Vessels Flashcards
Describe phenotypic heterogeneity
Arteries- ECs aligned in direction of flow, long and narrow cells, continuous endothelium with many tight junctions, no valves, specific markers- Ephrin B2
Veins- continuous endothelium, shorter wider cells not aligned in direction of flow, have valves, specific markers- Ephrin B4
Summarise endothelial function
Regulates vascular homeostasis Acts as both sensor and effector -blood flow regulation -permeability -homeostasis -neutrophil/leukocyte recruitment -hormone trafficking
Describe endothelial barrier function
Continuous non-fenestrated endothelium found in arteries, veins and capillaries if the brain, skin and heart- paracellular transport of water and small solutes, transcytosis- caveole smooth membrane invaginations and vesicles
Fenestrated continuous found in the glomerulus- pores have a diaphragm- permits greater transendothelial transport of fluids and solutes
Sinusoid found in the liver- large fenestrations that lack a diaphragm with a poorly formed basement membrane- high endocytic activity in clathrin-coated pots
Contain tight junctions (claudins, occludins, junctional adhesion molecules- JAMs)
Adherens- VE-cadherin
Gap junctions- connexins
What are VVOs?
Vesicular-vacuolar organelles
Form transcellular channels when they connect
A major route for transport of fluids and solutes across the endothelium particularly inflammatory situations
How does the endothelium provide a non-thrombogenic surface?
Tissue factor pathway inhibitor- blocks activation of extrinsic pathway
Thrombomodulin
Endothelial protein C receptor (EPCR)
Tissue-type plasminogen activator (t-PA)
Prostacyclin and NO
Tissue plasminogen activator
ATPase breaks ATP down into AMP so it cannot activate platelets
What is the procoagulant by activity of the endothelium?
Induction of tissue factors
Plasminogen activator inhibitor-1
von Willebrand factor release
Describe the maturation if blood vessels
- Endothelial cell tube
- Capillary- EC tube, pericyte, basement membrane
- Arterioles and venules- EC tube, internal elastic lamina, smooth muscle cell, basement membrane, external elastic lamina
- Lymphatics- endothelial cells and valves, initial lymphatic and collecting lymphatics
Describe vascular endothelial growth factor
VEGF is the master regulator of physiological and pathological cal angiogenesis
Selective for endothelial cells via three high-affinity receptors
Loss of one allele is embryonic lethal
Over-expression leads to severe abnormalities and death in utero
In the adult VEGF overexpression leads to oedema and tumour growth
Inhibition cause hypertension, proteinuria, bleeding gums
Summarise the regulation of vascular tone
Vasodilation- NO and prostacyclin
Vasoconstriction- endothelin1, thromboxane, hydrogen peroxide, superoxide anion
Describe NO synthesis pathway
L-arginine converted by coupled eNOS to make NO and L-citruline
Or L-arginine is used in protein synthesis and the degraded in to ADMA and excreted
eNOS- regulation of vascular tone, inhibition of smooth muscle cell proliferation, inhibition of platelet aggregation
Describe the regulators of eNOS activity
Phosphorylation
Association with co-factors- BH4
Cellular location- sequestration by calveolin1 in a less active state
Calmodulin binds with calcium after its release from calveolin, hsp90 binds, Akt and PKA phosphorylate it
Describe the activity of NO
Freely diffusible gas that acts as a signalling molecule
Local activity- short half-life
Activity limited by circulating haemoglobin
Prevents thrombosis
Anti-inflammation
Anti-oxidant
Inhibits smooth muscle proliferation and migration
Atheroprotective
Activates guanylate cyclase- produces cGMP reduces Ca2+ and activates phosphodiesterase, activates PKG which limits activation if the myosin light chain and therefore cross-bridging
What is the function of prostacyclin on vascular smooth nuclear relaxation?
Bind IP receptors
Activates adenylate cyclase
Increasing cAMP conc activates PKA
Decreases Ca conc limiting smooth muscle contraction
What can be a consequence of eNOS uncoupling?
Loss of BH4 uncoupled the eNOS dimer- decreases NO and promotes superoxide generation
Increased NADPH and xanthine oxidase activity increase superoxide production
Peroxynitrite (OONO-) leads to further uncoupling of eNOS after oxidative damage to eNOS and/or BH4
Describe vascular complication in diabetes mellitus
Central and peripheral vasculopathies- retinopathy, neuropathy, nephropathy
Increased risk of cardiovascular disease due to plaque rupture find atherothrombosis
Excessive O2- generation in mitochondria intimidates the vascular injury in response to hyperglycaemia
What is pre-eclampsia?
Maternal systemic syndrome caused by abnormal placentation in the first trimester
30% associated with intrauterine growth restriction of the foetus
Only cure is the delivery of the placenta
Leads to systemic endothelial activation, systemic inflammatory response
Splice variants of the flt1 gene are produced by the placenta and act to sequester VEGF and antagonise its activity in the maternal circulation
What nerves supply blood vessels?
Most vessels have only sympathetic innervation perivascular verve fibres in the adventia-media border and do not penetrate into the VSM
Vasoconstrictor- NA, ATP, NPY
Dilator- ACh, VIP and NO
Except from the cerebrum, heart, and reproductive tissue which also have parasympathetic
ACh, VIP NO
Describe varicosities
Granular vesicles contain NA ATP
Large opaque vesicles contain NYP
Release is as the action potential passes
NA production is matched to nerve activity
Increase nerve activity leads to increased enzymatic activity in the conversion of tyrosine to DOPA to dopamine to NA
There is also re-uptake via a NA transporter
After release they act on alpha1 and alpha2 receptors on the vessel and diffuse into the blood stream or broken down by COMT, act on pre-synaptic alpha2 inhibition receptors, re-uptake, break down by MAO
Describe the differential distribution of adrenoceptors
There are fewer alpha1 receptor in vessels as you approach the distal arterioles
Their constrictor influence is more easily blunted by local dilator influences
List some agonist as and antagonist to alpha receptors
A1 agonist- phenylephrine Antagonist- prazosin A2 agonist- clonidine Antagonist- yohimbine Non selective agonist- phenylephrine
Describe the effects of NA on VSM via A1 and A2 receptors
A1- stimulates PLC to convert PI to insP3 which acts triggers the release of more Ca from the sarcoplasmic reticulum
Stimulates voltage gated Ca channels leads to influx of Ca
A2- activation leads to the inhibition of K channels leading to depolarisation and activation of voltage gated Ca channels leading to an influx of calcium
Increased intracellular calcium leads to contraction
Describe the release and action of co-transmitter ATP
Purinergic receptors- P1- adenosine
P2- ATP ADP AMP
(P2X-1 on VSM and P2Y I. The endothelium)
ATP is metabolised by ecto-nucleotidases to AMP and then to adenosine by 5’ nucleotidases
Adenosine acts on pre-synaptic P1 receptors inhibiting ATP release
ATP primes the VSM for the action of NA
Describe neuropeptide Y
Discharged with long periods of high frequency activity
Produces slow and long-lasting constriction, particularly in the presence of NA
Y1 main receptor in VSM
Y2 pre-junctional, modulates NPY/ATP/NA release
Terminated by peptidases
Facilitated vasoconstriction evoked by ATP and NA
Released in order to prevent a catastrophic fall in ABP in the case of haemorrhage, dehydration, shock and heart failure
Describe the synthesis and release of ACh VIP and NO
Choline Acetyltransferase makes ACh
nNOS synthesises NO
Vesicle containing ACh and VIP act on VIP receptor on the smooth muscle
And on the muscarinic receptor on the endothelium to synthesis and release NO
NO acts on the smooth muscle
ACh is degraded by acetylcholine esterase and the choline is recycled
Describe the possible effects of nerve released ACh VIP and NO on VSM
NO acts on Guanine cyclase to produce cGMP which causes dilation
VIP actor to stimulate K channels to hyperpolarise cells and inhibit the activation of voltage gated Ca channels
List the influences on blood vessels in vivo
Myogenic tone (ABP) Sympathetic noradrenergic Endothelium derived substances Hormones, O2, CO2 etc in blood Substances released from tissue cells that accumulate in the interstitial space
Describe the hormonal influences on blood vessels
Catecholamines- adrenaline, noradrenaline- released from the adrenal medulla in response to environmental stressors, strenuous exercise, cold etc
NA- weak of all arterioles except cerebral- stimulates alpha2 receptors➡️ NO
Adrenaline- vasodilation in skeletal muscle only- beta2 receptors occur on VSM and endothelial cells- stimulates NO synthesis and increasing cAMP
-vasoconstriction in all other vessels (alpha receptors)
ADH- released by pituitary gland- acts on V1 receptors on VSM- vasoconstriction
Angiotensin2- acts in AT1 receptors on VSM- vasoconstriction
Summarise local influences on blood vessels
Blood flow is regulated by local mechanisms to support tissue metabolism- ‘flow-metabolism coupling’
Little variation in skin and kidney
Can vary a lot in skeletal muscle, cardiac muscle and brain
Describe the response to system hypoxia
Fall in PaO2 affects endothelial cells- Release adenosine➡️ stimulates NOS and COX
Fall in PaO2 affects RBCs- release ATP when O2 is offloaded via pannexin channels
ATP produces vasodilation via P2Y receptors on endothelial cells
ATP metabolised by ecto-nucleotidases to ADP, AMP and adenosine which acts on endothelial P1 receptors
Vasodilation increases blood flow and shear stress which further stimulates NO
Fall in PaO2 also inhibits vasoconstrictors
Describe the response to muscle hypoxia
Muscle contraction causes
1. K efflux- vasodilator
2. ATP release- metabolised by ectonucleotidases to adenosine
3. ATP degradation also produces inorganic phosphate
4. Synthesis and release of PGE2
Dilation caused by
K induced hyperpolarisation which closes Voltage gated Ca channels on VSM
Adenosine acting in VSM P1 receptors
PGE2 acting in EP receptors
RBC release O2 and ATP which may induce dilation via endothelial P2Y receptors
Vasodilation increases shear stress which stimulates NO production
Summarise the Berne hypothesis
Adenosine is manly responsible for matching O2 delivery to VO2 in cardiac muscle and coronary blood flow
Produced by endonucleotidases from AMP
Summarise cerebral circulation
Increased neuronal activity requires more O2 and glucose mostly by increasing delivery by vasodilation which has to be compensated by vasoconstriction elsewhere to prevent increased intracranial pressure
Responses to changes in local CO2 are thought to occur by stimulation of prostaglandin synthesis
Responses to local O2 occur via adenosine, prostaglandins and NO via the endothelium
Summarise glial cell function with cerebral blood flow
Interneurones release glutamate
Glial cells siphon up K and release near arterioles which cause the hyperpolarisation of VSM
Glial cells also synthesise and release prostaglandins and adenosine
What is the difference between angiogenesis and vasculogenesis?
Angiogenesis is the formation of blood vessels from other blood vessels, vasculogenesis is the formation of new blood vessels from mesodermal derived angioblasts
List the roles of angiogenesis
During development
Physiological processes- wound healing, menstruated cycle, adaption to increased muscle activity
Pathological processes- cancer, inflammatory, diabetic retinopathy
Describe the process of vasculogenesis
Endothelial progenitors (angioblasts) differentiate from mesodermal cells Angioblasts coalesce to form first embryonic vessels- dorsal aorta and cardinal vein Angioblasts from blood islands which fuse and remodel to form a primitive capillary plexus
Describe angiogenesis during development
Angiogenic remodelling of dorsal aorta, cardinal vein and vascular plexi give rise to arteries, veins and capillaries
Pericyte a are recruited to stabilise capillaries
Lymphatic endothelial cells sprout from veins and form they lymphatic system via lymphangiogenesis
What is the main cause of physiological angiogenesis?
Hypoxia- Hypoxia Inducible Factor (HIF)-1 is a transcription factor and a master regulator of oxygen homeostasis
Consists of alpha and beta subunits
Degradation of HIF-1alpha is regulated in response oxygen levels
In high levels proline hydroxylase hydroxylases proline residues on the alpha subunit so it is then recognised and bind by ubiquitin ligase which polyubiquitates it to be degraded
If there isn’t enough oxygen to drive the proline hydroxylation the alpha subunit associates with the beta subunit and activates its target genes eg. VEGF
What is the function of VEGF in endothelial cells?
VEGF-A is the potent driver of angiogenesis
VEGFR signals via activation of VEGFR-2 a receptor tyrosine kinase
Dimerisation of VEGFR-2 activates signalling pathways which affects- proliferation, survival, migration, vascular permeability
Summarise the modes of angiogenesis
Sprouting or splitting (intussusception)
Induced by shear stress- Intussusception involved the formation of a pillar which elongates splitting the vessel in two- rapid increase in capillarity and requires minimal endothelial proliferation
Tissue stretch- Sprouting involves the formation of a new capillary off another
Describe the stages of sprouting
VEGF regulated VE-cadherin and loosens endothelial junctions
Endothelial cells secrete proteases to remodel existing interstitial matrix
Selection of tip cell to guide the newly forming sprout- filopodia sense the extracellular environment and secrete VEGF
Pericyte detachment mediated by Angiopoeitin
Increased permeability allows the deposition of plasma proteins to make a provisional matrix
Notch signalling between the tip (delta-like 4) and stalk cells (notch) maintains their different specification
Stalk cells behind the top proliferate and extend the sprout- VEGFR-2 is down regulated
Stalk attracts pedicures to stabilise the newly formed vessels
Tip cells navigate in response to guidance signals and adhere to the extracellular matrix (mediated by integrins) to migrate
Sprouts from adjacent vessels grow towards each other
Formation of tight junctions, basement membrane deposition, pericytes maturation
Angiopoeitin signalling between endothelial cells and pericytes maintain quiescence of the vessel
Summarise pathological angiogenesis
Tumour growth and hyperplasia in inflammatory diseases increase the distance of cells from vessels and chronic hypoxia drives VEGF production plus inflammatory cytokines gives rise to a pro-angiogenic environment
Vessel formation is abnormal leading to leaky and poorly perfused vessels and persistent hypoxia
How would you modulate angiogenesis via VEGF?
Inhibit signalling using a monoclonal antibody eg. Bevacizumab, used in cancer s d diabetic retinopathy
Use a soluble receptor- VEGF trap
Small molecule inhibitors eg. Sunitinib, sorafenib used in cancer treatment
Antibody blocking binding if VEGF to the receptor eg. IMC 1C11
