Growth hormone/IGF-I and/or resistive exercise maintains myonuclear number in hindlimb unweighted muscles

David L. Allen 1 , Jon K. Linderman 2 , Roland R. Roy 3 , Richard E. Grindeland 2 , Venkat Mukku 4 , and V. Reggie Edgerton 1 , 3 1 Department of Physiological Science and 3 Brain Research Institute, University of California, Los Angeles, 90095-1527; 2 National Aeronautics and Space Administration-Ames Research Center, Moffett Field, 94035-1000; and 4 Genentech Incorporated, South San Francisco, California 94080 Received 6 January 1997; accepted in final form 15 July 1997. Allen, David L., Jon K. Linderman, Roland R. Roy, Richard E. Grindeland, Venkat Mukku, and V. Reggie Edgerton. Growth hormone/IGF-I and/or resistive exercise maintains myonuclear number in hindlimb unweighted muscles. J. Appl. Physiol. 83(5): 1857-1861, 1997. In the present study of rats, we examined the role, during 2 wk of hindlimb suspension, of growth hormone/insulin-like growth factor I (GH/IGF-I) administration and/or brief bouts of resistance exercise in ameliorating the loss of myonuclei in fibers of the soleus muscle that express type I myosin heavy chain. Hindlimb suspension resulted in a significant decrease in mean soleus wet weight that was attenuated either by exercise alone or by exercise plus GH/IGF-I treatment but was not attenuated by hormonal treatment alone. Both mean myonuclear number and mean fiber cross-sectional area (CSA) of fibers expressing type I myosin heavy chain decreased after 2 wk of suspension compared with control (134 vs. 162 myonuclei/mm and 917 vs. 2,076 µm 2 , respectively). Neither GH/IGF-I treatment nor exercise alone affected myonuclear number or fiber CSA, but the combination of exercise and growth-factor treatment attenuated the decrease in both variables. A significant correlation was found between mean myonuclear number and mean CSA across all groups. Thus GH/IGF-I administration and brief bouts of muscle loading had an interactive effect in attenuating the loss of myonuclei induced by chronic unloading. atrophy; unloading; remodeling; plasticity; countermeasures 0161-7567/97 $5.00 Copyright © 1997 the American Physiological Society