Expression of actin and myosin heavy chain genes in skeletal, cardiac and uterine muscles of young and old rats.

The steady-state levels of mRNA and transcription of alpha-skeletal actin (alpha-SKA) and adult myosin heavy chain (MHC) genes were measured in the skeletal, cardiac and uterine muscles of young (22-25 week) and old (123-135 week) female rats. The effects of 10(-8) M 17 beta-estradiol/dexamethasone/T3 alpha on their transcription were also studied. The data show that the alpha-SKA mRNA level is lower in the old skeletal muscle and uterus, but is higher in the old myocardium. The adult MHC mRNA level is not different in the three muscles of both the ages. The transcription of alpha-SKA gene is lower in the skeletal muscle and higher in the uterus of old rats. It is unaltered in the myocardium of old rats. The transcription of adult MHC gene is lower in the old uterus. The effects of hormones on transcription of both the genes are different in the three muscles. We show that the expression of alpha-SKA gene is tissue-specific and age-related. The over-expression of alpha-SKA gene in the old myocardium is possibly due to derepression of the gene caused by hypertrophy of cardiac myocytes, and continuous hemodynamic pressure overload on the old heart.