Immobilization atrophy and membrane properties in rat skeletal muscle fibres

Wet mass, resting membrane potential, frequency of miniature end-plate potentials and the concentration of [³H]ouabain-binding sites were studied after 7 days' immobilization of the rat soleus and extensor digitorum longus (EDL) muscles in the shortened or stretched position and after 3 and 7 days of remobilization. We observed that the loss of muscle mass by 37% in the rat soleus immobilized for 7 days in the shortened position is accompanied by a membrane depolarization of about 5 mV, a decrease in frequency of miniature end-plate potentials by 60 % and a decrease of [³H]ouabain binding by 25%. Only minor changes were found in stretched soleus and in shortened and stretched EDL. After 3 days of remobilization of stretched soleus the muscle mass, [³H]ouabain binding and miniature end-plate potential frequency recovered to control values but the resting membrane potential continued to decrease. All changes induced by immobilization disappeared on day 7 of remobilization.