GROWTH-HORMONE IGF-I AND OR RESISTIVE EXERCISE MAINTAINS MYONUCLEAR NUMBER IN HINDLIMB UNWEIGHTED MUSCLES/

In the present study of rats, we examined the role, during 2 wk of hin dlimb suspension, of growth hormone/insulin-like growth factor I (GH/I GF-I) administration and/or brief bouts of resistance exercise in amel iorating the loss of myonuclei in fibers of the soleus muscle that exp ress type I myosin heavy chain. Hindlimb suspension resulted in a sign ificant decrease in mean soleus wet weight that was attenuated either by exercise alone or by exercise plus GH/IGF-I treatment but was not a ttenuated by hormonal treatment alone. Both mean myonuclear number and mean fiber cross-sectional area (CSA) of fibers expressing type I myo sin heavy chain decreased after 2 wk of suspension compared with contr ol (134 vs. 162 myonuclei/mm and 917 vs. 2,076 mu m(2), respectively). Neither GH/IGF-I treatment nor exercise alone affected myonuclear num ber or fiber CSA, but the combination of exercise and growth-factor tr eatment attenuated the decrease in both variables. A significant corre lation was found between mean myonuclear number and mean CSA across al l groups. Thus GH/IGF-I administration and brief bouts of muscle loadi ng had an interactive effect in attenuating the loss of myonuclei indu ced by chronic unloading.