Skeletal muscle fiber types and myosin ATPase activity do not change with age or growth hormone administration.

Fischer 344 rats at various ages throughout the life span have been treated with growth hormone, clonidine, and insulin-like growth factor-I to restore circulating somatomedin levels in old animals to levels found in younger rats. The injections were intended to reverse (2-week treatments) or prevent (6-month treatments) deleterious effects of aging on skeletal muscle--specifically the loss of fast fibers during the latter part of the life span. However, measurements of myosin ATPase (and subsequent histochemical fiber type determinations) revealed that the previously reported age-related decrease in ATPase activities and fast fiber content did not occur in barrier-protected specific pathogen free rats or mice. None of the treatments used had a significant effect on the ATPase activity or fiber composition of soleus, extensor digitorum longus, or diaphragm muscles, although parallel determinations in collaborating laboratories verified that the hormones had major effects in other systems. Previously reported decreases with age in ATPase activity of heart muscle were confirmed in our experiments, and these decreases were shown to reflect a change in myosin isozyme composition of the hearts, both at the protein and the mRNA levels of gene expression.