Effect of growth hormone-secreting tumours on skeletal muscle cellularity in the rat.

Tumours secreting GH (GH1) or GH plus prolactin (GH3) were induced in young (1-week-old) and mature (17-week-old) female Wistar-Furth rats. Young animals were killed at 11 weeks and mature rats at 30 weeks of age. Induction of tumours increased serum GH concentrations and body and soleus muscle weights when compared with those of control rats. The soleus and extensor digitorum longus (EDL) muscles were examined in transverse section by electron microscopy. The percentages of myofibres with myonuclei and with satellite cell nuclei present in both the soleus and EDL muscles were generally greater in young rats with tumours than in young control rats. The percentage of myofibres in the EDL muscle with myonuclei present was higher in mature rats with GH1 tumours compared with age-matched controls. The presence of tumours did not affect the number of fibres in the soleus muscle of either young or mature rats. It was concluded that increased GH concentrations increased muscle weight by increasing the DNA content of myofibres and by myofibre hypertrophy. The rate of proliferation of satellite cells apparently exceeded the rate of incorporation of daughter nuclei into the fibres of the young tumour-bearing rats when compared with that of young controls. Thus, the factors regulating satellite cell proliferation and nuclear incorporation into the myofibre do not appear to be directly coupled.