Muscle is a major determinant of whole-body metabolic activity. The three types of muscle: skeletal, cardiac, and smooth, have both similar and unique biochemical properties related to metabolism and force production. However, the primary function of all muscle is to turn chemical energy into mechanical energy. This is accomplished through the breakdown of ATP. Muscle mass and activity are major determinants of the overall metabolic rate in both the basal and active state. Changes in muscle metabolism occur during prolonged or vigorous physical activity, which affects not only the metabolic rate, but also the relative rate of utilization of glucose and fatty acids as fuels.
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| Although skeletal muscle is mainly associated with locomotion and heat production, maintenance of skeletal muscle mass is also essential to provide protein reserves for gluconeogenesis during fasting. Muscle is also the major site of glucose and triglyceride disposal in the body following a meal. Through its GLUT-4 transporter and lipoprotein lipase activity, muscle removes excess fuels from the blood. Loss of muscle mass with age or in wasting diseases, such as AIDS and cancer, leads to glucose intolerance and is associated with increased mortality and morbidity.
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| ATP is used for muscle contraction
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| Muscle contraction involves the integration of several biochemical processes including: membrane ion flux, calcium release and re-uptake, and ATP hydrolysis and synthesis. ATP is required for the maintenance of ion gradients, restoration of intracellular calcium levels, and for the actual process of muscle shortening. The functional contractile unit of muscle, the sarcomere, relies on the interaction of two filamentous proteins, actin and myosin, for shortening. The head of the myosin protein has ATPase activity that hydrolyzes ATP, and when calcium is present results in sarcomere shortening. Resting ATP stores do not fluctuate a great deal during muscle contraction. Actively contracting muscle relies on the rapid synthesis of ATP from ADP by the creatine phosphate shuttle, with additional ATP production from both anaerobic and aerobic metabolism. This chapter will describe all three types of muscle, focusing primarily on skeletal muscle and pointing out unique features of biochemistry and regulation in cardiac and smooth muscle. Muscle will be examined from three classic points of view: its structure, the mechanism of mechano-chemical coupling, and its energy metabolism.
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