Age-related differences in tolerance to digoxin. Lower digoxin concentration in myocardial microsomal fraction in infant rats.

To clarify the mechanism responsible for age-related differences in tolerance to digitalis, we studied the potassium contents of serum and myocardium and the digoxin concentrations of serum, myocardium, and myocardial microsomal fraction in infant and adult rats. While serum potassium concentration was the same in both infant and adult rats, the potassium content of myocardium found in the infant rats was significantly higher than that in the adult ones. Digoxin concentration of serum in infant rats was lower than that in the adult ones after a bolus injection of the same dose of digoxin per kilogram of body weight was given. Digoxin concentration of microsomal fraction, obtained 1 hour after the administration of 0.2 mg/Kg of digoxin in infant rats, was lower than that obtained 1 hour after the administration of 0.1 mg/Kg of digoxin in adult ones even though the digoxin concentrations of serum and myocardium in infant rats were significantly higher than those obtained in adult ones. Thus, less sensitivity to digitalis found in infant rats may be attributable to the higher potassium content of myocardium and the lower digoxin concentration of microsomal fraction.