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The Journal of Cell Biology, Vol 57, 743-759, Copyright © 1973 by Rockefeller University Press

ARTICLE

AUTORADIOGRAPHIC LOCALIZATION OF NEW RNA SYNTHESIS IN HYPERTROPHYING SKELETAL MUSCLE



Charles K. Jablecki 1, John E. Heuser 1, and Seymour Kaufman 1

1 From the Laboratory of Neurochemistry, National Institute of Mental Health, and the Laboratory of Neuropathology and Neuroanatomical Sciences, National Institute of Neurological Diseases and Stroke, Bethesda, Maryland 20014.

Dr. Jablecki's present address is the Department of Neurology, Mayo Clinic, Rochester, Minnesota 55901. Dr. Heuser's present address is the Department of Biophysics, University College of London, London WC 1, England.

Work-induced growth of rat soleus muscle is accompanied by an early increase in new RNA synthesis. To determine the cell type(s) responsible for the increased RNA synthesis, we compared light autoradiographs of control and hypertrophying muscles from rats injected with tritiated uridine 12, 24, and 48 h after inducing hypertrophy. There was an increased number of silver grains over autoradiographs of hypertrophied muscle. This increase occurred over connective tissue cells; there was no increase in the number of silver grains over the muscle fibers. Quantitative studies demonstrated that between 70 and 80% of the radioactivity in the muscle that survived fixation and washing was in RNA. Pretreatment of the animals with actinomycin D reduced in parallel both the radioactivity in RNA and the number of silver grains over autoradiographs. Proliferation of the connective tissue in hypertrophying muscle was evident in light micrographs, and electron micrographs identified the proliferating cells as enlarged fibroblasts and macrophages; the connective tissue cells remained after hypertrophy was completed. Thus, proliferating connective tissue cells are the major site of the increase in new RNA synthesis during acute work-induced growth of skeletal muscle. It is suggested that in the analysis of physiological adaptations of muscle, the connective tissue cells deserve consideration as a site of significant molecular activity.

Submitted on October 19, 1972
Revised on February 5, 1973


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