The Role of Protein and Amino Acids in Sustaining and Enhancing Performance (1999) / Chapter Skim
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6 Regulation of Muscle Mass and Function: Effects of Aging and Hormones
6 Regulation of Muscle Mass and Function: Effects of Aging and Hormones
Pages 121-136
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From page 121... ...
Metabolic functions are crucial not only for locomotion but also for maintaining homeostasis of substrates in the circulation and providing amino acids for various body functions. Mitochondria in skeletal muscle convert energy from nutrients into adenosine triphosphate (ATP)
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From page 122... ...
Muscle mass is one determinant of muscle function, which is also dependent on the quality of distinct muscle proteins. Muscle functions include both contractile and metabolic aspects.
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From page 123... ...
With advancing age, a continuous loss of muscle efficiency occurs, indicating that muscle quality is declining. This study normalized muscle strength, quadriceps isokinetic strength, and leg extension for regional muscle mass (measured by Dual Photon X-ray)
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From page 124... ...
Of muscle mass and increased muscle weakness and fatigability, which results in substantial impairment of muscle function, has been coined sarcopenia of aging and may contribute substantially to morbidity of the elderly by restricting physical activity, increasing the risk of falls and fractures, and causing changes in body metabolism and composition, which results in increased incidence of noninsulin-dependent diabetes mellitus. It has been reported that in elderly in comparison with young subjects, there is a decline in the synthesis rate of mixed muscle protein both total and myofibrillar proteins (Welle et al., 1993; Yarasheski et al., 1993~.
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From page 125... ...
These results suggest that the aging process selectively affects the ATP-generating machinery of muscle and imply that any intervention should seek to restrict this loss of mitochondrial capacity. In addition, the reduced synthesis rate of myosin heavy chain is compatible with the notion that the ability to maintain adequate muscle protein quality declines with age, thereby potentially compromising the efficiency of the locomotive apparatus to extract mechanical energy from fuel stores.
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From page 126... ...
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From page 127... ...
(Figure 6-6~. In that study, which employed sampling of the femoral artery, femoral vein, and hepatic vein and administration of amino acid tracers in insulin-deprived patients with diabetes, it was demonstrated that insulin replacement inhibited protein breakdown in the leg, with no effect on protein synthesis, while it inhibited protein breakdown and synthesis in the splanchnic bed, indicating that insulin's anticatabolic effect is largely due to its inhibition of muscle protein breakdown (Nair et al., 1995~.
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From page 128... ...
Another pertinent question is whether insulin differentially affects the individual muscle proteins. Novel techniques that allow assessment of fractional synthesis and breakdown rates of specific proteins, such as mitochondrial proteins, myosin heavy chain, and myosin isoforms, have been, or are being, developed and will soon serve to clarify these issues.
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From page 129... ...
Studies in which IGF-I has been perfused locally indicate that IGF-I preferentially stimulates protein synthesis in muscle (Fryburg, 1994) , whereas systemically administered IGF-I only stimulates muscle protein synthesis when additional amino acids are supplied (Fryburg et al., 1995~.
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From page 130... ...
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From page 131... ...
Arteriovenous studies demonstrated that hyperthyroidism is associated with a net increase of muscle protein breakdown, although no changes were observed in hypothyroid patients (Morrison et al., 1988~. Direct measurement of muscle protein synthesis is required to determine the effect of thyroxine on muscle protein synthesis.
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From page 132... ...
Regulation of breakdown of specific muscle proteins is beyond current investigative capabilities because of the paucity of technology to quantify breakdown of specific muscle proteins. It is possible, however, to measure the regulation of the enzyme systems responsible for protein breakdown.
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From page 133... ...
1996. Effects of testosterone replacement on muscle mass and muscle protein synthesis in hypogonadal men.
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From page 134... ...
1996. Effects of amino acids on synthesis and degradation of skeletal muscle proteins in humans.
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From page 135... ...
We did not look at the dose effect or what is the effect of testosterone in healthy men on muscle protein synthesis. The UCLA group gave supraphysiological doses and measured the effect on muscle mass and strength.
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From page 136... ...
GAIL BUTTERFIELD: In terms of the mili~y's interest, do you find the kinds of changes that you observed in mitochondrial protein and myosin heavy chain, with a decrease in energy intake or a decrease in protein intake?
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