Effects of exogenous thyroxine and growth hormone on satellite cell and myonuclei populations in rapidly growing rat skeletal muscle.

We have examined the independent and combined effects of thyroxine (T4) and bovine growth hormone (bGH) on postweaning growth, satellite cell proliferation and accumulation of myonuclei in extensor digitorum longus (EDL) muscles in rats. Daily subcutaneous injections of T4 (1.5 micrograms/100 g BW), but not bGH (.25 IU/100 g BW) or T4 + bGH, resulted in elevated incidence of satellite cell nuclei and satellite cells per muscle fiber in transverse thin section, and a significantly (p less than .05) higher percentage of satellite cell nuclei and percentage satellite cells as a percentage of all muscle nuclei in transverse thin sections after one week of injections. The higher incidence of myonuclei per muscle fiber in transverse section in T4 injected rats at the end of the injection period (p less than .05) was interpreted to be the result of a higher concentration of satellite cells exhibiting normal or elevated mitotic activity during the early part of the injection period. The higher incidence of myonuclei per fiber was not caused by an increased mean fiber cross-sectional area in T4 injected rats. These histological observations relative to T4 injections were not accompanied by significantly greater weight, DNA content or estimates of total satellite cells per EDL muscle after either one or three weeks of hormone injections. These studies support the premise that T4 may be directly involved in the regulation of satellite cell proliferation and myonuclei accumulation, however, they do not support the premise that growth hormone exerts a similar influence during early muscle growth and development.