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

Maximum velocity of unloaded shortening (V-0) and myosin heavy (MHC) and light chain (MLC) isoform compositions were determined in 185 single fibers from the extensor digitorum long-us (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. V-0 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, V-0 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.