MYOSIN HEAVY-CHAIN EXPRESSION IN RODENT SKELETAL-MUSCLE - EFFECTS OF EXPOSURE TO ZERO-GRAVITY

This study ascertained the effects of 9 days of zero gravity on the re lative (percentage of total) and calculated absolute (mg/muscle) conte nt of isomyosin expressed in both antigravity and locomotor skeletal m uscle of ground control (CON) and flight-exposed (FL) rats. Results sh owed that although there were no differences in body weight between FL and CON animals, a significant reduction in muscle mass occurred in t he 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 examine d in the FL and CON groups. In the VI, there were trends for reduction s 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 II b and possibly type IIx MHC protein (P > 0.05). mRNA levels were consi stent 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 reduc tions in the absolute content of both type I and IIa MHCs that were of fset by calculated increases in type IIb MHC in both VI and red VL. Co llectively, these findings suggest that there are both absolute and re lative changes occurring in MHC expression in the ''red'' regions of a ntigravity skeletal muscle during exposure to zero gravity that could affect muscle function.