Lecture 6: Cardiac Metabolic Stress Flashcards
Metabolic stress can be categorised into:
Acute (ischemia)
Chronic (diabetes)
How important is ATP production for the heart?
Essential, needed 24/7 as the heart never stops intentionally.
How is ATP produced?
Aerobically (normal)
Anaerobically (lactate is a bi-product)
What are the substrates for ATP production?
Normally 60-70% FA
~ 30% carbs
What pathways are used in aerobic ATP production?
CAC
ETC
What pathway provide most ATP?
95% ATP is from ETC in mitochondria
Write some short notes on the CAC
- Pyruvate enters the mitochondria and is a substrate for the CAC
- The CAC produces ATP, NADPH and FADH2 for ETC
write some short notes on the ETC
- NADH and FADH2 produced by the CAC is used in the ETC at complex 1
- A transmembrane charge is set up (via transport of H ions at complex 1 to 4) which is then utilised to generate ATP at complex 5.
What are the determinate of oxygen supply to myocytes?
Diastolic pressure
Coronary Resistance
O2 carrying capacity
What is diastolic resistance in determining oxygen supply?
Diastolic Resistance:
- Coronary flow max in diastolic period
- Aortic diastolic pressure determines coronary perfusion (60mmHg normally) (coronary ostea)
What is coronary resistance in determining oxygen supply?
- Increased by vessel compression (max in systole)
- Vascular tone (auto regulation)
- Vessel Ostruction
What is O2 carrying capacity in determining oxygen supply?
- Depends on the Hb level and O2 saturation (Stable)
- Usually max O2 extraction in the heart
What are the pathways for anaerobic metabolism?
Glycolysis
Write some short notes on glycolysis
- Glucose yields net 2 ATP though glycolysis
- Glycogen yields net 3 ATP through glycolysis (glyconeogensis)
- Moderate kinetics
- Moderate extent
What is the real function of CAC?
- Fuels i.e acetyl CoA found in the cytosol, produce very little ATP through CAC.
- Most ATP from ETC
- Products of CAC create ETC transmembrane charge for the flow of ions and huge ATP generation
Describe how the cardiomyocyte obtains fuels for energy production:
- Glucose obtained via GLUT 1 & 4
- FA transported via translocase and goes straight to mitochondria for entry into the CAC.
What happens to glucose once it enters the myocyte?
It undergoes glycolysis and its metabolites enter the CAC
What regulates glucose entry into the cells?
Insulin regulates glucose entry by altering the expression of GLUT 4.
However Glut 1 is always present
(this is important in chronic metabolic stress caused in diabetes)
What is ATP essential for?
Sliding filament theory and SERCA operation (its a Ca ATPase)
Describe the important signalling pathway of insulin;
- Insulin (binds insulin receptor) and activates PI3K and AKT
- AKT causes GLUT 4 insertion into the membrane and activates mTOR
What is the function of mTOR?
mTOR leads to;
- Inhibition of autophagy
- Activation of protein synthesis (involved in hypertrophy pathway)
What can the ration of AMP/ATP lead to the activation of?
AMPK is activated when the ratio of AMP/ATP is high i.e High AMP and low ATP
What does AMPK do once activated?
AMPK
- Inhibits FA oxidation
- Activates (regulates) glycolysis (observed in disease states)
- Activates Autophagy
What are some types of metabolic stress on the heart?
Oxidative stress
Activation of autophagy
What is oxidative stress;
The imbalance between ROS and antioxidants.
ROS then are free to cause damage.
What does too much ROS lead to?
Too much ROS lead to ROS oxidising proteins and changing their structure
What is a major source of ROS?
NADPH Oxidase (enzyme) produces/ is an important source of superoxide.
How many steps are their in autophagy?
Five
What are the first two steps in autophagy activation?
1) Membrane buds off the ER
2) Initial proteins from membrane but extend that section of membrane and recruit additional proteins in to it.
What is the third step in autophagy activation?
ATTG8 is an important protein that binds to the cargo via an adaptive protein
What is the fourth step in autophagy activation?
The adaptive protein goes out and binds to the dysfunctional organelle/ protein. Where it binds and brings it back to ATTG8
Whats the fifth step in autophagy activation?
ATTG8 and adaptor protein and cargo binds to lysosome and is degraded.
Does autophagy degrade healthy proteins?
They also degrade healthy proteins for its amino acids to create new proteins when requested (not bad)
What can too much autophagy lead to?
Too much autophagy can lead to to lack of cell integrity and lead to cell death.
How are the sub types of autophagy defined?
Seperate pathways depending on what is being degraded i.e
- Macrophage (macromolecules)
- Mitophagy (mitochondria)
- Glycophagy (Glycogen)
What are the types of metabolic challenges for the heart?
Acute or chronic
What are some examples of acute metabolic stress?
- Hypoxia (ischemia)
- Sympathetic stimulation
- Nutrient withdrawal (fasting)
- Arrhythmias
What are some examples of chronic metabolic stress?
- Diabetes and insulin resistance
- Heart failure
- Chronic ischemia heart disease
- Hypertension
What example of acute metabolic stress is looked at?
Ischemia
What is schema?
An imbalance between oxygen supply and demand.
Defined as: Coronary flow is inadequate to maintain steady-state metabolism.
Doesn’t need to be fully blocked
What causes 90% of MI?
90% MI are caused by occur due to plaque rupture leading to thrombosis formation
How does MI pain present?
MI pain is similar to angina, persists with rest. (25% no pain)
Angina normally goes away with rest.
How is MI diagnosed using serum analysis?
Serum analysis detects intracellular molecules including;
- (Creatine Kinase, TnT, Tnl)
- Onset 4-8hrs
- Peak 24 hrs
- Baseline 48-72hrs
How does ischemia affect excitation coupling in the myocyte?
- An ischemic myocyte has low O2 therefore ATP production decreases.
- Low ATP leads to loss of SERCA, sarcolemma Ca ATPase and Na/K ATPase
- Anaerobic glycolysis occurs and its bi-products i.e lactic acid = H which decreases pH. (cellular acidosis)
How does cellular acidosis affect excitation contraction coupling?
- When H builds up the Na/H exchanger tries to remove H, thus Na builds up
- However eventually H equilibrates between ICF,ECF
- Excessive Na is removed by NCX which reverses.
- This leads to Ca build up
In summery, what happens to ionic distribution in myocyte ischemic insult.
- Ca overload
- Na overload
- Cellular acidosis
Thus leading mechanisms of myocyte death
What are the two pathways of myocyte death from myocardial hypoxia?
- Disruptive ionic distribution death pathway
or
- Cellular acidosis pathway
Describe the ion distribution disruption in a myocardial hypoxia insult;
Myocardial hypoxia
- Increased Na/H exchanger activity
- Decreased ATPase activity
- NCX reverses
- Decreased SR Ca release.
Leading to
- Ca overload
- High ECF K
- Na overload
What does a disruption in ionic distribution lead to?
- Ca overload
- High ECF K
- Na overload
Leads to;
- Lipase and protease activation. = cell death
or
- Altered resting membrane potential = Arrhythmias
Describe what occurs in cellular acidosis as a result of myocardial hypoxia;
Myocardial hypoxia;
- increased anaerobic metabolism
- Decreased cell pH (lactic acid)
Cellular acidosis - Chromatin clamping and protein denaturation
results in cell death
Look at the diagram
in your notes
Describe cellular events within an hour of MI
1-2 mins, Decreased ATP, Stops contraction
10 mins @ 50% ATP, Cell swelling, Decreased Resting Membrane potential, Arrhythmias
20-24mins Irreversible cell injury.
What happens between 1-24 hrs of MI in terms of cellular events?
1-3hrs Interstitial oedema and disruption
4-12hrs Neutrophil infiltration
18-24hrs Start necrotic process (phagocytosis onset)
What happens 2-4 days (early events) post MI in terms of cellular events?
Extensive phagocytosis of myocytes, fibroblasts (replace myocytes with dead collagen) and Connective tissue.
What are the late events post MI?
5-7 days; Restoration of dead tissue
1 week; Thinning and dilation of infarct zones, myocyte slippage.
7 weeks; Fibrosis and scarring complete.
(dilation because dead cells cannot produce tension)
What happens post ischemia that is vital?
post ischemia reperfusion
Why is post ischemia reperfusion vital?
Post ischemia reperfusion is a vital interventaiton to re-instate blood flow to the heart and restore oxygen availability
Can post ischemia reperfusion be bad?
Reperfusion can further increase heart dysfunction and cell death.
= Reperfusion injury
What affect does reperfusion have on excitation coupling?
Reperfusion results in the restoration of oxygen and thus ATP production.
- Ca ATPase active
- SERCA active
- External H is returned to normal.
What happens to metabolic processes in reperfusion?
Metabolic processes wont immediately come back online - will be inefficient, lots of ROS b/c everything is inefficient.
In reperfusion what happens when the external H is reduced?
The loss of external H causes a concentration gradient and H leaves the cell down this via the Na/H exchanger this causes Na overload in the cell
The result of this is reversed NCX goes into overdrive now causing Ca overload in early reperfusion.
What does Ca overload in early reperfusion lead to?
- Contracile dysfunction (myocyte cant relax)
- Fatal arrhythmias
- cardiomyocyte death
What contributes to cellular events in ischemia-reperfusion?
Metabolic signalling pathways
Describe the contribution of metabolic signalling pathways to ischemia;
Increase in AMP/ATP ratio thus leading to AMPK activation.
AMPK activation causes;
- Decreased FA oxidation
- Increased glycolysis (anaerobic)
- Increased autophagy
What else drives the increase in autophagy during ischemia?
increased autophagy is also driven by the decrease in mTOR concentration in an ischemic environment.
What happens to the metabolic signalling pathways in reperfusion?
- Autophagy decreases slightly (possibly)
- ATP levels rise slightly
- AMPK activity decreases slightly.
- mTOR increases slightly (possibly)
- ROS levels increase
Why dont metabolic signalling pathways return immediately to normal with reperfusion?
None of these signalling levels return to normal levels because restoration is a gradual task and does not occur instant with reperfusion
Whats an example of chronic metabolic stress?
Diabetes
Is diabetes metabolic stress impacting only on the heart?
No, it is a systemic metabolic distrubance
How does diabetes metabolically impact the heart?
- High glucose
- Low insulin (T1D) or high insulin (T2D)
- High free FA, triglycerides
- High fructose.
How does diabetes affect myocytes at a cellular level?
- Low glucose uptake
- Increased FFA
- Low glycolysis
- Increased ROS = oxidative stress
Describe how type 2 diabetes affects metabolic signalling
T2D; Impaired insulin resistance
Therefore less insulin/impaired
- Less glut 4 into membrane
- B/C AKT activity is decreased.
In type 2 diabetes what does lowered AKT activity also lead to?
Lowered AKT activity also results in a decrease in glyocogen synthesis which is meadiated by the insulin-AKT pathway
How does the heart respond to low glucose and insulin resistance?
The heart responds by increasing glycogen storage (GLUT 1 always present)
It is unknown how this occurs and its consequences
What can activate glycogen phosphorylase in a myocyte?
The b adrenergic pathway
How does the heart store glyocogen?
In both alpha and beta strucutral forms suggesting slow vs rapid release.
This is different from elsewhere in the body.
Whats another consequence of diabetes in the heart?
Protein glycation
What is protein glycation?
Glucose binds to proteins and form amadori products.
Over weeks/months amadori products become Advanced Glycation End Products (AGE’s) this is irrversible and modify protein function (very damaging)
In diabetes how else do AGEs effect the heart?
- Collagen cross linking AGE’s are evident, contributing to ventricular stiffness.
- Evidence there is glycation of SERCA and RYR in rat cardiomyocytes.
In diabetes what is form of metabolic stress?
Fructose toxicity
How are fructose levels high in diabetes?
- There is increased fructose by 30% over 30 years in parallel with increased insulin resistance.
- Increasing plasma fructose and cellular exposure.
Describe fructose metabolism
Not well regulated
Experimentally what does high dietary fructose lead to?
- Insulin resistance and diabetes
How does fructose create metabolic stress?
- It has the potential to bypass phosfructokinase regulation of glycolysis
- Enters the hexosamine biosynthesis pathway to produce o-glcNaCylation (reversible signalling regulatory modification of proteins)
What transports fructose into the cell?
Glut 5 only transports fructose and is present in myocardiocytes
In fructose dietary experiments, what was found?
Fructose did no produce
- Hypertension or obesity
Thus does not cause pressure or volume overload.
But did cause
- Insulin resistance and mild hyperglycemia
whats the cardiac morphology of high fructose?
- No change in heart size
- Increased interstitial fibrosis in the myocardium, myocyte loss ~4%
- Activated autophagy in fructose fed mouse hearts.
- increased oxidative stress (increased superoxide production)
How does the mouse experiments translate into human implications for the diabetes, fructose toxicity?
Bad implications for humans as the exposure to raised fructose or protein glycation etc can be over a much larger time scale.
