Age effect on expression of myosin heavy and light chain isoforms in suspended rat soleus muscle. J. Appl. Physiol. 86(5): 1483-1489, 1999.-This study was designed to test the hypothesis that myosin heavy (MHC) and light chain (MLC) plasticity resulting from hindlimb suspension (HS) is an age-dependent process. By using an electrophoretic technique, the distribution of MHC and MLC isoforms was quantitatively evaluated in the soleus muscles from 3-or 12-wk-old rats after 1-3 wk of HS treatment

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... s maintained. In normal 12-and 15-wk-old rats, the soleus muscles contained a predominance of MHC I (ϳ94%) with small amounts of MHC IIa , but not MHC IId or MHC IIb . The suspended muscles of adult rats were characterized by the appearance of MHC IIb and MHC IId , the latter reaching ϳ6% after 3 wk of HS treatment. In contrast to changes in MHC, HS did not induce a transition in the MLC pattern in the soleus muscles from adult rats. Compared with adult rats, in juveniles HS had a much more pronounced effect on the shift toward faster MHC and MLC isoform expression. The soleus muscles of 6-wk-old rats after 3 wk of HS were composed of 37.0% MHC I , 19.1% MHC IIa , 23.7% MHC IId , and 20.2% MHC IIb . Changes in MLC isoforms consisted of an increase in MLC 1f and MLC 2f concomitant with a decrease in MLC 2s . These results indicate the existence of a differential effect of HS on MHC and MLC transitions that appears to be age dependent. They also suggest that the suspended soleus muscles from young rats may acquire the intrinsic contractile properties that are intermediate between those in the normal soleus and typical fast-twitch skeletal muscles. development; hindlimb suspension; transition SKELETAL MUSCLE FIBERS can be envisaged as dynamic structures because they are capable of changing certain properties in response to altered functional demands. Generally, increased contractile activities, e.g., chronic stimulation, the removal of a synergist muscle, or heavy exercise training, are responsible for muscular hypertrophy, an increase in maximal tension, and/or a reduction in contraction speed (19, 22, 27) . In contrast, hindlimb suspension (HS), introduced by Morey (18) as a model for weightlessness during spaceflight, leads to 8750-7587/99 $5.00