Metabolism of muscle at rest


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Metabolism of muscle at rest

  1. 1. Dr. Gangadhar Chatterjee JRII, Dept of Biochemistry GGMC & JJH, Mumbai
  2. 2. Types of muscle fibers a. Skeletal muscle: multinucleated because of cell fusion during embryogenesis  transverse tubules (Ttubules), invaginations of the sarcolemma that tunnel from the surface toward the center of the muscle fiber  The contractile proteins actin and myosin,contained within the filaments; myosin within the thick filaments, actin within the thin filaments. B. Smooth Muscle Cells: -cells have a spindle shape with a central nucleus
  3. 3.  contain a single nucleus, display no striations under the microscope.  controlled involuntarily C. Cardiac Muscle Cells:  striated (contain fibers),but regulated involuntarily  quadrangular in shape  network with multiple other cells through tight membrane junctions and gap junctions.  allow the cells to act as a common unit and to contract and relax synchronously  depend on aerobic metabolism for energy needs because contain many mitochondria and very little glycogen
  4. 4. Properties of Muscle Fiber Types
  6. 6. Events leading to sarcoplasmic reticulum calcium release in skeletal muscle 1. Acetylcholine binds to receptors on the sarcolemma, leading to a change of conformation of the receptors ---act as an ion pore-- allows sodium to enter the cell and potassium to leave 2. 3. The membrane polarization A receptor in the T-tubules (the dihydropyridine receptor, DHPR) activated by membrane polarization such that activated DHPR physically binds to and activates the ryanodine receptor in the sarcoplasmic reticulum 4. The activation of the ryanodine receptor (calcium channel), leads to calcium release from the SR into the sarcoplasm.
  7. 7. overview of muscle contraction 1.A resting sarcomere. The troponin tropomyosin complex blocking the myosin-binding sites on the actin 2. Exposure of the active site. conformational change in the troponin molecule pulls the troponin away from the binding site. 3. Cross-bridge attachment 4. Myosin head pivoting:toward the center of the sarcomere (power stroke). 5. Detachment of the cross-bridge. 6. Reactivation of the myosin head.
  8. 8. Glycolysis and fatty acid metabolism in muscle cells  Phosphofructokinase 2 (PFK-2) is negatively regulated by phosphorylation in the liver  in skeletal muscle,PFK-2 not regulated by phosphorylation. because the skeletal muscle isozyme of PFK-2 lacks the regulatory serine residue, which is phosphorylated in liver.
  9. 9. FUEL UTILIZATION IN CARDIAC MUSCLE Normal Conditions  primarily uses fatty acids (60- 80%), lactate, and glucose (20–40%)  98% of cardiac ATP generated by oxidative means; 2% derived from glycolysis  Glucose transport into the cardiocyte occurs via both GLUT1 and GLUT4 transporters, although approximately 90% of the transporters are GLUT4.  Fatty acid oxidation in cardiac muscle cells is regulated by altering the activities of ACC-2 and malonyl CoA decarboxylase Ischemic Conditions  switches to anaerobic metabolism.  when oxygen reintroduced to the heart, the high rate of fatty acid oxidation in the heart is detrimental to the recovery of the damaged heart cells.  NADH accumulates in the mitochondria, leading to a reduced rate of NADH shuttle activity  increases the levels of mitochondrial acetyl CoA inhibits pyruvate dehydrogenase, leading to cytoplasmic pyruvate accumulation and lactate production.
  10. 10. FUEL UTILIZATION IN SKELETAL MUSCLE A. ATP and Creatine Phosphate  ATP,not a good choice as a molecule to store in quantity for energy reserves as many reactions are allosterically activated or inhibited by ATP levels.  problem solved by storing high-energy phosphate bonds in the form of creatine phosphate  Creatine synthesis begins in the kidney and completed in the liver
  11. 11. The synthesis of creatine Creatine phosphokinase reaction
  12. 12.  Creatine phosphate ,an unstable compound, spontaneously cyclizes, forming creatinine  amount of creatinine excreted each day is constant and depends on body muscle mass
  13. 13.  B. Fuel Utilization at Rest:  dependent on the serum levels of glucose, amino acids, and fatty acids  glucose converted to glycogen, and branched-chain amino acid metabolism will be high  balance between glucose oxidation and fatty acid oxidation exists, regulated by citrate.  At adequate energy, citrate leaves the mitochondria and activates ACC-2, which produces malonyl CoA. The malonyl CoA inhibits carnitine palmitoyl transferase1, thereby reducing fatty acid oxidation by the muscle.
  14. 14. C. Fuel Use during Starvation:  With dropin blood glucose as well as insulin levels the levels of GLUT4 transporters in the muscle membrane reduced and glucose use by muscle drops significantly.  In cardiac muscle, PFK-2 is phosphorylated and activated by insulin. The lack of insulin results in a reduced use of glucose by cardiomyocyte.  Fatty acids -- muscle’s preferred fuel under starvation conditions.
  15. 15. D. Fuel Utilization during Exercise  The rate of ATP utilization in skeletal muscle during exercise may be 100 times greater that that in resting skeletal muscles.  Anaerobic glycolysis especially important as a source of ATP in three conditions --at the initial period of exercise before exercise-stimulated increase in blood flow and substrate and oxygen delivery begin, allowing aerobic processes to occur. --muscle containing predominately fast-twitch glycolytic muscle fibers --strenuous activity, when the ATP demand exceeds the oxidative capacity of the tissue
  16. 16. 1. ANAEROBIC GLYCOLYSIS AT THE ONSET OF EXERCISE  During rest, most of the ATP required ,obtained from aerobic metabolism  amount of ATP present in skeletal muscle could sustain exercise for only 1.2 seconds if not regenerated.  the amount of phosphocreatine could sustain exercise for only 9 seconds if it not regenerated.  longer than 1 minute required for the blood supply to exercising muscle to increase significantly due to vasodilation
  17. 17. 2. ANAEROBIC GLYCOLYSIS IN THE TYPE IIB FASTTWITCH GLYCOLYTIC FIBER  These muscles contract rapidly and vigorously but only for short periods.  glycolytic capacity high because the enzymes of glycolysis present in large amounts  low levels of hexokinase in fast-twitch glycolytic fibers prevent muscle from drawing on blood glucose to meet this high demand for ATP, thus avoiding hypoglycemia.  tissues rely on endogenous fuel stores (glycogen and creatine phosphate) to generate ATP  conversion of glucose 1-phosphate to glucose 6-phosphate, and metabolism of glucose 6-phosphate to lactate.
  18. 18. 3. ANAEROBIC GLYCOLYSIS FROM GLYCOGEN  Glycogenolysis and glycolysis activated together because both PFK-1 (the rate-limiting enzyme of glycolysis) and glycogen phosphorylase b (the inhibited form of glycogen phosphorylase) are allosterically activated by AMP.  For compensation of the low ATP yield of anaerobic glycolysis, fasttwitch glycolytic fibers have a much higher content of glycolytic enzymes
  19. 19.  Muscle fatigue results from lowering of the pH of the tissue to approximately 6.4. o Both the lowering of pH and lactate production causes pain  Muscle glycogen stores used up in less than 2 minutes of anaerobic exercise.  Glycogen degradation in muscle not sensitive to glucagon (muscles lack glucagon receptors).  Unlike the liver form of glycogen phosphorylase, the muscle isozyme contains an allosteric site for AMP binding.  When AMP binds to muscle glycogen phosphorylase b, the enzyme is activated even though it is not phosphorylated.
  20. 20.  activation of muscle glycogen phosphorylase b further enhanced by the release of Ca2+from the sarcoplasmic reticulum.  during intense exercise, epinephrine release stimulates the activation of adenylate cyclase in muscle cells leading to activation of cAMP-dependent protein kinase  The hormonal signal is slower than the initial activation events triggered by AMP and calcium
  21. 21. Stimulation of glycogenolysis in muscle by epinephrine
  22. 22. Comparison of energy sources used during short-duration exercise and prolonged-duration exercise. Prolonged-duration exercise Short-duration exercise 6 seconds 10 seconds ATP stored in muscles is used first. ATP is formed from creatine phosphate and ADP (direct phosphorylation). 30–40 seconds End of exercise Glycogen stored in muscles is broken down to glucose, which is oxidized to generate ATP (anaerobic pathway). Hours ATP is generated by breakdown of several nutrient energy fuels by aerobic pathway.
  23. 23. 4. ANAEROBIC GLYCOLYSIS DURING HIGH-INTENSITY EXERCISE  though mitochondrial metabolism working at its maximum capacity, additional ATP needed for very strenuous, high-intensity exercise  Increased AMP levels activate PFK-1 and glycogenolysis, providing additional ATP from anaerobic glycolysis  most of the pyruvate formed by glycolysis enters the TCA cycle, remainder reduced to lactate to regenerate NAD for continued use in glycolysis.
  24. 24. 5. FATE OF LACTATE RELEASED DURING EXERCISE  lactate released from skeletal muscles during exercise can be used by resting skeletal muscles or by the heart, where NADH/NAD ratio lower than in exercising skeletal muscle.  Lactate dehydrogenase reaction proceed in the direction of pyruvate formation. The pyruvate that is generated then converted to acetyl CoA and oxidized in the TCA cycle  second potential fate -- return to the liver through the Cori cycle for glucose production.
  25. 25. MILD AND MODERATE-INTENSITY LONG-TERM EXERCISE A. Lactate Release Decreases with Duration of Exercise aerobic oxidation of glucose and fatty acids, which generates more energy per fuel molecule than anaerobic metabolism also produces acid at a slower rate than anaerobic metabolism B. Blood Glucose as a Fuel  During long periods of exercise, blood glucose levels maintained by the liver through hepatic glycogenolysis and gluconeogenesis.
  26. 26.  for up to 40 minutes of mild exercise, glycogenolysis mainly responsible for the glucose output of the liver.  after 40 to 240 minutes of exercise, increased utilization of fatty acids occur, which are being released from adipose tissue triacylglycerols (stimulated by epinephrine release).  induction of gluconeogenic enzymes by glucagon and cortisol (this only occurs in prolonged exercise)  increased supply of fatty acids to provide the ATP and NADH needed for gluconeogenesis and the regulation of gluconeogenic enzymes
  27. 27. Production of blood glucose by the liver from various precursors during rest and during prolonged exercise
  28. 28. C. Free Fatty Acids as a Source of ATP  longer the duration of the exercise, greater the reliance of the muscle on free fatty acids for generation of ATP  uses muscles that are principally slow-twitch oxidative fibers  resting skeletal muscle uses free fatty acids as a principle fuel. At almost anytime except the postprandial state (right after eating), free fatty acids are the preferred fuel for skeletal muscle.
  29. 29. Preferential utilization of fatty acids over glucose as a fuel depends on the following factors: 1.hormone-sensitive lipase 2. Inhibition of glycolysis by products of fatty acid oxidation Pyruvate dehydrogenase activity is inhibited by acetyl CoA, NADH, and ATP 3. Glucose transport may be reduced during long-term exercise due to downregulation of GLUT4 transporters in muscles. 4. oxidation of ketone body increases during exercise 5. Acetyl-CoA carboxylase (isozyme ACC-2) must be inactivated for the muscle to use fatty acids
  30. 30. The pattern of fuel utilization changes with the duration of the exercise.
  31. 31. D. Branched-Chain Amino Acids  Branched-chain amino acid oxidation estimated to supply a maximum of 20% of the ATP supply of resting muscle  Two purpose-1) generation of ATP 2) synthesis of glutamine, which effluxes from the muscle ( Transamination) E. The Purine Nucleotide Cycle  Exercise increases the activity of the purine nucleotide cycle, which converts aspartate to fumarate plus ammonia
  32. 32. The purine nucleotide cycle F. Acetate  Acetate, an excellent fuel for skeletal muscle, treated as veryshort-chain fatty acid.  activated to acetyl CoA in the cytosol and then transferred into the mitochondria via acetylcarnitine transferase, an isozyme of carnitine palmitoyl transferase
  33. 33. METABOLIC EFFECTS OF TRAINING ON MUSCLE METABOLISM  Training increases the muscle glycogen stores and increases the number and size of mitochondria.  The fibers thus increase their capacity for generation of ATP from oxidative metabolism and their ability to use fatty acids as a fuel.  Training to improve strength, power, and endurance of muscle performance is called resistance training.
  34. 34. Thank you