Two Na,K-ATPase isoenzymes in canine cardiac myocytes. Molecular basis of inotropic and toxic effects of digitalis

J Biol Chem. 1987 May 15;262(14):6842-8.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Digitalis Glycosides / pharmacology*
  • Digitalis Glycosides / toxicity
  • Digitoxigenin / pharmacology
  • Dogs
  • Heart Ventricles / drug effects
  • Heart Ventricles / enzymology
  • In Vitro Techniques
  • Isoenzymes / metabolism*
  • Kinetics
  • Microsomes / enzymology
  • Myocardial Contraction / drug effects
  • Myocardium / enzymology*
  • Ouabain / pharmacology
  • Sodium-Potassium-Exchanging ATPase / metabolism*
  • Ventricular Function

Substances

  • Digitalis Glycosides
  • Isoenzymes
  • Digitoxigenin
  • Ouabain
  • Sodium-Potassium-Exchanging ATPase