Functional Changes in Single Muscle Fibers Following Disuse Atrophy

The purpose of this investigation was to determine how muscle atrophy produced by the hindlimb suspension (HS) model alters the contractile function of slow and fast single fibers. Following 2 wks of HS, small bundles of fibers were isolated from the soleus, and the deep and superficial regions of the lateral and medial heads of the gastrocnemius, respectively. The bundles were placed in skinning solution and stored at -20°C until studied. Single fibers we\u27re isolated and suspended between a motor arm and force transducer, the functional properties were studied, and subsequently the fiber type was established by myosin heavy chain (MHC) analysis on 1-D SDS PAGE. Following HS, slow fibers of the soleus showed a significant reduction in fiber diameter (68±2 vs 41±1 um) and peak tension (1.37±0.01 vs 0. 99±0.06 Kg/cm2), while the Vmax increased ( 1.49±0.11 vs 1.92±0.14 fiber lengths/sec). A histogram showed two populations of fibers, one with Vmax values identical to control slow fibers, and a second with significantly elevated Vmax values. This latter group frequently contained both slow and fast MHC\u27s. The pCa-force relation of the soleus slow fibers was shifted to the right, consequently, the free ca2+ required for the onset of tension and for 50% of peak tension was significantly higher following HS.. Slow fibers isolated from the gastrocnemius following HS showed a significant reduction in diameter (67±4 vs 44±3 um) and peak tension (1.2±0.06 vs 0.96±0.07 Kg/cm2), but Vmax was unaltered (1 .70±0.13 vs 1.65±0.18 Kg/cm2). Fast fibers from the red gastrocnemius showed a significant reduction in diameter (59±2 vs 49±3 um), but no change in peak tension or Vmax. Fast fibers from the white superficial region of the medial head of the gastrocnemius were uneffected by HS. Collectively, these data suggest that HS produces a sequence of change beginning with fiber atrophy (slow and fast fibers) which is followed by a decline in peak tension (slow fibers) and finally an elevated Vmax