DIFFERENTIAL-EFFECTS OF A 6-DAY IMMOBILIZATION ON NEWBORN RAT SOLEUS MUSCLES AT 2 DEVELOPMENTAL STAGES

Our objective was to determine the effects of a six-day immobilization on the musculoskeletal system of the rat during postnatal development at two key periods when the states of innervation are known to be dif ferent. This work was undertaken on the soleus muscle since it is well known that postural slow muscles show marked changes after a period o f disuse. Thus, the soleus muscle was immobilized in a shortened posit ion either when the innervation was polyneuronal or monosynaptic, resp ectively from 6 to 12 and from 17 to 23 days. The muscle modifications were followed by ATPase staining and myosin heavy chain (MyHC) isofor m identification using monoclonal antibodies and SDS-PAGE. The functio nal properties of skinned fibre bundles were established by calcium/st rontium (Ca/Sr) activation characteristics. In control muscles the mat uration was characterized by a progressive increase of adult MyHCs (I and IIA) concomitant with a decrease in both the MyHC neo and the Ca a ffinity. Between 6 to 12 days, immobilization of the limb induced an i ncrease in histochemical type IIC fibres. Using antibodies we identifi ed new fibre types, classified as a function of their MyHC isoform co- expression. We observed an increase in expression of both MyHC neo and Ca affinity. From 17 to 23 days, the immobilization induced an increa se in Ca affinity and marked changes in the MyHC isoform composition: disappearance of MyHC neo and expression of the fast MyHC IIB isoform, which in normal conditions is never expressed in the soleus muscle. W e conclude that an immobilization imposed during polyneuronal innervat ion delays the postnatal maturation of the soleus muscle, whereas when the immobilization is performed under monosynaptic innervation the mu scle evolves towards a fast phenotype using a default pathway for MyHC expression. (C) Kluwer Academic Publishers.