Yajid, Fatima, Jacques G. Mercier, Béatrice M. Mercier, Hervé Dubouchaud, and Christian Préfaut. Effects of 4 wk of hindlimb suspension on skeletal muscle mitochondrial respiration in rats. J. Appl. Physiol. 84(2): 479-485, 1998.We investigated in rats the effect of 4 wk of hypodynamia on the respiration of mitochondria isolated from four distinct muscles [soleus, extensor digitorum longus, tibial anterior, and gastrocnemius (Gas)] and from subsarcolemmal (SS) and intermyofibrillar (IMF) regions of mixed hindlimb muscles that mainly contained the four cited muscles. With pyruvate plus malate as respiratory substrate, 4 wk of hindlimb suspension produced an 18% decrease in state 3 respiration for IMF mitochondria compared with those in the control group (P < 0.05). The SS mitochondria state 3 were not significantly changed. Concerning the four single muscles, the mitochondrial respiration was significantly decreased in the Gas muscle, which showed a 59% decrease in state 3 with pyruvate + malate (P < 0.05). The other muscles presented no significant decrease in respiratory rate in comparison with the control group. With succinate + rotenone, there was no significant difference in the respiratory rate compared with the respective control group, whatever the mitochondrial origin (SS, or IMF, or from single muscle). We conclude that 4 wk of hindlimb suspension alters the respiration of IMF mitochondria in hindlimb skeletal muscles and seems to act negatively on complex I of the electron-transport chain or prior sites. The muscle mitochondria most affected are those isolated from the Gas muscle.