Comparative ability of digoxin and an aminosugar cardiac glycoside to bind to and inhibit Na+,K+-adenosine triphosphatase. Effect of potassium

Cardenolide, Potassium, and Pyrethroid Insecticide Combinations Reduce Growth and Survival of Monarch Butterfly Caterpillars (Lepidoptera: Nymphalidae)

The monarch butterfly, Danaus plexippus L., has evolved to be insensitive to milkweed cardenolides via genetic modifications of Na+/K+-ATPase. There is concern for insecticide exposures near agriculture, with little information on monarch caterpillar toxicology. It is unclear how cardenolide insensitivity may affect the sensitivity of monarch caterpillars to pyrethroid insecticides. Additionally, potassium fertilizers may affect monarch caterpillar physiology and cardenolide sequestration. Here, we investigated the growth, survival, and development of caterpillars exposed to the cardenolide ouabain, bifenthrin, and potassium chloride (KCl) alone and in combination. Caterpillars were either exposed to 1) ouabain from third- to fifth-instar stage, 2) KCl at fifth-instar stage, 3) KCl and bifenthrin at fifth-instar stage, or 4) combinations of ouabain at third-instar stage + KCl + bifenthrin at fifth-instar stage. Caterpillar weight, diet consumption, frass, and survival were recorded for the duration of the experiments. It was observed that 1-3 mg ouabain/g diet increased body weight and diet consumption, whereas 50 mg KCl/g diet decreased body weight and diet consumption. Caterpillars feeding on KCl and treated with 0.2 µg/µl bifenthrin consumed significantly less diet compared to individuals provided untreated diet. However, there was no effect on survival or body weight. Combinations of KCl + ouabain did not significantly affect caterpillar survival or body weight following treatment with 0.1 µg/µl bifenthrin. At the concentrations tested, there were no effects observed for bifenthrin sensitivity with increasing cardenolide or KCl concentrations. Further studies are warranted to understand how milkweed-specific cardenolides, at increasing concentrations, and agrochemical inputs can affect monarch caterpillar physiology near agricultural landscapes.

Changes in digoxin pharmacokinetics associated with hepatic P-glycoprotein upregulation in rats with non-alcoholic fatty liver disease

BACKGROUND & OBJECTIVES

Upregulation of hepatic P-glycoprotein (P-gp) expression has been reported in patients with non-alcoholic fatty liver disease (NAFLD) and rodent models thereof. Here, we explored the changes hepatic P-gp expression and activity in a NAFLD rat model and the effects thereof on the pharmacokinetics of digoxin (a probe substrate of P-gp).

METHODS

Rats were fed a 1% (w/w) orotic acid-containing diet for 20 days to induce NAFLD; control rats received a normal diet. P-gp expression and biliary digoxin excretion were examined. The pharmacokinetics of digoxin were evaluated after it had been administered intravenously (10 μg·kg^-1 ) and orally (200 μg·kg^-1 ) to control and NAFLD rats.

RESULTS

The total areas under the plasma concentration-time curves (AUCs) of digoxin after intravenous and oral administration were significantly smaller (by 39.1% and 73.0%, respectively) in NAFLD rats because of faster biliary digoxin excretion, reflecting elevations of hepatic P-gp expression and activity. Notably, the steady-state volume of distribution rose by 98.2%, while extent of oral bioavailability fell by 55.5% in NAFLD rats.

CONCLUSION

This is the first study to report digoxin pharmacokinetic changes caused by hepatic P-gp upregulation in NAFLD. Further studies are needed to explore the clinical impact of enhanced P-gp-mediated biliary excretion on pharmacotherapies using P-gp substrates in patients with NAFLD.

Effects of digoxin on full-type hyperactivation in bovine ejaculated spermatozoa with relatively lower survivability for incubation with stimulators of cAMP signaling cascades

Previous researches of our laboratory reported that addition of cAMP analog cBiMPS and protein phosphatase inhibitor calyculin A (stimulators of cAMP signaling cascades) improved the capacity of incubation medium to induce full-type hyperactivation in bovine ejaculated spermatozoa. However, this modified medium was valid only for samples with relatively good survivability for incubation with stimulators of cAMP signaling cascades. Thus, it is necessary to make further modified medium for evaluation of potentials to exhibit full-type hyperactivation in bovine sperm samples with relatively lower survivability. Na⁺/K⁺-ATPase is an integral membrane protein and involved with the regulation of rodent sperm motility. To make further modification of the medium, we examined effects of Na⁺/K⁺-ATPase inhibition with digoxin on motility, full-type hyperactivation and protein tyrosine phosphorylation in bovine ejaculated spermatozoa with relatively lower survivability for incubation with stimulators of cAMP signaling cascades and also performed the immunodetection of bovine sperm Na⁺/K⁺-ATPase. The addition of Na⁺/K⁺-ATPase inhibitor digoxin to the incubation medium containing cBiMPS and calyculin A had the tendency to lessen the decreases in the percentages of motile spermatozoa in all of 12 samples after the incubation for 1-3 h and significantly increased the percentages of full-type hyperactivation in one group of 4 samples (Sample-A1) and another group of 4 samples (Sample-A2) after 1 and 2 h respectively, though it had no significant effects on full-type hyperactivation in the other group of 4 samples (Sample-B). In addition, incubation time-related changes in the sperm protein tyrosine phosphorylation (a good marker for sperm capacitation) were correlated with those in the percentages of full-type hyperactivation in Sample-A1 containing digoxin. Immunodetection showed that Na⁺/K⁺-ATPase is present in the middle and principal pieces of the flagella, indicating that Na⁺/K⁺-ATPase has possible relations with sperm motility. These results obtained with bull ejaculated spermatozoa with relatively lower survivability indicate that incubation method using digoxin is useful to evaluate potentials of sperm samples to exhibit full-type hyperactivation, that digoxin has effects on suppressing reduction of sperm motility, and that prolonged incubation with digoxin induces reduction of capacitation state which may suppress the maintenance of full-type hyperactivation.

How many endobains are there?

Oxidative metabolism is very active in brain, where large amounts of chemical energy as ATP molecules are consumed, mostly required to maintain cellular Na+/K+ gradients through the participation of the sodium pump (Na+,K+-ATPase), whose activity is selectively and potently inhibited by the alkaloid ouabain. Na+/K+ gradients are involved in nerve impulse propagation, in neurotransmitter release and cation homeostasis in the nervous system. Likewise, enzyme activity modulation is crucial for maintaining normal blood pressure and cardiovascular contractility as well as renal sodium excretion. The present article reviews the progress in disclosing putative ouabain-like substances, examines their denomination according to different research teams, tissue or biological fluid sources, extraction and purification, assays, biological properties and chemical and biophysical features. When data is available, comparison with ouabain itself is mentioned. Likewise, their potential action in normal physiology as well as in experimental and human pathology is summarized.

Kinetics of Na+, K+-ATPase inhibition by an endogenous modulator (II-A)

We have previously reported the isolation by gel filtration and anionic exchange HPLC of two brain Na+, K+-ATPase inhibitors, II-A and II-E, and kinetics of enzyme interaction with the latter. In the present study we evaluated the kinetics of synaptosomal membrane Na+, K+-ATPase with II-A and found that inhibitory activity was independent of ATP (2-8 mM), Na+ (3.1-100 mM), or K+ (2.5-40 mM) concentration. Hanes-Woolf plots showed that II-A decreases Vmax in all cases; KM value decreased for ATP but remained unaltered for Na+ and K+, indicating respectively uncompetitive and noncompetitive interaction. However, II-A became a stimulator at 0.3 mM K+ concentration. It is postulated that brain endogenous factor II-A may behave as a sodium pump modulator at the synaptic region, an action which depends on K+ concentration.

Na+,K+-ATPase interaction with a brain endogenous inhibitor (endobain E)

Na+,K+-ATPase activity of rat brain synaptosomal membranes was evaluated in the presence of an inhibitory fraction II-E (termed endobain E), isolated by gel filtration and anionic exchange HPLC of a rat brain soluble fraction. We studied endobain E aging, analyzed its inhibitory potency in the absence or presence of ouabain as well as its ability to block high affinity [3H]ouabain binding to cerebral cortex membranes. Similar loss of endobain E activity was observed when samples were stored either dried or in solution. Endobain E fraction inhibited synaptosomal membrane Na+,K+-ATPase activity in a concentration-dependent manner and the slope of the corresponding curve strongly resembled that of ouabain. Assays performed in the presence of endobain E and ouabain indicated that the inhibitory effect was additive or less than additive, depending on their respective concentrations during preincubation and/or incubation. High affinity [3H]ouabain binding to cerebral cortex membranes proved concentration-dependent from 0.10 to 0.50 mg protein per ml; binding inhibition by endobain E was independent of protein concentration within the above range. [3H]ouabain binding inhibition by endobain E was concentration-dependent over a 10-fold range, an effect similar to that found for Na+,K+-ATPase inhibition. The extent of endobain E effect on Na+,K+-ATPase inhibition was much higher (90-100%) than that on [3H]ouabain binding blockade (50%). Findings suggest some type of interaction between endobain E and ouabain inhibitory mechanisms and favour the view that the former behaves as an endogenous ouabain.

Kinetics of Na+, K+-ATPase inhibition by a rat brain endogenous factor (II-E)

Previous work from this laboratory led to the isolation by gel filtration and anionic exchange HPLC of a rat brain fraction named II-E, which highly inhibits synaptosomal membrane Na+, K+-ATPase activity. In this study we evaluated the kinetics of such inhibition and found that inhibitory potency was independent of Na+ (1.56-200 mM), K+ (1.25-40 mM), or ATP (1-8 mM) concentration. Hanes-Woolf plots indicated that II-E decreases Vmax but does not alter KM value, and suggested uncompetitive inhibition for Na+, K+ or ATP. However, II-E became a stimulator at 0.5 mM ATP concentration. It is postulated that this brain factor may modulate ionic transport at synapses, thus participating in central neurotransmission.

Dose-dependent reversal of digoxin-inhibited activity of an in vitro Na+K+ATPase model by digoxin-specific antibody

We investigated the potency of digoxin-specific Fab fragments to reverse digoxin-induced Na+K+ATPase inhibition in rat brain microsomes according to (a) the extent of initial inhibition of Na+K+ATPase and (b) the neutralizing dose of antibody. Mathematical analysis of the digoxin concentration-Na+K+ATPase inhibition curve supports the existence of 2 digoxin sensitive Na+K+ATPase isoforms. The IC50 was 1.3 x 10(-4) M and 2.5 x 10(-8) M for the low (alpha 1) and high (alpha 2) digoxin affinity isoenzyme, respectively. The reversal of digoxin-induced Na+K+ATPase inhibition was dependent on the digoxin-specific Fab concentration. The maximal effect was observed when the Fab:digoxin ratio was stoichiometrical and addition of an excess of antibodies did not result in a complete reversal of inhibition at the 4 digoxin concentrations studied. This simple and rapid in vitro model will be a useful tool to predict the efficacy of a new generation of antibodies.

Affinity and dose-dependent digoxin Na+K+ATPase dissociation by monoclonal digoxin-specific antibodies

The effect of three monoclonal digoxin-specific antibodies and of polyclonal Digidot as reference on digoxin dissociation from rat brain Na+K+ATPase microsomes was studied to determine the role of the affinity constant (Ka) and dose of the antibody on the rate of digoxin dissociation from Na+K+ATPase. Stoichiometrical doses of 1C10, 6C9, 9F5 IgG, and Digidot (Ka = 6 10(9), 3.1 10(8), 2.5 10(7), and 8.5 10(9) M-1, respectively) resulted in digoxin dissociation related to Ka. When the IgG:digoxin molar ratio increased from 0.25 to 10, digoxin dissociation from Na+K+ATPase sites also increased according to the Hill equation, allowing comparative parameters among the three antibodies to be determined. 1C10 IgG was 2- and 10-fold more efficacious than 6C9 and 9F5, respectively. This in vitro model appears to be a useful predictive screening assay before in vivo experimentation.