W2 - Chapter 3 - Fuel Sources for Muscle & Exercise Metabolism Flashcards
What surrounds the myofibrils of a muscle?
Sarcoplasmic reticulum
How is energy stored in the sarcoplasm?
Triacylglycerol droplets
Glycogen
PCr
small pool of aa
ATP
What are myofibrils composed of?
Thick + thin filaments
What ATPase in the muscle breaks ATP down?
Myosin ATPase
G-actin
Its monomers are polymerised into long strands of fibrous actin (F-actin)
F-actin
2 F-actin strands twist together to form thin filament backbone.
What spirals around the F-actin chains?
Rod-shaped tropomyosin molecules
3 sub units of troponin
Troponin - I
Troponin - T
Troponin - C
What does Troponin - I bind to?
Actin
What does Troponin - T bind to?
Tropomyosin
What does Troponin - C bind to?
Ca2+
Define a sarcomere
Smallest contractile unit of a muscle fibre + is the region between 2 z-lines.
What happens when there’s a sufficient amount of Ca2+ + ATP present in the muscle?
Actomyosin is formed
When does sliding of the filaments in the muscle happen?
When myosin heads form cross-bridges w/ active sites of actin.
What happens in the muscle without Ca2+
Tropomyosin blocks myosin binding sites on actin.
How is Ca2+ released from the sarcoplasmic reticulum and into the sarcoplasm of a muscle?
AP along sarcolemma + down T-tubules
What does Ca2+ do once released into the sarcoplasm of a muscle?
Bind to troponin = change shape = tropomyosin moves away from myosin binding sites.
How is muscle excitation initiated?
By arrival of a nerve impulse at muscle membrane via motor end plate.
What do the activated/cocked myosin heads do?
Bind to actin
Myosin head then changes to a bent shape = head pulls on thin filament = slides towards centre of sarcomere.
== Power stroke
What also happens during the power stroke?
ADP + Pi are released from myosin head.
What happens when a new ATP binds to the myosin head at the ATPase activity site?
Myosin cross-bridge detaches from actin.
Energy released per mole of PCr
43kJ/mol
What does it mean that PCr has a higher free energy of hydrolysis than ATP?
Its P is donated directly to ADP.
When is PCr broken down?
When ATP content ⬇️
What is an additional pathway to regenerate ATP when ATP + PCr stores are depleted?
Kinase reaction
– ONLY important during high intensity exercise.
When can the total adenylate pool decline rapidly?
If AMP conc. of cell ⬆️ during muscle force generation
What is the principal reason for why the total adenylate pool declines rapidly when AMP conc of cell ⬆️ during muscle force generation?
By deamination of AMP to IMP
ALSO, by dephospho rylation of AMP –> adenosine.
When does the deamination of AMP to IMP occur?
Under low ATP:ADP
When is adenine nucleotide loss important to muscle function?
During cond of metabolic crisis
i.e max exercise or later stages of prolonged submax exercise when glycogen stores become depleted.
What carries glucose across cell membrane?
Transporter protein - GLUT4
What happens once the glucose molecule has been transported into the muscle cell?
- Irreversibly phosphorylated
2. Catalyzed by hexokinase
What does attaching a P group to glucose to prod G-6-P mean?
Now trapped in cell
Do the liver + skeletal muscle contain the G-6-P enzyme?
No, just liver.
When is hexokinase inhibited?
By accumulation of G-6-P
What enzyme helps conversion of glucose-1-phosphate to G-6-P?
Phosphoglucomutase
IN MITOCHONDRIA
Reaction for pyruvate to Acetyl-CoA
Pyruvate + CoA + NAD+ – (Pyruvate dehydrogenase) –> AcetylCoA
What does insulin promote in regards to fat?
Triacylglycerol synthesis
What does insulin inhibit in regards to fat?
Lipolysis
When does adipose tissue bf increase?
During prolonged exercise at around 50% of vo2 Max
What does sympathetic vasoconstriction during intense exercise cause?
⬇️ in adipose tissue bf = accumulation of FA w/in adipose tissue + limits entry of FFAs + glycerol into circulation.
Where is glycerol readily soluble?
Plasma
What can happen to the glycerol in the plasma?
Taken up into the liver + phosphorylated to G-3-P
What can glycerol-3-phosphate be used for in the liver?
To form triacylglycerol
OR
oxidised to dihydroxyacetone P
What can happen to dihydroxyacetone P
Enter glycolytic pathway
OR
be converted to glucose
How are most FAs in plasma transported?
Loosely bound to albumin
What is the uptake of FA by muscle directly related to?
Plasma FA conc
What must FFAs do to be taken up by muscle?
Dissociate from albumin
Then pass through:
- Endothelial lining of capillary
- Interstitial fluid
- Sarcolemma
What happens to the FA-albumin complex at the endothelial lining of the blood capillary?
Binds to albumin-binding proteins (ABPs)
= allow FAs to be released from albumin = facilities entry into muscle
When will FA uptake into muscle only occur?
IF ICF FFA conc is less than that in the true aqueous solution in ECF
How is the low intracellular FFA conc maintained?
By the cytoplasmic FA-binding protein
Where does beta-oxidation of FAs occur?
Mitochondria
What does beta-oxidation of FAs do?
Removes 2-C units from FA chain in the form of ACoA.
– ACoA can then enter TCA cycle.
How are the fatty ACoA molecules in the muscle sarcoplasm transported into the mitochondria?
w/ carnitine.
Where is carnitine synthesised?
Liver
Where is carnitine usually abundant?
Tissues that are able to oxidise FAs
What enzyme regulates the transport of FA by carnitine?
Carnitine acyl-transferase (CAT)
What are the forms of CAT found in the muscle?
CAT-I
CAT-II
CAT-I in the muscle
Where is it located?
What does it do?
Outer surface of membrane
Generates acyl-carnitine
CAT-II in the muscle
Where is it located?
What does it do?
Inner surface of inner mitochondrial membrane
Regenerates ACoA + free carnitine
What are CAT-I and CAT-II also referred to as?
Carnitine palmitoyl transferase I + II
CPT 1 + CPT 2
Why has carnitine been promoted as a dietary supplement?
Weight loss by ⬆️ fat oxidation
⬆️ endurance exercise by promoting fat use = sparing limited glycogen stores.
What must happen before aa can be oxidised?
Removal of amino group
Activators of liver glycogen to plasma glucose
Glucagon
Epinephrine
Norepinephrine
Inhibitors of liver glycogen to plasma glucose
Insulin
Activators of muscle glycogen to glucose-1-phosphate
AMP
Pi
Ca2+
Epinephrine
Inhibitors of muscle glycogen to glucose-1-phosphate
ATP
Activators of muscle protein to aa
Cortisol
Inhibitors of muscle protein to aa
Insulin
Activators of pyruvate to ACoA
Ca2+
ADP
CoA
NAD+
Activators of adipose tissue + muscle triacylglycerol to FAs
Epinephrine
Glucagon
Cortisol
Inhibitors of adipose tissue + muscle triacylglycerol —> FAs
Insulin
What are the catecholamines?
Epinephrine
Norepinephrine
Where is insulin secreted from?
Beta cells of pancreatic islets
Where is glucagon secreted from?
Alpha cells of pancreatic islets
Where is epinephrine secreted from?
Adrenal medulla
Where is norepinephrine secreted from?
Sympathetic nerve endings
Where is cortisol secreted from?
Adrenal cortex
Where is the growth hormone secreted from?
Anterior pituitary gland
Metabolic changes in response to endurance training
⬆️ fat oxidation
⬇️ muscle glucose + glycogen use
Small ⬆️ in circulating hormones
⬆️ FFA transporters in muscle membrane
⬇️ Lactate accumulation