Two Na,K-ATPase isoenzymes in canine cardiac myocytes. Molecular basis of inotropic and toxic effects of digitalis.
Canine cardiac myocytes contain two distinct molecular forms of the Na,K-ATPase catalytic subunit. They are resolved by gel electrophoresis and identified using immunological techniques. The apparent molecular weights of the catalytic subunits are 95,000 (alpha) and 98,000 (alpha +). As judged by [3H]ouabain-binding measurements and Na,K-ATPase assays, the two forms are active and differ by a factor of 150 in their respective affinity for digitalis (ouabain and digitoxigenin). The dissociation constant of the high affinity form (alpha +) is KD, 2 nM, and that of the low affinity molecular form (alpha) is KD, 300 nM. According to both enzymatic and binding assays, up to 70% of maximum inhibition is caused by occupation of the high affinity sites (alpha +). Inasmuch as the pharmacological and toxic concentrations of digitalis in dog are 1 and 200 nM, respectively, and as maximum inhibition of Na+ pump in vivo should not exceed 80% to avoid toxicity (Akera, T. and Brody, T. (1982) Annu. Rev. Physiol. 44, 375-388), it appears that the high affinity molecular form (alpha +) is the pharmacological receptor exclusively related to positive inotropy, whereas the low affinity form (alpha) is mainly associated with toxicity.
D000818:Animals; D004071:Digitalis Glycosides / Q000494:pharmacology* / Q000633:toxicity; D004073:Digitoxigenin / Q000494:pharmacology; D004285:Dogs; D006352:Heart Ventricles / Q000187:drug effects / Q000201:enzymology; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000201:enzymology*; D010042:Ouabain / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D016276:Ventricular Function
D004071:Digitalis Glycosides; D007527:Isoenzymes; D004073:Digitoxigenin; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase