Contractile properties and protein isoforms of single skeletal muscle fibers from 12-and 30-month-old Fischer 344 Brown Norway F1 hybrid rats

This study characterizes single skeletal muscle fiber contractile properties and myosin heavy chain (MHC) isoform compositions from the soleus (SOL) and deep portion of the lateral head of the gastrocnemius (RG) muscles of 12- and 30-month-old Fischer 344 Brown Norway F1 hybrid rats (FBN F1). Thirty months of age is approximately the age of 50% survival for the FBN F1 rat. For type I MHC individual fibers from the SOL of 30-month-old animals, the diameter was 88+/-2 mu m, peak active force was 4.4+/-0.2 x 10(-4) N, peak specific tension (P-0) was 76+/-5 kN/m(2), and maximal unloaded shortening velocity (V-0) was 0.98+/-0.09 fl/s. The type I MHC fibers from the SOL of 12-month-old animals had similar properties with the exception of Po which was 92+/-4 kN/m(2) and V-0 which was 1.65+/-0.12 fl/s. Contractile properties of the RG MHC type I fibers were not significantly different from MHC type I fibers from the SOL in both age groups. The V-0 of the RG type IIa MHC fibers from the 12-month animals (4.05+/-0.36 fl/s) and 30-month animals (3.55+/-0.41 fl/s) and of fibers co-expressing type I MHC and type IIa MHC from 12-month animals (4.21+/-0.55 fl/s) and 30-month animals (2.22+/-0.27 fl/s) were significantly faster than that of MHC type I fibers of the respective age group. In conclusion, skeletal fibers from the 12- and 30-month-old FBN F1 rats demonstrate fiber-type specific properties with a close relationship between the MHC isoform composition and the V-0.