Background: NORs are loops of DNA that transcribe to ribosomal RNA, and their number reflects the rDNA transcription, and, in this way, cell proliferation.
Experimental design: The effect of muscle immobilization and remobilization on the number of nucleolar organizer regions (NORs) in the muscle cells and fibroblasts of rat calf muscles was studied histologically using a silver-staining technique (AgNOR).
Results: In muscle-cell nuclei, immobilization of 3 weeks produced a significant decrease in the number of AgNORs per nucleus. This phenomenon was reversible since, free remobilization of 4 weeks and especially intensified remobilization (free mobilization of 1 week plus treadmill running of 3 weeks) produced significant increase in the AgNOR counts. The treadmill running also produced a significant increase in the number of AgNORs in the muscle cells of contralateral (nonimmobilized) limbs. In intramuscular fibroblast nuclei, immobilization caused, in turn, a steady increase in the AgNOR count, the change being significant after 3 weeks. Free remobilization returned the count towards controls, and intensified remobilization returned it to the control level.
Conclusions: Previous studies have shown that in the muscle cells immobilization decreases and remobilization (training) increases intracellular protein synthesis and that in the fibroblasts the opposite happens (immobilization increases and remobilization decreases intramuscular connective tissue formation). Our study gives indirect evidence that the number of NORs is a marker of ribosomal protein synthesis activity in non-proliferating cells. The cell-proliferation activity was proved to be zero using a proliferating-cell-nuclear-antigen technique.