Age-dependent changes in the number of [3H]ouabain-binding sites in rat soleus muscle

The influence of age on the number of (Na+K+)-ATPase units in skeletal muscle has been assessed by measurements of [3H]ouabain binding in vitro and in vivo to rat soleus muscle. In vitro measurements showed that from the 2nd to the 28th day of life, the number of [3H]ouabain-binding sites increases from 120 to 580 pmol/g wet wt. This is followed by a decrease, until a plateau between 150 and 200 pmol/g is reached around 150 days after birth. 60 min after intraperitoneal injection of [3H]ouabain (12.5 mumol/kg body weight), the soleus muscles of 28-day-old rats had accumulated 2.4-times more 3H-activity per g wet wt. than muscles of 85-day-old rats and the 3H-activity in plasma was 54% lower. The results may explain the low sensitivity to digitalis glycosides found in infants as compared to premature or adult individuals.