Substrate profile in rat soleus muscle fibers after hindlimb unloading and fatigue

Abstract

Limb muscles from rats flown in space and after hindlimb unloading (HU) show 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 slow-twitch fibers and to assess the substrate utilization patterns in single slow type I fibers isolated from control and HU animals. A second objective 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 isolated from the freeze-dried soleus and gastrocnemius muscles. HU increased the 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 substrate profile was shifted toward that observed in fast fibers. For example, fiber 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 type IIa fiber. With contractile activity, the type I fiber from the HU animal showed a greater utilization of glycogen and accumulation of lactate compared with the control type I fiber. HU had no effect on the ability of crude homogenate or mitochondria fractions from the soleus or gastrocnemius to oxidize pyruvate or palmitate. The increased fatigability after HU may have resulted from an elevated glycolysis producing an increased cell lactate and a decreased pH.

Effect of prior chronic contractile activity on mitochondrial function and apoptotic protein expression in denervated muscle

“…In contrast to chronic contractile activity, chronic muscle disuse results in a rapid loss of skeletal muscle mass (27) and an impairment in oxidative metabolism (12). These changes in muscle composition are facilitated by rapid increases in muscle protein degradation via the preferential expression of proteins that promote muscle atrophy (23).…”

Lactate dehydrogenase expression at the onset of altered loading in rat soleus muscle

“…In summary, the onset of altered loading induced a differential expression of LDH-A and -B in the rat soleus muscle, favoring rapid energy production. Long-term altered loading is associated with myofiber conversion; however, the rapid changes in LDH at the onset of altered loading may be involved in other physiological processes.atrophy; hypertrophy; disuse; hindlimb suspension; glycolytic enzymes THE LOADING DEMANDS PLACED on skeletal muscle can dramatically affect fiber cross-sectional area, phenotype, and metabolic profile (14,30,40). These adaptations are specific to the type of loading stimulus applied.…”

“…Functionally overloading the rat soleus muscle for 3 days alters gene expression for immune function, inflammation, and cell cycle regulation (9). Brief periods of muscle unloading initiate signaling that decreases muscle mass that includes a decrease in protein synthesis (26) and satellite cell proliferation (11).Increasing or decreasing muscle loading for extended periods can reduce the soleus muscle's ability to oxidize longchain fatty acids (14,41,49) and is likely related to a reduced expression of 3-hydroxyacyl-CoA dehydrogenase (41), fatty acid transport proteins (9, 49), and fatty acid binding proteins (49). It has been shown that functional overload can decrease many markers related to glycogen metabolism in both slow and fast muscles (4, 35).…”

“…atrophy; hypertrophy; disuse; hindlimb suspension; glycolytic enzymes THE LOADING DEMANDS PLACED on skeletal muscle can dramatically affect fiber cross-sectional area, phenotype, and metabolic profile (14,30,40). These adaptations are specific to the type of loading stimulus applied.…”

“…Increasing or decreasing muscle loading for extended periods can reduce the soleus muscle's ability to oxidize longchain fatty acids (14,41,49) and is likely related to a reduced expression of 3-hydroxyacyl-CoA dehydrogenase (41), fatty acid transport proteins (9,49), and fatty acid binding proteins (49). It has been shown that functional overload can decrease many markers related to glycogen metabolism in both slow and fast muscles (4,35).…”