Mature male rats (n = 16) were assigned to either 14 days of head-down suspension with one hindlimb supported (HDS) or to control cages (C) of similar dimensions. Hindlimb support during HDS preserved the muscle mass-to-body mass ratio (mg/100 g) compared with C conditions for the soleus (48.3 ± 1.0 to 41.7 ± 1.0), plantaris (98.4 ± 3.4 to 103.3 ± 4.1), and gastrocnemius (484.7 ± 18.5 to 507.2 ± 13.9). However, the muscle mass-to-body mass ratio was significantly lower for the soleus (28.9 ± 1.5), plantaris (83.9 ± 3.6), and gastrocnemius (411.9 ± 24.2) muscles from the freely hanging hindlimbs compared with the contralateral muscles from the supported hindlimbs or muscles from C animals. Citrate synthase activity (μmol · g-1 · min-1) was significantly lower in soleus muscles from HDS rats in both the supported (19.4 ± 2.3) and freely hanging (20.0 ± 1.6) hindlimbs compared with C (28.5 ± 3.1), whereas soleus muscle glycogen concentration (mg/g) was significantly higher in the freely hanging limbs from HDS rats (5.90 ± 0.31) but not in the supported limbs (3.80 ± 0.61) compared with C (4.34 ± 0.50). Doppler flow probes were used to determine that iliac blood flow to freely hanging hindlimbs was significantly decreased after 48 (-19 ± 5%) and 72 (-20 ± 6%) h of HDS compared with presuspension values. In addition, iliac vascular resistance was significantly elevated at most time points during the 72 h of HDS in the freely hanging limbs but not the supported hindlimbs. Collectively, these results indicate that hindlimb support during 3-14 days of HDS will sustain the hindlimb muscle mass-to-body mass ratio for antigravity muscles and maintain soleus glycogen concentration and iliac blood flow but will not maintain citrate synthase activity in soleus muscles.
