Ontogenetic, gravity-dependent development of rat soleus muscle

We tested the hypothesis that rat soleus muscle fiber growth and changes in myosin phenotype during the postnatal, preweaning period would be largely independent of weight bearing. The hindlimbs of one group of pups were unlo aded intermittently from postnatal day 4 to day 21: the pups were isolated from the dam for 5 h during unloading and returned for nursing for 1 h. Con trol pups were either maintained with the dam as normal or put on an altern ating feeding schedule as described above. The enlargement of mass (similar to3 times), increase in myonuclear number (similar to1.6 times) and myonuc lear domain (similar to2.6 times), and transformation toward a slow fiber p henotype (from 56 to 70% fibers expressing type I myosin heavy chain) obser ved in controls were inhibited by hindlimb unloading. These properties were normalized to control levels or higher within 1 mo of reambulation beginni ng immediately after the unloading period. Therefore, chronic unloading ess entially stopped the ontogenetic developmental processes of 1) net increase in DNA available for transcription, 2) increase in amount of cytoplasm sus tained by that DNA pool, and 3) normal transition of myosin isoforms that o ccur in some fibers from birth to weaning. It is concluded that normal onto genetic development of a postural muscle is highly dependent on the gravita tional environment even during the early postnatal period, when full weight -bearing activity is not routine.