There are three types of muscle cells: smooth, skeletal, and cardiac. In all types of muscle, contraction occurs via an actin/myosin sliding filament system, which is regulated by oscillations in intracellular calcium levels.
Muscle cells use stored glycogen and circulating glucose, fatty acids, and amino acids as energy sources. Muscle glycolysis is regulated differently from the liver, with the key difference being the regulation of phosphofructokinase-2 (PFK-2). Muscle PFK-2 is not inhibited by phosphorylation; cardiac PFK-2 is actually activated by phosphorylation by a number of protein kinases. Thus, under conditions in which liver PFK-2 is inactive, and glycolysis is running slowly, muscle glycolysis is either unaffected or even stimulated, depending on the isoform of PFK-2 being expressed.
Although muscle cells do not synthesize fatty acids, they do contain an isozyme of acetyl CoA carboxylase (ACC-2) to regulate the rate of fatty acid oxidation. ACC-2 produces malonyl CoA, which inhibits carnitine palmitoyltransferase I, thereby blocking fatty acid entry into the mitochondria. Muscle also contains malonyl CoA decarboxylase, which catalyzes the conversion of malonyl CoA to acetyl CoA and carbon dioxide. Thus, both the synthesis and degradation of malonyl CoA are carefully regulated in muscle cells to balance glucose and fatty acid oxidation. Both allosteric and covalent means of regulation are employed. Citrate activates ACC-2, and phosphorylation of ACC-2 by the adenosine monophosphate (AMP)-activated protein kinase inhibits ACC-2 activity. Phosphorylation of malonyl CoA decarboxylase by the AMP-activated protein kinase activates the enzyme, further enhancing fatty acid oxidation when energy levels are low.
Muscles use creatine phosphate to store high-energy bonds. Creatine is derived from arginine and glycine in the kidney, and the guanidinoacetate formed is methylated (using S-adenosyl methionine) in the liver to form creatine. The enzyme creatine phosphokinase (CPK) then catalyzes the reversible transfer of a high-energy phosphate from adenosine triphosphate (ATP) to creatine, forming creatine phosphate and adenosine diphosphate (ADP). Creatine phosphate is unstable and spontaneously cyclizes to form creatinine, which is excreted in the urine. The spontaneous production of creatinine occurs at a constant rate and is proportional to body muscle mass. Thus, the amount of creatinine excreted each day (the creatinine clearance rate) is constant and can be used as an indicator of the normalcy of the excretory function of the kidneys.
Skeletal muscle cells can be subdivided into type I and type II fibers. Type I fibers are slow-twitch fibers that use primarily oxidative metabolism for energy, whereas the type II fibers (fast-twitch) use glycolysis as their primary energy-generating pathway.
Glucose transport into muscle cells can be stimulated during exercise because of the activity of the AMP-activated protein kinase. Fatty acid uptake into exercising muscle is dependent on the levels of circulating fatty acids, which are increased by epinephrine release.
THE WAITING ROOM
Renee F., a 9-year-old girl, complained of a severe pain in her throat and difficulty in swallowing. She had chills, sweats, headache, and a fever of 102.4°F. When her symptoms persisted for several days, her mother took her to her pediatrician, who found diffuse erythema (redness) in her posterior pharynx (throat), with yellow exudates (patches) on her tonsils. Large, tender lymph nodes were present under her jaw on both sides of her neck. A throat culture was taken, and therapy with penicillin was begun.
Although the sore throat and fever improved, 10 days after the onset of the original infection, Renee F.’s eyes and legs became swollen and her urine suddenly turned the color of Coca-Cola. Her blood pressure was elevated. Protein and red blood cells were found in her urine. Her serum creatinine level (see Chapter 3 for how creatinine is measured) was elevated at 1.8 mg/dL (reference range, 0.3 to 0.7 for a child). Because the throat culture grew group A β-hemolytic streptococci, the doctor ordered an antistreptolysin O titer, which was positive. As a result, a diagnosis of acute poststreptococcal glomerulonephritis was made. Supportive therapy, including bed rest and treatment for hypertension, was initiated.
Seth D., a 3-year-old boy, was brought to the pediatrician by his parents because they noticed he was having difficulty running and going up stairs. He did not start walking until 16 months, and his parents noticed he is not running like other children his age. He also tends to walk on his toes and uses his hands to help him stand from a sitting position. On physical examination, the physician noted a lack of muscle strength, mostly in the lower proximal muscle groups and large calf muscles. The physician suspected that Seth D. had Duchenne Muscular Dystrophy (DMD), and to confirm the diagnosis his physician ordered a creatine kinase level, which was elevated, and then sent a blood sample to the human genetics laboratory for sequencing of the DMD gene.
I. Muscle Cell Types
Muscle consists of three different types: skeletal, smooth, and cardiac (Fig. 45.1). The metabolism of each is similar, but the functions of the muscles are quite different.
A. Skeletal Muscle
Skeletal muscles are those muscles that are attached to bone and facilitate the movement of the skeleton. Skeletal muscles are found in pairs, which are responsible for opposing, coordinated directions of motion on the skeleton. The muscles appear striated under the microscope and are controlled voluntarily (you think about moving a specific muscle group, and then it happens).
Skeletal muscle cells are long, cylindrical fibers that run the length of the muscle. The fibers are multinucleated because of cell fusion during embryogenesis. The cell membrane surrounding the fibers is called the sarcolemma, and the sarcoplasm is the intracellular milieu, which contains the proteins, organelles, and contractile apparatus of the cell. The sarcoplasmic reticulum is analogous to the endoplasmic reticulum in other cell types and is an internal membrane system that runs throughout the length of the muscle fiber. Another membrane structure, the transverse tubules (T-tubules), are thousands of invaginations of the sarcolemma that tunnel from the surface toward the center of the muscle fiber to make contact with the terminal cisterns of the sarcoplasmic reticulum. Because the T-tubules are open to the outside of the muscle fiber and are filled with extracellular fluid, the muscle action potential that propagates along the surface of the muscle fiber’s sarcolemma travels into the T-tubules and to the sarcoplasmic reticulum.
The striations in skeletal muscle are attributable to the presence and organization of myofibrils in the cells. Myofibrils are thread-like structures consisting of thin and thick filaments. The contractile proteins actin and myosin are contained within the filaments—myosin in the thick filaments and actin in the thin filaments. The sliding of these filaments relative to each other, using myosin-catalyzed ATP hydrolysis as an energy source, allows for the contraction and relaxation of the muscle (see online Fig. 20.1).
Muscle fibers can be classified as either fast-twitch or slow-twitch. The slow-twitch fibers, or type I fibers (also called slow-oxidative fibers), contain large amounts of mitochondria and myoglobin (giving them a red color), utilize respiration and oxidative phosphorylation for energy, and are relatively resistant to fatigue. Compared with fast-twitch fibers, their glycogen content is low. The slow-twitch fibers develop force slowly but maintain contractions longer than fast-twitch muscles.
The fast-twitch fibers, or type II, can be subdivided as type IIa or type IIb. Type IIb fibers (also called fast-glycolytic fibers) have few mitochondria and low levels of myoglobin (hence, they appear white). They are rich in glycogen and use glycogenolysis and glycolysis as their primary energy source. These muscles are prone to fatigue because continued reliance on glycolysis to produce ATP leads to an increase in lactic acid levels, resulting in a drop in the intracellular pH. As the pH drops, the ability of the muscle to produce ATP also diminishes. However, fast-twitch muscle can develop greater force than slow-twitch muscle, so contractions occur more rapidly. Type IIa fibers (also called fast-oxidative glycolytic fibers) have properties of both type I and IIb fibers and thus display functional characteristics of both fiber types. The properties of types I, IIa, and IIb fibers are summarized in Table 45.1.
|TYPE II FIBERS|
|TYPE I FIBERS||TYPE IIA||TYPE IIB|
|Slow-twitch (slow speed of contraction)||Intermediate-twitch (fast speed of contraction)||Fast-twitch (fast speed of contraction)|
|Slow-oxidative (low glycogen content)||Fast-oxidative glycolytic fibers (intermediate glycogen levels)||Fast-glycolytic (high glycogen content)|
|High myoglobin content (appear red)||High myoglobin content (appear red)||Low myoglobin content (appear white)|
|Small fiber diameter||Intermediate fiber diameter||Large fiber diameter|
|Increased concentration of capillaries surrounding muscle (greater oxygen delivery)||Increased oxidative capacity on training||Limited aerobic metabolism Low mitochondrial content|
|High capacity for aerobic metabolism||Intermediate resistance to fatigue||More sensitive to fatigue compared with other fiber types|
|High resistance to fatigue||Least efficient use of energy, primarily glycolytic pathway|
|Used for prolonged, aerobic exercise||Used for sprinting and resistance tasks|
Muscles are a mixture of the different fiber types, but depending on the function, a muscle may have a preponderance of one fiber type over another. Type I fibers are found in postural muscles such as the psoas in the back musculature or the soleus in the leg. The ratio of type I to type II varies with the muscle. The triceps, which functions phasically, has 32.6% type I, whereas the soleus, which functions tonically, has 87.7% type I. Type II fibers are more prevalent in the large muscles of the limbs that are responsible for sudden, powerful movements. Extraocular muscles would also have more of these fibers than type I.
B. Smooth Muscle Cells
Smooth muscle cells are found in the digestive system, blood vessels, bladder, airways, and uterus. The cells have a spindle shape with a central nucleus (see Fig. 45.1B). The designation “smooth” refers to the fact that these cells, which contain a single nucleus, display no striations under the microscope. The contraction of smooth muscle is controlled involuntarily (the cells contract and relax without any conscious attempt to have them do so; examples of smooth muscle activity include moving food along the digestive tract, altering the diameter of the blood vessels, and expelling urine from the bladder). In contrast to skeletal muscle, these cells have the ability to maintain tension for extended periods, and do so efficiently, with a low use of energy.
C. Cardiac Muscle Cells
The cardiac cells are similar to skeletal muscle in that they are striated (contain fibers), but like smooth muscle cells, they are regulated involuntarily (we do not have to think about making our heartbeat). The cells are quadrangular in shape (see Fig. 45.1C) and form a network with multiple other cells through tight membrane junctions and gap junctions. The multicellular contacts allow the cells to act as a common unit and to contract and relax synchronously. Cardiac muscle cells are designed for endurance and consistency. They depend on aerobic metabolism for their energy needs because they contain many mitochondria and very little glycogen. These cells thus generate only a small amount of their energy from glycolysis using glucose derived from glycogen. A reduced flow of oxygen-rich blood to the heart muscle may lead to a myocardial infarction (heart attack). The amount of ATP that can be generated by glycolysis alone is not sufficient to meet the energy requirements of the contracting heart.
II. Neuronal Signals to Muscle
For an extensive review of how muscle contracts or a detailed view of the signaling to allow muscle contraction, consult a medical physiology book. Only a brief overview is presented here.
The nerve–muscle cell junction is called the neuromuscular junction (Fig. 45.2). When appropriately stimulated, the nerve cell releases acetylcholine at the junction, which binds to acetylcholine receptors on the muscle membrane. This binding stimulates the opening of sodium channels on the sarcolemma. The massive influx of sodium ions results in the generation of an action potential in the sarcolemma at the edges of the motor end plate of the neuromuscular junction. The action potential sweeps across the surface of the muscle fiber and down the transverse tubules to the sarcoplasmic reticulum, where it initiates the release of calcium from its lumen, via the ryanodine receptor (Fig. 45.3). The calcium ion binds to troponin, resulting in a conformational change in the troponin–tropomyosin complexes so that they move away from the myosin-binding sites on the actin. When the binding site becomes available, the myosin head attaches to the myosin-binding site on the actin. The binding is followed by a conformational change (pivoting) in the myosin head, which shortens the sarcomere. After the pivoting, ATP binds the myosin head, which detaches from the actin and is available to bind another myosin-binding site on the actin. As long as calcium ion and ATP remain available, the myosin heads will repeat this cycle of attachment, pivoting, and detachment (Fig. e-45.1) . This movement requires ATP, and when ATP levels are low (such as occurs during ischemia), the ability of the muscle to relax or contract is compromised. As the calcium release channel closes, the calcium is pumped back into the sarcoplasmic reticulum against its concentration gradient using the energy-requiring protein SERCA (sarcoplasmic reticulum Ca2+ ATPase), and contraction stops. This basic process occurs in all muscle cell types, with some slight variations between cell types.
III. Glycolysis and Fatty Acid Metabolism in Muscle Cells
The pathways of glycolysis and fatty acid oxidation in muscle are the same as has been previously described (see Chapters 22 and 30). The difference between muscles and other tissues is how these pathways are regulated.
Phosphofructokinase 2 (PFK-2) is negatively regulated by phosphorylation in the liver (the enzyme that catalyzes the phosphorylation is the cyclic adenosine monophosphate [cAMP]–dependent protein kinase). However, in skeletal muscle, PFK-2 is not regulated by phosphorylation. This is because the skeletal muscle isozyme of PFK-2 lacks the regulatory serine residue which is phosphorylated in the liver. However, the cardiac isozyme of PFK-2 is phosphorylated and activated by a kinase cascade initiated by insulin. This allows the heart to activate glycolysis and to use blood glucose when blood glucose levels are elevated. The AMP-activated protein kinase also activates cardiac PFK-2 (kinase activity) as a signal that energy is low.
Fatty acid uptake by muscle requires the participation of fatty acid–binding proteins and the usual enzymes of fatty acid oxidation. Fatty acyl CoA uptake into the mitochondria is controlled by malonyl CoA, which is produced by an isozyme of acetyl CoA carboxylase (ACC-2; the ACC-1 isozyme is found in liver and adipose tissue cytosol and is used for fatty acid biosynthesis). ACC-2 (a mitochondrial protein, linked to carnitine palmitoyltransferase 1 [CPT1] in the outer mitochondrial membrane) is inhibited by phosphorylation by the AMP-activated protein kinase (AMP-PK) so that when energy levels are low, the levels of malonyl CoA drop, allowing fatty acid oxidation by the mitochondria. In addition, muscle cells also contain the enzyme malonyl CoA decarboxylase, which is activated by phosphorylation by the AMP-PK. Malonyl CoA decarboxylase converts malonyl CoA to acetyl CoA, thereby relieving the inhibition of CPT-I and stimulating fatty acid oxidation (Fig. 45.4). Muscle cells do not synthesize fatty acids; the presence of acetyl CoA carboxylase in muscle is exclusively for regulatory purposes. Mice that have been bred to lack ACC-2 have a 50% reduction of fat stores compared with control mice. This was shown to be attributable to a 30% increase in skeletal muscle fatty acid oxidation resulting from dysregulation of CPT1, brought about by the lack of malonyl CoA inhibition of CPT1.
IV. Fuel Utilization in Cardiac Muscle
A. Normal Conditions
The heart uses primarily fatty acids (60% to 80%), lactate, and glucose (20% to 40%) as its energy sources. Ninety-eight percent of cardiac ATP is generated by oxidative means; 2% is derived from glycolysis. The lactate used by the heart is taken up by a monocarboxylate transporter in the cell membrane that is also used for the transport of ketone bodies. However, ketone bodies are not a preferred fuel for the heart; the heart prefers to use fatty acids.
Lactate is generated by red blood cells and working skeletal muscle. When the lactate is used by the heart, it is oxidized to carbon dioxide and water, following the pathway lactate to pyruvate, pyruvate to acetyl CoA, acetyl CoA oxidation in the tricarboxylic acid (TCA) cycle, and ATP synthesis through oxidative phosphorylation. An alternative fate for lactate is its utilization in the reactions of the Cori cycle in the liver.
Glucose transport into the cardiocyte occurs via both GLUT 1 and GLUT 4 transporters, although approximately 90% of the transporters are GLUT 4. Insulin stimulates an increase in the number of GLUT 4 transporters in the cardiac cell membrane, as does myocardial ischemia. This ischemia-induced increase in GLUT 4 transporter number is additive to the effect of insulin on the translocation of GLUT 4 transporters to the plasma membrane.
Fatty acid uptake into cardiac muscle is similar to that for other muscle cell types and requires fatty acid–binding proteins and carnitine palmitoyltransferase I for transfer into the mitochondria. Fatty acid oxidation in cardiac muscle cells is regulated by altering the activities of ACC-2 and malonyl CoA decarboxylase. Under conditions in which ketone bodies are produced, fatty acid levels in the plasma are also elevated. Because the heart preferentially burns fatty acids as a fuel rather than the ketone bodies produced by the liver, the ketone bodies are spared for use by the nervous system.
B. Ischemic Conditions
When blood flow to the heart is interrupted, the heart switches to anaerobic metabolism. The rate of glycolysis increases, but the accumulation of protons (via lactate formation) is detrimental to the heart. Ischemia also increases the levels of free fatty acids in the blood and, surprisingly, when oxygen is reintroduced to the heart, the high rate of fatty acid oxidation in the heart is detrimental to the recovery of the damaged heart cells. Fatty acid oxidation occurs so rapidly that NADH accumulates in the mitochondria, leading to a reduced rate of NADH shuttle activity, an increased cytoplasmic NADH level, and lactate formation, which generates more protons. In addition, fatty acid oxidation increases the levels of mitochondrial acetyl CoA, which inhibits pyruvate dehydrogenase, leading to cytoplasmic pyruvate accumulation and lactate production. As lactate production increases and the intracellular pH of the heart drops, it is more difficult to maintain ion gradients across the sarcolemma. ATP hydrolysis is required to repair these gradients, which are essential for heart function. However, the use of ATP for gradient repair reduces the amount of ATP available for the heart to use in contraction, which, in turn, compromises the ability of the heart to recover from the ischemic event.