Fuel utilization skeletal muscle

The preferential utilization of fatty acids over glucose as a fuel in skeletal muscle depends on the following factors ... 

Ketone body oxidation also increases during exercise. Their utilization as a fuel is dependent on their rate of production by the liver. Ketone bodies are, however, never a major fuel for skeletal muscle (muscles prefer free fatty acids). 

Skeletal muscle utilizes glucose as a fuel, forming both lactate and CO2. It stores glycogen as a fuel for its use in muscular contraction and synthesizes muscle protein from plasma amino acids. Muscle accounts for approximately 50% of body mass and consequently represents a considerable store of protein that can be drawn upon to supply amino acids for gluconeogenesis in starvation. 

Influence of Aerobic Exercise on Fuel Utilization by Skeletal Muscle... 

Layman—Biochemical adaptations in skeletal muscle induced by exercise training/Ronald L. Terjungand David A. Hood—Influence of aerobic exercise on fuel utilization/Michael N. Goodman—[etc.]... 

Muscle can utilize a variety of fuels-glucose, fatty acids, and ketone bodies. Skeletal muscle varies widely in its energy demands and the fuels it consumes, in line with its wide variations in activity. 

Fig. 2.3 Oxidation of fuels in exercising skeletal muscle. Exercising muscle uses more energy than resting muscle, and, therefore fuel utilization is increased to supply more ATP.

As Otto Shape runs, his skeletal muscles increase their use of ATP and their rate of fuel oxidation. Fatty acid oxidation is accelerated by the increased rate of the electron transport chain. As ATP is used and AMP increases, an AMP-dependent protein kinase acts to facilitate fuel utilization and maintain ATP homeostasis. Phosphorylation of acetyl CoA carboxylase results in a decreased level of malonyl CoA and increased activity of carnitine palmitoyl CoA transferase I. At the same time, AMP-dependent protein kinase facilitates the recruitment of glucose transporters into the plasma membrane of skeletal muscle, thereby increasing the rate of glucose uptake. AMP and hormonal signals also increase the supply of glucose 6-P from glycogenoly-sis. Thus, his muscles are supplied with more fuel, and all the oxidative pathways are accelerated. 

Fig. 42.3. Interorgan amino acid exchange after an overnight fast. After an overnight fast (the postabsorptive state), the utilization of amino acids for protein synthesis, for fuels, and for the synthesis of essential functional compounds continues. The free amino acid pool is supported largely by net degradation of skeletal muscle protein. Glutamine and alanine serve as amino group carriers from skeletal muscle to other tissues. Glutamine brings NH4 to the kidney for the excretion of protons and serves as a fuel for the kidney, gut, and cells of the immune system. Alanine transfers amino groups from skeletal muscle, the kidney, and the gut to the liver, where they are converted to urea for excretion. The brain continues to use amino acids for neurotransmitter synthesis.

The actions of epinephrine and norepinephrine in the liver, the adipocyte, the skeletal muscle cell, and the a and 3 cells of the pancreas direcdy influence fuel metabolism (Fig. 43.6). These catecholamines are counterregulatory hormones that have metabolic effects directed toward mobilization of fuels from their storage sites for oxidation by cells to meet the increased energy requirements of acute and chronic stress. They simultaneously suppress insulin secretion, which ensures that fuel fluxes will continue in the direction of fuel utilization rather than storage as long as the stressful stimulus persists. 

---