Contractile function of single muscle fibers after hindlimb suspension.

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-twitch single muscle fibers. After 2 wk 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 degrees C until studied. Single fibers were 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 sodium dodecyl sulfate polyacrylamide gel electrophoresis. After HS, slow-twitch fibers of the soleus showed a significant reduction in fiber diameter (68 +/- 2 vs. 41 +/- 1 micron) and peak tension (1.37 +/- 0.01 vs. 0.99 +/- 0.06 kg/cm2), whereas the maximal shortening speed (Vmax) increased [1.49 +/- 0.11 vs. 1.92 +/- 0.14 fiber lengths (FL)/s]. A histogram showed two populations of fibers: one with Vmax values identical to control slow-twitch fibers and a second with significantly elevated Vmax values. This latter group frequently contained both slow and fast MHC protein isoforms. The pCa-force relation of the soleus slow-twitch 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 after HS. Slow-twitch fibers isolated from the gastrocnemius after HS showed a significant reduction in diameter (67 +/- 4 vs. 44 +/- 3 microns) 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 FL/s). Fast-twitch fibers from the red gastrocnemius showed a significant reduction in diameter (59 +/- 2 vs. 49 +/- 3 microns) but no change in peak tension or Vmax. Fast-twitch fibers from the white superficial region of the medial head of the gastrocnemius were unaffected by HS. Collectively, these data suggest that the effects of HS on fiber function depend on the fiber type and location. Both slow-twitch type I and fast-twitch type IIa fibers atrophied; however, only slow-twitch fibers showed a decline in peak tension, and the increase in Vmax was restricted to a subpopulation of slow-twitch soleus fibers.

Effect of swim exercise training on human muscle fiber function.

This study examined the effect of a typical collegiate swim-training program and an intensified 10-day training period on the peak tension (Po), negative log molar Ca2+ concentration (pCa)-force, and maximal shortening speed (Vmax) of the slow-twitch type I and fast-twitch type II fibers of the deltoid muscle. Over a 10-wk period, the swimmers averaged 4,266 +/- 264 m/day swimming intermittent bouts of front crawl, kicking, or pulling. The training program induced an almost twofold increase in the mitochondrial marker enzyme citrate synthase. Po of the single fibers was not altered by either the training or 10-day intensive training programs, and no significant differences were observed in the Po (kg/cm2) of type I compared with the type II fibers. The type II fiber diameters were significantly larger than the type I fibers (94 +/- 4 vs. 80 +/- 2 microns), and although fiber diameters were unaffected by the training, the 10-day intensive training significantly reduced the type II fiber diameter. The type I fibers from the trained swimmers showed pCa-force curves shifted to the right such that higher free Ca2+ levels were required to elicit a given percent of Po (for values less than 0.5 Po). The activation threshold (pCa) for the onset of tension and the pCa required to elicit one-half maximal tension were not altered by the training in either fiber type. Fiber Vmax (measured by the slack test) was fivefold higher in type II compared with type I fibers (4.85 +/- 0.50 vs. 0.86 +/- 0.04 fiber lengths/s). The exercise-training program significantly increased and decreased the Vmax of the slow and fast fibers, respectively. The 10 days of intensified training produced a further significant decrease in the Vmax of the type II fibers. After a period of detraining, the Vmax of both fiber types returned to the control level. The force-velocity relation was not significantly altered in either fiber type by the swim training; however, the intensified training significantly depressed the velocity of the type II fiber at all loads studied. The Vmax changes with exercise training are likely explained by an exercise-induced expression of fast myosin in slow fibers and slow myosin in fast fibers.