Regulation of Blood Flow Flashcards by Graeme Miller

What are the functions of the vascular endothelium?

Barrier function (diffusion / transport)
Clotting system e.g produces von Willebrand Factor
Cell adhesion - venule side, macrophage entry to damaged tissue
Structural components for basement membrane
Produces many vasoactive substances

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What are the vasoactive substances produced by the vascular endothelium?

Vasodilators:

Nitric Oxide (NO) - endothelium derived relaxing factor (EDRF)

Prostacyclin (PGI2)

Endothelium derived hyperpolarising factor (EDHF). E.g epoxyeicosatrienoic acids (EET)

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What are the vasoconstrictors produced by the vascular endothelium?

Endothelin 1

Angiotensin 2

Endothelium derived contracting factor - EDCF

Angiotensin converting enyme (catalyses the conversion of angiotensin 1 to angiotensin 2, which is a vasoconstrictor, it also catalyses the degradation of bradykinin which is a vasodilator - net effect is vasoconstriction)

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How is NO produced?

It is produced by L-arginine by NO synthase

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What are the three types of NOS?

NO (synthase)

eNOS - endothelial cell - constitutive

nNOS - some neural cells - constitutive

iNOS

inflammatory cells normally

Basically iNOS is made in other tissues in response to a stimulus such as anaphylactic shock, infection or endotoxins. It is an oxidising agent that kills pathogens.

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What is the role of NO?

Controls regional blood flow and blood pressure

NO pathway is tonically active - it is always switched on in endothelial cells of resistance vessels

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What is one of the main stimuli for eNOS?

Sheer wall stress.

There are other chemical mediators such as acetylcholine, thrombin, bradykinin. The endothelial cells have receptors for these mediaters on the cell membrane

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How does sheer stress result in NO release and relaxation?

Sheer stress causes endothelial cell depolarisation which opens up voltage gated calcium channels. If the signalling molecules are used (acetylcholine, thrombin, bradykinin) the calcium is released from intracellular stores via inositol triphosphate secondary messenger system (IP3).

This calcium then binds to the calcium binding protein called calmodulin, this complex activates eNOS. The enzyme eNOS produces NO from L - arginine.

The NO needs to travel from the endothelium (tunica intima) to the smooth muscle layer (tunica media).

NO is highly diffusible. Diffuses into the smooth muscle cells - and though the membrane of these cells. Ultimately switches on secondary messenger system cGMP - decrease in intracellular calcium and therefore causes relaxation.

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How does prostacyclin result in reduced smooth muscle contraction?

Prostacyclin - PGI2

The main stimulus for this is calcium, calcium dependant.

Calcium switches on an enzyme called phospholipase A2. This breaks down phospholipids producing arachidonic acid. Cyclooxygenase uses arachidonic acid as a substrate to produces a lot of prostaglandin mediators. In endothelial cells the main one is prostacyclin - binds to its receptor (doesn’t diffuse through membranes) - activates secondary messenger cAMP - switches on potassium channel (when it opens potassium flows down its concentration gradient (there is more potassium inside the cell compared to outside the cell). Potassium flows out of the cell and causes a hyperpolarisation - means that you are less likely to activate voltage gated channels that lead to contractility.

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How does EET result in relaxation of smooth muscle?

EET ( which is an endothelial derived hyperpolarising factor)

This is derived from p450 enzymes in the endothelial cells. Switches on potassium channels - hyperpolarisation and therefore less likely to activate voltage gated calcium channels - reducing contractility.

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How can potassium be implicated in the relaxation of smooth muscle?

Potassium

Potassium might be able to be released through endothelial cells in to the smooth muscle cells (gap junctions between endothelial cells and vascular smooth muscle cells). Nano tubes is what looks like what is binding these cells.

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How might the effects of NO be antagonised?

Using NOS inhibitors - these are anologues of L-Arginine that don’t get broken down.

eg L-NAME or L-NMMA

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How does the role of EDHF and NO change as vessels get smaller?

EDHF becomes more important as the vessels get smaller?

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How does the EDHF / NO ratio change with age?

EDHF : NO ratio declines with age

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How does NO interact with PGI 2 and EDHF?

When NO and PGI2 systems are co-activated there are synergistic effects, i.e. resulting vasodilation is more marked that expected

NO system suppresses EDHF where both are present

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How does vascular smooth muscle respond when there is an increase in blood pressure?

Vascular smooth muscle increases contraction

when blood pressure increases and relaxes when pressure is reduced

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Here this graph shows is that as blood pressure increases there is a brief moment where the vessel diameter increases, but quickly after the vascular smooth muscle contracts to maintain a constant vessel diameter

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How does membrane potential change when transmural pressure increases?

Membrane potential increases (becomes more positive and goes towards depolarisation) - this activates voltage gated calcium channels - calcium travels into the cell from outside the cell.

How can drugs be used to prevent the myogenic respoonse?

Complete Ca2+ channel blockade prevents myogenic responses, but membrane depolarisation survives
Specific blockers of voltage-sensitive L-type Ca2+ channels also prevent myogenic responses

Summary of myogenic response

What are the proposed mechanisms for the depolarisation response to increase blood pressure?

Stretch sensitive cation channels - it has been proven that there are some stretch sensitive sodium channels which can assist in increasing depolarisation. However when we run ion substitute experiments it appears that sodium does not have an important role in increasing depolarisation.

(ii) Phospholipase A (PLA2) activation and HETE formation (hydroxyeicosatetraenoic acid)

How does HETE work?

When vessel wall tension increases - activates phospholipase A 2. This increases production of arachidonic acid, this is the substrate of some enzymes (cytochrome P450 4A) - this produces HETE. This closes calcium activated potassium channels. This results in the membrane becoming more positive and reaches depolarisation opens voltage gated calcium channels - influx - contraction. Negative feedback because calcium will open potassium channels.

What is the experimental advice supporting HETE mechanism?

Phospholipase A 2 reduce the myogenic response in some vessels

Inhibitors of the cytochrome P450 enzyme reduce the myogenic response.

Directly adding HETE results in inhibition of calcium activated potassium channels

Unrelated calcium channel blockers impair the myogenic response.

Why does vessel diameter slowly increase in this diagram?

Flow mediated NO response

How do the vasorelaxation pathways compete with HETE?

Prostacyclin and EET both come from the modification of arachidonic acid. Cytochrome P450 4A also uses arachidonic acid to make 20-HETE P450s contain a haem group NO binds well to haem groups - inactivates the enzyme

How does training affect the levels of NO and endothelin in the plasma?

Training increases the levels levels of NO Training decreases levels of endothelin (a vasoconstrictor)

What is the effect of nerve stimulation on the vessel smooth muscle?

Contracts - if there is no flow If there is flow then the effect is blunted - NO release causes vasodilation.

What are the potential metabolic vasodilators?

Decreased Oxygen Increased CO2 Decrease in pH Increase in lactate Increase in potassium Increase in phosphate Increase in adenosine Increase in osmolarity (potassium is especially important for electrically active tissues such as the brain and muscle) pCO2 is important for the brain

What is the probable mechaism for vessel dilation in response to decreased oxygen?

There is decrease in ATP and an increase in ADP - therefor the ATP sensitive potassium channels open in vascular smooth muscle - therefore there is hyperpolarisation and muscle relaxation.

Looking at the graph, explain why oxygen cannot be the only metabolic vasodilator responsible for the vasodilation?

The area in between the two dotten lines indicates muscle contraction. There is sustained vasodilation for the duration of the contraction - if oxygen was the primary vasodilator then we would expect the O2 levels to be decresed for the entire time (in order to produce the desired vasodilation). However Oxygen levels return to normal when whilst the arteries are still dilated.

How is CO2 implicated in metabolic vasodilation?

Acidosis dilates isolated vessels by opening the kATP channels. Acidosis derived in part from CO2 via carbonic acid But skeletal muscle and heart muscle are relatively insensitive to changes in blood pCO2.

Why is decreased pH dismissed as a primary vasodilator?

pH changes are too small in order to account for the large increase in muscle blood flow that is possible.

Why is lactate dismissed as a primary vasodilator?

Lactate infusion into resting muscle circulation to match levels found in venous outflow from active muscle does not cause vasodilation McArdle’s syndrome - lack of glycogen phosphorylase, so cannot use glycogen for anaerobic glycolysis. During muscle activity, no change in pH or venous lactate outflow - but still get fairly normal vasodilation.

Which tissues might be influenced by potassium as a vasodilator?

Tissues that are electrically active and release potassium - Brain, skeletal muscle and potentially the heart.

Why is potassium dismissed as a primary vasodilator?

Topical application of K+ to skeletal muscle produces vasodilation too slowly to account for the rapid increase in blood flow in active muscles For brain: K+ increases with neural activity are transient because of rapid uptake by glial cells, therefore, cannot account for sustained vasodilation.

Why was inorganic phosphate considered as a metabolic vasodilator?

You get a build up of phosphate when muscle is active. So you get a venous outflux of inorganic phosphate

Why isn't inorganic phosphate considered as a primary metabolic vasodilator?

Pi infusion into resting human limbs to raise venous levels to those found in exercise did not alter limb blood flow.

What receptors does adenosine act on?

Why don't adenosine levels correlate well with blood flow?

Might be cofounded by the fact that tissues have rapid adenosine reuptake mechanisms.

What happens when you inject adenosine deaminase into the heart, skeletal muscle or brain?

Adenosine deaminase rapidly breaks down adenosine - reduces maximum hyperaemic response activity by 20-40%

Why doesn't osmolarity bear any effect in brain vasodilation?

Osmolarity is tightly regulated

Is osmolarity a primary mediator in vasodilation?

May be implicated in skeletal muscle whereby osmolarity varies directly with repetitive stimulation. However Hyperosmotic solution infused into resting limbs to mimic venous outlflow levels during exercise does not cause sufficient lood flow rise.

How does indomethacin effect blood flow?

Indomethacin is a COX inhibitor - COX makes prostaglandin PGE 2 which is a potent vasodilator - released by actvie tissues. As a result indomethacin results in a small decrease in skeletal muscle blood flow during exercise For cerebral circulation, indomethacin partially inhibits the vasodilator response to CO2

What is the effect of Kinins on blood flow?

Kinins e.g. bradykinin. Skeletal muscle during exercise increases kinin release, which are powerful vasodilators. However, little is known about their potential physiological role in the control of blood flow.

Conclusion of metabolic vasodilators

No single local metabolic vasodilator yet discovered, although adenosine may be important in some vascular beds. There may be multi-factor effects; additive or synergistic interactions between multiple mediators.