EFFECTS OF EXOGENOUS GROWTH-HORMONE ON SKELETAL-MUSCLE OF YOUNG FEMALE RATS

We examined the effects of exogenous growth hormone (GH) treatment on the soleus and rectus femoris muscles of young female rats. Rat GH (1. 8 IU/mg) was administered for 3 weeks by subcutaneous injection, twice a day, at doses of 0.5, 0.6, and 0.8 mg/day during the 1st, 2nd, and 3rd week, respectively. Final body weight, as well as wet and dry weig ht, of the soleus and rectus femoris muscles were significantly greate r in the GH-treated group, compared to controls. Muscle weight to body weight ratios did not differ between the two groups. The fiber type c omposition of the soleus muscle was determined by histochemical staini ng for myosin ATPase activity. No statistically significant difference was found between the GH-treated and the control groups in the percen tages of fiber types. However, GH treatment significantly increased th e cross-sectional area of type II fibers of the soleus muscle. These r esults suggest that, in young female rats, acceleration of body weight gain by homologous GH administration is accompanied by a proportional hypertrophy of skeletal muscle mass, Increased muscle mass is due to hypertrophy of muscle fibers. Type II muscle fibers appear to be more sensitive to GH stimulation.