Effect of age, potassium depletion and denervation on specific displaceable [3H]ouabain binding in rat skeletal muscle in vivo

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Abstract
Following i.p. injection of [3H]ouabain in rats, the isotope is rapidly distributed in blood plasma available for binding to the Na-K-ATPase in the plasma membranes of most tissues. In skeletal muscle tissue excised and washed 4 .times. 30 min in ice-cold buffer, 95% of the 3H activity retained was [3H]ouabain using a specific binding assay. The [3H]ouabain bound to soleus and extensor digitorum longus (edl) muscles in vivo and retained following wash-out in the cold showed the same saturation characteristics as those determined when binding took place in vitro. In soleus and edl muscles obtained form 28-day-old rats, the number [3H]ouabain binding sites measured in vivo was 583 .+-. 19 and 720 .+-. 22 pmol/g wet wt., respectively, i.e., in good agreement with previous and present results obtained in vitro. In vivo measurements showed that 7 days after denervation, the number of [3H]ouabain binding sites in soleus and edl muscles was reduced by 22 and 13%, respectively. In the age interval from 28 to 85 days, the number of [3H]ouabain binding sites in soleus decreased by 58%. Following i.p. injection of [3H]ouabain, the 85-day-old rats showed a more pronounced and sustained rise in plasma 3H activity, which in part can be due to the reduced capacity for [3H]ouabain binding in skeletal muscle. K depletion induced by the administration of K-deficient diet for 3 wk reduced [3H]ouabain binding by 63% in soleus muscles. In the K-depleted animals, the plasma 3H activity measured 15 min after i.p. injection of [3H]ouabain was 77% higher than in controls receiving the same dose per kg body wt. Increased digitalis toxicity due to increasing age or K depletion evidently is related to reduced binding capacity for digitalis glycosides in skeletal muscle.