Effects of reduced joint mobility and training on Na, K‐ATPase and Ca‐ATPase in skeletal muscle

In guinea pigs, the ankle joint was partly immobilized in a position reducing dorsiflection to 105 degrees (as compared to the normal value of 30 degrees). When compared with the contralateral unrestrained leg, this led to a significant atrophy and a decrease in contractile force (-23%) of the gastrocnemius muscle. This was associated with a significant decrease in the total concentration of [3H]ouabain binding sites in gastrocnemius and plantaris muscle reaching minimum (-19% and -23%) after 3 weeks, but no evidence of degenerative changes. Total contents of Ca and Ca-ATPase were increased by 27% and 22%, respectively. After 4 to 5 weeks of reduced mobility, the concentration of [3H]ouabain binding sites in gastrocnemius muscle returned to control level. The lowest concentration of [3H]ouabain binding sites reached during reduced mobility was 258 +/- 13 pmol/g wet wt., and the maximum value attained following 3 weeks of reduced mobility and 3 weeks of training by running was 498 +/- 25 pmol/g wet wt., i.e, 93% higher. In soleus, training produced an increase of 25%. Clinically, it is important to realize that movable braces cannot prevent the development of muscular atrophy. The observed spontaneous recovery of the Na,K-pump concentration may partly explain why patients using movable casts show a better capacity for physical performance than those treated with complete immobilization. In conclusion, the total concentration of Na,K-pumps in guinea pig skeletal muscle undergoes downregulation and upregulation as a function of contractile activity as well as muscle length under conditions mimicking the constraints on mobility frequently used in the clinical treatment of lesions.