Lecture 07 Vascular Physiology 1 Flashcards
1. How blood vessel contraction is regulated physiologically 2. How do we target these mechanisms therapeutically in different diseases?
Why is regulation of blood vessel contraction important? Give examples.
changes in blood pressure
important for directing blood supply to where oxygen and nutrients are needed in the body
exercise, digestion, thermoregulation
What happens when vascular physiology goes wrong?
atherosclerosis and hypertension
What initiates atherosclerosis?
dysfunctional blood vessels
What causes hypertension?
regulation of blood vessel contraction goes wrong
What are important physiological factors and functions of arteries?
strong - withstand high blood pressure
muscular - contract and relax
permeable - supply nutrients, remove waste and provide gas exchange
What are the main resistance vessels?
smaller muscular arteries
What are the layers of the artery?
tunica externa
external elastic membrane
tunica media - smooth muscle, elastin and collagen matrix
internal elastic membrane
tunica intima - endothelium and in larger arteries connective tissue
What is the function of the external/Adventitia elastic membrane?
very strong fibrous tissue
acts to maintain vessel shape
What is the function of elastin?
allows the vessel wall to stretch (elasticity)
What is the function of the smooth muscle?
contracts and relaxes to determine the size of the artery
What controls the state of the smooth muscle?
circulating hormone and local mediators from endothelium and sympathetic nerves
What is the function of endothelium?
located directly in contact with the blood to response to circulating factors, e.g. hormones
How do endothelium and vascular smooth muscle cells (VSMCs) communicate?
direct contact - gap junctions coupling cells together
also diffusion of mediators from endothelium
How do VSMCs communicate? Why is this important?
gap junctions between VSMCs allow effective calcium transport to allow coordinated contraction of the vessel
Why is endothelium so important?
it is the largest organ in the body lining every blood vessel
dysfunctional or activated endothelium leads to disease states
is the first line to react to circulating factors, blood cells and pathogens
What is an indicator that endothelia cells are healthy or inactive?
healthy glycocalyx
What does the glycocalyx consist of?
carbohydrate/sugar chains protruding from the apical surface of the endothelium
Why is the glycocalyx important?
acts as an anti-coagulant
prevents circulating cells from binding to adhesion molecules found on the surface of the endothelium
What can cause shedding of the glycocalyx?
injury, infection or inflammation
oxLDL (lipid)
disturbed blood flow (oscillatory shear stress)
What happens when the glycocalyx is damaged?
adhesion molecules on endothelium become exposed
circulating glycans on blood cells (monocytes, neutrophils, platelets) bind to adhesion molecules
blood cells roll along the artery wall, transmigrating
this initiates atherosclerosis
Where does disturbed blood flow occur? What does it cause?
branches of arteries
arteries bend
In healthy endothelium, what neurotransmitters can stimulate endothelium?
acetylcholine
histamine
bradykinin
serotonin
What happens in healthy endothelium signalling?
endothelium stimulated by neurotransmitters that bind to various receptors
this mediates an increase in intracellular calcium which activates endothelial Nitric Oxide Synthesis
eNOS converts arginine to nitric oxide and citrulline
What effect does nitric oxide (NO) have on VSMCs? How does this effect occur?
relaxation
activates guanylyl cyclase which increases cGMP
activating PKG which activates myosin phosphatase
myosin phosphatase promotes relaxation of the muscle
What stimulants can activate endothelial cells in unhealthy endothelium?
interleukin-1 (IL-1)
endotoxin (bacterial cell wall)
thrombin (platelets)
What can stimulants of unhealthy endothelium activate?
endothelin-1 (ET-1)
ROS
What does endothelin-1 (ET-1) do in VSMCs?
contraction
What can disturbed blood flow in unhealthy endothelium activate?
ROS ICAM-1 VCAM-1 IL-8 COX-2
What do ICAM-1 and VCAM-1 expression lead to? What effect does this have on the endothelium?
increased expression of adhesion molecules at the cell surface and shedding of the glycocalyx
increased monocyte, neutrophil and platelet interactions promoting transmigration across the artery wall and atherosclerosis
How does calcium signalling cause contraction of the VSMCs?
increased intracellular calcium forms a complex with calmodulin
this complex binds to myosin light chain kinases which phosphorylate myosin leading to activation and contraction
How does contracted VSMCs relax?
myosin phosphatase is constitutively active and works to dephosphorylate myosin
What is the cells source of calcium?
calcium store in the sarcoplasmic reticulum
How is low intracellular calcium normally maintained?
calcium-ATPase pumps on the plasma membrane and SR membrane
What are the two routes VSMC contraction can occur?
GPCRs and calcium channels
What can stimulate GPCRs?
enodthelin A/B TP (prostanoid) AT1 (angiotensin 1) histamine NA
How does activation of GPCRs lead to calcium release?
active GPCR activates IP3 which signals calcium channels on the SR to open increasing intracellular calcium levels
What are the different types of calcium channels?
voltage sensitive (L-type) receptor operated (e.g. P2X) TRP channels store operated (Ora1)
What are the three mediators of VSMC relaxation?
cGMP
cAMP
potassium channels
How does cGMP mediate VSMC relaxation?
activation of PKG
PKG acts on myosin phosphatase which dephosphorylates myosin leading to relaxation
What mediated increased cGMP?
increased NO from endothelium which activates guanylyl cyclase
How does cAMP mediate VSMC relaxation?
increased cAMP decreases calcium concentration
prevents myosin light chain kinase from working and no contraction to occur
What mediates increased cAMP?
beta-agonists, adenosine and prostaglandins
bind to Gs proteins which increase cAMP through adenylyl cyclase activation
How does potassium mediate VSMC relaxation?
potassium efflux from the cell leads to hyperpolarisation
this leads to decreased calcium preventing myosin light chain kinase from working and no contraction to occur
What mediates increased open probability of the potassium channel?
beta-agonists via the beta-gamma G-protein
Give two examples of potassium channels
BK channels (large conductance) SK channels (small conductance)
What endothelial mediators lead to VSMC contraction?
prostanoids
endothelin-1
angiotensin II
What is the function of nitric oxide (NO)?
an endothelial-derived vasodilator responsible for regulating blood pressure and regional blood flow
What is dysregulation of NO linked to?
cardiovascular risk factors
What impairs eNOS?
smoking
high glucose and insulin
oxLDL
How does smoking affect NO dysregulation?
reduction in NO bioavailability
interferes with eNOS acetylation
loss of eNOS anchored to the membrane
How does hyperglycaemia affect NO dysregulation?
reduced eNOS phosphorylation
reduced bioavailability in NO
may act via loss of coupling in the Akt pathway
How does oxLDL affect NO dysregulation?
depletion cholesterol from caveolae causing loss of eNOS
displacement of ENOS from caveola through binding to CD36 scavenger receptors
What does disturbed eNOS function lead to?
hypercholesterolaemia
What does NO dysregulation lead to?
inability to regulate blood pressure and hypertension
What risk factors are linked to ageing and disease?
atherosclerosis damage to glycocalyx calcification loss of elastin decreased NO increased blood pressure and hypertension
What does atherosclerotic plaques cause?
increased separation of endothelium from VSMCs
loss of regulation of artery contractility
raised blood pressure
loss of elasticity - reinforcing loss of relaxation
Damage to the glycocalyx is caused by?
hyperglycaeia hyperlipidaemia smoking sepsis inflammation
How are endothelial-derived Prostanoids produced?
increased intracellular calcium or ROS activate COX1/2 enzymes which convert arachidonic acid into prostanoids
Give three examples of Prostanoid derivatives
thromboxane A2 (TxA2) prostaglandin E2 (PGE2) prostaglandin I2 (PGI2)
By what mechanism does Thromboxane A2 (TxA2) affect VSMCs?
thromboxane A2 binds to thromboxane receptor (TP) on VSMC plasma membrane
TP receptor couples to PLC activating and producing IP3 which signal calcium release from the SR
increased intracellular calcium leads to muscle contraction
By what mechanism does Prostaglandin E2 (PGE2) affect VSMCs?
prostaglandin E2 binds to prostaglandin E2 receptors (EP1-4)
some activate adenylyl cyclase via Gs GPCRs which leads to a reduction in cAMP leading to muscle contraction
other inhibit adenylyl cyclase via Gi GPCRs which increases the cAMP levels, which activates of PKA
PKA activates myosin phosphatase leading to muscle relaxation
What is the effect of PGE2 dependent on?
the relative expression of the EP receptor subtypes on VSMCs
By what mechanism does Prostaglandin I2 (PGI2) affects VSMCs?
binds to IP receptors (IP-R) on VSMC plasma membranes
IP GPCR receptor couples to activation of adenylyl cyclase leading to an increase in cAMP which activates PKA
PKA activates myosin phosphatase leading to muscle relaxation
What is the precursor of Endothelin-1 (ET-1)? What upregulates this precursor?
big endothelin
inflammatory or pathogenic
What enzyme mediates the breakdown of big endothelin to endothelin-1
endothlin converting enzyme (ECE)
By what mechanism does endothelin-1 act on VSMCS?
ET-1 act on either ET alpha or beta (ETA/ETB) GPCR-Gq receptors which couples with PLC leading to activation and production of IP3
acting of calcium channels in SR membranes to increase intracellular calcium leading to muscle contraction
By what mechanism does endothelin-1 acts on endothelial cells?
negative feedback
ET-1 acts on ETB receptors on endothelial cell plasma membrane which increases intracellular calcium causing production of NO from arginine via eNOS
NO inhibits ECE down-regulating the production of endothelin-1
How is Angiotensin II produced?
ACE enzyme converts angiotensin I into angiotensin II
Where are ACE enzymes predominantly expressed?
expressed at the membrane of pulmonary and renal vasculature endothelial cells
By what mechanism does Angiotensin II act on VSMCs?
angiotensin II binds to AT1 receptors on VSMC plasma membranes which can activate PLC leading to increased IP3 which activates calcium channels on the SR membrane
increased intracellular calcium leads to muscle contraction
AT1 receptors can also activate MAPK signalling
What does MAPK singalling lead to in VSMCs?
makes VSMCs more contractile in their responses
more persistent changes in VSMC responses
What effect does smoking have on vascular function?
damages glycocalyx on endothelin which increases adhesion, activation of endothelin and initiation of atherosclerosis
reduces the bioavailability of NO and stops eNOS attachment to the membrane
What effect does hyperlipidaemia (high oxLDL) have on vascular function?
depletion of cholesterol from endothelial caveoli, reducing the function of eNOS
fatty plaque build up which separate the connections between endothelial cells and VSMCs
increased endothelin production leading to increased VSMC contraction
What effect does hyperglycaemia have on vascular function?
excessive insulin dampens the Akt pathway on eNOS activity
increased damaged to the glycocalyx
increased endothelin production from endothelial cells leading to increased VSMC contraction
What effect does ageing have on vascular function?
loss of elastin/stretch
loss of compliance if arteries
What effect does infection have on vascular function?
pathogens and inflammatory/immune response activate endothelium
recreation of leukocytes to the artery wall weaken atherosclerotic plaques
these stimuli generally activate pathways leading to increased VSMC contraction
