On the binding of a 3-alpha-methylated digitoxigenin-glucoside to ouabain receptors in heart muscle homogenate.
3-alpha-Methyl substituted digitoxigenin-3-beta-glucoside (methyl-dtg-gluc) displays unusual features, e.g. a high dissociation rate constant from its binding site leading to rapid reversibility of the inotropic and toxic effects, and a flat dose-response curve attaining higher inotropic maxima thus indicating an increased therapeutic index in animal experiments. In order to check whether or not methyl-dtg-gluc is specifically bound to the same receptors as classic cardiac glycosides we compared binding of ouabain and of methyl-dtg-gluc to guinea-pig heart muscle homogenate. For both compounds specific binding required the addition of ATP (2.5 mM). The binding curve for ouabain yielded half maximum binding at 1.3 x 10(-7) M and maximum number of binding sites of 6 pmole/mg protein; the corresponding values for methyl-dtg-gluc amounted to 1.4 x 10(-6) M and 6 pmole/mg protein, respectively. A mutual competition could be demonstrated between the two compounds. Since the provided data are equilibrium values they do not exclude higher turnover rates of methyl-dtg-gluc in comparison with ouabain at a given glycoside-ATPase complex concentration which can be expected from the fast dissociation rate constant of the methyl-dtg-gluc-ATPase complex. The results are briefly discussed with respect to the molecular mole of action of cardiac glycosides.
D000818:Animals; D004073:Digitoxigenin / Q000031:analogs & derivatives* / Q000378:metabolism; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008566:Membranes / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D011955:Receptors, Drug / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase
D011955:Receptors, Drug; C028996:cardiac glycoside receptors; D004073:Digitoxigenin; C035601:3 alpha-methyldigitoxigenin glucoside; D000254:Sodium-Potassium-Exchanging ATPase