Single muscle fiber enzyme shifts with hindlimb suspension and immobilization

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.

Reloading of atrophied rat soleus muscle induces tenascin-C expression around damaged muscle fibers

“…A main focus of previous research concerning the adaptations of mammalian skeletal muscle to changes in muscle loading was on contractile (34,45), metabolic (22,25,64,66), and cytoskeletal proteins (17,28). Here we show for the first time that expression of the two ECM proteins, tenascin-C and fibronectin, in antigravitational rat skeletal muscle is loading dependent.…”

Fiber type-specific expression of GLUT4 in human skeletal muscle: influence of exercise training.

“…Because large proportions of the skeletal muscles in the rat have a relatively homogenous fiber type composition and because analysis can be done on the entire muscle, this approach is probably valid in this animal. However, great caution should be taken in interpretation because fibers of one type in one muscle can be very different from fibers of the same type in another muscle (32)(33)(34). This is well illustrated in the study by Chi et al (32), who found 2-3 times higher activity of both glycolytic and oxidative enzymes in type I fibers from rabbit tibialis-anterior muscle compared with type I fibers from the soleus muscle.…”

Can we optimise the exercise training prescription to maximise improvements in mitochondria function and content?

“…However, while this has consistently been observed in humans, it has not always been observed in rats, with some studies reporting an increase in CS activity [73], or a tendency for an increase in total mitochondrial volume density [74], in response to hind-limb suspension. It has been suggested that these findings could be reconciled if the non-mitochondrial cell protein decreased to a greater extent than the mitochondrial fraction [73].…”

“…However, while this has consistently been observed in humans, it has not always been observed in rats, with some studies reporting an increase in CS activity [73], or a tendency for an increase in total mitochondrial volume density [74], in response to hind-limb suspension. It has been suggested that these findings could be reconciled if the non-mitochondrial cell protein decreased to a greater extent than the mitochondrial fraction [73]. However, decreases in myofibril volume density are quite small (< 10%) and seem unlikely to explain why hind-limb suspension induced an almost doubling of CS activity in type I oxidative rat skeletal muscle fibres [73], while detraining leads to CS activity returning close to control values in human type I fibres [66].…”

“…It has been suggested that these findings could be reconciled if the non-mitochondrial cell protein decreased to a greater extent than the mitochondrial fraction [73]. However, decreases in myofibril volume density are quite small (< 10%) and seem unlikely to explain why hind-limb suspension induced an almost doubling of CS activity in type I oxidative rat skeletal muscle fibres [73], while detraining leads to CS activity returning close to control values in human type I fibres [66].…”