Differential effect of potassium on the action of digoxin and digoxigenin in guinea-pig heart

Hetero-Tricyclic Lead Scaffold as Novel PDE5A Inhibitor for Antihypertensive Activity: In Silico Docking Studies

Objective:

To screen the phytochemicals for phosphodiesterase 5A (PDE5A) inhibitory potential and identify lead scaffolds of antihypertensive phytochemicals using in silico docking studies.

Methods:

In this perspective, reported 269 antihypertensive phytochemicals were selected. Sildenafil, a PDE5A inhibitor was used as the standard. In silico docking study was carried out to screen and identify the inhibiting potential of the selected phytochemicals against PDE5A enzyme using vLife MDS 4.4 software.

Results:

Based on docking score, π-stacking, H-bond and ionic interactions, 237 out of 269 molecules were selected which have shown one or more interactions. Protein residue Gln817A was involved in H-boding whereas Val782A, Phe820A and Leu804A were involved in π-stacking interaction with ligand. The selected 237 phytochemicals were structurally diverse, therefore 82 out of 237 molecules with one or more tricycles were filtered out for further analysis. Amongst tricyclic molecules, 14 molecules containing nitrogen heteroatom were selected for lead scaffold identification which finally resulted in three different basic chemical backbones like pyridoindole, tetrahydro-pyridonaphthyridine and dihydro-pyridoquinazoline as lead scaffolds.

Conclusion:

In silico docking studies revealed that nitrogen-containing tetrahydro-pyridonaphthyridine and dihydro-pyridoquinazoline tricyclic lead scaffolds have emerged as novel PDE5A inhibitors for antihypertensive activity. The identified lead scaffolds may provide antihypertensive lead molecules after its optimization.

GEPSI: A Gene Expression Profile Similarity-Based Identification Method of Bioactive Components in Traditional Chinese Medicine Formula

The identification of bioactive components in traditional Chinese medicine (TCM) is an important part of the TCM material foundation research. Recently, molecular docking technology has been extensively used for the identification of TCM bioactive components. However, target proteins that are used in molecular docking may not be the actual TCM target. For this reason, the bioactive components would likely be omitted or incorrect. To address this problem, this study proposed the GEPSI method that identified the target proteins of TCM based on the similarity of gene expression profiles. The similarity of the gene expression profiles affected by TCM and small molecular drugs was calculated. The pharmacological action of TCM may be similar to that of small molecule drugs that have a high similarity score. Indeed, the target proteins of the small molecule drugs could be considered TCM targets. Thus, we identified the bioactive components of a TCM by molecular docking and verified the reliability of this method by a literature investigation. Using the target proteins that TCM actually affected as targets, the identification of the bioactive components was more accurate. This study provides a fast and effective method for the identification of TCM bioactive components.

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.

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

We compared the abilities of digoxin and aminogalactose digitoxigenin (ASI-222) to bind to, or inhibit, purified dog heart Na+,K+-ATPase in the presence of 1, 10, or 80 mM potassium chloride. Changing the potassium concentration from 1 to 10 mM increased the dose producing 50% inhibition of enzyme activity (IC50) by 9- and 2.5-fold for digoxin and ASI-222 respectively. Raising the potassium concentration to 80 mM increased the IC50 for digoxin 3-fold but did not alter significantly the IC50 for ASI-222. Equilibrium binding studies showed that this enzyme exhibited a single class of specific binding sites for both digoxin and ASI-222. Raising the potassium concentration did not affect the maximum number of binding sites (Bmax) but increased the apparent dissociation constant (KD) for digoxin. Potassium differentially affected the affinity and number of binding sites for ASI-222; raising the potassium concentration from 1 to 10 mM did not affect the Bmax or the KD, but raising it to 80 mM increased both. The effect of i.v. infusion of potassium chloride upon cardiac upon cardiac arrhythmias produced by i.v. infusion of digoxin or ASI-222 in anesthetized dogs was also determined. Infusion of potassium chloride reversed the cardiac arrhymias due to digoxin to normal rhythm, but not those due to ASI-222. In conclusion, the interaction of digoxin and the polar digitalis agent, ASI-222, with dog heart Na+,K+-ATPase was differentially affected by potassium. These agents also also produced cardiac arrhythmias, which were differentially affected by potassium.

Phospholipid composition of cardiac (Na+ + K+)-ATPases from various species

There is a difference in phospholipid composition of cardiac (Na+ + K+)-ATPase preparations between species which are sensitive to ouabain and those which are not. Sphingomyelin is higher and phosphatidylcholine is lower in the enzymes from sensitive species than in those from insensitive ones. Lysophosphatidylcholine is detectable only in the latter preparations.

Effects of K+ on the interaction between cardiac glycosides and Na,K-ATPase

Inhibition of Na,K-ATPase by cardiac glycosides is at least partially antagonized by K+. The kinetics of the antagonism, however, appear complicated because K+ is capable of reducing both association and dissociation rate constants for the glycoside-enzyme interaction. In order to better understand the effect of K+, inhibition of partially purified Na,K-ATPase obtained from rat brain, guinea-pig heart and rat heart by ouabain, digoxin, digoxigenin, dihydrodigoxin and cassaine were compared in the presence of 1, 3 or 10 mM K+. Higher concentrations of K+ caused a parallel shift to the right in the concentration-inhibition curves for these compounds. For ouabain or digoxin, the extent of the shift was minimal with rat brain enzyme, intermediate with guinea-pig heart enzyme and more substantial with rat heart enzyme. For digoxigenin, dihydrodigoxin or cassaine, the extent of the shift was substantial in all enzyme preparations. These results could not be explained from either the affinity of the enzyme for the compound or its lipid solubility alone. The concentrations of these compounds required to cause a 50 percent inhibition of enzyme activity were markedly different with rat brain enzyme, but relatively similar with rat heart enzyme. The effects of K+, which depend on the source of the enzyme and chemical structures of the compounds, have to be considered in studies on comparative effects of various compounds on Na,K-ATPase, [3H]ouabain binding, sodium pumping and the force of myocardial contraction.

The influence of reduced serum potassium level on the toxicity of some cardenolides in guinea pigs

Recent experiments on the pharmacological properties of the semisynthetic cardiotonic steroid strophanthidin-3-bromoacetate (SBA) have challenged the well-known potassium digitalis antagonism in isolated heart muscle preparations. In order to establish these results in vivo, the minimum lethal doses (LDmin) of ouabain (OUA), digoxin (DO), digotoxin (DT), k-strophanthidin (STR) and SBA were determined by the infusion toxicity method in guinea pigs at normokalemia and hypokalemia. The experimentally induced decrease of the serum potassium concentration (5.0 mmoles/l vs. 3.3 mmoles/l) significantly reduced the LDmin of DO (1.42 vs. 1.05 mumoles/kg), DT (1.78 vs. 1.24 mumoles/kg) and STR (20.16 vs. 15.98 mumoles/kg), whereas the LDmin of OUA (0.37 vs. 0.34 mumoles/kg) was not altered. Contrary, the LDmin of SBA was even slightly, but not significantly increased during hypokalemia (16.77 vs. 19.04 mumoles/kg). In addition, from the experimental data an optimum time of infusion (Topt), corresponding to the LDmin, can be derived, which is equivalent to the time for optimum "utilization" of the drug. The obtained sequence: STR less than OUA less than DO less than DT less than SBA represents the well-known differences in the onset of the pharmacological action in man resp. animal. Hypokalemia in general resulted in a shortening of the Topt, thus indicating a more rapid "utilization" of the drug tested. The above differences of the cardenolide action at reduced serum potassium concentration may be dependent on the recently reported divergent influence of potassium on the association- resp. dissociation rate constants for the interaction of these drugs with their binding site at the Na+-K+-ATPase.