MYONUCLEAR NUMBER AND MYOSIN HEAVY-CHAIN EXPRESSION IN RAT SOLEUS SINGLE MUSCLE-FIBERS AFTER SPACEFLIGHT

The effects of 14 days of spaceflight on myonuclear number, fiber size , and myosin heavy chain (MHC) expression in isolated rat soleus muscl e fiber segments were studied. Single soleus muscle fibers from rats f lown on the Spacelab Life Sciences-2 14-day mission were compared with those from age-matched ground-based control rats by using confocal mi croscopy and gel electrophoresis. Spaceflight resulted in a significan t reduction in the number of fibers expressing type I MHC and an incre ase in the number of fibers expressing type IIx or IIa MHC. Spacefligh t also resulted in an increase in the percentage of fibers coexpressin g more than one MHC and in the reexpression of the neonatal isoform of MHC in some fibers. Fiber cross-sectional area was significantly redu ced in pure type I MHC-expressing fibers and in fibers coexpressing ty pe I+II MHC but not in fibers expressing one or more type II MHC in th e flight rats. The number of myonuclei per millimeter was significantl y reduced in type I MHC-expressing fibers from the flight rats but was not significantly different in type I+II and type II MHC-coexpressing fibers. Fibers expressing neonatal MHC were similar in size to contro l fibers but had significantly fewer myonuclei per millimeter than fli ght fibers not expressing neonatal MHC. In type I MHC-expressing fiber s, the reduction in fiber cross-sectional area was greater than the re duction in myonuclear number; thus the average cytoplasmic volume per myonucleus was significantly lower in flight than in control fibers. T he reduction in both myonuclear number and fiber size of fibers expres sing type I MHC after 14 days of spaceflight supports the hypothesis t hat changes in the number of myonuclei may be a contributing factor to the reduction in fiber size associated with chronic unloading of the musculature.