title: The influence of potassium ion (K+) on digoxin-induced inhibition of porcine cerebral cortex Na+ / K+-ATPase.
abstract: The in vitro influence of potassium ion modulations, in the concentration range 2 mM-500 mM, on digoxin-induced inhibition of porcine cerebral cortex Na+ / K+-ATPase activity was studied. The response of enzymatic activity in the presence of various K+ concentrations to digoxin was biphasic, thereby, indicating the existence of two Na+ / K+-ATPase isoforms, differing in the affinity towards the tested drug. Both isoforms showed higher sensitivity to digoxin in the presence of K+ ions below 20 mM in the medium assay. The IC50 values for high/low isoforms 2.77 x 10(-6) M / 8.56 x 10(-5) M and 7.06 x 10(-7) M / 1.87 x 10(-5) M were obtained in the presence of optimal (20 mM) and 2 mM K+, respectively. However, preincubation in the presence of elevated K+ concentration (50-500 mM) in the medium assay prior to Na+ / K+-ATPase exposure to digoxin did not prevent the inhibition, i.e. IC50 values for both isoforms was the same as in the presence of the optimal K+ concentration. On the contrary, addition of 200 mM K+ into the medium assay after 10 minutes exposure of Na+ / K+-ATPase to digoxin, showed a time-dependent recovery effect on the inhibited enzymatic activity. Kinetic analysis showed that digoxin inhibited Na+ / K+-ATPase by reducing maximum enzymatic velocity (Vmax) and Km, implying an uncompetitive mode of interaction.
authors.affiliation: Institute of Chemistry, School of Medicine, University of Belgrade, Belgrade, Serbia & Montenegro.
authors.forename: Danijela
authors.initials: D
authors.lastname: Krstić
authors.forename: Nenad
authors.initials: N
authors.lastname: Tomić
authors.forename: Katarina
authors.initials: K
authors.lastname: Krinulović
authors.forename: Vesna
authors.initials: V
authors.lastname: Vasić
mesh_terms: D000251:Adenosine Triphosphatases / Q000737:chemistry; D000818:Animals; D002540:Cerebral Cortex / Q000201:enzymology*; D004077:Digoxin / Q000737:chemistry*; D004305:Dose-Response Relationship, Drug; D015195:Drug Design; D004353:Drug Evaluation, Preclinical; D004791:Enzyme Inhibitors / Q000737:chemistry* / Q000494:pharmacology*; D020128:Inhibitory Concentration 50; D007477:Ions; D008956:Models, Chemical; D011188:Potassium / Q000737:chemistry*; D020033:Protein Isoforms; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors* / Q000737:chemistry*; D013552:Swine
chemical_list: D004791:Enzyme Inhibitors; D007477:Ions; D020033:Protein Isoforms; D004077:Digoxin; D000251:Adenosine Triphosphatases; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium