Abstract
This study ascertained the effects of 9 days of zero gravity on the relative (percentage of total) and calculated absolute (mg/muscle) content of isomyosin expressed in both antigravity and locomotor skeletal muscle of ground control (CON) and flight-exposed (FL) rats. Results showed that although there were no differences in body weight between FL and CON animals, a significant reduction in muscle mass occurred in the vastus intermedius (VI) (P < 0.05) but not in the vastus lateralis (VL) or the tibialis anterior. Both total muscle protein and myofibril protein content were not different between the muscle regions examined in the FL and CON groups. In the VI, there were trends for reductions in the relative content of type I and IIa myosin heavy chains (MHCs) that were offset by increases in the relative content of both type IIb and possibly type IIx MHC protein (P > 0.05). mRNA levels were consistent with this pattern (P < 0.05). The same pattern held true for the red region of the VL as examined at both the protein and mRNA level (P < 0.05). When the atrophy process was examined, there were net reductions in the absolute content of both type I and IIa MHCs that were offset by calculated increases in type IIb MHC in both VI and red VL. Collectively, these findings suggest that there are both absolute and relative changes occurring in MHC expression in the "red" regions of antigravity skeletal muscle during exposure to zero gravity that could affect muscle function.
Highlights
We reported that 14 days of rodent hindlimb suspension decreased the relative level of expression of the type I myosin heavy chains (MHCs) while increasing the relative content ofboth the type Ila and IIb MHC isoforms in the antigravity vastus intermedius (VI) as studied at both the protein and mRNA levels of analyses (5)
The key findings of this study indicate that, after as litt le as 9 days in zero gravity, both atrophy and subtle alterations in MHC expression, as examined at both the mRNA and protein levels, occur in rodent skeletal muscles known to be involved in both antigravity and locomotor function
Because both the t otal and myofibril protein content were not different between CON and FL groups, the net loss in muscle protein that occurred in muscles such as the VI appeared to be uniform across cellular fractions
Summary
When the atrophy process was examined, there were net reductions in the absolute content ofboth type I and Ila MHCs that were offset by calculated increases in type Ilb MHC in both VI and red VL These findings suggest that there are both absolute and relative changes occurring in MHC expression in the "red" regions of antigravity skeletal muscle during exposure to zero gravity that could affect muscle function. We report that 9 days of zerogravity exposure induced an altered pattern suggesting a reduction in both the relative and absolute content of type I and Ila MHC protein that was partially offset by small net increases in type llb and possibly type Ilx MHC content in the high-oxidative red VI and vastus lateralis (VL) skeletal muscles. No changes occurred in either the fast low-oxidative VL or in the nonweight-bearing tibialis anterior
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