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converted into mechanical energy by the enzymatic (ATPase) action of myosin and actin in the sarcomere (Figure 6-1). In this process, a major proportion of the energy loss inevitably appears as heat. Under resting conditions, muscle accounts for 20 to 30 percent of total resting energy expenditure, the variability of which to a large extent is determined by differences in muscle metabolism (Zurlo et al., 1990). Under conditions of cold exposure and shivering thermogenesis, the function of muscle as a "heater" for the body and the resultant energy loss become still more conspicuous. In addition, skeletal muscle supplies amine acids for synthesis of proteins in other tissues (crucial during wound healing), for the immune functions, and for gluconeogenesis (alanine and glutamine) under catabolic conditions. Skeletal muscle also oxidizes glucose and fatty acids and stores large amounts of glycogen postprandially.

To fulfill these requirements optimally, muscle tissue function relies on a variety of factors as illustrated in Figure 6-1. First, the contractile myofibrillar apparatus consisting of actin and myosin filaments must exist in sufficient quality and quantity, and nervous activation of the contractile elements must take place in a controlled fashion. Second, the quality and quantity of mitochon-drial proteins (mostly enzymes) must be sufficiently maintained to ensure efficient ATP production. Also critical, that the metabolic demands of the muscle cell be met to ensure a continuous ATP production (i.e., supplies of fuel substrates such as glucose and fatty acids and oxygen must be adequate) and that the metabolic by-products such as carbon dioxide are removed efficiently and in a timely manner. The quality of all proteins, both structural and metabolically active (such as myosin, actin, and mitochondrial enzymes), are maintained by a continuous remodeling process involving protein breakdown and protein

FIGURE 6-1 Muscle mass and quality are determined by a remodeling process. 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. Two metabolic processes critical for contractility are mitochondrial ATP production and subsequent hydrolysis of ATP in the sarcomere (by myosin and actin) to release chemical energy for contractile function.

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