Substrate profile in rat soleus muscle fibers after hindlimb unloading andfatigue

Limb muscles from rats flown in space and after hindlimb unloading (HCT) sh ow an increased fatigability, and spaceflight has been shown to result in a reduced ability to oxidize fatty acids. The purpose of this investigation was to determine the effects of HU on the substrate content in fast- and sl ow-twitch fibers and to assess the substrate utilization patterns in single slow type I fibers isolated from control and HU animals. A second objectiv e was to assess whether HU altered the ability of the heart or limb muscle to oxidize pyruvate or palmitate. After 2 wk of HU, single fibers were isol ated from the freeze-dried soleus and gastrocnemius muscles. HU increased t he glycogen content in all fiber types, and it increased lactate, ATP, and phosphocreatine in the slow type I fiber. After HU, the type I fiber substr ate profile was shifted toward that observed in fast fibers. For example, f iber glycogen increased from 179 +/- 16 to 285 +/- 25 mmol/kg dry wt, which approached the 308 +/- 23 mmol/kg dry wt content observed in the post-HU t ype IIa fiber. With contractile activity, the type I fiber from the HU anim al showed a greater utilization of glycogen and accumulation of lactate com pared with the control type I fiber. HU had no effect on the ability of cru de homogenate or mitochondria fractions from the soleus or gastrocnemius to oxidize pyruvate or palmitate. The increased fatigability after HU may hav e resulted from an elevated glycolysis producing an increased cell lactate and a decreased pH.