Myosin heavy chain mRNA and protein distribution in immobilized rat skeletal muscle are not affected by testosterone status

The effects of testosterone treatment and gonadectomy on myosin heavy chain (MHC) messenger RNA (mRNA) and protein expression after 1 week's immobiliz ation were studied in male rat gastrocnemius muscle. In the testosterone-tr eated group silastic testosterone capsules were implanted subcutaneously be fore immobilization. The gonadectomized animals were castrated at 5 weeks o f age. One group of eugonadal animals served as the immobilized control gro up, and another as the sedentary control group. Immobilization was performe d at 9 weeks of age by bilateral hindlimb casting. The body and muscle mass es, and the amount of type tla MHC mRNA decreased significantly (P < 0.01) in the immobilized animals by similar to 30, 40 and 50%, respectively, rega rdless of the serum testosterone levels which ranged from 1.1 +/- 0.4 to 59 +/- 14 nmol L-1. In the immobilized testosterone-treated group the proport ion of type IIx MHC mRNA increased to 14% of the total MHC mRNA (P= 0.02, c ompared with control). The MHC protein distribution did not change signific antly. There were no significant differences in any parameters between the three immobilized groups. In conclusion, neither the lack nor excess of tes tosterone significantly altered the changes caused by immobilization. There fore, it seems that lack of mechanical loading is a far more important dete rminant of MHC expression than the male sex hormone status.