The importance of endothelial function on health and disease Flashcards
What is the vascular system composed of
Arteries -> arterioles -> capillaries -> venules -> veins
What are the different types of arterioles
Conduit (elastic)- recoil. Muscular (distributing)- dilate. Terminal (end) arteries
What is the typical blood vessel structure
Endothelium, tunica initma, internal elastic lamina, tunica media, external elastic lamina, tunica adventitia.
What is the tunica media composed of
Smooth muscle cells
What is the tunica adventitia’s function
It provides structure
What does the tunica intima contain
The endothelium and basement mebrane
What do capillaries consist of
Single cells
Which direction is the endothelium elongated in
The direction of flow
Is the endothelium flat
No, it protrudes into the lumen of the cell
Where is Von Willebrand factor produced
In endothelial cells
What is PGI2
Prostacyclin, it is antithrombotics and acts by inhibiting aggregation of platelets
Where is PGI2 released from
Endothelial cells
What do endothelial cells have an obligatory role in in relation to arterial smooth muscle
Obligatory role in relaxation of arterial smooth muscle by acetlycholine
What happens when acetylcholine is introduced to an artery with its endothelium intact
Relaxation is induced
What affect does histamine have on coronary arteries
The greater the concentration of histamine the more the muscle contracts resulting in an increase in perfusion pressure
Why do vessels have a basal level of tone
To respond to stimuli. To vasodilate allowing increased localised blood flow and tissue perfusion (such as that required by increased metabolic demand- high tone in skeletal muscle creates a “reserve”, potential for increased capacity
What presents a blood vessel constricting too much
Basal release of EDRF
What is the stimulus for basal release
Flow- the endothelial cell surface senses haemodynamic mechanical forces, such as shear stress, with transduction into intracellular signalling events leading to release of EDRF
What is EDRF
Endothelial dervied relaxing factor
What does EDRF ensure
There is always tone in the vessel wall= vessel can respond to stimuli
What was EDRF shown to be
Nitric oxide (NO)
Describe stimulation of NO prodction
Agonist stimulated
Describe No
NO is freely diffusible, soluble, lipophilic gas secreted by the endothelium as it is produced (not stored)
What is NO formed by
NO formed in the endothelium by calcium sensitive constitutive enzyme endothelial Nitric Oxide synthase (eNOS) from its precursor L-arginine, Tetrahydrobioptern (BH4) is as essential cofactor in this process
What stimulates activation of eNOS
Agonist stimulated activation of eNOS followsa rise in inracellular calcium through release from intracellular calcium stores and calcium influx
What is NO production stimulated by
Flow stimulayted by both calcium dependent and independent activation of eNOS
Describe the main transcriptional and post-transcriptional mechanisms in response to shear stress
Shear stress evokes nitric oxide (NO) production. This is mediated by an increase in eNOS protein activity (1) that occurs within seconds and implicates cytosolic calcium and eNOS protein phosphorylations. Later increases in transcription (2) and eNOS mRNA stability (3) allow to maintain an increased NO production when the stimulus is prolonged
Describe flow sensing mechanical forces in NO production
The endothelial cell perceives mechanical tensions such as shear stress through membrane changes and glycoalyx movement which are linked to the cytoskeleton, leading to activation of ion channels that increase Ca2+ permeability. The cytoskeleton is linked to stretch mediated calcium channels
Describe NO mechanisms of action regulating smooth muscle contraction
NO increases production of cyclic guanosine monophosphate (cGMP) which acts through a number of mechanisms to reverse vasoconstriction. cGMP-dependent protein kinases (PKGs) elicit multiple phosphorylations of cellular proteins that result in lowering of cellular calcium either through decreased mobilisation from intracellular stores or decreased entry from the extracellular
Summarise NO production and mechanisms of action
An increase in calcium in the endothelium results in relaxation (vasodilation). An increase in calcium in muscle results in contraction as calcium stimulates the contraction response in muscle. After smooth muscle has contracted calcium goes back into sarcoplasmic reticulum
What are the targets of endothelial derived nitric oxide
Decreased smooth muscle contraction, decreased smooth muscle proliferation, decreased platelet aggregation, decreased LDL oxidation, decreased expression of adhesion molecules. decreased monocyte and platelet adhesion, decreased vasoconstrictor production (endothelin)
What is the function of the endothelium
As a monolayer of cells that covers the internal surface of all blood vessels, providing a vast interface between the circulating blood and the vessel wall, the endothelium if involved in many mechanisms of vacular regulation
What mechanisms of vascular regulation is the endothelium involved in
Synthesis and release of factors, vascular tone, haemostasis, angiogenesis, permeability, inflammatory response
What diseases are all, in part, causes by endothelial dysfucntion
Coronary artery disease, hypertension, stroke
Describe a healthy vascular endothelium
In a tightly regulated state of balance between pro and antioxidants, vasodilators and vasoconstrictors, pro and anti-inflammatory molecules and pro and anti-thrombotic signals
What does a diseased or dysfunctional endothelium lose
The tightly regulated balance and displays pro-oxidant, vasoconstrictor, pro-inflammatory and pro-thrombotic properties
Describe the effect of oxidative stress on endothelial dysfunction and vascular disease
Endothelial oxidative stress reduces the effects of or the bioavailabilty of NO.
What may eNOS produce under conditions of substrate or cofactor depletion
Superoxide (O2-)
When does increased O2- occur
In disease associated with diabetes and hypertension
What effect does O2- have on nitric oxide
Reduces bioavailability of nitric oxide by chemical interaction forming peroxynitrite: Nitric oxide (NO) + superoxide (O2-) -> peroxynitrite (ONOO-)
What does peroxynitirite do
Interacts with lipids, DNA and proteins triggering cellular responses ranging from modulation of cell signalling to overwhelming oxidative injury
Describe the effects of peroxynitirite
Modification of mediator pathways and cellular signalling molecules, membrane channel inhibition, impairment of enzyme cofactors, activation (gain of function) of specific enzymes, protein aggregation, cytosolic enzyme inhibition, antioxidant depletion, antioxidant enzyme inhibition
Describe how young endothelial cells behave
In younger endothelial cells eNOS has adequate cofactor availability e.g. tetrahydrobioprotein (BH4) and produces NO. Reactive oxygen species e.g. superoxide and hydrogen peroxide produced by the mitochondrial electron transport chain (such as NADPH oxidase) are quenches by endogenous antioxidant enzymes (superoxide dismutade (SOD) and catalse)
Describe how old endothelial cells behave
In older endothelial cells ROS produced in the mitochondria increases NOX mediated O2, this O2 quenches NO bioavailability through conversion of NO to peroxynitrite as well as by uncoupling eNOS via reductions in BH4 availability. In the face of unchanged antioxidant defences these effects lead to a reduction in NO bioavailability and a pro-oxidant phenotype in the aged endothelium
What occurs in disease states associated with diabetes and hypertension
Similar endothelial oxidative stress and impaired NO bioavailability and a pro-oxidant phenotype in the aged endothelium
Explain how vascular changes in hypertension mimic those found in arteries with observed ageing
Increased media:lumen ratio, vascular remodelling, increased stiffness, vascular inflammation, calcification
Describe the effect of endothelium derived contracting factors
Endothelial cells can evoke contraction (constriction) of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factors (EDCFs), usually prostanoids
When are EDCF-mediated responses exacerbated
When the production of NO is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes)
What to EDCFs contribute to
Blunting of endothelium-dependent vasodilatations in ages subjects and essential hyperintensive patients
Describe how atherosclerosis results in endothelial dysfunction
Atherosclerotic plaques occur at specific sites where flow patterns are disturbed. Atherosclerosis typically develops at arterial branches and sites of curvature that are regions of low and disturbed wall shear stress, with complex changes in speed and direction. Continuing work reveals that responses of vascular endothelial cells (ECs) to mechanical stimuli are central to disease initiation and progression
What risk factors does atherosclerosis depend on
Hyperlipidemia, smoking, hypertension and diabetes
Describe laminar blood flow
Normal physiologic vascular function and vascular homeostasis, thromboresistance, barrier function
Describe disturbed flow
Decreased eNOS production, decreased vasodilatation, decreased endothelial cell repair, increased ROS- endothelial permeability to lipoproteins, leukocyte adhesion, apoptosis, SMC proliferation and collagen deposition
What is atherosclerosis
Chronic inflammatory disease of arteries
What does the initiation and progession of atherosclerosis involve
Circulatign factors such as LDLs and triglycerides, inflammatory activation of the cells of the vascular wall
What happens to LDL in atherosclerosis
It undergoes oxidative modifiction within the intima and becomes oxidsed LDL (oxLDL)
What do oxLDL and other atherogenic signals lead to
Expresssion of adhesion molecules allowing circulating monocytes to attach and migrate into the intima
What happens when circulating monocytes migrate into the intima
Circulating monocytes differentiate in macrophages
What do macrophages do
Uptake the oxLDL in order to reduce the lesion and become foam cells
What do foam cells do
Contribute to the inflammation by releasing pro-inflammatory factors to recruit more leukocytes to the site of the lesion
What happens to the lesion in relation to foam cells
It progresses and foam cells accumulate becoming apoptotic
What happens when foam cells become apoptotic
Lipid is deposited and inflammatory signals grow
What does the chronic inflammatory response result in
Further influx of inflammatory cells
What does smooth muscle cell differentiation result in
Them to migrate and proliferate from the media into the intima
What forms the fibrous cap
Proliferated smooth muscle cells in the intima secrete extracellular matrix proteins which form the fibrous cap
What are the preferential locations atherosclerosis occurs at
Curvatures and bifurications
What happens at atherosclerosis hotspots
There is a focal region of decreased shear stress around a curvature resulting in decreased eNOS, decreased endothelial repair, decreased cytoskeleton/ cellular alignment in direction of flow, increased reactive oxygen species, increased leukocyte adhesion, increased lipoprotein permeability and increased inflammation
What do the factors at atherosclerotic hotspots result in
Plaque formation amplifying the region of disturbed flow
What are endothelial contracting factors
Thromboxane A2, superoxide anion, endothelin-1, angiotensin II, prostanoids, H2O2
What are relaxing endothelial factors
Adenosine, PGI2, NO, epoxy eicosatrienicacids (EETs), H2O2
What is EDHF
Endothelial derived hyperpolarisation factor= hyperpolarisation= calcium can’t enter cell= relxation
What is EDHF dependent on
Gap junctions, it passes through gap junctions
Describe the structure of gap junctions
Specialised mebrane structures that connect the cytoplasm of adjacent cells. They are intercellular channels, some form gap junction plaques at points of cell-cell contact
What is the function of gap junctions
Metabolic coupling and electrical coupling
Describe metabolic coupling of gap junctions
Permit the free passage between cells of small metbolites, nucleoties and 2nd messengers (up to molecular weight on about 1000 daltons, IP3, cAMP)
Describe electrical coupling of gap junctions
Ions can flow through them gap junctions permit propagation of electrical impulse and change in membrane potential to pass from cell to cell
Describe the biological importance of EDHF
Inverse relationship between NO and EDHF-type response. Backup role to NO becoming more important in disease states.
What dominates in resistance vessels
EDHF
What are ACh-evoked responses in myometrial arteries during pregnancy dependent on
Gap junctions
What are EDHF responses mediated by in hypertension
Increased EC-SMC coupling
What is the mechanism of EDH initiation
Similar to NO (increase in endothelial Ca2+) and EDH (not NO) is potentiated by H2O2
How is EDHF potentiated by H2O2 a compensatory mechanism in situations where NO bioavailabilty is compromised by increased superoxide production
Superoxide is dismutased to H2O2
Summarise the endothelium
Monolayer forming between the lumen of all blood vessels. Provides vast interface between blood and tissue. Releases vasoactive agents that control numerous functions e.g. vascualr tone, haemostasis. Endothelial dysfunction leads to increase in vascular smooth muscle cotraction (vasospam) and atherosclerosis
