Age effect on expression of myosin heavy and light chain isoforms in suspended rat soleus muscle

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 distribut ion of MHC and MLC isoforms was quantitatively evaluated in the soleus musc les from 3- or 12-wk-old rats after 1-3 wk of HS treatment was maintained. In normal 12- and 15-wk-old rats, the soleus muscles contained a predominan ce of MHCI (similar to 94%) with small amounts of MHCIIa, but not MHCIId or MHCIIb. The suspended muscles of adult rats were characterized by the appe arance of MHCIIb and MHCIId, the latter reaching similar to 6% after 3 wk o f HS treatment. In contrast to changes in MHC, HS did not induce a transiti on 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 g-wk-ol d rats after 3 wk of HS were composed of 37.0% MHCI, 19.1% MHCIIa, 23.7% MH CIId, and 20.2% MHCIIb. Changes in MLC isoforms consisted of an increase in MLC1f and MLC2f concomitant with a decrease in MLC2s. These results indica te 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 sole us muscles from young rats may acquire the intrinsic contractile proper tie s that are intermediate between those in the normal soleus and typical fast -twitch skeletal muscles.