The effect of various cardenolides and bufadienolides with different cardiac activity on the 86-rubidium uptake of human erythrocytes

Identifying toxic effects and metabolic perturbations of Duttaphrynus melanostictus skin extracts in human erythrocytes

Background

Skin secretions of toads are widely used in medicine all over the world for their antiviral, anti-infective, and cardiotonic properties. Because these secretions are mostly employed to combat blood parasite infection, it is important to understand their potential toxic effects on human erythrocytes. Therefore, the objective of the current investigation was to elucidate the effects of Duttaphrynus melanostictus (Schneider) skin extracts on the physiology of human erythrocytes.

Methods

Toads captured from their natural habitat were separated into three groups according to their body size. Hydroalcoholic extracts of toad skin were prepared by reflux heating. These extracts were then evaluated for their hemolytic and hemoglobin denaturation potential. The effects of the extracts on cytosolic and membrane-bound enzymes of human erythrocytes were assessed.

Results

The hemolysis and hemoglobin denaturation caused by these extracts correlated positively with the respective toad sizes. Extracts from medium and large toads led to increased osmotic fragility even at near iso-osmotic concentrations. Biochemical analysis of hemolysate showed that the treatment induced a shift of metabolic flux toward the glutathione pathway. Analysis of membrane-bound enzymes revealed a significant decrease in the activity of Na+/K+ ATPase and acetylcholinesterase. SDS-PAGE analysis of the erythrocyte membrane did not show the band of tropomodulin for the cells treated with 1000 𝜇g/ml extract from large toads.

Conclusions

In conclusion, the present study demonstrates that the toxicity of toad skin secretions aggravates with the size of the animal and interferes with the physiology of human erythrocytes, leading to their membrane disruption and rapid lysis.

Whither digitalis? What we can still learn from cardiotonic steroids about heart failure and hypertension

Cloning of the "Na⁺ pump" (Na⁺,K⁺-ATPase or NKA) and identification of a circulating ligand, endogenous ouabain (EO), a cardiotonic steroid (CTS), triggered seminal discoveries regarding EO and its NKA receptor in cardiovascular function and the pathophysiology of heart failure (HF) and hypertension. Cardiotonic digitalis preparations were a preferred treatment for HF for two centuries, but digoxin was only marginally effective in a large clinical trial (1997). This led to diminished digoxin use. Missing from the trial, however, was any consideration that endogenous CTS might influence digitalis' efficacy. Digoxin, at therapeutic concentrations, acutely inhibits NKA but, remarkably, antagonizes ouabain's action. Prolonged treatment with ouabain, but not digoxin, causes hypertension in rodents; in this model, digoxin lowers blood pressure (BP). Furthermore, NKA-bound ouabain and digoxin modulate different protein kinase signaling pathways and have disparate long-term cardiovascular effects. Reports of "brain ouabain" led to the elucidation of a new, slow neuromodulatory pathway in the brain; locally generated EO and the α2 NKA isoform help regulate sympathetic drive to the heart and vasculature. The roles of EO and α2 NKA have been studied by EO assay, ouabain-resistant mutation of α2 NKA, and immunoneutralization of EO with ouabain-binding Fab fragments. The NKA α2 CTS binding site and its endogenous ligand are required for BP elevation in many common hypertension models and full expression of cardiac remodeling and dysfunction following pressure overload or myocardial infarction. Understanding how endogenous CTS impact hypertension and HF pathophysiology and therapy should foster reconsideration of digoxin's therapeutic utility.

Ouabain, endogenous ouabain and ouabain-like factors: The Na+ pump/ouabain receptor, its linkage to NCX, and its myriad functions

In this brief review we discuss some aspects of the Na⁺ pump and its roles in mediating the effects of ouabain and endogenous ouabain (EO): i) in regulating the cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) via Na/Ca exchange (NCX), and ii) in activating a number of protein kinase (PK) signaling cascades that control a myriad of cell functions. Importantly, [Ca²⁺]_CYT and the other signaling pathways intersect at numerous points because of the influence of Ca²⁺ and calmodulin in modulating some steps in those other pathways. While both mechanisms operate in virtually all cells and tissues, this article focuses primarily on their functions in the cardiovascular system, the central nervous system (CNS) and the kidneys.

Quinidine-digoxin interaction: effect of quinidine on 86Rb-uptake of human erythrocytes

Co-administration of quinidine results in a marked increase in serum digoxin concentration (SDC). The implication of this increase in SDC in regard to an increased digoxin effect on the heart is controversial. The 86Rb-erythrocyte-assay represents a perfect model to study whether quinidine interferes with digoxin at the Na-K-ATPase (the so called glycoside receptor) and thus presumably with the inotropic effect of the glycoside. The inhibitory effect of digoxin (0-150 ng/ml) on the 86Rb-uptake was measured in the absence and presence of quinidine in therapeutic or higher concentrations (0-60 microgram/ml). Addition of quinidine produced no effect on digoxin-induced inhibition of Na-K-ATPase activity. Together with our clinical observations these results strongly suggest that the increased SDC during concomitant quinidine therapy actually reflects an increased digoxin effect on the heart.

Stability in vitro of methylproscillaridin

The in vitro stability of methylproscillaridin has been compared with that of proscillaridin, the activities of the glycosides being assayed by the 86Rb-technique. After incubation in gastric juice at pH 1,2, and 3, the activity half life of each glycoside was proportional to pH and was approximately 0.25 h, 2.5 h, and 25 h, respectively. The inactivation rate in pure hydrochloric acid at pH 2 did not differ from that in gastric juice of the same pH. The glycosides were stable in bile and enteric juice. In faeces, methylproscillaridin was stable and proscillaridin was inactivated with a half life of 32 h. It is concluded that the difference in biological availability between the two glycosides cannot be explained by differences in gastrointestinal stability.

The effects of digoxin and beta-methyldigoxin on the heart rate of decompensated patients with atrial fibrillation

Eighteen patients with atrial fibrillation were given digoxin 0.13 mg twice daily for 3 weeks and beta-methyldigoxin 0.10 mg twice daily for another 3 weeks. At the end of each 3 week period an exercise test was performed and the effects on the heart rate of the two drugs were compared. No difference in heart rate was obtained at rest, whereas the heart rate after 6 min of exercise was higher during treatment with digoxin (131 beats/min) than when the patients were taking beta-methyldigoxin (124 beats/min). There were no significant differences between digoxin and beta-methyldigoxin in their effects on the ECG (R-R intervals, T-wave, Q-T duration). The plasma concentrations of the two glycosides were determined by radioimmunoassay and by 86Rb-uptake inhibition assay. Comparable plasma concentration values (1.0 ng/ml for digoxin, 1.1 ng/ml for beta-methyldigoxin, mean values) were obtained by radioimmunoassay, but the 86Rb-technique gave significantly higher values (mean 1.5 ng/ml) for beta-methyldigoxin. It is concluded that beta-methyldigoxin is equal to digoxin for producing slowing of the heart rate in patients with atrial fibrillation.

Pharmacokinetics and pharmacodynamics of methyl proscillaridin in healthy man

The aim of this study was to obtain data about the pharmacological properties of a new glycoside derivative in man. Plasma concentrations and ECG parameters were measured after oral and intravenous administration of a single dose of 1.2 mg methyl proscillaridin in 16 healthy volunteers, using a strictly randomized, two-period change-over design. Glycoside concentrations were measured using a modified 86Rb-erythrocyte-assay. QT-duration, corrected for frequency (QTc), was the principal variable measured in the ECG. By either route, there was a maximum plasma level after 1 hour, which had decreased to a minimum at 3 hours, followed by a second peak at 4 to 10 hours (orally greater than iv). From 10 to 72 hours the concentrations decreased with a median t 1/2 of 23.3 hours (iv) and 33.0 hours (orally). Comparison of the ratio of plasma concentrations following oral and iv administration resulted in a bioavailability of 69% using the 48 hour plasma levels, and 59% using the areas under the concentration-time curves. The mean QTc was maximally shortened to 28 msec at 1 hour after iv and to 19 msec at 10 hours after the oral dose. A distinct similarity between time-concentration and time-QTc curves was seen after the initial distribution phase, both after oral and intravenous administration. The new derivative shows a rapid elimination. Its bioavailability is reasonably high.

Studies on a plasma cardiac glycoside assay based upon displacement of 3-H-ouabain from Na+-K+-ATPase

We tested an assay system introduced for plasma glycoside measurements, basing on the displacement of 3-H-ouabain from Na+-K+-ATPase by unlabeled glycoside. ATPase preparations from hog, cat and guinea pig were used. Displacements were performed using 20 cardiac glycosides, genins and derivatives with different cardiac activity. Most of the glycosides and derivatives do not induce a continuous 3-H-ouabain displacement from the ATPase, but a very steep increase of unbound 3-H-ouabain between 10-minus 7 and 2 times 10-minus 7 M. Therefore this assay system shows a satisfactory discrimination only in a short concentration range. This behavior and a relatively low sensitivity make the ATPase displacement assay problematic for clinical and pharmacokinetical plasma glycoside measurements.

In vitro stability of proscillaridin A

In an in vitro study, proscillardin A was found to be rapidly inactivated at low pH. More than 50 per cent of its activity, measured by 86Rb assay, was lost after incubation for 15 minutes at pH 1 and 37 degrees C. Compared with proscillaridin, the rate of inactivation of digoxin was lower in these experiments. The rapid inactivation of proscillaridin might be of clinical importance when treating patients with this glycoside.