SHORTENING VELOCITY AND ATPASE ACTIVITY OF RAT SKELETAL-MUSCLE FIBERS- EFFECTS OF ENDURANCE EXERCISE TRAINING

Mechanical properties were measured in single skinned fibers from rat hindlimb muscle to test the hypothesis that the fast type IIb fiber ex hibits a higher maximal shortening velocity (V-o) than the fast type I Ia fiber and that the difference is directly attributable to a higher myofibrillar adenosinetriphosphatase (ATPase) activity in the type IIb , fiber. Additional measurements were made to test the hypotheses that regular endurance exercise increases and decreases the V-o of the typ e I and IIa fiber, respectively, and that the altered V-o is associate d with a corresponding change in the fiber ATPase activity. Rats were exercised by 8-12 wk of treadmill running for 2 h/day, 5 day/wk, up a 15% grade at a speed of 27 m/min. Fiber V-o was determined by the slac k test, and the ATPase was measured fluorometrically in the same fiber . The myosin isozyme profile of each fiber was subsequently determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The mea n +/- SE V-o (7.9 +/- 0.22 fiber lengths/s) of the type IIb fiber was significantly greater than the type IIa fiber (4.4 +/- 0.21 fiber leng ths/s), and the higher V-o was associated with a higher ATPase activit y (927 +/- 70 vs. 760 +/- 60 mu M.min(-1).mm(-3)). The exercise progra m induced cardiac hypertrophy and an approximately twofold increase in the mitochondrial marker enzyme citrate synthase. Exercise had no eff ect on fiber diameter or peak tension per cross-sectional area in any fiber type, but, importantly, it significantly increased (23%) both th e V-o and the ATPase activity of the slow type I fiber of the soleus. The increased V-o was highly correlated with (r = 0.76) and probably c aused by the elevated fiber ATPase. Possible causes of the increased f iber V-o and ATPase include an exercise-induced increase in the number of slow fibers expressing fast myosin light chains (from 39 to 83%) a nd a small increase in the number of hybrid fibers containing both slo w and fast myosin heavy chains. The contractile properties of the fast type IIa and IIb fibers of the gastrocnemius muscle were not signific antly altered by the exercise program.