Muscle hypertrophy, protein synthesis increase

These experimental models indicate that muscle hypertrophy occurs through increases in protein synthesis and suggest that weightlifting should require increased dietary protein. The confusion is derived from the interpretation of the quantity of protein needed to meet this increased need. The RDA s state that "there is little evidence that muscular activity increases the need for protein, except by the small amount required for the development of muscles during conditioning." The amount has been... 

Mechanism of Action. P-Agonists stimulate skeletal muscle growth by accelerating rates of fiber hypertrophy and protein synthesis, but generally do not alter muscle DNA content in parallel with the increases in protein accretion (133—135). This is in contrast to the effects of anaboHc steroids and ST on skeletal muscle growth. Both of the latter stimulate fiber hypertrophy and muscle protein synthesis, but also increase muscle DNA content coincident with increased protein accretion. Whether the P-agonists decrease muscle protein degradation is equivocal. 

As the model suggests, the dietary need for amino acids is determined by the rates of depletion of the free amino acid pool by oxidation or synthesis of protein. During steady state conditions, the contribution to the free pool from dietary intake and protein breakdown should be exactly balanced by the flux out of the pool to synthesis and oxidation. Any condition that increases deposition of protein in the body or the rate of amino acid oxidation should produce an increased need for protein. For example, muscle hypertrophy is dependent on a positive balance of the protein turnover process. If synthesis of protein exceeds the catabolism of protein, then muscle mass will increase and the free amino acid pool would be depleted. Thus, a net increase in protein requires an increase in intake or a decrease in oxidation. Likewise, the same arguments hold for an increase in oxidation of amino acids. 

Exercise is known to have acute catabolic effects on muscle protein turnover. During exercise protein snythesis is depressed which leads to protein catabolism. However, the impact of a relatively short exercise bout on 24-hour protein needs is unclear. Anaerobic exercise can produce hypertrophy of specific muscles depending on the type of training utilized. The hypertrophy is due to a positive balance in protein turnover which appears to be produced by an increase in the rate of protein synthesis after exercise. The increased need for protein during anaerobic exercise is unlikely to be more than 7 grams per day. 

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). 

Training to improve strength, power, and endurance of muscle performance is called resistance training. Its goal is to increase the size of the muscle fibers (hypertrophy of the muscle). Muscle fibers can develop a maximal force of 3 to 4 kg/cm of muscle area. Thus, if one can increase their muscle size from 80 to 120 cm, the maximal resistance that could be lifted would increase from 240 to 360 kg. Hypertrophy occurs by increased protein synthesis in the muscle and a reduction in existing protein turnover. 

Simply put, muscle size is determined by an intricate balance of musde protein synthesis and breakdown (Figure 6.1). Both processes are critical muscle protein synthesis makes new proteins while protein breakdown removes damaged ones. When synthesis of new proteins exceeds protein breakdown over time - due to increases in muscle protein synthesis, decreases in breakdown, or both - net protein balance is positive and muscle cell size increases. This is termed muscle hypertrophy. An increase in muscle cell size can easily be identified following a resistance exerdse training paradigm complemented with adequate nutritional intake (e.g., essential amino acids). Alternatively, when protein breakdown... 

Figure 6.1 Simplified model of protein turnover represented by a plank resting on a fulcrum. Muscle protein balance tips in the positive direction (light shade) when protein synthesis is enhanced, breakdown is decreased, or both, resulting in muscle-mass increases (hypertrophy). This is commonly seen with resistance exercise and essential amino acid supplementation. Muscle protein balance tips toward the negative direction (dark shade) when protein breakdown is accelerated, synthesis is decreased, or both, resulting in muscle-mass decreases (atrophy). This can be seen in models of disuse such as bedrest/ physical inactivity and limb immobilization.

GH also stimulates protein accretion, and this action in skeletal muscle appears to be mediated predominantly by IGF-I (Clemmons, 2009 Etherton and Bauman, 1998). Three specific IGF-I effects culminate to increase protein accretion in skeletal muscle, i.e. stimulation of proliferation of satellite cells, promotion of their assembly into myotubes and finally stimulation of hypertrophy of the myofiber by increasing protein synthesis and reducing protein degradation (Clemmons, 2009). It is important to mention that the hyperinsulinemia that is seen in GH-treated animals does not play a... 

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