A comparison of the affinities of rat (Na+ + K+)-ATPase isozymes for cardioactive steroids, role of lactone ring, sugar moiety and KCl concentration.
Binding experiments at equilibrium were performed to study pharmacological properties of isozymes of (Na+ + K+)-ATPase from rat tissues. Experiments were performed on brain (alpha 3 isozyme), kidney (alpha 1 isozyme) and heart microsomes (alpha 1 and alpha 2 isozymes). Affinity of series of ouabain and digoxin derivatives was studied in competition experiments. It was observed that: (i) ouabain and digoxin had higher affinity (P less than 0.01) for alpha 3 isozyme (Kd of 0.071 +/- 0.004 and 0.066 +/- 0.001 microM, respectively) than for alpha 1 isozyme (Kd of 15.9 +/- 0.8 and 1.78 +/- 0.46 microM, respectively) and alpha 2 isozyme (Kd of 0.26 +/- 0.04 and 0.15 +/- 0.06 microM, respectively); (ii) saturation of the C20-C22 bond on the C17 beta lactone ring present in ouabain and digoxin markedly decreased the drug affinity for all isozymes (P less than 0.01); and (iii) suppression of the C3 beta osidic chain decreased the affinity of ouabain and digoxin to a higher extent for alpha 2 and alpha 3 than for alpha 1 (P less than 0.01). The presence of 10 mM KCl in the incubation medium decreased ouabain affinity for the alpha 1 isozyme to a much higher extent (Kd increase of about 20-fold) than for the other isozymes (Kd increase of about 2-fold). The results show that the isozymes of (Na+ + K+)-ATPase from rat tissue are differently sensitive to changes in the substituents of the cardioactive steroids and to the presence of 10 mM KCl.
Noël
F
F
Laboratoire de Pharmacodynamie Générale et de Pharmacologie, Université Catholique de Louvain, Brussels, Belgium.
Fagoo
M
M
Godfraind
T
T
D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000201:enzymology; D004077:Digoxin / Q000494:pharmacology; D007527:Isoenzymes / Q000037:antagonists & inhibitors; D007668:Kidney / Q000201:enzymology; D007783:Lactones; D008861:Microsomes / Q000201:enzymology; D009206:Myocardium / Q000201:enzymology; D010042:Ouabain / Q000378:metabolism / Q000494:pharmacology*; D011189:Potassium Chloride / Q000494:pharmacology; D011485:Protein Binding; D051381:Rats; D011919:Rats, Inbred Strains; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013329:Structure-Activity Relationship
D007527:Isoenzymes; D007783:Lactones; D010042:Ouabain; D011189:Potassium Chloride; D004077:Digoxin; D000254:Sodium-Potassium-Exchanging ATPase