The effect of androgen status on skeletal muscle myosin heavy chain mRNA and protein levels in rats recovering from immobilization

Immobilization rapidly alters skeletal muscle. The aim of the present study was to determine whether testosterone administration or, in contrast, hypogonadism affects the recovery of muscle mass and myosin heavy chain (MHC) profile at both the mRNA and protein level, after 1 week of immobilization. Male rats were assigned to one of five groups: control (C), hindlimb-immobilized (IMM), and recovery (REC; where animals were allowed 2 weeks of free cage-activity after immobilization). The recovery group was further divided to eugonadal (REC-C), castrated (REC-GDX), and a testosterone-treated (REC-T). In all groups except REC-T, the body masses after immobilization were smaller than in C, although after immobilization the body mass in REC-T recovered at a slower rate than in the other two REC groups. The gastrocnemius mass and the amount of type IIa MHC mRNA decreased during immobilization, but the control levels were regained after recovery. The amount of type IIb mRNA was reduced in REC-GDX compared to C and IMM. The changes in the relative distribution of MHC mRNA were in line with these results. After recovery, the proportion of type IIx MHC protein increased and type IIb protein decreased, although in REC-T the changes were not statistically significant. The proportion of type IIa MHC protein increased only in REC-GDX. In summary, during recovery from immobilization it seems that muscle mass increases and the MHC mRNA and protein profile tend to change toward a slower phenotype, primarily as a result of the decrease in type IIb MHC. However, these changes occur rather independently of the testosterone status. Accepted: 9 June 2000