Recovery of reinnervating rat muscle after cast immobilization

We evaluated the recovery of the reinnervating rat plantar flexor muscles after different periods of casting and then decasting the lower extremities. Four groups of 4-month-old, female Wistar rats underwent bilateral crush-denervation of the sciatic nerve at the sciatic notch. Two weeks after nerve crush, the hind legs of three groups of rats were immobilized with bilateral casts at the knee and ankle joints and the fourth group was a control group. Of the three casted groups, one was mobilized after 1 week and another group after 3 weeks of casting. The third experimental group remained casted until the end of 6 weeks. Six weeks after the nerve crush, all groups were evaluated for muscle weights of the soleus, plantaris, and gastrocnemius; absolute amounts of the myofibrillar, sarcoplasmic, and stromal proteins in the gastrocnemius; the fiber diameters and percent composition of type I and type II fibers in the soleus and plantaris; and the isometric contractile properties of the soleus muscle. Compared with the denervated control group, the experimental groups revealed the following: (i) Four weeks of casting caused a reduction in wet weight (range 30.6 to 40.4%, P less than 0.01) in the soleus, plantaris, and gastrocnemius muscles. Decasting led to an earlier recovery of the soleus than of the plantaris and gastrocnemius muscles. (ii) The myofibrillar protein returned to control values with 3 weeks of decasting but the stromal protein remained significantly elevated and the sarcoplasmic protein significantly depressed regardless of the period of mobilization. (iii) Except for the type I fibers in the plantaris, the remainder of the muscle fibers in the soleus and plantaris decreased in size due to casting. Only the type I muscle fibers of the soleus increased in size with longer periods of mobilization. (iv) Four weeks of casting significantly altered the maximum isometric twitch tension (-42.3%), contraction time (+17.1%), maximum tetanic tension (-38.1%), and half-fatigue time (+40.5%) in the soleus. The reinnervating soleus muscle appears to recover from the effects of casting sooner than the plantaris or gastrocnemius muscles.