CYTOPLASM-TO-MYONUCLEUS RATIOS FOLLOWING MICROGRAVITY

The cytoplasmic volume-to-myonucleus ratio in the tibialis anterior an d gastrocnemius muscles of juvenile rats after 5.4 days of microgravit y was studied. Three groups of rats (n = 8 each) were used. The experi mental group (space rats) flown aboard the space shuttle Discovery (NA SA, STS-48), while two ground-based groups, one hindlimb suspended (su spended rats), one non-suspended (control), served as controls. Single fibre analysis revealed a significant decrease in cross-sectional are a (mu m(2)) in the gastrocnemius for both the space and the suspended rats; in the tibialis anterior only the suspended rats showed a signif icant decrease. Myonuclei counts (myonuclei per mm) in both the tibial is anterior and gastrocnemius were significantly increased in the spac e rats but not in the suspended rats. The mean myonuclear volume (indi vidual nuclei: mu m(3)) in tibialis anterior fibres from the space rat s, and in gastrocnemius fibres from both the space and the suspended r ats, was Significantly lower than that in the respective control group . Estimation of the total myonuclear volume (mu m(3) per mm), however, revealed no significant differences between the three groups in eithe r the tibialis anterior or gastrocnemius. The described changes in the cross-sectional area and myonuclei numbers resulted in significant de creases in the cytoplasmic volume-to-myonucleus ratio (mu m(3) x 10(3) ) in both muscles and for both space and suspended rats (tibialis ante rior; 15.6 +/- 0.6 (space), 17.2 +/- 1.0 (suspended), 20.8 +/- 0.9 (co ntrol): gastrocnemius; 13.4 +/- 0.4 (space) and 14.9 +/- 1.1 (suspende d) versus 18.1 +/- 1.1 (control)). These results indicate that even sh ort periods of unweighting due to microgravity or limb suspension resu lt in changes in skeletal muscle fibres which lead to significant decr eases in the cytoplasmic volume-to-myonucleus ratio.