MYONUCLEAR LOSS IN ATROPHIED SOLEUS MUSCLE-FIBERS

Background: A skeletal muscle fiber consists of many successive ''terr itories,'' each controlled by the nucleus residing in that territory. Because nuclei appear to control a specific amount of territory (nucle ar domain), nuclei must be added to accommodate an increase in fiber s ize. Because growth and hypertrophy require the addition of nuclei to fibers, it is of interest to determine whether atrophy causes a decrea se in myonuclear number. This study compared the myonuclear population in the soleus muscles of rats that had undergone atrophy due to 10 da ys of spaceflight in the space shuttle. Endeavour, with muscles of gro und-based control animals (10 rats each). Methods: Myofibrillar ATPase activity was used to determine the major skeletal muscle fiber types in control rats and those having spent 10 days in space, and dystrophi n antibodies were used to label the sarcolemma to identify underlying myonuclei. Results: Type I and II fibers were atrophied after the flig ht, but type I fibers were atrophied twice as much as type II. Myonucl ei were counted in identified and measured fibers, and the distributio n normalized to number per millimeter of fiber circumference; this was significantly greater in type II than in type I fibers in both groups of rats. However, although the muscle fibers from flight animals were significantly atrophied, the normalized number of nuclei were identic al between control and flight animals, indicating that nuclei decrease d in numbers as the fibers atrophied. Conclusion: The nuclear domain i s under strict control, and a decrease in the domain, as induced by at rophy, will cause nuclear degeneration and loss, which maintains a rel atively constant size of the nuclear domain. (C) 1997 Wiley-Liss, Inc.