PLASTICITY OF MYONUCLEAR NUMBER IN HYPERTROPHIED AND ATROPHIED MAMMALIAN SKELETAL-MUSCLE FIBERS

Although a mammalian skeletal muscle fiber may contain thousands of my onuclei, the importance of this number or the potential to modulate it in adult muscle has not been clearly demonstrated. Using immunohistoc hemistry and confocal microscopy, we examined the plasticity of myonuc lear number and fiber size in isolated fast and slow fiber segments fr om adult cat hindlimb muscles in response to chronic alterations in ne uromuscular activity and loading. Compared with slow fibers in the sol eus of control cats, myonuclear number in presumably transformed fast fibers was 32% lower and fiber size was decreased 73% after eliminatio n of neuromuscular activation for 6 mo by spinal isolation. Slow fiber s in the soleus of spinal-isolated cats had smaller cross-sectional ar eas, whereas myonuclear number was not significantly different than th at in the control cats. Myonuclear number in fast plantaris fibers was more than threefold higher and fiber size was 2.8-fold higher after 3 mo of functional overload compared with the plantaris of control cats . Compared with control slow plantaris fibers, myonuclear number and f iber size also increased in overloaded slow plantaris fibers. These re sults demonstrate that changes in myonuclear number are associated wit h changes in myosin type and suggest that modulations in the amount of available DNA may be a factor in regulating cytoplasmic volume of mus cle fibers in response to chronic changes in neuromuscular activity.