EFFECTS OF BOVINE GROWTH-HORMONE ANALOGS ON MOUSE SKELETAL-MUSCLE STRUCTURE

Skeletal muscles of transgenic mice expressing altered bovine growth h ormones (bGH) have been compared with those of nontransgenic mice to d etermine whether muscle fiber type-specific responses or histopatholog ies are associated with the altered gene. The slow soleus and predomin antly fast gastrocnemius muscles were prepared for myofibrillar ATPase activity (to determine muscle fiber type) and histological examinatio n from mice that were either giant (M4 line), larger than normal (M11 line), dwarf (G119K line), or nontransgenic (NTC). No histopathology w as observed in any of the muscles. Although body weights were signific antly different between all four lines of mice, only the giant M4 mice had significantly larger muscle fibers than the other lines of mice, while neither the G119K nor M11 lines were significantly different fro m the NTC for either muscle. No fiber type-specific differences were n oted. These results suggest that the different muscles are the product of differences in numbers of muscle fibers expressed in the G119K and M11 lines of mice; the increase in body mass matched the fiber size g rowth only in the giant M4 line. Therefore, the altered bGH genes may be acting on fetal liver and myoblast/myotube GH receptors to change t he GH and IGF-I regulated pattern of muscle development, and eventuall y, to determine the adult muscle fiber numbers.