Metabolic Properties of Muscles I

Question 1

Name the characteristics of Type I Fibers

• Twitch?
• metabolism?
• myoglobin content?
• fiber diameter?
• fatigue?
• 3 others

Answer 1

• slow-twitch (slow speed of contraction)
• Slow-oxidative (low glycogen content)
• High myoglobin content
• small fiber diameter
• high resistance to fatigue
• increased concentration of capillaries surrounding muscle
• high capacity for aerobic metablism
• used for prolonged aerobic exercise

Question 2

Name the characteristic of Type II A fibers

1. Twitch?
2. metabolism?
3. myoglobin content?
4. fiber diameter?
5. fatigue?
6. 1 other

Answer 2

1. Intermediate-twitch (fast speed of contraction)
2. Fast-oxidative glycolytic fibers (intermediate glycogen levels)
3. High myoglobin content (appear red)
4. intermediate fiber diameter
5. ntermediate resistance to fatigue
6. increased oxidative cpacity on training

Question 3

Name the characteristic of Type II B fibers

1. Twitch?
2. metabolism?
3. myoglobin content?
4. fiber diameter?
5. fatigue?
6. 3 other

Answer 3

1. Fast-twitch (fast speed of contraction)
2. fast-glycolytic (high glycogen content)
3. low myoglobin content (appear white)
4. large fiber diameter
5. more sensitive to fatigue compared with other fiber types
6. limited aerobic metabolism (low mitochondrial content)
7. least efficient use of energy primarily glycolytic pathwy
8. used for sprinting and resistance tasks

Question 4

Explain the process from the release of acetylcholine to the release of calcium

Answer 4

Acetylchoine is released from a neuron

it binds to the acetylcholine receptor on the sarcolema

this stimulates the vltage-gated Na+ channels to open

This allows a rush of Na+ into the cell and K+ out of the cell

The Na+ rushes down the T-tubules,

this stimulates Ca2+ release from the sarcoplasm

Question 5

What does it mean that both actin and myosin have ATPase activity?

Answer 5

They are able to hydorlyze ATP

Question 6

What are the accessory proteins associated with actin and myosin?

Answer 6

• alpha-actinin
• tropomyosin
• troponin
• titin
  
  • largest known protein
  • connects Z-line to M-line
  • responsible for elasticity of muscle

Question 7

Explain the composition of one thin filament. What two proteins associate with the thin filament and what are their roles?

Answer 7

• G-actin polymerize to form F-actin
• Two F-actin helically intertwine to form one thin filament
• Tropomyosin covers the myosin bindign sites of the 7 G-actins
• Troponin heterdimeric: TnT, TnC, and TnI
  
  • mutations in thes proteins cause inherited cardiomyopathy known as Amish Nemaline Myopathy

Question 8

What are the components of the crossbridge portion of the myosin filament?

Answer 8

• Actin binding site
• ATP binding site
• light chains

Question 9

What two enzymes can break the heavy chains of myosin? What do the sites of these digestions signify?

Answer 9

Trypsin and Papain

The sites of digestion signify the point of the hinge

Question 10

What is the difference in filament states when cytosolic calcium is low? When cytosolic calcium is high?

Answer 10

When cytosolic calcium is low, muscles are relaxed because torponin is in a configuraiton to allow tropomyosin to coer the myosin bindign site.

When cytosolic calcium is high, the muscles are activated. The calcium binds to the troponin, moving the tropomyosin off of the myosin binding sites. This allows teh cross bridge to bind to the actin

Question 11

Explain the cross bridge cycle.

What happens during Rigor Mortis?

Answer 11

• The cross bridge is in its energized form when it contains ADP and Pi, at this stage (when calcium is high), the crossbridge will bind to actin
• The ADP and Pi are released form the Myosin, generating force that produces movement
• ATP will bind to the crossbridge, forcing it to dissociate from the actin
• The ATPase activity of the crossbridge converts ATP to ADP +Pi, and the whole process starts over again
• Durig Rigor Mortis, there is no more ATP to bind the the crossbridge, allowing it to release the actin. Therefore it remains in the bound state, giving the characteristic stiff presentation.

Question 12

What is the name for calcium release channels in the SER and ER?

What is the function of the molecule they bind?

Answer 12

Ryanodine receptors

Ryanodine inhibits SR calcium release and acts as a paralytic agent

Question 13

What is the funciton of DHP receptors?

Answer 13

Dihydropyridine receptors serve two functions

1. voltage sensor which undergoes fast transition to control release of calcium from sarcoplasmic reticulum
2. slow activating calcium channel

Question 14

What are the 4 functions of ATP in muscle fibers?

Answer 14

1. hydrolysis by sodium potassium ATPase
   
   1. to maintain sodium/potassium gradient
2. Calcium ATPases
   
   1. bringing the calcium back to the sarcoplasmic reticulum
3. Myosin ATPase
   
   1. energizes the crossbridge
4. Binding of ATP to actin site
   
   1. dissociates myosin from actin

Question 15

What are the 3 ways the muscle fibers generate ATP?

Answer 15

1. oxidative phosphorylation
2. glycolysis
3. Creatine phosphate
   
   1. provides a source for ATP phosphorylation by creatine kinase

Question 16

Explain the difference in PFK-2 regulation in the liver, skeletal muscle and cardiac muscle

Answer 16

• liver
  
  • negatively regulated by phosphorylation
• skeletal muscle
  
  • not regulated by phosphorylation (lacks regulatory serine residue)
• cardiac
  
  • phosphorylated and activated by a kinase cascate initiated by insulin
  • allows heart to activate glycolysis adn use blood glucose when blood glucose levels are elevated
  • also activated by AMPK as a signal that energy is low

Question 17

Why do muscles cells contain an isozyme of acetyl coenzyme A carboxylase (ACC-2)

Answer 17

although muscle cells do not synthesize fatty acids, they contain this enzyme to regulate the rate of fatty acid oxidation

Question 18

Regulation of fatty acyl-CoA entry into muscle mitochondria

Answer 18

1. ACC-2 converts acetyl-CA to malonyl-CoA, which inhibits caritine palmitoyltransferase I (CPTI- transfers fatty acids through outer membrane of mitochondria), thereby blocking fatty acyl-CoA entry into mitochondria
2. As energy levels drop, AMP levels rise, which activates AMPK, which phosphorylates and inactivates ACC-2, and also phosphorylates adn activates malonyl-CoA decarboxylase (MCoADC)
3. The decarboxylase converts malonyl-CoA to acetyl-CoA, relieving the inhibition of the CPTI and allowing fatty acyl-CoA entry into the mitochondria
4. This allows muscles to generate ATp via oxidation of fatty acids

Question 19

What is the major source of energy used by the heart?

Answer 19

fatty acids (60-80%) , lactate and glucose (20-40%)

98% cardiac ATP is generated by oxidative means

Question 20

What is the name for the transporter that takes up lactate into the heart?

Which cells produce lactate?

How is lactate used?

Answer 20

monocarboxylate transporter (also used for the transport of ketone bodies)

red blood cells and skeletal muscles produce lactate

lactate is used through oxidative phosphorylation or the Cori cycle in the liver

Question 21

What are the names for the transporters that brign Glucose into the cardiocyte?

Answer 21

• GLUT 1 and GLUT 4 (90%)
• GLUT4 numbers are stiumulated by insulin and myocardial ischemia

Question 22

Explain the ischemic conditon of the heart

(11 steps)

Answer 22

1. Interrupted heart blood flow
2. Heart switches to anaerobic metabolism
3. Glycolysis increases, protons accumulate (via lactate formation– detrimental to the heart)
4. Rapid fatty acid oxidation (NADH accumulates in the mitochondria)
5. Reduced NADH shuttle activity, an increased cytoplasmic NADH level (increased lactate formation, whcih generates mroe protons)
6. faty acid oxidation increases the levels of mitochondrial acetyl-CoA
7. Inhibits pyruvate dehygrogenase (cytoplasmic pyruvate accumlation and lactate production)
8. increased lactate production –> intracellular pH of the heart drops
9. difficult to maintain ion gradients across the sarcolemma
10. ATP hydrolysis is required to repair these gradients (essential for heart function)
11. use of ATP for gradient repair reduces the amount of ATP available for the heart to use contraction (compromises the ability of the heart to recover from the ischemic event)

Question 23

What is the class of drug that reduce the extensive fatty acid oxidation in the heart after an ischemic episode? What is the purpose of this drug?

Provide an example.

Answer 23

pFOX (partial fatty acid oxidation) inhibitors

It allows glucose oxidation adn reduces the lactate bulidup in teh damaged heart muscle

An example is trimetazidine (TMZ), which partiall inhibits mitochondrial long-chain beta-ketoacy coenzyme A thiolase

TMZ is also used to decrease symptos of angina

Question 24

Skeletal muscles are capable of completely oxidizing the carbon skeletons of which amino acids?

Answer 24

alanine, aspartate glutamate, valine, leucine, and isoleucine

Question 25

Why is it not a good idea to keep large stores of ATP? What is the body's solution to this problem?

Answer 25

Many reactions are allsterically activated or inhibited by ATP levels Muscle cells store high-energy phosphate bonds in the form of creatine phosphate

Question 26

Where is creatine derived from?

Answer 26

Arginine and Glycine in the kidney and the guanidinoacetate formed is methylated in teh liver to form creatine

Question 27

Describe creatinine production and excretion. What is it proportional to? What clinical information can this tell us?

Answer 27

creatine phosphate is unstable adn cyclizes to form creatinine, which is excreted in teh urine the produciton of creatinine occurs at a constant rate proportional to muscle mass, and is used as in idicator of the function of the kidneys

Question 28

What is muscle fuel at rest dependent on?

Answer 28

serum levels of glucose, AA, and FA

Question 29

Describe the fuel use of muscle cells when blood glucose and amino acids are elevated.

Answer 29

* glucose converted to glycogen * branched chain aa metabolism will be high * fatty acids will be used for acetyl-CoA production (satisfy energy needs of muscles) * balance between glucose and fatty acid oxidation, regulated by **citrate** * when the muscle cell has adequate energy, citrate leaves the mitochondria and activates ACC-2, which produces malonyl-CoA * malonyl-CoA inhibits CPTI * reducing fatty acid oxidation * malonyl-CoADC is inactive b/c AMPK is not active * muscle regulates it oxidation of glucose and fa oxidation through monitoring cytoplasmic citrate levels

Question 30

Describe muscle cell fuel use durign starvation (8 steps)

Answer 30

1. blood glucose levels drop 2. insulin levels drop 3. reducted GLut 4 transporters int eh muscle membrane (glucose use by muscle drop significantly) 4. Glucose conserved for use by the nervouse system and RBCs 5. Cardiac muscle PFK-2 is phosphorylated and activated by insulin (lack of insulin results in a reduced use of glucose by these cells as well) 6. Pyruvate dehydrogenase is inhibited (by the high levels of acetyl-CoA and NADh beign produced by fatty acid oxidation) 7. Fatty acids muscle's preferred fuel under starvation conditions (AMPK is active at lower ATP levels, ACC-2 is inhibited, and malonyl-CoADC is activated, CPTI activate) 8. The lack of glucose reduces in the glycolytic rate --\> no glycogen synthesis (inactivation fo glycogen synthase by epinephrine-stimulatd phosphorylation)

Question 31

Describe what happens to muscle cell fuel use during prolonged starvation

Answer 31

1. muscle proteolysis is induced for gluconeogenesis (by the liver in part by cortisol release) 2. Muscle use fatty acids for its own eneryg needs under these condition

Question 32

What are the 3 conditions in which anaerobic glycolysis is an important source of ATP?

Answer 32

1. during the initial period of exercise 2. exercise by muscles containing predominately fast-twitch glycolytic muscle fibers 3. durign strenuous activity

Question 33

About how long does it take for blood supply to muscles to significantly increase? How quickly is the amount of ATP already present in skeletal muscle used up?

Answer 33

It takes about 1 minutes for blood to get there the ATP could sustain exercise for 1.2 seconds, and only 9 seconds after use of the stored creatine phosphate

Question 34

Whya re glycogenolysis and glycolysis activated together durign exercise?

Answer 34

PFK-1 (rlm of glycolysis) and glycogen phosphorylase b (th einhibited form of glycogen phosphorylase) are allosterically activated by AMP

Question 35

Why is AMP an ideal activator?

Answer 35

its concentrations are normally kepy low by the adenylate kinase (myokinase in muscles), thus when ATP levels decrease slightly, the AMP concetration increases manifold

Question 36

What does epinephrine stimulate in muscles cells?

Answer 36

Epinephrine stimulates glycogenolysis

Question 37

When additional ATP is needed durign strenuous, high-intensity exercise, what is activated by the increased AMP levels?

Answer 37

PFK-1 is activaed as is glycogenolysis (providing additional ATP from anaerobic glycolysis)

Question 38

What are the two fates for lactate released from exercisign muscles?

Answer 38

1. lactate can be converted to pyruvate and enter the TCA cycle in resting skeletal muscle or cardiac muscle 2. return to the liver through the Cori cycle where it will be converted to glucose