Effects of insulin and exercise on rat hindlimb muscles after simulated microgravity

This study was designed to examine insulin- and exercise-stimulated glucose uptake and metabolism in the hindlimb muscles of rats after conditions of simulated microgravity. To simulate microgravity, male Sprague- Dawley rats were suspended in a head-down (45°) position with their hindlimbs non-weight bearing (SUS) for 14 days. In addition, rats were assigned to suspension followed by exercise (SUS-E), to cage control (CC), or to exercising control (CC-E) groups. Exercise consisted of five 10-min bouts of treadmill running at the same relative intensity for the CC-E and SUS-E rats (80-90% of maximum O₂ consumption). Hindlimb perfusion results indicated that glucose uptake for the entire hindquarter at 24,000 μU/ml insulin (maximum stimulation) was significantly higher in the SUS (8.9 ± 0.5 μmol · g^-1 · h^-1) than in the CC (7.6 ± 0.4 μmol · g^-1 · h^-1) rats, signifying an increased insulin responsiveness. Glucose uptake at 90 μU/ml insulin was also significantly higher in the SUS (48 ± 4; % of maximum stimulation over basal) than in the CC (21 ± 4%) rats. In addition, exercise-induced increases in glucose uptake for the hindlimbs (133%) and glucose incorporation into glycogen for the plantaris (8.4-fold), extensor digitorum longus (5.4-fold), and white gastrocnemius (4.8-fold) muscles were greater for the SUS-E rats than for the CC-E rats (39% and 1.9-, 1.9-, and 3.0-fold, respectively). Therefore, suspension of the rat with hindlimbs non- weight bearing leads to enhanced muscle responses to insulin and exercise when they were applied separately. However, insulin action appeared to be impaired after exercise for the SUS-E rats, especially for the soleus muscle.