Muscle protein-synthesis activity

The uniqueness of leucine can be further demonstrate by examining muscle protein synthesis. We have shown that leucine has the ability to stimulate protein synthesis in muscles during catabolic conditions such as starvation (25). Using a large dose of leucine, protein synthesis can be stimulated 50 in muscles from starved rats. The significance of this effect remains controversial however, the in vitro activity is clear and emphasizes the unique metabolic potential of leucine. 

Anabolic steroids decrease catabolism and increase skeletal muscle protein synthesis. Whether this results in muscular hypertrophy or hyperplasia, or a combination of these, is unclear and probably depends upon the muscle studied. Different muscle types contain different cytosolic receptor numbers and, therefore, the response to anabolic steroids varies. Anabolic steroids initiate an increase in RNA polymerase activity and the synthesis of either structural or contractile proteins. In some muscles, anabolic steroids may increase the ratio of fast twitch to slow twitch fibers (Nimmo et al 1982, Snow et al 1982). Increased activity of enzymes involved in energy metabolism may also occur. However, the total glycogen content may remain unchanged (Hyyppa et al 1997). The effects are most profound in females and castrated males (Snow 1993). 

Increased uptake of amino acids into muscle with consequent activation of muscle protein synthesis and... 

Chronic, heavy, daily alcohol consumption is associated with decreased muscle strength, even when adjusted for other factors such as age, nicotine use, and chronic illness. Heavy doses of alcohol also can cause irreversible damage to muscle, reflected by a marked increase in the activity of creatine kinase in plasma. Muscle biopsies from heavy drinkers also reveal decreased glycogen stores and reduced pyruvate kinase activity. Approximately 50% of chronic heavy drinkers have evidence of type 11 fiber atrophy. These changes correlate with reductions in muscle protein synthesis and serum camosinase activities. Most patients with chronic alcoholism show electromyographical changes, and many show evidence of a skeletal myopathy similar to alcoholic cardiomyopathy. 

Svanberg, E., Zachrisson, H., Ohlsson, C., Iresjo, B.M., and Lundholm, K.G., Role of insulin and IGF-I in activation of muscle protein synthesis after oral feeding. Am J Physiol, 270, E614, 1996. 

Escobar, J., J.W. Frank, A. Suryawan, H.V. Nguyen, S.R. Kimball, L.S. Jefferson and T.A. Davis, 2005. Physiological rise in plasma leucine stimulates muscle protein synthesis in neonatal pigs hy enhancing translation initiation factor activation. Am. J. Physiol. Endocrinol. Metah. 288, E914-E921. 

Potassium is required for enzyme activity in a few special cases, the most widely studied example of which is the enzyme pymvate kinase. In plants it is required for protein and starch synthesis. Potassium is also involved in water and nutrient transport within and into the plant, and has a role in photosynthesis. Although sodium and potassium are similar in their inorganic chemical behavior, these ions are different in their physiological activities. In fact, their functions are often mutually antagonistic. For example, increases both the respiration rate in muscle tissue and the rate of protein synthesis, whereas inhibits both processes (42). 

Loss of muscle protein in trauma is caused by increased degradation rather than decreased synthesis. The degradation is controlled by changes in the levels of glucocorticoids, insulin and the proinflammatory cytokines TNFa and IL-1. The proteolytic enzyme complex that degrades the protein is the proteasome (Chapter 8). The mechanism by which the enzyme is activated is not known, but increased activities of the enzymes involved in ubiquitina-tion of proteins and an increase concentration of ubiquitin may play a role (Chapter 8). 

The mechanism of the action of nitrates is not completely known, though it is reasonably likely that within smooth muscle cells, nitrates are transformed into nitrites, which then release NO. This, in turn, reacts with guanylatecyclase, causing increased synthesis of guanosine 3, 5 -monophosphate (cyclic GMP). As a result, a GMP-requiring protein kinase is activated, which results in less phosphorylation of muscle protein. Dephosphorylated muscle proteins are less able to contract, which ultimately results in a reduction of the heart s need for oxygen. 

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