Maximum shortening velocity and myosin isoforms in single muscle fibers from young and old rats.

Maximum velocity of unloaded shortening (Vo) and myosin heavy (MHC) and light chain (MLC) isoform compositions were determined in 185 single fibers from the extensor digitorum longus (EDL) and soleus muscles in 3- to 6- and 20- to 24-mo-old rats. In the soleus, fibers expressing the type I MHC isoform dominated in young and old animals. In the EDL, most fibers in the young animals expressed type IIb MHC or a combination of types IIx and IIb (type IIxb), whereas in the old animals type IIxb MHC fibers predominated. Vo was significantly (P < 0.01) lower (0.59 +/- 0.28 ml/s, n = 55) in soleus fibers from old than from young animals (1.12 +/- 0.46 ml/s, n = 48), despite the fact that all fibers expressed the type I MHC and slow MLC isoforms. In the EDL, Vo values in single fibers did not differ between young (2.18 +/- 0.58 ml/s, n = 43) and old animals (2.10 +/- 0.53 ml/s, n = 39). The mechanism underlying age-related slowing in soleus fibers is not known, but it has been suggested that there could be more than one beta/slow MHC isoform and that there is an age-related transition within these isoforms.