Theme 3: Vascular Biology

Question 1

What is the key to endothelial cell function?

Answer 1

Cytoplasmic calcium changes, just like in smooth muscle cells.

Question 2

What is the difference between smooth muscle cells and endothelial cells in terms of calcium signalling?

Answer 2

Endothelial cells:

• Do not have VGCCs
• Do not have ryanodine receptors

Question 3

What are the main ways in which cytosolic calcium can increase in endothelial cells?

Answer 3

• Spontaneously -> Calcium puffs via IP₃R
• Agonist stimulation (e.g. ACh) -> Increases frequency of spontaneous entrance

Calcium can also enter via cell surface channels (not VGCCs though) and via gap junctions.

Question 4

What are the three mechanisms by which endothelial cells can drive vasodilation in smooth muscle cells?

Answer 4

1. Release of prostacyclin (PGI₂)
2. Release of nitric oxide (EDRF)
3. Release of endothelium derived hyperpolarizing factor (EDHF)

Question 5

Is endothelial dysfunction important in cardiovascular disease?

Answer 5

Yes, and the impact of impaired endothelial cell function is apparent in microcirculation before obstructive disease in large arteries.

Question 6

In which blood vessels is it especially important to study endothelial function?

Answer 6

Resistance arteries, because they are the vessels that provide the most resistance and are therefore linked to cardiovascular diseases.

Question 7

The endothelium enables … vasodilation.

Answer 7

Co-ordinated

Question 8

Which of the three mechanisms of endothelium-driven vasodilation is predominant in resistance arteries?

Answer 8

Endothelium derived hyperpolarizing factor (EDHF)

NO and PGI₂ have much less effect.

Question 9

How do we know EDHF is the dominant vasodilator mechanism in resistance arteries? Give experimental evidence.

Answer 9

(Garland, 1992):

• Plotted a graph of tension against ACh concentration -> The ACh is an agonist at endothelial cells that promotes vasodilation
• Block of NO synthase or prostacyclin synthesis has no effect on the curve
• This showed that the other two vasodilator mechanisms are not significant in resistance arteries
• Addition of oxyhaemoglobin (which mops up NO) had no effect

Question 10

Why is EDHF more effective in small arteries? Give experimental evidence.

Answer 10

(Bowles, 1997):

• Found that there was heterogeneity of L-type calcium current density in coronary smooth muscle
• Plotted current against voltage for conduit, small artery and large arteriole vessels
• Smaller vessels had higher currents and therefore higher density of L-type calcium channels

Question 11

How does EDHF transfer hyperpolarization to the smooth muscle layers? Give experimental evidence.

Answer 11

• There are two possible mechanisms for this:
  
  • Release of a diffusible factor (an EDHF)
  • Passive spread of hyperpolarization
• They are not mutually exclusive and if both the relative importance may vary between arteries
• The channels responsible for hyperpolarisation are potassium channels, so the first thing to determine is what potassium channels are involved and then the next thing is where these potassium channels are found
• (Waldron, 1994):
  
  • Used noradrenaline to induce contraction in small arteries
  • Progressively added increasing concentrations of ACh to drive relaxation and measured voltage
  • Repeated the experiment in the presence of Ca²⁺-activated K channel blockers (apamin and ChTX)
    
    • Apamin: Selective block of SK_Ca (small conductance)
    • Charybdotoxin: Blocks IK_Ca (intermediate conductance) and BK_Ca (big conductance)
  • In the presence of apamin and ChTX individually, hyperpolarisation was inhibited partly
  • In the presence of both at once, hyperpolarisation was fully inhibited
• In further experiments, iberiotoxin was used to selectively block big conductance channels, but this had no effect on hyperpolarisation. Overall, these results suggest that SK_Ca and IK_Ca are responsible for hyperpolarisation.
• (Mistry, 1998):
  
  • Found that there is no SK_Ca in isolated smooth muscle cells, only BK_Ca
• This shows that it is potassium channels in the endothelium that produce hyperpolarisation, which then transfers to the smooth muscle cells, although is not sufficient to know whether there is a diffusible factor or passive spread of hyperpolarisation.
• (Edwards, 1998):
  
  • Carried out intracellular recordings from endothelial cells
  • Found that apamin and ChTX blocked the hyperpolarising effect of ACh
  • Also showed that ouabain plus barium inhibited hyperpolarisation of smooth muscle cells (barium is an inhibitor of inward recitifier potassium channels)
• This led to the idea that potassium release into the extracelllular space occurs via SK_Ca and IK_Ca, which then activates the sodium-potassium pump and inward rectifier potassium channels on smooth muscle to cause hyperpolarisation.

Question 12

What is the diffusible EDHF that underlies vasodilation?

Answer 12

K⁺ released from endothelial cells

Question 13

What are the two main pieces of evidence for the idea that diffusion of K⁺ between endothelial cells and SMCs acts as an EDHF?

Answer 13

• K_IR-channels inhibited by barium
• Na⁺/K⁺-ATPase blocked by ouabain

Both of these lead to reduced vasodilation, but do not completely block it, which is indication that this is not the only mechanism.

Question 14

Aside from K⁺ release into the extracellular space acting as the EDHF, what is the other mechanism by which EDHF works?

Answer 14

Hyperpolarizing current spreads through gap junctions between endothelium and smooth muscle (myoendothelial gap junctions).

Question 15

Give some experimental evidence for the importance of myoendothelial gap junctions in endothelial-driven vasodilation in small resistance arteries.

Answer 15

(Mather, 2005):

• Anti-connexin antibodies (Cx40 Ab) had no effect on the rise in endothelial cells [Ca²⁺]_i but inhibits dilatation in response to ACh

Question 16

Give a summary of how EDHF works to produce vasodilation in small resistance arteries.

Answer 16

• Endothelial cell IP₃-evoked Ca²⁺ release leads to opening of potassium channels
• These lead to hyperpolarisation of the endothelial cell, which spreads to smooth muscle cells by two mechanisms:
  
  • Through myoendothelial gap junctions
  • Through potassium release into the extracellular space, leading to hyperpolarisation of SMCs
• Hyperpolarisation drives reduced calcium entry, leading to vasodilation

Question 17

Give some experimental evidence for the importance of IK_Ca and SK_Ca in endothelial cells of small resistance arteries.

Answer 17

(Brahler, 2009):

• Arterial blood pressure is raised (by 10-15mmHg) in freely moving mice deficient in endothelial cell potassium channels
• Therefore, there is an important basal influence of K_Ca channels normally to suppress blood pressure

Question 18

Describe how signalling between endothelial cells and smooth muscle cells is bidirectional.

Answer 18

• Smooth muscle cells can feedback on endothelial cells when calcium enters them (during contraction)
• This calcium moves via the gap junctions to the endothelial cells
• This leads to NO release and potassium channel opening -> This leads to vasodilation
• Overall, this bi-directional negative feedback leads vasomotion

Question 19

Give experimental evidence for feedback from arterial smooth muscle cells onto endothelial cells.

Answer 19

(Garland, 2017):

• Stimulated smooth muscle cells using BayK, which is a VGCC agonist, so it promotes Ca²⁺ entry into smooth muscle cells
• 3nM BayK led to an increase in calcium events in the endothelial cells compared to baseline
• 30nM BayK showed signs of vasomotion
• This is evidence for the idea that calcium in the smooth muscle cells can travel through myoendothelial gap junctions and feedback onto endothelial cells

Question 20

Give a summary of local myoendothelial circuit.

Answer 20

Question 21

Summarise the main properties of vascular smooth muscle cells.

Answer 21

1. Small spindle shaped cells approximately 5µm diameter, 100-300µm long- so large surface area to volume ratio
2. Electrically interconnected but less extensively than endothelial cells
3. Calcium key for contraction and cells can maintain sustained, slow contraction- Myosin Light Chain phosphorylation key, regulation through Kinase (MLCK) and phosphatase pathways (MLCP) i.e. by RhoKinase
4. Cytoplasmic calcium regulated by internal stores and importantly by influx from extracellular space
5. Membrane potential (voltage, mV) dependent and independent paths regulate calcium entry
6. Mainly quiescent cells, spontaneous electrical activity very rare in arteries when healthy

Question 22

Draw how cytosolic calcium is increased in smooth muscle cells and how this then leads to contraction.

Answer 22

Question 23

Describe how sensitisation of the contractile apparatus to calcium occurs in smooth muscle cells.

Answer 23

• G-protein activation leads to increased rho-kinase activity
• Rho-kinase inhibits MLC phosphatase
• This maintains MLC in the phosphorylated form, which is more sensitive to calcium
• Thus, this leads to increased contraction

Question 24

Draw a summary of the main ion channels present in smooth muscle cells.

Answer 24

Question 25

Summarise how opening and closing of potassium channels affects vasodilation in arterial smooth muscle cells.

Answer 25

Question 26

Name some ion channels that, when opened, will lead to depolarisation of smooth muscle cells.

Answer 26

• Chloride -> E_Cl = -20mV
• Sodium -> E_Na = +79mV
• Calcium -> E_Ca = +126mV

Each of these will lead to depolarisation when opened, since the resting membrane potential is usually more negative than these equilibrium potentials.

Question 27

What is unusual about chloride in arterial smooth muscle cells and what is the effect of this?

Answer 27

• Arterial smooth muscle cells concentrate chloride using the NKCC and anion exchanger (AE)
• This means that the equilibirum potential for chloride is much less negative (around -20mV) than in other cells (usually close to the equilibrium potential of potassium)
• This means that opening of chloride channels leads to depolarisation, rather than hyperpolarisation

Question 28

Give some experimental evidence for chloride channels in smooth muscle cells leading to depolarisation.

Answer 28

(Byrne, 1987):

• Used carbachol on isolated smooth muscle cells and perfomed patch clamping on them
• The carbachol is an agonist of a GPCR that leads to opening of chloride channels
• This leads to inward currents
• Plotted an I/V graph that shows that the equilibirum potential for chloride here is around 0mV (which is less negative than the usual -20mV)
• The downside of using isolated SMCs in solution is that SMCs usually act as a syncytium so this is not representative. Also, it is difficult to control conditions.

Question 29

Describe the different types of chloride channels in vascular smooth muscle cells. How do they function?

Answer 29

• The chloride channels are of two main types:
  
  • TMEM16A
  • Bestrophin
• These are both calcium-activated, leading to depolarisation of the membrane -> This in turn leads to opening of VGCCs and entry of more calcium
• This provides an explanation for how GPCRs can lead to opening of chloride channels (by increasing intracellular calcium)
• Bestrophin channels are also activated by cGMP, providing another route for modulation

Question 30

Give some experimental evidence for the role of chloride channels in vascular smooth muscle cells.

Answer 30

(Heinze, 2014):

• Knocked out the calcium-activated chloride channel TMEM16A in vascular SMCs, intermediate cells, and pericytes
• This led to elimination of calcium-activated chloride currents and caused lower systemic blood pressure and a decreased hypertensive response following vasoconstrictor treatment.
• This suggests that TMEM16A plays a general role in arteriolar and capillary blood flow and is a promising target for the treatment of hypertension.

Question 31

Are sodium currents important in depolarising arterial smooth muscle cells? Give experimental evidence.

Answer 31

• Evidence for their involvement is limited, although there is some
• (Keatinge, 1968):
  
  • Carried out sucrose gap recordings (a technique no longer used) on isolated vascular smooth muscle cells
  • Used a calcium-free solution where the calcium was replaced with a non-permeant similar ion
  • Observed sodium-based voltage spikes under these conditions, which were lost when the calcium in the solution was replaced
  • This suggests that sodium currents could play a role in arterial smooth muscle cells, but are inhibited by physiological concentrations of calcium
• (Ho, 2013):
  
  • Used veratridine to block Na_v channels in arterial smooth muscle cells, which led to reduced vasoconstriction
  • It was suggested that Na⁺ influx drives reverse mode NCX, Ca²⁺ entry and Cl^-_Ca activation to cause depolarisation
• Another route for Na+ entry is via TRPM4 and possibly other ‘non-selective’ cation channels

Question 32

Are calcium currents important in depolarising arterial smooth muscle cells? Give experimental evidence.

Answer 32

• There is no evidence specifically in arterial smooth muscle, but there is evidence from other types of smooth muscle
• (Bulbring, 1963):
  
  • Studied action potentials in guinea pig taenia coli muscles
  • Found that these were calcium-based
  • Their frequency was increased by stretch

Question 33

Define:

• Calcium sparks
• Calcium puffs
• Calcium sparklets
• TRPV4 sparklets

Answer 33

• Calcium sparks -> Spontaneous intracellular release through RyR
• Calcium puffs -> Spontaneous intracellular release through IP₃R

Sparks and puffs both interact to generate cell-wide and rapid Ca²⁺ waves.

• Calcium sparklets -> Entry of extracellular calcium through individual VGCCs
• TRPV4 sparklets -> Entry of extracellular calcium through individual TRPV4 channels

Question 34

What are the two ways that calcium can affect vasoconstriction?

Answer 34

1. Direct action -> Promoting vasoconstriction
2. Indirect -> Suppressing vasoconstriction

Question 35

Give an example of how calcium entry into smooth muscle promotes vasoconstriction.

Answer 35

The Bayliss effect (myogenic tone)

Question 36

In what vessels is the Bayliss effect most pronounced?

Answer 36

• Small resistance arteries and arterioles
• This could be explained by a different distribution of ion channels

Question 37

What calcium events lead to relaxation of arterial smooth muscle and how?

Answer 37

Calcium sparks:

• Calcium sparks activate BK_Ca (calcium-dependent potassium channels) on the membrane, leading to hyperpolarisation
• This reduces Ca²⁺ influx via VGCCs, relaxing arterial smooth muscle

This is dependent on the specific arrangement of proteins in the smooth muscle cell (since the calcium sparks are very local).

Question 38

Summarise basic myogenic mechanisms in arterial smooth muscle.

Answer 38

Question 39

What is the main function of calcium sparks causing vasodilation?

Answer 39

This allows negative feedback modulation of myogenic tone and agonist-driven vasoconstriction.

Question 40

What allows increased blood flow to muscles during exercise?

Answer 40

Mostly the dilation of skeletal muscle microcirculation

Question 41

What are the main mechanisms controlling skeletal muscle microcirculation to match blood supply to demand?

Answer 41

1. Mechanisms limiting blood flow:

• Myogenic tone
• Vasoconstrictors

2. Mechanisms increasing blood flow:

• Endothelial cell-dependent mechanisms (myoendothelial feedback)
• Metabolic + Conducted vasodilation

Question 42

Describe how the different factors influencing flow in the circulation change between large arteries and capillaries.

Answer 42

• Flow-induced vasodilation is only seen in large arteries
• Voltage-dependent processes are seen in smaller vessels

Question 43

What is myogenic tone in the skeletal muscle microcirculation? Give experimental evidence.

Answer 43

• Myogenic tone is the contraction of small vessels in response to increases in pressure in the lumen
• Experimental evidence:
  
  • Used an isolated cannulated cremaster muscle arteriole ex vivo
  • Suddenly increased the pressure from 5mmHg to 80mmHg
  • This led to an increase in the diameter, followed by a return to baseline diameter
  • The vessel diameter was then plotted against the luminal pressure at increments from 5 to 80mmHg, both with and without removal of calcium to show active and passive processes

Question 44

What underlies myogenic tones in skeletal muscle arteries? Give experimental evidence.

Answer 44

Myogenic tone is driven by depolarisation and calcium entry:

• Plotted a graph of % artery diameter against luminal pressure (normalised to 100% at 70mmHg) -> Plotted a line for the passive diameter (when calcium is removed) against the active diameter (normal)
• Also plotted smooth muscle cell membrane potential against luminal pressure
• At 75mmHg, there was around 50% tone (i.e. the active diameter was half of the passive diameter)
• Nifedipine (L-type calcium channel blocker) blocked the tone but not the depolarisation -> This suggests that calcium entry is involved in myogenic tone, but L-type calcium channels are not involved in the depolarisation

The depolarisation drives increased calcium entry into SMCs and increased phosphorylation of MLC:

• Utilised FURA-2 as an indicator of intracellular calcium, which increased proportionally to diameter during the myogenic response
• Found that MLC phosphorylation was increased equally at 3 different time points during the myogeni response

Question 45

Compare the effect of noradrenaline (a vasoconstrictor) and the myogenic effect on skeletal muscle arteries. Give experimental evidence.

Answer 45

• Added a concentration of NA that eventually produced the same level of contraction as the myogenic response did
• Found that there were lower levels of calcium in the NA experiment than the myogenic experiment -> This suggests that while myogenic contraction involved voltage-dependent calcium entry, this might be less so the case in the NA experiment
• In both experiments, MLC phosphorylation was increased at the start of the response, but in the myogenic response it stayed constant, whilei in the NA response it decreased over time
• At the point where the contraction was the same, MLC phosphorylation was lower in the NA group, which suggests there may be a different mechanism at play here enabling the same level of contraction

The main point is that the response to a vasoconstrictor is not sustained, while myogenic tone is sustained.

Question 46

Describe how endothelial-dependent agonists lead to vasodilation.

Answer 46

See previous lecture about NO, prostacyclins and EDHF

Question 47

Is the myogenic response in skeletal muscle modulated by endothelial agonists?

Answer 47

• At normal pressure, no. Use of various blockers of the ACh response to block NO and calcium-dependent potassium channels produced no change in myogenic tone.
• However, when the luminal pressure is lowered to 5mmHg, the number of calcium events inside endothelial cells doubled
• Any reduction in myogenic tone at this low pressure could be blocked using a TRPV4 antagonist, suggesting these play a role in this process
• The TRPV4 channels are close to holes in the internal elastic lamina and are aligned with IK_Ca channels, which enable hyperpolarisation and relaxation of the SMCs

Question 48

What channels are found near holes in the internal elastic lamina of skeletal muscle arteries?

Answer 48

• TRPV4
• IK_Ca

Question 49

How does vasomotion occur in skeletal muscle arteries?

Answer 49

• There is bi-directional feedback between the endothelial cells and smooth muscle cells in arteries

See the flashcards from earlier lecture.

Question 50

Give experimental evidence for the channels found on endothelial cells in skeletal muscle arteries.

Answer 50

• Studied a preparation of endothelial cells
• Addition of phenylephrine or Bay K 8644 (L-type calcium channel agonist) did not produce an increase in spontaneous activity -> This suggests there are no calcium channels involved
• Addition of ACh did lead to an increase in spontaneous activity -> This shows this is a viable preparation and that there are muscarinic receptors present

Question 51

If there are no calcium channels on endothelial cells in skeletal muscle arteries, why does addition of Bay K 8644 (L-type calcium channel agonist) lead to calcium events in the endothelial cells?

Answer 51

• Calcium channels are present on SMCs in the arteries
• There can be influx of calcium from these SMCs into the endothelial cells

Question 52

Describe the main ways that arterial smooth muscle cells can influence the endothelial cells. Give experimental evidence.

Answer 52

• Application of phenylephrine (alpha-1 agonist), potassium (for depolarisation of membrane to open VGCCs) and BayK (L-type calcium channel agonist) to smooth muscle cells all led to an increase in endothelial cell calcium events
• This response was blocked by nifedipine in all cases except with the high concentration phenylephrine
• This suggests that VGCCs and alpha-1 receptors are in SMCs
• The VGCCs lead to an increase in intracellular calcium, which enters the endothelial cells
• The alpha-1 receptors lead to an increase in intracellular IP₃, which enters the endothelial cells and triggers calcium release

Question 53

What is responsible for endothelial cell-driven feedback dilation and vasomotion in response to a vasoconstrictor in skeletal muscle arteries? Give experimental evidence.

Answer 53

IK_Ca channels on the endothelial cells:

• Addition of 3nM BayK led to slight vasoconstriction and then slight feedback vasodilation
• Addition of 30nM BayK led to more pronounced vasoconstriction and then feedback vasomotion
• When TRAM-34 (IK_Ca blocker) was added, there was no feedback vasodilation or vasomotion in response to BayK

Question 54

Summarise what initiates myogenic tone in skeletal muscle arteries and what counteracts it.

Answer 54

• Luminal pressure initiates myogenic tone
• It is counteracted by endothelial-cell dependent feedback, but this is only apparent at very low pressures (such as during intense vasoconstriction)
  
  • Therefore, there is a high degree of resting myogenic tone

Question 55

Describe the principle of conducted vasodilation in arteries.

Answer 55

• Potassium released from endothelial cells can activate (inward rectifier) potassium channels on adjacent endothelial cells.
• Hence, there is spread of the hyperpolarisation.
• This enables the arterioles to act as a syncytium.

Question 56

Give some experimental evidence for how conducted vasodilation in skeletal muscle arteries works.

Answer 56

• Measured the myogenic tone and membrane potential of an endothelial cell
• Applied ACh at progressively further distances from the endothelial cell
• At each distance, hyperpolarisation and vasodilation were simultaneously increased

Question 57

Describe how skeletal muscle contraction can affect arterial tone to match blood supply to the demand.

Answer 57

• Skeletal muscle cells have K_ATP channels that are activated by low ATP and there is also general release of potassium -> This potassium efflux can activate inward rectifier potassium channels on smooth muscle cells and endothelial cells in the arteries
• ADP and adenosine are vasodilators
• Some ACh from the NMJ can reach the arteries, leading to vasodilation

Question 58

Give experimental evidence for the channels that are important for how skeletal muscle contraction can affect arterial tone to match blood supply to the demand.

Answer 58

• Studied the module inflow arteriole (which supplies the skeletal muscle) and the upstream arteriole
• In response to skeletal muscle contraction, both dilated
• Although the module arteriole dilated by a higher percentage, since the upstream arteriole is larger, it may be responsible for most of the increased flow in the module arteriole
• Use of a K_ATP channel blocker inhibited the vasodilation in the module arteriole
• This suggests that K_ATP channels are important in this vasodilation, since low ATP opens the K_ATP channels, leading to hyperpolarisation and vasodilation

Question 59

Compare how the application of nitric oxide and skeletal muscle contraction affect upstream arteries.

Answer 59

• NO does not lead to conducted dilation
• Skeletal muscle contraction does lead to conducted dilation due to released of vasoactive factors

Question 60

Draw a summary of the anatomy of the coronary arteries.

Answer 60

Question 61

Draw the vasculature of the left anterior descending coronary artery.

Answer 61

Question 62

Does angiography enable the microcirculation to be visualised?

Answer 62

No, the limit of resolution is around 300 micrometers.

Question 63

Draw a schematic diagram of the main types of artery in the wall of the heart.

Answer 63

Question 64

Describe how coronary blood flow changes at different coronary perfusion pressures.

Answer 64

• Usually, there is greater blood flow through endocardial vessels than epicardial vessels
• However, at low pressures there is greater flow through epicardial vessels since the blood reaches these first
• Adenosine can be added to remove the effect of autoregulation and find the maximal blood flow at any given perfusion pressure
• The difference between the autoregulated and maximal blood flow is the coronary flow reserve

Question 65

What is coronary flow reserve and what is a typical value?

Answer 65

• The maximum increase in blood flow through the coronary arteries above the normal resting volume.
• It can be tested by adding adenosine to fully dilate the vessels
• Typical value = Around 5ml/min/g

Question 66

Give some experimental evidence for the important of coronary reserve flow.

Answer 66

Women’s Ischaemia Syndrome Evaluation (WISE) study:

• Compared women with high CRF (>2.32) and low CRF (<2.32)
• Those with low CRF had much lower event-free survival over the 6 years of follow-up
• The effect was preserved when looking at just women without coronary artery disease

Question 67

What are the different factors that influence blood flow in the coronary arteries?

Answer 67

• Luminal pressure initiates the myogenic response
• Circulating and released vasoactive molecules
• Physical factors

Question 68

What initiates conducted vasodilation in coronary arteries and how does it work?

Answer 68

Initiated by:

• Factors released from blood cells (e.g. ATP released from RBCs during hypoxia)
• Factors released from cardiac myocytes (e.g. adenosine, K⁺)

The endothelium acts as the conduit for the conducted vasodilation.

Question 69

Describe how arterial constrictive and dilation responses can be studied.

Answer 69

Question 70

Give experimental evidence for what underlies myogenic tone in coronary arteries.

Answer 70

• Plotted a graph of vessel diameter against pressure
• Showed that inhibition of nitric oxide synthase (by L-NAME) led to increased myogenic response
• This suggests that NO is important for the myogenic response in coronary arteries, unlike in skeletal muscle
• Block of IK_Ca and SK_Ca channels had no effect on myogenic tone
• This suggests that NO plays more of a role in controlling myogenic tone and that calcium-dependent potassium channels have no effect

Spontaneous endothelial cell calcium events were linked to endothelial cell-dependent feedback onto smooth muscle cells.

Question 71

Compare how myogenic tone and endothelial cell-driven dilation work in coronary arteries. Give experimental evidence.

Answer 71

Myogenic tone:

• The vasoconstriction in response to luminal pressure
• Remains stable at a given pressure
• Blocked by nifedipine -> Suggests importance of calcium entry into smooth muscle cells

EC-dependent dilation:

• Can be induced by agonists, such as bradykinin
• Involves some NO and mainly calcium-activated K⁺ channels on endothelial cells

Question 72

Describe the responses of coronary arteries to acetylcholine and noradrenaline.

Answer 72

• Acetylcholine drives vasodilation
• Noradrenaline drives vasoconstriction -> This is the opposite to most vascular beds

Question 73

Give some experimental evidence for conducted vasodilation in coronary arteries.

Answer 73

• Measured the myogenic tone at a specific point along the artery
• Applied a vasoactive factor at progressively further distances from the point
• When bradykinin was added, there were signs of conducted vasodilation
• When adenosine was added, there was some slight conducted vasodilation
• When an NO donor was added, there was no conducted vasodilation
• When potassium ions were added, there was some conducted vasodilation
• Barium (inward rectifier potassium channel blocker) blocked conducted vasodilation in all the experiments, and also reduced local vasodilation with bradykinin and the potassium ions

Question 74

Give some experimental evidence for the importance of gap junctions in conducted vasodilation in coronary arteries.

Answer 74

Question 75

What is the importance of K⁺ in vasodilation of coronary arteries? Give experimental evidence.

Answer 75

• Application of pinacidil (K_ATP channel opener) led to an increase in the diameter of an isolated mouse papillary muscle preparation
• However, in a culture of human coronary microvascular endothelial cells, pinacidil did not hyperpolarise these cells
• Hence, K_ATP channels are likely present in the ventricular myocytes and/or smooth muscle cells, but not the endothelial cells
• Increased potassium concentration in the endothelial cell culture led to hyperpolarisation that was blocked by barium

Question 76

Give a summary of coronary microcirculation vasodilation.

Answer 76

• Ischaemia in ventricular tissue leads to opening of K_ATP channels in the myocytes -> This leads to release of potassium from the cells
• This potassium activates inward rectifier potassium channels on endothelial cells, leading to hyperpolarisation
• This then leads to conducted vasodilation via potassium leading to opening of inward rectifier potassium channels along the capillary

Question 77

Give a summary of the different receptors that agonists can act on in the coronary microcirculation to influence vasodilation.

Answer 77

Question 78

Give a summary comparison of vasodilation in skeletal muscle and coronary arteries.

Answer 78

Question 79

What percentage of resting cardiac output does the brain receive?

Answer 79

Around 15%

Question 80

Draw a graph of blood flow against arterial blood pressure for cerebral arteries. What does this tell us?

Answer 80

• The graph shows that blood flow remains constant over a range of pressures
• This is a sign of myogenic tone enabling autoregulation of blood flow

Question 81

Draw a summary diagram of how cerebral artery myogenic tone works.

Answer 81

Overall, increased pressure leads to stretch and depolarisation of the membrane and calcium entry, which drives contraction.

Question 82

What things have been implicated in the transduction mechanism of cerebral artery myogenic tone?

Answer 82

• Good evidence for TRPC6 and TRPM4 -> These are TRP channels in the membrane
• Vasoconstrictor action of an arachidonic acid derivative, 20-HETE

Question 83

What is 20-HETE and how is it made?

Answer 83

• A vasoconstrictor that contributes to myogenic tone
• It is a derivative of arachidonic acid

Question 84

How does 20-HETE produce vasoconstriction in the cerebral arteries? Give experimental evidence.

Answer 84

(Harder, 2011):

• Found that higher concentrations of 20-HETE lead to increased currents through voltage-gated calcium channels
• Also found that higher concentrations of 20-HETE suppressed the outward K_Ca current via action of PKC

Question 85

What is the main source of 20-HETE?

Answer 85

Astrocytes

Question 86

Give some experimental evidence for the importance of TRPC6 channels in cerebral artery myogenic tone.

Answer 86

(Welsh, 2002):

• Used antisense oligodeoxynucleotides to TRPC6 to decrease TRPC6 expression
• Increased the pressure incrementally from 40mmHg to 100mmHg
• Compared to control, the antisense experiment showed greatly attenuated constriction -> This is evidence of reduced myogenic tone
• Cation currents were also reduced in the antisense experiment

Question 87

Describe how TRPC6 channels are involved in myogenic tone in the cerebral arteries.

Answer 87

• Stretch activates GPCRs, which signal via phospholipase C
• This activates TRPC6, which enables cation currents that depolarise the cell

Question 88

Describe how TRPM4 channels are involved in myogenic tone in cerebral arteries.

Answer 88

• TRPM4 channels are cation channels selective for sodium
• They enable depolarisation of the cell, which activates VGCCs
• They are regulated by cytoplasmic calcium

(Gonzales, 2010):

• Performed perforated patch-clamp recordings of transient inward cation currents (TICCs) in the same cell
• Found that these were activated at negative potentials
• The TICCs disappear following treatment with TRPM4 siRNA
• 9-phenanthrol (a blocker of TRPM4) led to hyperpolarisation and dilation of the cerebral arteries
• HOWEVER, 9-phenanthrol was also shown to be an activator of endothelial IK_Ca and SK_Ca channels so this makes the mechanism of action unclear

Question 89

What limits basal myogenic tone in cerebral arteries? Give experimental evidence.

Answer 89

(Perez, 1999):

• Recorded spontaneous transient outward currents (STOCs) alongside calcium sparks in the cerebral arteries.
• The sparks and STOCs were at the same time.
• This is evidence for calcium sparks activating STOCs through K_Ca channels.
• This outward potassium current hyperpolarises the cell and suppresses further calcium influx via LTCCs.

Question 90

What is the neurovascular unit?

Answer 90

• As pial arterioles penetrate the brain parenchyma, they gradually lose innervation from extracellular nerves originating from peripheral ganglia.
• The vascular tone of parenchymal arterioles is chiefly regulated by signals derived from neurons and astrocytes that encase much of the surface of the arterioles.
• This concept of coupling between the arterioles and surrounding cells is called the neurovascular unit.

Question 91

Describe the concept of conductive vasodilation in cerebral vasculature.

Answer 91

• A signal of need for increased downstream blood flow can arise in levels 1 or 2
• It is conducted up to higher levels
• This then results in vasodilation here

Question 92

Describe how conductive vasodilation may be initiated and propagated in cerebral arterioles. Give experimental evidence.

Answer 92

(Longden, 2011):

• Imaged a cerebral arteriole that was linked to an astrocyte
• Electrical field stimulation (EFS) of the astrocyte led to arteriole dilation
• RBCs entered arteriole, showing increased flow
• This dilatation was sensitive to K_Ca channel blockers and K_IR channel blockers (barium)
• Thus, the suggested mechanism is that EFS (a proxy for nerve stimulation of the astrocyte) leads to an increase in astrocytic calcium, which opens K_Ca channels on the astrocyte. The increased extracellular potassium leads to opening of K_IR, which leads to hyperpolarisation and dilation.

(Knot, 1996):

• Demonstrated the importance of K_IR channels on smooth muscle cells
• Increased extracellular K⁺ evokes hyperpolarization and relaxation of cerebral arteries
• This is blocked by Ba²⁺ (K_IR blocker)
• This is independent of endothelium (on smooth muscle)
• Not all arteries have K_IR channels on their smooth muscle cells (e.g. mesenteric arteries)

Hence, conductive vasodilation in arterioles is mediated by neurons and astrocytes releasing potassium onto the arterioles and further upstream. There is likely to be retrograde spread of the potassium upstream.

However, there is also more recent evidence that glutamate release from excitatory nerves onto astrocytes and neurons can also stimulate vasodilation via release of arachidonic acid derivatives, mainly PGE₂ onto the smooth muscle. They could also release NO.

(Dabertrand, 2013):

• Found evidence that contradicted the importance of PGE₂ in vasodilation
• In parenchymal arterioles, PGE₂ produced vasoconstriction and not vasodilation

Question 93

Describe the properties of inward rectifier potassium channels (K_IR). What is the physiological effect of this?

Answer 93

• Inward rectifier refers to the idea that the channels are much better at allowing potassium into the cell than out of the cell
• However, most of the time, the extracellular potassium concentration means that K_IR does not show an conductance (figure C)
• When the extracellular potassium is increased, the curve shifts so that the K_IR shows outwards conductance in the physiological range (figure D)
• Under physiological conditions, the curve is never in a position where K_IR shows inwards conductance (hence the name is misleading)
• The result of this is that the graph for observed membrane potential against [K⁺]_o is not as predicted by the Nernst equation (figure B) -> This is because when at low [K⁺]_o, there is no conductance through the K_IR, so the membrane potential does not tend towards the equilibrium potential
• What all this means is that when there is an increase in extracellular potassium (e.g. when an adjacent cell releases potassium), there is often an outward potassium current through K_IR, hyperpolarising the cell -> In smooth muscle cells, this leads to vasodilation

Question 94

How can potassium released from astrocytes adjacent to cerebral arteries have a biphasic effect?

Answer 94

Vasodilation:

• When astrocytes release potassium, it leads to hyperpolarisation of the vascular smooth muscle cells by two mechanisms:
  
  • Activation of K_IR channels
  • Activation of the Na⁺/K⁺-ATPase
• This leads to closing of VGCCs and thus vasodilation

Vasoconstriction:

• When astrocytes release a LOT of potassium (due to enhanced neuronal activity), it leads to a situation where the potassium concentration is beyond the range where K_IR is activated
• Instead, the potassium leads to depolarisation as predicted by the Nerst equation

Question 95

How can glucocorticoids affect neurovascular coupling?

Answer 95

• Stress-induced glucocorticoid signaling remodels neurovascular coupling by reducing cerebrovascular inwardly rectifying K+ channel function
• This leads to reduced hyperpolarisation in response to extracellular potassium, so that relaxation is blunted

Question 96

Describe how conductive vasodilation may be initiated and propagated in cerebral capillaries. Give experimental evidence.

Answer 96

(Chen, 2014):

• Showed the importance of endothelial cells in this process (showing that smooth muscle cells are not required, as they are not present in capillaries)
• Optically studied the area of the cerebral cortex of rats that was activated by a somatosensory stimulus
• Found that a stimulus led to an increase in blood flow to the region for 15 seconds
• Used light dye damage to endothelial cells -> This blocked the propagation of the vasodilation, suggesting that the endothelial cells are responsible for the conductive vasodilation

(Longden, 2017):

• Suggested that the mechanism of spread of vasodilation must be a retrograde spread of hyperpolarization from capillaries to arterioles
• Application of 10mM K⁺ to capillaries led to marked vasodilation in the upstream arterioles, which was preceded by hyperpolarisation of the arterioles
• This was blocked by barium (K_IR blocker)
• The effect was also abolished by K_IR knockout
• The results were also replicated in vivo using fluorescently-labelled RBCs as a marker of flow

Question 97

What is the importance of pericytes in conductive vasodilation in cerebral vasculature?

Answer 97

• It has been suggested that active neurons and astrocytes produce vasodilators (mainly PGE₂) that affect pericytes wrapped around capillaries
• This pericyte dilation may spread to the upstream arterioles

The mechanism is shown in the diagram.

Question 98

Name one situation where HPV is useful and one where it is harmful.

Answer 98

• Useful: V/Q matching
• Harmful: HAPE

Question 99

How does HAPE occur?

Answer 99

When there is uneven HPV, there can be very high pressures in certain capillaries, leading to oedema.

Question 100

Describe an isolated lung preparation.

Answer 100

• A rat is prepared in a way that allows the gases flowing into the lungs and lung perfusion to be controlled
• The pulmonary arterial blood pressure can also be measured

Question 101

Describe how the pulmonary arterial pressure changes with repeated hypoxic challenge to the lungs.

Answer 101

The blood pressure increases with each challenge, showing a degree of summation until the blood pressure reaches an optimum response.

Question 102

Where is oxygen sensed to induce hypoxic pulmonary vasoconstriction? Give experimental evidence.

Answer 102

(Bergofsky, 1968):

• Created a setup with an isolated lung where the gases entering the lung could be varied and the blood gases could be varied using a disc oxygenator
• Varied the alveolar, arterial and venous (with the flow reversed) oxygenation to see the effect on pulmonary vascular resistance
• Only the alveolar hypoxia induced a large increase in pulmonary vascular resistance -> This suggests the main sensor for HPV is in the airways
• Arterial hypoxia had small effect on pulmonary vascular resistance

Question 103

What is the location of hypoxic pulmonary vasoconstriction (i.e. the arteries, capillaries or veins)? Give experimental evidence.

Answer 103

(Hakim, 1982):

• Created a model where the arterial and venous pressure could be measured, as well as where the arterial and venous segments could be constricted
• In this model, it is assumed that the arterial and venous segments are rigid, while the capillaries are compliant so they can buffer changes in pressure
• In the control experiment, arterial occlusion led to sharp fall in arterial pressure followed by a plateauing of the arterial pressure -> This is because the capillaries buffer the extra pressure until the capillary pressure is equal to the arterial pressure. So the plateau pressure is the capillary pressur.e
• Similarly, in the control experiment, venous occlusion led to a sharp increase in venous pressure, followed by a slower increase once the capillary pressure equals the venous pressure
• During hypoxia, the arterial pressure is higher, but the venous and capillary pressures remain unchanged -> This shows that HPV is driven by arterial contraction

Question 104

What is myography?

Answer 104

• A technique used to study pressure changes in single blood vessels
• A section of artery is mounted with a force transducer (to measure force generation) and micrometer (to apply basal stretch or contraction)

Question 105

Describe HPV in an isolated artery. How is it different from the tissue-wide response?

Answer 105

• The vasoconstriction occurs in two phases: an initial sharp acute increase followed by a smaller gradual increase
• There is a staircase effect of the phase 2 response after repeat hypoxia
• The tissue-wide response does not feature a phase 1, only a phase 2 -> The reason for this difference has not been explained

Question 106

How does HPV occur?

Answer 106

(Harder, 1985):

• Did microelectrode recordings of a pulmonary artery smooth muscle
• Hypoxia led to a gradual depolarisation and firing of action potentials in the smooth muscle
• These were blocked by the verapamil, showing they were calcium-dependent

(Weir, 1992):

• Used patch-clamping to study the physiology of individual smooth muscle cells
• Confirmed that hypoxia led to progressive depolarisation
• Showed that this depolarisation was underlied by a delayed-rectifier K⁺ current that is reduced during hypoxia
• However, this potassium current was only seen at high voltages

(Gurney, 1997):

• Used a different voltage-clamp protocol to plot a voltage-current graph for a smooth muscle cell under control and hypoxic conditions
• This showed evidence of a potassium current (named I_Kn) that was suppressed by hypoxia at a range of voltages

(Olschewski, 2006):

• Found that the I_Kn current is likely via TASK-1 channels
• This is because anandamide (TASK-1 blocker) led to a reduction in the I_Kn current
• siRNA transfection confirmed this

This all gives rise to the electrical model of HPV, where hypoxia inhibits potassium currents, leading to depolarisation, increased LTCC currents and vasoconstriction.

Question 107

What is some evidence against the electric model of HPV?

Answer 107

• Myography was used to study the hypoxic response in pulmonary smooth muscle
• The phase 1 and 2 responses were still present (only slightly attenuated) in the presence of nifedipine
• When the tissue was pre-depolarised using a solution of 80mmM potassium (which would inactivate the LTCCs), there was still a clear phase 1 and 2 response
• This is indicative of the fact that the response might not rely on extracellular calcium influx, but internal calcium release

Question 108

Is there calcium sensitisation in HPV?

Answer 108

(Robertson, 1995):

• Measured tension and intracellular calcium during the HPV response
• Found that calcium stayed constant during the phase 2 response, but tension gradually increased
• This is indicative of some form of calcium sensitisation

Question 109

Draw the 3 component model of HPV.

Answer 109

Component 3 is calcium sensitisation.

Question 110

Describe the properties of the 3 components of the HPV response.

Answer 110

• Component 1
  
  • Inhibited by cyclopiazonic acid (Calcium ATPase inhibitor) -> Not clear why
  • Independent of external calcium
• Component 2
  
  • Inhibited by 8-Br-cADPR (RyR blocker)
  • Independent of external calcium
• Component 3
  
  • Inhibited by removal of endothelium
  • Dependent on external calcium

Question 111

Give a summary of the 3 component model of HPV.

Answer 111

Component 1:

• Assumed to involve SR calcium release
• Thought to be due to the functioning of the calcium ATPase

Component 2:

• Due to cyclic ADP-ribose activation of the RyR

Component 3:

• Involves oxygen-sensing in the endothelium and release of a factor
• This factor leads to inhibition of smooth muscle myosin phosphatase
• A possible factor would be endothelin-1

In this model, the sensor of hypoxia could be mitochondrial activity, which affects production of cADPR. Another mechanism could be the production of ROS.

Question 112

Draw the electron transport chain with the classic inhibitors of each stage.

Answer 112

Question 113

Describe how the mitochondria may be involved in oxygen-sensing in HPV. Give experimental evidence.

Answer 113

(Rounds, 1981):

• Found that various poisons of the ETC led to increases in pulmonary artery pressure
• This suggests that the mitochondria may affect HPV because they link to metabolism

(Archer, 1993):

• Proposed a ROS model, since ROS are a product of the ETC
• Hence, ROS could be an indicator of oxygen levels
• Chemiluminescence showed that ROS were decreased in hypoxia
• Antimycin and rotenone (inhibitors of the ETC) led to decreases in ROS and an increase in pulmonary artery pressure -> They are irreversible poisons so sensitivity to hypoxia was lost
• Cyanide (also an inhibitor of the ETC) similarly led to a decrease in ROS and an increase in pulmonary artery pressure -> Cyandine is reversible so hypoxic sensitivity returned

(Chandel, 2000):

• Found that myxothiazol and antimycin (both complex III inhibitors) had different effects on HPV
• Myxothiazol led to ablation of HPV, while antimycin did not
• This was proposed to be because they act at different points on complex III, so antimycin enables the intermediate semiquinone to be stabalised, enabling ROS production
• This led to the idea that ROS production at complex III was indicator of hypoxia, underlying HPV
• However, this was contradictory to the idea that ROS goes down during hypoxia, while this model suggests ROS would go up -> This required further investigation

(Desireddi, 2010):

• Attempted to measure ROS production during hypoxia
• Used RoGFP as an indicator -> This is a bit of a surrogate measure since the RoGFP relies on glutathione, which is part of the antioxidant defense mechanism but not superoxide or hydrogen peroxide per se
• This showed that ROS was increased in hypoxia, which could be ablated using catalase

Question 114

Is complex IV involved in oxygen-sensing in the pulmonary vasculature?

Answer 114

• It usually has a very high affinity for oxygen, so its function would not be compromised by physiological levels of hypoxia
• However, the carotid body has an unusual complex IV that has an altered oxygen affinity -> Evidence for this in the pulmonary vasculature is currently lacking
• This could be due to different isoforms
• (Sommer, 2017):
  
  • Found that knockout of complex IV isoform 2 led to a lack of hypoxic response in isolated vessels
  • However, oxygen sensitivity did not change in the knockout mice

Question 115

Give some experimental evidence for the possible involvement of H₂S in oxygen-sensing in mitochondria in the pulmonary vasculature.

Answer 115

• H₂S has different effects in different vessels -> In some it causes vasodilation and in some it causes vasoconstriction
• However, the effects of hypoxia and H₂S tend to be the same in each type of vessel
• (Olsen, 2010):
  
  • Showed that H₂S degradation is oxygen-dependent
  • Also showed that oxygen-sensitivity of H₂S degradation is unusually sensitive in the lungs
• However, critics have argued that cystathionine-gamma-lyase inhibition does not affect HPV

Question 116

Give some experimental evidence for the possible involvement of HIF in HPV.

Answer 116

(Robbins, 2008):

• 8 hour exposure to hypoxia led to increase baseline pulmonary arterial pressure and also increased PAP after acute hypoxia
• Iron affects the response, suggesting that HIF may play a role

Chuvash polycythaemia also features increased HPV.

Question 117

What makes th epulmonary circulation unique compared to other organs?

Answer 117

It has to take all of the blood with every cycle.

Question 118

Draw a classic setup for the study of vertical distribution of blood flow in the lungs.

Answer 118

Question 119

Give some experimental evidence for the vertical distribution of blood flow in the lungs.

Answer 119

(West, 1964):

• Utilised the setup on the previous flashcard
• Used xenon¹³³ to study blood flow to different heights in the lungs
• First plotted count rate against height -> However, this is affected by the volume of lung at each height
• Next plotted the same after rebreathing the gas into a bag for 2 minutes -> This allows the xenon to equilibriate throughout the lungs, meaning that the count rate is an indicator of lung volume, not blood flow
• Dividing the graphs by each other gives a plot of blood flow per unit volume at each height
• This showed that blood flow decreases with height
• Repeated this at a lower inflow pressure (16cmH₂O) and found that blood flow stopped at a lower height

Question 120

Draw the 3 zone model of the lung. Why is this relevant?

Answer 120

Alveolar pressure can change and affect these zones -> Positive pressure ventilation will increase the size of zones one and two.

Question 121

Is gravity an important influence on pulmonary blood flow distribution?

Answer 121

• West’s classic xenon experiments suggest so
• However, other studies have not found this
• (Prisk, 1994) found that some inhomogeneity of pulmonary blood flow appears to persist in space, in the absence of gravity

Question 122

How can pulmonary vascular flow be measured in vivo?

Answer 122

• Doppler echocardiography can be used to estimate pulmonary artery systolic pressure (PASP) -> This is done by looking at the speed of tricuspid valve regurgitation during ventricular contraction
• Swan-Ganz catheterization

Question 123

How does pulmonary arterial pressure change with increasing exercise workload? Give experimental evidence.

Answer 123

(Grünig, 1997):

• Found that pulmonary pressures remain relatively constant with increasing workload
• During moderate exercise, distension and recruitment of pulmonary vessels means that PVR falls, and pulmonary pressures can remain relatively constant.

Question 124

Give some experimental evidence for how pulmonary arterial and pulmonary venous pressures affect pulmonary vascular resistance.

Answer 124

(Borst, 1956):

• Created a setup including the heart and lungs of anaesthetised dogs connected to a system that allows the left atrial pressure (a proxy for pulmonary venous pressure) and pulmonary arterial pressure to be altered
• Plotted the PVR against left pulmonary arterial pressure
• At low pulmonary venous pressures, a small change in pulmonary arterial pressure leads to a large change in PVR
• But at high pulmonary venous pressures, a large change in pulmonary arterial pressure leads to a small change in PVR
• This shows how the system begins to act more like a rigid tube at high transmural pressure (i.e. it becomes less distensible)

Question 125

What is the main indicator of genetic pulmonary arterial hypertension?

Answer 125

Thicker, less distensible vessel walls

Question 126

What does pulmonary arterial hypertension lead to?

Answer 126

Cor pulmonale (right heart failure), which leads to:

• Right ventricular hypertrophy and dilatation
• Peripheral oedema

Question 127

Give some experimental evidence for the existence of HPV.

Answer 127

(Smith, 2009)

Question 128

What underlies HAPE? Give experimental evidence.

Answer 128

(Grunig, 1997):

• Compared the response to 4 hours of hypoxia in control patients and those who had previously experienced HAPE
• The pulmonary arterial systolic pressure increased much more in the HAPE group than the control group
• This suggests HAPE may be driven by HPV

In particular, uneven HPV may lead to stress failure of capillaries.

Question 129

What is a treatment for HAPE?

Answer 129

Inhaled beta-2 agonists (salmeterol) reduce HAPE in susceptible individuals, perhaps by enhancing alveolar fluid clearance.

Question 130

Describe what happens in the pulmonary circulation during chronic hypoxia. Give experimental evidence.

Answer 130

(Chen, 1995):

• Chronic hypoxia causes pulmonary hypertension and right ventricular hypertrophy in mice.
• There are signs of thickening of the pulmonary arteries.

This can be prevented by endothelin antagonists, or by inactivation of the hypoxia-inducible factor transcription factors.

(Groves, 1987):

• Volunteers were decompressed in a hypobaric chamber for 40 days to a barometric pressure (PB) of 240 Torr, equivalent to the summit of Mt. Everest.
• Serial measurements of the pulmonary vascular pressure gradient were then taken at PB 760 (sea level), 347, and 282/240 Torr.
• At higher altitudes, the pressure gradient increased more with cardiac output and cardiac output could be increased less
• This is also evidence of increased rigidity of the pulmonary vascular system
• The ‘rigidity’ of the pulmonary circulation at altitude might limit exercise capacity by placing a strain on the right ventricle

Question 131

Do pulmonary vasodilators improve exercise capacity at high altitude? Give experimental evidence.

Answer 131

(Naeiji, 2010):

• Found that some of the decrease in exercise capacity during acute and chronic hypoxia can be reversed using sitaxsentan (endothelin receptor antagonist)

Question 132

Is HIF important in pulmonary vascular responses to hypoxia? Give experimental evidence.

Answer 132

(Yu, 1999):

• HIF-1 deficiency in mice (HIF1α+/- ) confers protection against hypoxic pulmonary hypertension

(Smith, 2006):

• HIF upregulation in humans with Chuvash polycythaemia causes pulmonary hypertension

(Groves, 1993):

• Tibetans seem to have selected against high pulmonary pressures at altitude, possibly contributing to remarkable exercise performance
• This population has been at altitude for the longest, suggesting that this response is unwanted

(Petousi, 2014):

• There is blunted HPV in Tibetans

Question 133

Does hypercapnia affect pulmonary vascular resistance? Give experimental evidence.

Answer 133

(Dorrington, 2010):

• Studied the effect of hypercapnia and hypocapnia on pulmonary arterial pressure
• Hypercapnia increased the pressure compared to control, while hypocapnia decreased it

This suggests that alveolar P_CO2 may be at least as important as P_O2 for the acute matching of perfusion to ventilation in the human lung.

Question 134

Describe how ST elevation and depression can happen.

Answer 134

Question 135

Describe different measurements of coronary flow.

Answer 135

Question 136

Describe how flow through the coronary arteries can be measured.

Answer 136

Question 137

Describe how index of microcirculatory resistance (IMR) can be calculated.

Answer 137

TMN = Transit time of the saline bolus

Question 138

What can contrast-enhanced magnetic resonance imaging be used to identify? How does this work?

Answer 138

• It can be used to identify areas of ischaemic damage (i.e. scar tissue) in the heart.
• Gadolinium is used as the contrast. It is washed out of tissue very quickly, but not in the case of scar tissue. Hence, 10 minutes after a bolus of gadolinium is given, areas of scar tissue will still retain the contrast.