L7 - Physiology of the Vasculature I Flashcards
Why are blood vessels so important?
Changes in blood vessel underly the majority of cardiovascular diseases
Blood vessels supply nutrients, remove waste, provide gas exchange
What 4 diseases are caused by issues in blood vessels?
Stroke - Block in blood vessels supplying or within the brain Myocardial infarction - Block in coronary artery Hypertension - Dysfunctional blood vessels - high blood pressure Pulmonary hypertension - Narrowing of pulmonary arteries
How does blood supply change during exercise?
↑ supply to muscles, lungs, heart
Driven by the sympathetic nervous system
How does blood supply change during digestion?
↑ supply to GI tract
How does blood supply change during thermoregulation?
↑ supply to skin to allow heat loss
What 2 disease are caused by dysfunction in artery walls~?
Atherosclerosis
- Initiated by dysfunctional blood vessels
Hypertension
- High blood pressure, due to blood vessel contraction regulation going wrong
Why do we need to understand vascular physiology?
Arterial contraction or relaxation results in changes in blood pressure
Understanding mechanisms → drug targets
Where does most control of blood vessel contraction and relaxation occur?
Arteries/arterioles
They are the main resistance vessels – small and muscular
What are the 4 layers of an artery wall?
Endothelium
Tunica media
Tunica intima
Tunica externa
What is the structure of the endothelium?
Thin layer of cells in direct contact with the blood – line the blood vessel
In direct contact with the blood to respond to circulating factor
What is the structure of the tunica media?
Smooth muscle, elastin, collagen matrix
Smooth muscle cells contract/relax to determine artery size
What is the structure of the tunica intima?
Connective tissue
Only in larger arteries - provides structural strength
What is the structure of the tunica external?
Elastic membrane - strong fibrous tissue to maintain vessel shape
Vessel wall stretches due to elasticity
What two things is smooth muscle contraction controlled by?
Circulating hormones
Local mediators released from the endothelium and sympathetic nerves
What are in contact with VSMCs?
Endothelial cells are directly in contact with VSMCs
Neurones are deeper in the artery wall to act directly on VSMCs
How do endothelial cells and VSMCs communicate?
Some direct contact between EC and VSMCs occurs via gap junctions
Where do gap junctions mostly occur?
Between VSMCs
Allow waves of calcium signalling across many cells so the artery wall contracts in a coordinated way
What happens if the endothelium is dysfunctional or activated?
You get disease
- Dysregulated blood pressure
- Initiate the events underlying atherosclerotic plaque formation
How do the endothelium and blood interact?
Endothelium is the first line to react to circulating factors, blood cells, pathogens
Have mechanical receptors on their surface that respond to blood flow - sheer stress
What is glycocalyx?
Sugar coating on surface of endothelium
Intact in non-activated endothelium
Anti-coagulant
- Prevents circulating cells from binding to
adhesion molecules on endothelial surface
What 3 things does the shedding of the glycocalyx occur in response to?
Injury, infection, inflammation (matrix metalloproteases released from immune cells)
oxLDL (lipid)
Disturbed blood flow (oscillatory shear stress)
What does shedding of the glycocalyx expose?
Adhesion molecules
What do adhesion molecules bind to once exposed and what does this lead to?
Adhesion molecules bind to glycans on circulating blood cells (monocytes, neutrophils and platelets)
Allows rolling/transmigration of blood cells along artery wall
- Key initiating events in atherosclerosis
What is blood flow like in straight vessels?
Blood flow exerts high shear stress force against the wall
What is blood flow like at branches/bends in vessels?
Blood flow becomes disturbed
Where do fatty plaques/activated endothelium mostly occur and why?
At the regions of disturbed blood flow
- Shedding of the glycocalyx
- Slowing of blood cells circulating in that region
- Activated endothelium
- Attracts monocytes and neutrophils to enter the artery wall
- Atherosclerotic plaque formation
What signalling occurs within healthy endothelial cells?
Stimulated by Ach, histamine, bradykinin and 5-HT
Bind to GPCRs to mediate an increase in intracellular Ca
Activates eNOS which converts arginine to NO and citrullin
- NO is freely able to diffuse to act on adjacent cells
What does NO released from endothelial cells do to VSMCs?
Causes relaxation
What factor does high shear stress increase in endothelial cells?
NO
What other factors are there that don’t increase intracellular Ca but produce other signalling molecules that activate endothelium and cause a contraction of VSMCs?
Interleukin-1 - proinflammatory cytokine
Endotoxin - bacterial cell wall
Thrombin - from platelets
Disturbed blood flow
What do IL1, endotoxin, thrombin and disturbed blood flow all lead to an increase in?
Endothelin-1 or ROS –> act on adjacent VSMCs
Adhesion molecule expression –> increased monocyte/neutrophil/platelet interactions with endothelial cell –> transmigration across artery –> atherosclerosis
IL-8 production
COX-2 activity
On which side are factors secreted from endothelial cells to act on VSMC’s?
Basolateral surface (into arterial wall)
How is Ca maintained in a resting VSMC?
Intracellular Ca maintained low by Ca ATPase pumps in plasma membrane or sarcoplasmic reticulum
What is the main way to cause an increase in Ca in VSMCs?
Activating Ca channels in plasma membrane Ca release from SR stores Ca now present in cytoplasm Binds to calmodulin to form a complex Activates myosin light chain kinase Phosphorylates myosin to activate it Actin cross-bridge cycle starts
What is the alternative way to cause an increase in Ca in VSMCs?
Also have cell surface receptors that couple to increased IP3 levels
Activate receptors on the SR membrane to cause intracellular Ca release
Then activation of the myosin phosphorylation-contraction pathway
What are the two differences between smooth muscle and skeletal muscle?
In VSMCs
- Myosin needs to be phosphorylated to be active
- Myosin phosphatase is always active - cell tends to be relaxed
- Converts active myosin to inactive
What are the two pathways that lead to VSMC contraction?
Increased IP3 production
Increased intracellular Ca
What are the 5 GPCRs involved in VSMC contraction?
Endothelin A/B TP (prostanoid) AT1 (angiotensin) Histamine Noradrenaline (alpha-adrenergic)
What are the 4 Ca channels involved in VSMC contraction?
Voltage sensitive (L-type) Receptor operated (P2X) TRP channels Store operated (Ora1)
What 3 signalling molecules are involved in VSMC relaxation?
cGMP
cAMP
Activation of K channels
How does increased cGMP lead to VSMC relaxation?
Produced by guanylyl cyclase
Reduced intracellular Ca
Activate PKG –> activate myosin phosphatase –> more inactive myosin
Hydrolysed by phosphodiesterase’s
How does increased cAMP lead to VSMC relaxation?
Produced by adenylyl cyclase
Reduced intracellular Ca Hydrolysed by phosphodiesterase’s
How does activation of K channels lead to VSMC relaxation?
K efflux causes hyperpolarisation
Reduced intracellular calcium
What are 3 K channels involved in VSMC relaxation?
BK channels (large conductance) SK channels (small conductance) Beta-agonists (via bg G-protein)
How does NO also play a role in VSMC relaxation?
Released from endothelium
Activates guanylyl cyclase
How do Gs coupled receptors also play a role in VSMC relaxation?
Activates adenylyl cyclase E.g. - Beta-agonists - Adenosine - Prostaglandins
Overall what 5 things does the apical side of the endothelium respond to?
Circulating hormones Thrombin from platelets – injury Cytokines and infectious stimuli Shear stress Sympathetic neurotransmitters These result in intracellular signaling in the endothelial cell either dependent on independent on Ca
Overall what 4 mediators regulate VSMCs?
Nitric oxide
Prostanoids
Endothelin
Angiotensin II
When is NO produced?
Produced by endothelium in response to a rise in Ca
What does NO activate?
Freely permeable and diffuses to act on the VSMCs
Activates guanylyl cyclase –> cGMP –> PKG –> myosin phosphatase –> VSMC relaxation
What does NO regulate?
Blood pressure and regional blood flow
What increases NO production?
Many vasodilators (e.g. ACh)
What regulates eNOS?
Shear stress regulates eNOS in resistance vessels via Akt
Phosphorylation of specific residues on eNOS regulates its sensitivity to calcium-calmodulin
What does cGMP directly decrease?
Intracellular Ca
What are caveoli?
Cholesterol rich microdomains
Caveoli contain clusters of eNOS enzymes
This means NO is produced close to cell surface to rapidly act on nearby VSMCs
What does eNOS form complexes with?
eNOS is acetylated and complexes with Golgi and caveolae
What 3 things is eNOS impaired by?
Smoking
High glucose and insulin
oxLDL
How does smoking affect eNOS?
Interferes with eNOS acetylation –> loss of eNOS anchored to membrane
Reduces NO bioavailability
How does high glucose and insulin affect eNOS?
Reduces eNOS phosphorylation
Reduces NO bioavailability
How does oxLDL affect eNOS?
Depletes cholesterol from caveolae –> loss/displacement of eNOS
Disturbs eNOS function –> NO production dysregulated
When are prostanoids produced?
By endothelium in response to a rise in Ca or ROS
What do prostanoids activate
Activate COX 1/2 enzymes
Convert arachidonic acid to PGH2
What is PGH2 converted into?
Converted into prostanoid derivatives depending on the enzymes present
- Thromboxane A2
- Prostaglandin E2
- Prostaglandin I2
PGH2 –> thromboxane A2 overview
Act on prostanoid thromboxane receptor (TP)
TP GPCR receptor –> PLC activation and IP3 production –> intracellular Ca release –> muscle contraction
PGH2 –> prostaglandin E2 overview
Acts on prostaglandin E2 receptors (EP1-4) to either
Activate adenylyl cyclase –> increased cAMP –> PKA –> myosin phosphatase –> relaxation
Inhibit adenylyl cyclase –> decreases cAMP –> contraction
PGE2 effect depends on relative level of expression of EP receptor subtypes on the VSMCs - varies in different regions of the vasculature
PGH2 –> prostaglandin I2 overview
Acts on the IP receptor
IP GPCR receptor –> adenylyl cyclase activation –> increased cAMP –> relaxation
Endothelial cells produce ET-1 when exposed to?
IL-1 Thrombin Glucose Ox-LDL Insulin Angiotensin II
How do endothelial cells produce endothelin?
Big endothelin is cleaved to ET-1 by Endothelin converting enzyme
oET-1 acts on VSMC at both ET A and B GPCR Gq receptors –> PLC activation and IP3 production –> intracellular Ca –> contraction
What is the ET-1 negative feedback mechanism?
ET-1 acts back on endothelium via ETB receptors to block ECE activity
This increases Ca inside endothelium –> increases NO –> opposes VSMC contraction
How is the ET-1 negative feedback mechanism of interest therapeutically?
Can specifically block ETA receptors and not ETB receptors to preferentially downregulate production of ET-1 and oppose its effects by maintaining the ETB negative feedback loop
How is angiotensin II produced?
By the action of ACE on endothelial cells
Where is ACE predominantly expressed?
On endothelial cells of pulmonary and renal vasculature
What does ACE convert?
Converts circulating angiotensin –> angiotensin II –> AT1 receptors –> PLC activation and IP3 production –> intracellular Ca –> contraction
What else do AT1 receptors also activate?
MAPK pathways in VSMC –> more persistent changes in signaling –> VSMCs more contractile
How does ageing affect endothelial cells?
Cause changes where endothelial cells no longer able to regulate the VSMCs
All lead to increased blood pressure
What 5 changes can ageing cause which all lead to an increase in blood pressure?
Atherosclerosis Damage to glycocalyx Calcification Loss of elastin Decrease in NO
What is atherosclerosis?
Build up on plaque over decades
Endothelial cells become physically more separated from the VSMCs.
- Loss of the coupling between endothelium and VSMCs
- Loss of regulation of the contractility of the artery
Results in raised blood pressure over decades
- Increase in blood pressure –> more strain on arteries –> loss of elasticity –> reinforce the loss of function, particularly in relation to relaxation
What is damage to the glycocalyx caused by?
Hyperglycaemia
Hyperlipidaemia
Smoking
Sepsis and inflammation
What is calcification?
Arteries become stiffer – less able to contract and relax
What causes a decrease in NO?
Diet
Smoking
Hypoxia
