Single soleus muscle fiber function after hindlimb unweighting in adult and aged rats.

This investigation compared how hindlimb unweighting (HU) affected the contractile function of single soleus muscle fibers from 12- and 30-mo-old Fischer 344 Brown Norway F1 Hybrid rats. After 1 wk of HU, functional properties of single permeabilized fibers were studied, and, subsequently, the fiber type was established by myosin heavy chain (MHC) analysis. After HU, the relative mass of soleus declined by 12 and 19% and the relative mass of the gastrocnemius declined by 15 and 13% in 12- and 30-mo-old animals, respectively. In 12-mo-old animals, the peak active force (5.0 +/- 0.2 x10(-4) vs. 3.8 +/- 0.2 x10(-4) N) and the peak specific tension (92 +/- 4 vs. 78 +/- 3 kN/m2) were significantly reduced in the MHC type I fibers by 24 and 15%, respectively. In 30-mo-old animals, the peak active force declined by 40% (4.7 +/- 0.2 x10(-4) vs. 2.8 +/- 0. 3 x10(-4) N) and the peak specific tension declined by 30% (79 +/- 5 vs. 55 +/- 4 kN/m2). The maximal unloaded shortening velocity of the MHC type I fibers increased in 12-mo-old animals (from 1.65 +/- 0.12 to 2.59 +/- 0.26 fiber lengths/s) and in 30-mo-old animals (from 0.90 +/- 0. 09 to 1.50 +/- 0.10 fiber lengths/s) after HU. Collectively, these data suggest that the effects of HU on single soleus skeletal muscle fiber function occur in both age groups; however, the single MHC type I fibers from the older animals show greater changes than do single MHC type I fibers from younger animals.