Differentiation of myosin in soleus and extensor digitorum longus muscle in differnt animal species during development.

CA2+-ATPase activity and light chains of myosin prepared from fast and slow muscles of rat guinea-pig and rabbit were studied during development from embryonic to old age to establish further correlation with the well-known developmental changes in contraction properties of these muscles. The changes involve the slow soleus muscle much more than the fast extensor digitorum longus muscle. Myosin-ATPase activity of the soleus muscle before or at birth is higher than in the muscle of adult animals. Myosin from the soleus muscle of embryos or newborn animals reveals light chains of myosin of both fast and slow type (with a preponderance of light chains of fast type in 26-days-old rabbit embryos). During postnatal development the amount of light chains of the fast type decreases, that of the slow type increases. Myosin from the soleus muscle of adult animals contains only light chains of the slow type. However, myosin from the soleus muscle of 30-months-old rats exhibits high myosin ATPase activity and contains light chains of myosin of both slow and fast type as in perinatal development. This is in agreement with the shortening of contraction time observed in this muscle in very old age. Thus developmental differentiation of myosine in the soleus muscle is followed by a trend of levelling out of the differences between fast and slow muscles of senescent animals. No such "biphasic" development is observed with respect to the fast extensor digitorum longus muscle.