L-Thyroxine effects upon ATPase activities of several subcellular fractions of liver of the rat and the guinea pig.

The effect of thyroxine administration upon ATPase activity of several subcellular fractions of livers from rats and guinea pigs has been studied. To determine a patho-physiological dose of levo thyroxine [T4] for guinea pigs, a dose-response curve was examined of T4 effect upon oxidative phosphorylatin of guinea pig liver mitochondria. Maximum stimulation of mitochondrial respiration without uncoupling of oxidative phosphorylation was found with 15 microgram of T4 per 100 g body weight per day. This dose of T4 stimulated Mg++ activated ATPase of plasma membranes of guinea pigs and slightly stimulated Mg++ activated ATPase of guinea pig liver nuclear membranes. Rat liver nuclear membrane ATPase was not responsive to thyroxine at doses from 5 to 150 microgram per 100 g body weight. T4 significantly stimulated Ca++ or Mg++ ATPase of mitochondria and microsomes from both rat and guinea pig liver. Microsomes from both species were maximally activated by Mg++ and no significant additional stimulation with Ca++ was found. Mitochondrial ATPase from both species showed significantly greater Ca++ plus Mg++ ATPase activity than did Mg++ alone. Ca++ activated ATPase was approximately equal to dinitrophenol stimulated mitochondrial ATPase. Maximum activation of microsomal ATPase in both species was found with 1 mM calcium. We conclude that at physiological-intracellular concentrations of Ca++ and Mg++, thyroxine probably stimulates Mg++ activated microsomal ATPase and Ca++ activated mitochondrial ATPase. A potential role of Ca++ as a moderator of thyroxine stimulated activity in mitochondria and the relation of calcium to other metabolic reactions that are thyroxine sensitive is discussed.