Single muscle fiber enzyme shifts with hindlimb suspension and immobilization

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Abstract
The purpose of this investigation was to determine how models of weightlessness, hindlimb suspension (HS), and hindlimb immobilization (HI) affect the metabolic enzyme profile in the slow oxidative (SO), fast oxidative glycolytic (FOG), and fast glycolytic (FG) fibers of rat hindlimb. After 1, 2, or 4 wk of HS or HI, single fibers were isolated from freeze-dried soleus and gastrocnemius muscles; a small section of each fiber was run on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels to identify fiber type, and the remaining piece was assayed for either lactate dehydrogenase (LDH) and citrate synthase (CS) or phosphofructokinase (PFK) and beta-hydroxyacyl-CoA dehydrogenase (beta-OH-acyl-CoA). Two weeks of HS induced an almost twofold increase in the activity of CS (2.13 +/- 0.13 vs. 3.60 +/- 0.26 mol.kg dry wt-1.h-1) in the SO fiber of the soleus, and the activity stayed high at 4 wk. Although the FOG fiber had significantly higher CS activity (3.85 +/- 0.29) than either the SO or FG (1.59 +/- 0.16 mol.kg dry wt-1.h-1) fiber, neither fast fiber type was altered by HS. The glycolytic enzymes LDH and PFK were both elevated in the SO fiber after HS. The increase in LDH occurred by 1 wk (14.80 +/- 1.51 vs. 8.83 +/- 0.78), whereas the activity of PFK was not significantly changed until 4 wk (1.16 +/- 0.13 vs. 0.68 +/- 0.05 mol.kg dry wt-1.h-1). The control FG fiber had the highest LDH (44.30 +/- 2.29) and PFK (2.40 +/- 0.16) activities, followed by the FOG fiber (LDH, 34.10 +/- 2.83; PFK, 1.62 +/- 0.17 mol.kg dry wt-1.h-1); however, the activities of these glycolytic enzymes in the fast fiber types were unaltered by HS. The activity of beta-OH-acyl-CoA was not affected by HS in either the slow or fast fiber types. HI showed qualitatively similar changes to those observed with HS; however, the enzyme shifts developed with a slower time course. In conclusion, both HS and HI shifted the SO fiber enzyme pattern toward that of the control FOG fiber; however, a complete conversion from the SO to FOG fiber did not occur within the 4-wk treatment period.

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