Summary
The cytoplasmic volume-to-myonucleus ratio in the tibialis anterior and gastrocnemius muscles of juvenile rats after 5.4 days of microgravity was studied. Three groups of rats (n=8 each) were used. The experimental group (space rats) was flown aboard the space shuttle Discovery (NASA, STS-48), while two ground-based groups, one hindlimb suspended (suspended rats), one non-suspended (control), served as controls. Single fibre analysis revealed a significant decrease in cross-sectional area (μm2) in the gastrocnemius forboth the space and the suspended rats; in the tibialis anterior only the suspended rats showed a significant decrease. Myonuclei counts (myonuclei per mm) in both the tibialis anterior and gastrocnemius were significantly increased in the space rats but not in the suspended rats. The mean myonuclear volume (individual nuclei: μm3) in tibialis anterior fibres from the space rats, and in gastrocnemius fibres from both the space and the suspended rats, was significantly lower than that in the respective control group. Estimation of the total myonuclear volume (μm3 per mm), however, revealed no significant differences between the three groups in either the tibialis anterior or gastrocnemius. The described changes in the cross-sectional area and myonuclei numbers resulted in significant decreases in the cytoplasmic volume-to-myonucleus ratio (μm3×103) in both muscles and for both space and suspended rats (tibialis anterior; 15.6±0.6 (space), 17.2±1.0 (suspended), 20.8±0.9 (control): gastrocnemius; 13.4±0.4 (space) and 14.9±1.1 (suspended) versus 18.1±1.1 (control)). These results indicate that even short periods of unweighting due to microgravity or limb suspension results in changes in skeletal muscle fibres which lead to significant decreases in the cytoplasmic volume-to-myonucleus ratio.
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Kasper, C.E., Xun, L. Cytoplasm-to-myonucleus ratios following microgravity. J Muscle Res Cell Motil 17, 595–602 (1996). https://doi.org/10.1007/BF00124357
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DOI: https://doi.org/10.1007/BF00124357