain this link, this study investigated the effect of resibufogenin (RBG), a marinobufagenin antagonist, on leptin-induced increases in blood pressure and proteinuria during pregnancy in rats. Four groups of Sprague-Dawley rats, aged 12 weeks, were given either normal saline (CONTROL) or 120 μg/kg/day of leptin (LEP), or 120 μg/kg/day of leptin+30 μg/kg/day of resibufogenin (L + RBG) or 30 μg/kg/day of resibufogenin (RBG) from Day 1-20 of pregnancy. Systolic blood pressure and urinary protein excretion (UPE) were measured during the study period. Animals were euthanized on day 21 of pregnancy and vascular cell adhesion molecule 1, (VCAM-1), soluble intracellular cell adhesion molecule 1 (sICAM-1), E-selectin and endothelin-1 (ET-1) were estimated in the serum. SBP, UPE, VCAM-1, sICAM-1 and ET-1 were significantly higher only in the LEP group when compared with those in CONT and in L + RBG and RBG groups. The prevention by RBG of leptin-induced increases in SBP, proteinuria, and endothelial activation during pregnancy seem to suggest a potential role for MBG in leptin-induced adverse effects on blood pressure, urinary protein excretion and endothelial activity during pregnancy in the rat.

Simultaneous determination of resibufogenin and its eight metabolites in rat plasma by LC-MS/MS for metabolic profiles and pharmacokinetic study

BACKGROUND

Resibufogenin is one of the main active compounds of Venenum Bufonis and exhibits diverse pharmacological activities. It is brought into focus for its potency in heart failure and cancer therapy.

PURPOSE

The purpose of this study was to establish a convenient and effective method which was used to simultaneously determine the resibufogenin and its metabolites in rat plasma for further understanding the metabolic profiles of resibufogenin in vivo and pharmacokinetic study by LC-MS/MS.

METHODS

The analytes were separated on a BEH C18 column with a mobile phase of water containing 0.05% formic acid and acetonitrile under gradient elution at a flow rate of 0.4 ml/min. Resibufogenin and its eight metabolites were quantified in positive electrospray ionization and MRM mode with transitions of m/z 385.5→349.2 for resibufogenin; m/z 513.7→145.3 for IS (internal standard); m/z 401.23→365.21, m/z 417.23→285.21 and m/z 385.24→349.21 for three main metabolites (hydroxylated-resibufogenin; dihydroxylated-resibufogenin and 3-epi-resibufogenin, respectively).

RESULTS

This method was successfully validated with a good linearity over the concentration ranges of 1-200 ng/ml for resibufogenin and the correlation coefficients was more than 0.990. The lower limit of quantification was 1 ng/ml and the precision and accuracy values were less than 15%. The method was applied to study the metabolic profiles of resibufogenin in rat plasma after oral administration of 20 mg/kg. The results indicated that the metabolic reactions of resibufogenin were mainly hydroxylation, dihydroxylation, dehydrogenation and isomerization. Totally eleven metabolites were identified, among which eight were successfully quantified.

CONCLUSION

The results could provide further research foundation for the mechanisms study of activity and toxicity in vivo and facilitate the appropriate clinical application of resibufogenin.

Involvement of the Bufadienolides in the Detection and Therapy of the Acute Respiratory Distress Syndrome

PURPOSE

The acute respiratory distress syndrome (ARDS) represents a major challenge for clinicians as well as basic scientists. The mortality rate for ARDS has been maintained within the range of 40-52%. The authors have examined the involvement of the "cardiotonic steroids" in the pathogenesis and therapy of ARDS. We have studied the possible role of the bufadienolide, marinobufagenin (MBG), in the pathogenesis of ARDS in both a rat model of ARDS and in patients afflicted with that disorder. In addition, the potential therapeutic benefit of an antagonist of MBG, resibufogenin (RBG), in an animal model has been evaluated.

METHOD

A syndrome resembling human ARDS was produced in the rat by exposing the animals to 100% oxygen for 48 h. In other animals, RBG was administered to these "hyperoxic" rats, and the serum MBG was measured. In human ICU patients, urinary samples were examined for levels of MBG, and the values were compared to those obtained from other ICU patients admitted with diagnoses other than ARDS.

RESULTS

(1) Exposure of rats to hyperoxia produced a histologic picture which resembled that of human ARDS. (2) Serum levels of MBG in the "hyperoxic" rats substantially exceeded those obtained in animals exposed to ambient oxygen levels and were reduced to normal by RBG. (3) In ARDS patients, substantial elevations in urinary MBG were obtained compared to those in non-ARDS ICU patients.

CONCLUSIONS

MBG may serve as an important biomarker for the development of ARDS, and RBG may represent a preventative/therapy in this disorder.

Resibufogenin and cinobufagin activate central neurons through an ouabain-like action

Cinobufagin and resibufogenin are two major effective bufadienolides of Chan su (toad venom), which is a Chinese medicine obtained from the skin venom gland of toads and is used as a cardiotonic and central nervous system (CNS) respiratory agent, an analgesic and anesthetic, and as a remedy for ulcers. Many clinical cases showed that Chan su has severe side-effects on the CNS, causing shortness of breath, breathlessness, seizure, coma and cardiac arrhythmia. We used whole-cell recordings from brain slices to determine the effects of bufadienolides on excitability of a principal neuron in main olfactory bulb (MOB), mitral cells (MCs), and the cellular mechanism underlying the excitation. At higher concentrations, cinobufagin and resibufogenin induced irreversible over-excitation of MCs indicating a toxic effect. At lower concentrations, they concentration-dependently increased spontaneous firing rate, depolarized the membrane potential of MCs, and elicited inward currents. The excitatory effects were due to a direct action on MCs rather than an indirect phasic action. Bufadienolides and ouabain had similar effects on firing of MCs which suggested that bufadienolides activated neuron through a ouabain-like effect, most likely by inhibiting Na⁺/K⁺-ATPase. The direct action of bufadienolide on brain Na⁺ channels was tested by recordings from stably Na_v1.2-transfected cells. Bufadienolides failed to make significant changes of the main properties of Na_v1.2 channels in current amplitude, current-voltage (I-V) relationships, activation and inactivation. Our results suggest that inhibition of Na⁺/K⁺-ATPase may be involved in both the pharmacological and toxic effects of bufadienolide-evoked CNS excitation.

Marinobufagenin regulates permeability and gene expression of brain endothelial cells

Marinobufagenin (MBG) is a cardiotonic steroid that increases in the circulation in preeclampsia. Preeclampsia and eclampsia are associated with cerebral edema. Therefore, we examined the effects of MBG on human brain microvascular endothelial cells (HBMEC) in vitro. MBG enhanced the permeability of HBMEC monolayers at 1-, 10-, and 100-nM doses, but had no effect at 0.1 nM. Agilent Human Gene Expression microarrays were utilized in these studies. MBG treatment (10 nM for 12 h) downregulated concentrations of the soluble VEGFR transcript sFLT by 59% but did not alter those of FLTv3 mRNA (determined by quantitative PCR). When treated and control HBMEC transcriptomes were interrogated on microarrays, 1,069 genes appeared to be regulated by MBG. Quantitative RT-PCR confirmed that MBG treatment upregulated ENKUR mRNA concentrations by 57%. Its protein product interacts with calmodulin and calcium channel proteins. MBG treatment downregulated several genes whose protein products are involved in cell adhesion (ITGA2B, FERMT1, CLDN16, and TMEM207) and cell signaling (GRIN2C, SLC8A1, and ESR1). The level of downregulation ranged from 22 to 66%. Altogether, MBG actively enhanced the permeability of HBMEC monolayers while downregulating genes involved in adhesion. MBG treatment had variable effects on ENKUR, GRIN2C, and SLC8A1 genes, all associated with calcium transport. These studies provide the basis for future investigations of MBG actions in normal physiology and disease.

Marinobufagenin is an upstream modulator of Gadd45a stress signaling in preeclampsia

Preeclampsia (PE) is a hypertensive disorder of pregnancy, in which marinobufagenin (MBG), a circulating cardiotonic steroid, is increased. The Gadd45a stress sensor protein is an upstream modulator of the pathophysiological changes observed in PE. However, the effects of MBG on Gadd45a stress signaling remain unknown. We examined the expression of Gadd45a, the sFlt-1 receptor, and p38, as well as caspase 3 and 8 activities in placental samples from four groups of rats. These were: normal pregnant (NP, n=8); pregnant rats which received weekly injections of desoxycorticosterone acetate and 0.9% saline as their drinking water (PDS, n=9); normal pregnant rats injected with MBG (NPM, n=8); and PDS rats injected with resibufogenin (RBG), an in vivo antagonist of MBG (PDSR, n=8). Utilizing human cytotrophoblast (CTB) cells, we examined the effect of MBG on these stress signaling proteins in vitro. Placental Gadd45a expression, caspase 3 and 8 activities, sFlt-1 concentrations, and sFlt-1 receptor expression were significantly higher in PDS and NPM compared to NP and PDSR rats. Gadd45a protein was significantly upregulated in the CTB cells when MBG was present in concentrations ≥1nM. Treatment with MBG (≥1nM) also significantly arrested cell cycle progression and activated the expression of the Gadd45a-mediated stress signaling proteins. Inhibition of Gadd45a through RNAi-mediation attenuated MBG-induced CTB cell stress signaling. In conclusion, MBG is involved in the alteration in Gadd45a stress signaling both in vivo and in vitro and RBG prevents these changes when administered in vivo.

Resibufogenin prevents the manifestations of preeclampsia in an animal model of the syndrome

BACKGROUND AND OBJECTIVES

We have developed a rat model of preeclampsia which is based upon excessive volume expansion and includes hypertension, proteinuria and intrauterine growth restriction. In this model, the urinary excretion of the circulating steroid inhibitor of Na +/ K+ ATPase, marinobufagenin, is increased prior to the development of hypertension and proteinuria. An analogue of marinobufagenin, resibufogenin, successfully treats the hypertension and proteinuria.

METHODS

We administered resibufogenin early in pregnancy in this model, prior to the development of the syndrome.

RESULTS

We found that resibufogenin not only prevented the advent of hypertension and proteinuria, but also the development of intrauterine growth restriction.

DISCUSSION

These results may have relevance to the human condition.

Marinobufagenin, resibufogenin and preeclampsia

The bufodienolides are cardiac glycosides which have the ability to inhibit the enzyme, Na(+)/K(+) ATPase (sodium potassium adenosine triphosphatase). They are cardiac inotropes, cause vasoconstriction (and, potentially, hypertension) and are natriuretic. Evidence has accrued over time which supports the view that they are mechanistically involved in volume expansion-mediated hypertension. In this communication, the authors summarize data which support the view that the bufodienolides and, in particular, marinobufagenin (MBG) are involved in the pathogenesis of preeclampsia. In a rat model of the syndrome, MBG causes hypertension, proteinuria, intrauterine growth restriction and increased weight gain. All of these phenotypic characteristics are prevented by an antagonist to MBG, resibufogenin (RBG). The "preeclamptic" animals also develop a vascular leak syndrome, resulting in hemoconcentration. Abnormalities in the MAPK (mitogen-activated protein kinase) system play a role in the mechanism by which MBG produces the abnormalities in the pregnant rat. Studies to discover the relevance of these findings to human preeclampsia are currently underway in several laboratories and clinics.