Myonuclear number and myosin heavy chain expression in rat soleus single muscle fibers following spaceflight. Allen, D. L., W. Yasui, T. Tanaka, Y. Ohira, S. Nagoaka, C. Sekiguchi, W. E. Hinds, R. R. Roy, and V. R. Edgerton. Department of Physiological Science, University of California, Los Angeles, Los Angeles, CA 90024-1568, Department of Physiology and Biomechanics, National Institute of Fitness and Sports, Kanoya City, 891-23 Japan
APStracts 2:0411A, 1995.
The effects of 14 days of spaceflight on myonuclear number, fiber size, and myosin heavy chain (MHC) expression in isolated rat soleus muscle fiber segments were studied. Single soleus muscle fibers from rats flown on the SLS-2 14-day mission were compared to those from age-matched, ground-based controls using confocal microscopy and gel electrophoresis. Spaceflight resulted in a significant reduction in the number of fibers expressing type I MHC and an increase in the number of fibers expressing type IIx MHC or IIa MHC. Spaceflight also resulted in an increase in the percentage of fibers co-expressing more than one MHC, and in the re-expression of the neonatal isoform of MHC in some fibers. Fiber cross-sectional area was significantly reduced in pure type I MHC-expressing fibers and in fibers co -expressing type I + II MHC but not in fibers expressing one or more type II MHC in the flight rats. Myonuclei/mm was significantly reduced in type I MHC-expressing fibers from the flight rats but was not significantly different in type I + II and type II MHC co -expressing fibers. Fibers expressing neonatal MHC were similar in size to control fibers, but had significantly fewer myonuclei/mm than flight fibers not expressing neonatal MHC. In type I MHC-expressing fibers, the reduction in fiber cross-sectional area was greater than the reduction in myonuclear number; thus the average cytoplasmic volume per myonucleus was significantly lower in flight than control. The reduction in both myonuclear number and fiber size of fibers expressing type I MHC following 14 days of spaceflight supports the hypothesis that changes in the number of myonuclei may be a contributing factor to the reduction in fiber size associated with chronic unloading of the musculature. 

Received 12 January 1995; accepted in final form 8 September
1995.
APS Manuscript Number A44-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 31 October 95