Contractile properties and myosin heavy chain composition of newborn rat soleus muscles at different stages of postnatal development.

This study was undertaken to correlate some of the functional characteristics with the myofibrillar composition in myosin heavy chain isoforms on newborn and adult rat soleus muscles. The following postnatal ages were chosen in order to determine the role of innervation in the establishment of the mature muscle phenotype: before (postnatal day 6), when (postnatal day 12), and after (days 17 and 23) the monosynaptic innervation appeared. The steady state of definitive innervation was controlled on adult muscles (i.e. approximately 13 weeks). Muscle maturation was followed by ATPase staining and fibre diversity was observed at postnatal day 12. The functional properties of skinned bundles isolated from newborn rats were determined by Calcium/Strontium activation characteristics (Tension/pCa and pSr relationships). From postnatal days 6 to 17, the Soleus bundles exhibited Calcium/Strontium activation characteristics intermediate between slow and fast fibre populations previously described in muscles. At day 23, the Calcium/Strontium activation characteristics of the soleus were closer to those of a slow type. Moreover, we observed a decrease in Ca affinity concomitant with the installation of the monosynaptic innervation, and an increase of the slow type I during postnatal development. Finally, this work reported a greater correlation between the Calcium/Strontium activation parameters and the myosin heavy chain isoform composition at the postnatal days when the mature monosynaptic innervation pattern occurred.