Muscle

There are three types of muscle: – skeletal, cardiac, and smooth muscle – each with a unique physiologic role

All muscles function to convert chemical energy to mechanical energy, but the various types of muscle differ in their mechanism of initiation of contraction, rate of force development, duration of contraction, ability to adapt to their environment, and substrate utilization. Muscle accounts for about 40% of total body mass, and muscle metabolism is a major determinant of whole-body metabolic rate in both the basal and active state. Changes in skeletal muscle metabolism occur with physical activity and are directly related to the required force output and duration of activity. These factors also affect the muscle's relative utilization of glucose and fatty acids for fuel. Besides locomotion, skeletal muscle is also a source of body heat, provides amino acids for hepatic gluconeogenesis during fasting, and is a major site of glucose and triglyceride disposal following a meal. Because of its critical role in the regulation of systemic fuel flux and metabolism, loss of muscle mass has a profound effect on overall metabolism. Advancing age, sepsis, and wasting diseases, such as AIDS and cancer, are conditions associated with loss of muscle mass, and this loss is associated with increased morbidity and mortality.

The primary focus of this chapter will be on skeletal muscle, supplemented by discussion of similarities and differences in skeletal, cardiac and smooth muscle structure, function and metabolism. The chapter will begin with a discussion of the mechanism of muscle contraction, proceed to the signaling that initiates the contractile process, and then examine energy metabolism essential for contraction.

Actin and Myosin account for over 75% of muscle protein.

The sarcomere may shorten by as much as 70% in length during muscle contraction (see Fig. 20.2). The components effecting the contraction are the thick and thin filaments. The thick filament is composed of myosin and titin protein, and the thin filament is mainly made up of actin, with associated proteins, tropomyosin and troponins. The thin filament also has some interaction with titin. Thick and thin filaments extend in opposite directions from both sides of the M- and Z-lines, respectively, and overlap and slide past one another during the contractile process (see Fig. 20.2). The M- and Z-lines are, in effect, base plates for anchoring the myosin and actin filaments. In striated muscle, increased thick–thin filament intercalate during contraction, causing the H-zone (myosin only) and I-bands (actin only) to shrink. In smooth muscle, thick and thin filaments are anchored at structures called dense bodies that are further anchored by intermediate filaments. Although all three muscle types contain the same proteins, each muscle type expresses tissue-specific isoforms; the cardiac actin and troponins, for example, differ slightly from those in skeletal muscle.

Interaction between actin and myosin during muscle contraction is dependent on cytoplasmic Ca⁺⁺ concentration

Myosin is one of the largest proteins in the body, with a molecular mass of approximately 500 kDa, and accounts for more than half of muscle protein (Table 20.2). Under the electron microscope, myosin appears as an elongated protein with two globular heads. It is the primary component of the thick filament in muscle. Each myosin molecule is made up of two heavy chains (approx. 200 kDa) and four light chains (approx. 20 kDa). The heavy chain can be subdivided into the helical tail and globular head regions; the four light chains are bound to the globular heads. Structural analysis by limited proteolysis indicates that there are two flexible hinge regions in the myosin molecule (Fig. 20.3): one where the globular head attaches to the helical region and the other further into the helical region. The myosin filaments are associated through their helical regions and extend outward from the M-line toward the Z-line of each myofibril (see Figs 20.2 and 20.3). The hinge regions allow the myosin heads to interact with actin and provide the flexibility needed for reversible interactions and conformational changes during muscle contraction.

There are several features of myosin that are essential for muscle contraction.

Troponins modulate the interaction between actin and myosin

Calcium activation of muscle contraction in striated muscle involves thin filament-associated proteins, tropomyosin and the troponins. Tropomyosin is a fibrous protein that extends along the grooves of F-actin, each molecule contacting about seven G-actin subunits. Tropomyosin has a role in stabilizing F-actin and coordinating conformational changes among actin subunits during contraction. In the absence of Ca²⁺, tropomyosin blocks the myosin-binding site on actin.

A complex of troponin proteins is bound to tropomyosin: Tn-T (tropomyosin-binding), Tn-C (calcium-binding) and Tn-I (inhibitory subunit). Calcium binding to Tn-C, a calmodulin-like protein, induces changes in Tn-I which shift the interaction between tropomyosin and actin, exposing the myosin-binding site on F-actin and permitting actin–myosin interactions. For a description of the diagnostic use of cardiac troponin measurements see Box at end of this chapter.

Titin modulates the passive tension of muscle

Titin is the largest protein in the human body, with more than 34,000 amino acids and a mass of 3800 kDa. Structurally, titin spans half the length of the sarcomere, with its N-terminus anchored to the Z-line, and its C-terminus to the thick filament at the M-line. Titin has an elastic, extensible PEVK domain (rich in Pro, Glu, Val and Lys) that contributes to passive myocardial and skeletal muscle tension, and a kinase domain that participates in intracellular signaling. Depending on the skeletal muscle, titin may account for more than half of the passive tension of the muscle, and contributes a spring-like property to the sarcomere – when a muscle is stretched, potential energy is stored in the PEVK domain, which re-coils during relaxation. Mutations in one region of titin may cause a genetic disease of the heart (e.g. hypertrophic cardiomyopathy), while a mutation elsewhere in the gene causes a disease of skeletal muscle only (e.g. limb girdle muscular dystrophy).

The sliding filament model describes how a series of chemical and structural changes in the actomyosin complex can induce sarcomere shortening

The contractile response depends on reversible, Ca²⁺-dependent cross-bridge formation between the myosin head and its binding site on actin. A conformational change in the hinge regions of myosin occurs after cross-bridge formation, providing the power stroke for muscle contraction (Fig. 20.4). This conformational change, the relaxation of the high-energy form of myosin, is accompanied by dissociation of ADP and Pi. After the stroke is completed, the binding and hydrolysis of ATP restore the high-energy conformation. The stability of the contracted state is maintained by multiple and continuous Ca²⁺-dependent actin–myosin interactions, so that slippage is minimized until calcium is removed from the sarcoplasm, allowing dissociation of the actomyosin complex and muscle relaxation.

Higher myosin-ATPase activity increases cross-bridge cycling, which allows for increased rate of contraction. Different myosin isoforms have varying levels of ATPase activity, with fast muscles having higher rates of myosin-ATPase activity. Isoforms of actin and myosin are also found in the cytoskeleton of nonmuscle cells, where they have roles in diverse processes such as cell migration, vesicle transport during endocytosis and exocytosis, maintenance or changing of cell shape, and anchorage of intracellular proteins to the plasma membrane.

T-tubules transmit electrochemical signals for efficient muscle contraction

Skeletal muscle contraction is initiated by neuronal stimulation at the neuromuscular endplate. As described previously (see Fig. 8.4), this stimulus leads to depolarization of the electrochemical gradient across the muscle plasma membrane (sarcolemma). The depolarization, caused by an influx of Na⁺, propagates rapidly along the sarcolemma membrane and signals a voltage-gated calcium release from the sarcoplasmic reticulum (SR), a membrane-bound, calcium-sequestering compartment inside the muscle cell. The influx of Ca²⁺ from the SR to the sarcoplasm initiates cross-bridge formation and excitation–contraction coupling (see Fig. 20.4). In striated muscle, depolarization is transmitted into the muscle fiber by invaginations of the plasma membrane, called transverse tubules (T tubule) (Fig. 20.5). The transmission of depolarization through the highly branched T-tubule network, which interacts closely with the SR, leads to rapid, concerted release of calcium from the SR into the sarcoplasm. In order for depolarization to occur again, sodium must be actively pumped out of the cytosol, by Na⁺/K⁺-ATPase pumps located in the sarcolemma. The rate of muscle repolarization is affected by both the rate and density of these pumps. Higher levels of Na⁺/K⁺-ATPase activity are found in fast contracting muscles, and increased Na⁺/K⁺-ATPase pump density is an important adaptation to exercise.

Skeletal, cardiac, and smooth muscle differ in their mechanism of neural stimulation, and have different structural adaptations for propagating the depolarization. Skeletal muscle contraction is volitional and fibers are innervated by motor nerve endplates that originate in the spinal cord; acetylcholine functions as the neurotransmitter (see Chapter 41). The neuromuscular junction is a special structural feature of skeletal muscle that is not found in cardiac or smooth muscle. Each individual fiber is innervated by only one motor nerve, and all the fibers innervated by one nerve are defined as a motor unit. Motor unit control and synchronization is the basis for coordinated whole-muscle contraction. Skeletal muscle cramping is a nonvoluntary muscle contraction resulting from alterations in neuromuscular control and/or from electrolyte imbalances after excessive fluid loss typically during intense exercise in hot, humid conditions.

Cardiac muscle is striated and contracts rhythmically under involuntary control. The general mechanism of contraction of heart muscle is similar to that in skeletal muscle; however, the sarcoplasmic reticulum is less developed and the transverse tubule network is more developed in the heart. The heart is more dependent on, and actually requires, extracellular calcium for its contractile response (see Fig. 8.4); the influx of extracellular calcium enhances Ca²⁺ release from the SR. Lacking direct neural contact, cardiac myocytes propagate depolarization from a single node, the SA node, throughout the myocardium. The depolarization is passed cell to cell along specialized membrane structures called intercalated disks. Cardiac muscle is also more responsive to hormonal regulation. For example, cAMP-dependent protein kinases phosphorylate transport proteins and Tn-I, mediating changes in the force of contraction in response to epinephrine.

Smooth muscle can respond to both neural and circulating factors. Unlike skeletal muscle, neural input to smooth muscle innervates bundles of smooth muscle cells that cause both phasic (rhythmic) and tonic (sustained) contractions of the tissue. Smooth muscle can also be induced to depolarize by ligand–receptor interactions at the sarcolemma. This is called pharmacomechanical coupling, and is the basis for many drugs that target smooth muscle contraction or relaxation. Nitric oxide donors, such as amyl nitrite and nitroglycerin, used for treatment of angina, relax vascular smooth muscles, increasing the flow of blood to cardiac muscle.

Creatine phosphate is a high energy phosphate buffer used for rapid regeneration of ATP in muscle

A metabolic reality for skeletal muscle is that high force output can only be maintained for a short period of time. Contractions at or near maximal power levels depend on high myosin-ATPase activity and rapid ATP resynthesis by substrate-level phosphorylation using the high-energy compound creatine phosphate (creatine-P). Creatine (see Table 9.2) is synthesized from arginine and glycine and is phosphorylated reversibly to creatine-P by the enzyme creatine (phospho)kinase (CK or CPK) (Fig. 20.6). CK is a dimeric protein and exists as three isozymes: the MM (skeletal muscle), BB (brain) and MB isoforms. The MB isoform is enriched in cardiac tissue.

The level of creatine-P in resting muscle is several-fold higher than that of ATP (Table 20.3). Thus, ATP concentration remains relatively constant during the initial stages of exercise. It is replenished not only by the action of CK but also by adenylate kinase (myokinase) as follows:

Creatine phosphate stores decline rapidly during the first minute of high power output muscle contraction. As creatine phosphate stores are depleted, the muscle becomes unable to sustain the high force output, and contractile force rapidly declines. At this point, muscle glycogenolysis becomes a major source of energy. Calcium entry into muscle, in addition to its role in activation of myosin-ATPase-dependent contraction, also leads to formation of a Ca²⁺–calmodulin complex, which activates phosphorylase kinase, catalyzing the conversion of phosphorylase b to phosphorylase a. AMP also allosterically activates muscle phosphorylase and phosphofructokinase-1, accelerating glycolysis from muscle glycogen (see Chapter 13).

A further decline in force occurs as pyruvate and lactate gradually accumulate in the contracting muscle, resulting in a decrease in muscle pH. Force will then decline to a level that can be maintained by aerobic metabolism of fatty acids. Maximal aerobic power is about 20% of maximal power output, and about 50–60% of maximal aerobic power can be sustained for long periods of time.

Fatty acids are the major source of energy in muscle during prolonged exercise

The availability and utilization of oxygen in working muscle are major limitations for maintaining continuous physical activity. Long-duration contractile activity requires adequate oxygen delivery and the capacity for the muscle to utilize the oxygen delivered. Oxygen delivery to muscle is affected by the red blood cell and hemoglobin concentrations in blood, the number of capillaries within the muscle and heart pump capacity. Highly oxidative muscle has a higher capillary density than glycolytic muscle, and muscle capillary density increases with endurance exercise training. Muscle oxygen utilization is also directly related to the number and size of muscle mitochondria. Muscles subjected to continual contractile activity, such as postural muscles, have more mitochondria than infrequently contracted muscle. A standard observation in muscle subjected to increased contractile demands is an elevation in oxidative enzyme activity.

At rest or at low intensities of physical work, oxygen is readily available and the aerobic oxidation of lipid predominates as the main source of ATP synthesis. However, at higher work intensities, oxygen availability for lipid catabolism can become limiting, and subsequently the muscle work rate decreases. During the first 15–30 minutes of exercise, there is a gradual shift from glycogenolysis and glycolysis to aerobic metabolism of fatty acids. Perhaps this is an evolutionary response to deal with the fact that lactate, produced by glycolysis, is more acidic and less diffusible than CO₂. As exercise continues, epinephrine contributes to activation of hepatic gluconeogenesis, providing an exogenous source of glucose for muscle. Lipids gradually become the major source of energy in muscle during long-term, lower-intensity exercise when oxygen is not limiting.

Fats burn in the flame of carbohydrates; glycogen is required for efficient metabolism of lipids in muscle

Marathon runners typically ‘hit the wall’ when muscle glycogen reaches a critically low level. Glycogen is the storage form of glucose in skeletal muscle and its muscle concentration can be manipulated by diet, e.g. by carbohydrate loading prior to a marathon run. Fatigue, which can be defined as an inability to maintain the desired power output, occurs when the rate of ATP utilization exceeds its rate of synthesis. For efficient ATP synthesis, there is a continuing, but poorly understood, requirement for a basal level of glycogen metabolism, even when glucose is available from plasma and when fats are the primary source of muscle energy. Carbohydrate metabolism is important as a source of pyruvate, which is converted to oxaloacetate by the anaplerotic, pyruvate carboxylase reaction. Oxaloacetate is required to maintain the activity of the TCA cycle, for condensation with acetyl-CoA derived from fats. To some extent, muscle glycogen can be spared and performance time increased during long-term vigorous physical activity by increasing the availability of circulating glucose, either by gluconeogenesis or by carbohydrate ingestion, e.g. bread or Gatorade^®. Increased utilization of fatty acids during early stages of exercise is an important training adaptation to regular vigorous physical activity – it serves to spare glycogen stores.

Strength or resistance training increases muscle mass

A change in daily use of skeletal muscle has a profound effect upon its functional capacity. Both an increase and a decrease in daily activity level can change muscle structure, force production capacity, and fatigability. From a biochemical point of view, these changes are primarily caused by changes in tissue perfusion and metabolic enzymes, and thus the muscle's ability to take up glucose, utilize fat as an energy source, and generate ATP. The amount and intensity of daily physical activity occurs on a continuum, and muscle adaptation to this occurs in response to the specific stress placed upon it. For the sake of simplicity, and because this is how most research studies are designed, we can separate increased use (exercise training) into two categories: strength and aerobic training. The primary purpose of strength training, also called resistance training, is to increase the ability of a specific muscle, or group of muscles, to produce force. This is typically conducted through a small number of repetitions of one exercise movement against a resistance that only allows the muscle to contract through a full range of motion a very limited number of times (e.g. 6 to 8 repetitions of a bicep curl). The primary purpose of aerobic training, also called endurance training, is to increase endurance and decrease fatigue during prolonged, lower-intensity physical activity, e.g. running or walking. This is achieved through a high number of repetitions of muscle contractions at a low resistance. Each muscle contraction in strength training might be 75–90%, while in an aerobic training session it might be 15–20%, of the maximal voluntary force production of that muscle. The biochemical changes in response to these types of exercise are distinct.

Strength training has minimal effects on muscle biochemistry. The increase in force production capacity that occurs with strength training is due to increased cell size, i.e. hypertrophy. The hypertrophy of individual muscle cells occurs as a result of an increase in structural and sarcomeric proteins. With more myofibrils and sarcomeres (the contractile units of muscle) comes an increase in force production capability. When glycolytic enzymes are examined and normalized to the increased cell size, there is no change with strength training. When mitochondrial enzyme activity is normalized to the increased cell size of strength training, there is usually a slight decrease, suggesting that while force production capacity increases, ATP production capacity (at least based on the size of the cell) has slightly decreased. In terms of contraction speed and sarcomere cross-bridge cycling, this is primarily determined by myosin-ATPase activity, which remains relatively unchanged in response to resistance training.

Endurance or aerobic training increases the oxidative metabolic capacity of muscle

In response to aerobic training, the primary biochemical change is an increase in capacity to metabolize fat, supported by increases in mitochondrial number, size and enzymes. All muscle fiber types (fast and slow) will increase their concentration and activity of citrate synthase and cytochrome-c by 2–3-fold, resulting in increased ATP production at a given workload (i.e. exercise intensity), so that muscle can then rely more on fat oxidation and less on anaerobic metabolism. The shift toward aerobic metabolism delays muscle fatigue; there are only minor effects on glycolytic enzymes in response to aerobic training, and the effects on cell size due to aerobic training are also minimal. Small shifts in myosin-ATPase composition may also occur, leading to a slower muscle phenotype (slower cross-bridge formation during contraction) due to aerobic training. Increases in glucose utilization as a result of increased expression of GLUT-4 and hexokinase also develop more in response to aerobic training, as opposed to strength training, but it is easy to see, considering the amount of skeletal muscle in the body, how blood glucose is decreased in a person with diabetes by an exercise program. It should also be noted that nearly all of these adaptations will occur in reverse in response to any form of de-training, whether that be due to cessation of an exercise program or bed rest due to injury or disease. Decreased use of muscle causes it to become much less metabolically efficient; unfortunately this de-adaptation becomes apparent within a few days following cessation of exercise. Other factors induced by endurance training include changes in cardiac output, increases in capillary density, and increases in glycogen stores. Of critical importance to health and medicine is the continuum within which these adaptations occur and the fact that small changes may impact many chronic diseases, including diabetes, atherosclerosis and cancer cachexia. Further, as changes occur relative to the original status of the muscle, older sedentary persons will see responses in muscle biochemistry comparable to those observed in younger persons. Thus, regardless of age, sedentary individuals who start even a moderate exercise program are likely to see substantial biochemical adaptations and health benefits. Much research is still ongoing in these areas in an attempt to understand the molecular genetic and signaling pathways that bring about these responses and understanding how they might be modified after injury or disease.