Bretylium, an organic quaternary amine, inhibits the Na,K-ATPase by binding to the extracellular K-site

Probing the extracellular access channel of the Na,K-ATPase

When the Na,K-ATPase pumps at each turnover two K(+) ions into the cytoplasm, this translocation consists of several reaction steps. First, the ions diffuse consecutively from the extracellular phase through an access pathway to the binding sites where they are coordinated. In the next step, the enzyme is dephosphorylated and the ions are occluded inside the membrane domain. The subsequent transition to the E1 conformation produces a deocclusion of the binding sites to the cytoplasmic side of the membrane and allows in the last steps ion dissociation and diffusion to the aqueous phase. The interaction and competition of K(+) with various quaternary organic ammonium ions have been used to gain insight into the molecular mechanism of the ion binding process from the extracellular side in the P-E2 conformation of the enzyme. Using the electrochromic styryl dye RH421, evidence has been obtained that the access pathway consists of a wide and water-filled funnel-like part that is accessible also for bulky cations such as the benzyltriethylammonium ion, and a narrow part that permits passage only of small cations such as K(+) and NH4(+) in a distinct electrogenic way. Benzyltriethylammonium ions inhibit K(+) binding in a competitive manner that can be explained by a stopper-like function at the interface between the wide and narrow parts of the access pathway. In contrast to other quaternary organic ammonium ions, benzyltriethylammonium ions show a specific binding to the ion pump in a position inside the access pathway where it blocks effectively the access to the binding sites.

[Animal in vivo model of arrhythmia for genes target identification for 5-amino-exo-3-azatricyclo[5.2.1.0(2,6)]decan-4-one]

The goal of the current work is to study the molecular mechanisms underlay the action of 5- amino-exo-3-azatricyclo[5.2.1.0(2,6)]decan-4-one (P-11) with combined antiarrhythmic, nootropic, anti-inflammatory and anaesthetic activities. The aconitine-induced experimental rat model of cardiac arrhythmia has been used in our study. Aconitine was administered once intravenously in a dose 50 microg/kg whereas experimental animal group received P-11 in a dose 0.3 mg/kg (the compound was injected intravenously 2 min before acute aconitine treatment). Expression macroarray (Atlas Rat cDNA Expression Array, #7738-1; BD Biosciences) was used to identify the target genes for P-11 compound. Comparative analysis of changes in the status of expression of genes in the heart of rats induced by P-11 against the simulated in vivo arrhythmia identified 16 genes that reproducibly alter the level of expression.These genes encode the extracellular matrix proteins (glypican 1, Gpc1; tissue inhibitor of metalloproteinase 2, 3, Timp2, Timp 3); intracellular signaling molecules (rho GTPase activating protein 7, Dlc1; protein tyrosine phosphatase 4a1, Ptp4a1; phosphodiesterase 4D, PDE4D; PI3-kinase regulatory subunit alpha, PIK3R1; guanine nucleotide binding protein alpha 12, Gna12) and protein of intermediate junctions (junction plakoglobin, Jup), proteins involved in glycolysis (phosphofructokinase I, Pfk1) and hemostasis (tissue plasminogen activator, Plat), plasma membrane transporters (Solute carrier family 16, member 1, Slc16a1; ATPase, Na+/K+ transporting, Atp1a), and ets. (c-fos protooncogene, c-fos; telomerase protein component 1, tlp; Annexin 1, anxa 1). Thus, the data about the selective effect of P-11 on genes whose products are involved in the aritmogenesys mechanisms, allow us to consider this compound as a promising means of pathogenetically oriented pharmacotherapy of cardiac arrhythmias.

P-type ATPases as drug targets: tools for medicine and science

P-type ATPases catalyze the selective active transport of ions like H+, Na+, K+, Ca2+, Zn2+, and Cu2+ across diverse biological membrane systems. Many members of the P-type ATPase protein family, such as the Na+,K+-, H+,K+-, Ca2+-, and H+-ATPases, are involved in the development of pathophysiological conditions or provide critical function to pathogens. Therefore, they seem to be promising targets for future drugs and novel antifungal agents and herbicides. Here, we review the current knowledge about P-type ATPase inhibitors and their present use as tools in science, medicine, and biotechnology. Recent structural information on a variety of P-type ATPase family members signifies that all P-type ATPases can be expected to share a similar basic structure and a similar basic machinery of ion transport. The ion transport pathway crossing the membrane lipid bilayer is constructed of two access channels leading from either side of the membrane to the ion binding sites at a central cavity. The selective opening and closure of the access channels allows vectorial access/release of ions from the binding sites. Recent structural information along with new homology modeling of diverse P-type ATPases in complex with known ligands demonstrate that the most proficient way for the development of efficient and selective drugs is to target their ion transport pathway.

Red blood cell Na pump: Insights from species differences

The red blood cell membrane is specialized to exchange chloride and bicarbonate; usually the pH gradient, the chloride ratio, and the membrane potential are tightly coupled. We review the evidence that led to the ability to separately vary inside and outside pH in red cells. The effect of pH on Na pump activity and on the selectivity of the inside and the outside transport sites is reviewed. In red blood cells, at high pH, the outside site is not selective. An increase in protons leads to an increase in K(+) affinity, thus making the site more selective. The pK for this site is different in rats and humans; because of the high conservation of residues in these two species, there are only a few possible residues that can account for this difference. On the inside, work from unsided preparations suggests that, at high pH, the transport site is highly selective for Na(+). Once again, an increase in protons leads to an increase in K(+) affinity, but now the result is a less selective site. During their maturation, reticulocytes lose many membrane proteins. The type and fractional loss is species dependent. For example, most reticulocytes lose most of their Na pumps, retaining about 100 pumps per cell, but animals from the order Carnivora lose all their pumps. We review some of the evidence that PKC phosphorylation of N-terminus serines is responsible for endocytosis in other cell types and species variation in this region.

Study on cardioactive effects of brazilein

Brazilein (6a,7-dihydro-3,6a,10-trihydroxy-benz[b]indeno[1,2-d]pyran-9(6H)-one) is a compound obtained in a large amount from Caesalpinia sappan ethanol extracts with a high purity of about 98%. In isolated cardiac tissues, we found that brazilein exhibited a positive inotropic action in a concentration-dependent manner with little effect on heart rate and coronary perfusion. To study its possible mode of action, isolated rat hearts were treated with propranolol. This treatment did not alter the cardiotonic effect of brazilein, suggesting that this effect does not involve stimulation of beta-adrenoceptors. On the other hand, an analysis of the interaction between Na(+),K(+)-ATPase and brazilein was carried out. Albino guinea pig erythrocytes (mainly alpha1-Na(+),K(+)-ATPase isoforms) enriched with Na(+),K(+)-ATPase isoforms were utilized to compare the inhibition promoted by brazilein with that of classical inhibitors such as the cardiac glycoside deslanoside. Analysis of inhibition curves revealed that unlike deslanoside, brazilein had a relatively low affinity for erythrocyte isoforms and failed to completely inhibit the Na(+),K(+)-ATPase activity. The extent of the maximum inhibition rate was about 50%. The inhibitory effect of brazilein was not antagonized by 10 mmol/l K(+), as observed with deslanoside. Electrocardiogram research in vivo showed that brazilein did not induce the ventricular arrhythmias observed with deslanoside, suggesting that brazilein might have a less adverse effect and higher therapeutic index than cardiac glycosides. In light of all the above-mentioned observations, it can be concluded that brazilein, a molecule with a non-steroidal skeleton, produced its positive inotropic effect through inhibiting Na(+),K(+)-ATPase and could thus serve as a structural paradigm to develop new inotropic drugs.

Kinetic characterization of tetrapropylammonium inhibition reveals how ATP and Pi alter access to the Na+-K+-ATPase transport site

Current models of the Na(+)-K(+)-ATPase reaction cycle have ATP binding with low affinity to the K(+)-occluded form and accelerating K(+) deocclusion, presumably by opening the inside gate. Implicit in this situation is that ATP binds after closing the extracellular gate and thus predicts that ATP binding and extracellular cation binding to be mutually exclusive. We tested this hypothesis. Accordingly, we needed a cation that binds outside and not inside, and we determined that tetrapropylammonium (TPA) behaves as such. TPA competed with K(+) (and not Na(+)) for ATPase, TPA was unable to prevent phosphoenzyme (EP) formation even at low Na(+), and TPA decreased the rate of EP hydrolysis in a K(+)-competitive manner. Having established that TPA binding is a measurement of extracellular access, we next determined that TPA and inorganic phosphate (P(i)) were not mutually exclusive inhibitors of para-nitrophenylphosphatase (pNPPase) activity, implying that when P(i) is bound, the transport site has extracellular access. Surprisingly, we found that ATP and TPA also were not mutually exclusive inhibitors of pNPPase activity, implying that when the cation transport site has extracellular access, ATP can still bind. This is consistent with a model in which ATP speeds up the conformational changes that lead to intracellular or extracellular access, but that ATP binding is not, by itself, the trigger that causes opening of the cation site to the cytoplasm.