Potential antihypertensive targets in vascular smooth muscles Flashcards
What affects blood pressure?
Cardiac Output x systemic resistance = blood pressure
What affects the cardiac outpit?
Stroke volume x heart rate = Cardiac output
What is isoprenaline?
Isoprenaline is a beta-adrenoreceptor (stimulate cardiomyocytes and pacemaker cells/relaxes arteries) agonist – has no/little change in blood pressure, heart rate to goes up
How does noradrenaline act?
NA acts preferentially on the alpha adrenoceptors – causes constriction of blood vessels, heart rate increases slightly then decreases, blood pressure goes up
Give an overview of the RAAS system
- Renin converts angiotensinogen into Angiotensin I
- ACE enzyme converts ANG I into ANG II whilst ACE 2 converts ANG I into ANG 1-9
- ANG II can bind to AT1/2
- ANG II is converted into ANG III by aminopeptidase A
- ANG III can also bind to AT1/2
Why do some antihypertensive target angiotensin II activity?
- Antihypertensives usually target angiotensin II activity – ACE is targeted or by blocking the receptors angiotensin II works on
- This affects peripheral resistance, sympathetic nervous discharge, aldosterone release
What do ANG II and ANG III have in similar?
Ang II and Ang III exhibit similar affinity for AT1 and AT2 receptors.
They also have a similar affinity for a non-AT1, non-AT2 angiotensin-binding site
How does the brain RAS (renin-angiotensin system) control BP
- increase in vasopressin release from the posterior pituitary into the bloodstream
- activation of sympathetic premotor neuron activity at the level of the rostral ventrolateral medulla (RVLM)
- inhibition of the baroreflex at the level of the nucleus of the solitary tract (NTS) – leads to vagal stimulation
What is EC33
Aminopeptidase A (APA) inhibitor - has a high affinity for APA and blocks it
What does central administration of EC33 and an intravenous dose of EC33 cause? and what does this tell us?
- Central administration of EC33 blocked the pressor effect of intracerebroventricular Ang II in SHR.
- BUT high intravenous dose of EC33 did not change BP in hypertensive rats.
- Suggests that Ang II converted to AngIII in the brain
What does intracerebroventricular infusion of APA and APN (aminopeptidase N; converts Ang III into Ang IV) do to blood pressure?
- Intracerebroventricular infusion of APA significantly increases BP.
- Intracerebroventricular infusion of APN (converts Ang III into Ang IIII) in SHR rats decreases BP
How is EC33 administered?
As a pro-drug RB150
- it crosses the intestinal, hepatic, and blood-brain barriers
- it then gets cleaved into 2 EC33 molecules (inhibits the conversion of Ang II into Ang III)
What is EC33 like as a drug?
- Has a good blood pressure reducing affect
- in people that had higher blood pressure, there was a greater reduction in BP
- Agrees with experimental models of hypertension where firibastat acted as an antihypertensive agent and not as a hypotensive agent (didn’t reduce BP but prevented it from increasing
What has new research regarding RAS and hypertension tells us?
Growing evidence confirms involvement of the brain RAS in the development of hypertension.
Targeting this system with novel agents, such as the first-in-class APA inhibitor prodrug RB150/firibastat, has been shown to be effective.
What ion channels are present on smooth muscle cells and what do they do?
Calcium channles and potassium channels:
- Ca channels become more open when the membrane potential become less active
- K channels oppose them
VDCCs open more with depolarisation
• Cation (Ca) influx
• Anion efflux
How do smooth muscle cells accumulate chloride ions?
Smooth muscle cells ACTIVELY accumulate Chloride ions:
• NK2Cl transporter
• Cl- / HCO3 exchanger
Also found in renal tubules and are targets for diuretics
What are the different mechanisms of chloride ions?
Different Cl ions opened by different mechanisms
- Agonist-gated: Glycine/GABA-A receptors
- Voltage-gated: opened by depolarisation (found in skeletal muscles)
- Calcium-gated: opened by a rise in calcium
- Mechano-sensitive: opened by membrane stretch
- Cyclic nucleotide gated: opened by direct binding of cyclic AMP
What happens when NK2Cl transporter and Cl- / HCO3 exchanger are inhibited?
When inhibited, arteries become less contractile – the vasoconstrictor opens Cl channels, allows Cl efflux, causes depolarisation which causes Ca channels to open, Ca influx
What are Ca-activated Cl channels encoded by?
Ca activated Cl channels encoded by ANO1
How does VGCC open?
Vasoconstrictor activates phospholipase C, liberates Ca, Ca will activate ANO1 Cl channel, Cl efflux and there will membrane depolarization. This will cause VGCC to open, Ca influx and blood vessels will constrict
Where is the ANO1 gene present?
ANO1 gene present in the left anterior descending artery and the pulmonary artery – in smooth muscles and cardiomyocytes
ANO1 plays a role in increasing BP and is a target for drugs that want to lower BP
What do TMEM16A/ANO1 blockers do?
TMEM16A/ANO1 blockers are anti-contractile:
In the presence of the blockers, there’s less contraction when a vessel contracting drug is introduced
What are the different types of vascular smooth muscle cell K channels?
There’s a lot of different K channels • Inward-rectifying channels • Ca activated K channels • Some affected by ATP blocks • Voltage sensitive K channels (kv1.2/1.5/2.1/7): open by depolarisation
What are voltage-sensitive (Kv) potassium channels like?
• Dominant expression
o Kv 1.2 & 1.5 (correolide sensitive, PSORA)
o Kv 2.1 & 2.2 (stromatoxin sensitive)
o Little evidence for Kv 3.x / 4.x = transient currents
- Assemble with various subunits (KCNE, KCHIPS, Kv9s)
- Kv1.x and 2.x shown to go down AND up in hypertension
What is the effect of potassium channels?
Potassium channels brake cellular excitability.
K efflux leads to membrane hyperpolarisation, reduces chances of Ca channels opening, less Ca influx = less cross-bridge formation through reduced MLCK activation, relaxed smooth muscle cell
What is the KCNQ family, what genes and proteins are in the family?
- Genes: KCNQ1, 2, 3, 4 & 5
- Protein: Kv7.1, 7.2, 7.3, 7.4 & 7.5
- All form tetrameric voltage-gated potassium channels.
What can modify KCNQ family genes/proteins?
• Modifiers:
o KCNE gene products
o PIP2 levels
o Pharmacological agents
What is the function of Kv7.1?
Kv7.1 contributes to late repolarising phase of cardiac action potential
What are some agents modulate Kv7 channels?
Retigabine
• Activates Kv7 channels – enhances the channels, keeps them open for longer
• Promotes membrane hyperpolarisation
• developed as anti-convulsant
• causes human mesenteric arteries to relax
Kv7 blockers:
• Narrow blood vessels/contracts them
• Reverses the relaxation by Kv7 enchancers
What happens when you increase pressure in the cerebral arteries?
In cerebral arteries, the more you increase the pressure, the more they contract against it
The blockers/enhancers have the effect in the cerebral arteries as well
Which endogenous dilators work through Kv7 channels?
Endogenous dilators includes: • B-adrenoceptor agonist • Adenosine • CGRP • ANP • Perivascular fat derived molecules
How can Kv7 channels be targeted in hypertension?
Kv7 activators as anti-hypertensives / enhance receptor-mediated dilatation
Kv7 activity is downregulated in hypertension – harder to target
Why is calcium removal important and how is it done?
Calcium has to be removed for the muscles to relax – can be done through
• SERCA
• Na/Ca exchanger – pump out the Ca
What are Na+ / Ca2+ exchangers
• Plasma membrane transporter
• 3 genes (NCX1-3)
o NCX2 /3 = brain and skeletal muscle
o VSM = NCX1.3 and 1.7 predominantly
• Bidirectional exchanger
o Regulated by membrane potential
o Affected by Na and Ca gradients
3Na come in for 1Ca – unless intracellular [Na] is too high, it goes out, Ca comes in, can lead to arterial contraction
What is seen in patients with idiopathic pulmonary arterial hypertension?
Higher level of NCX protein and mRNA
What is seen with reduced expression of NCX?
o reduced increase in arterial tension and reduced radioactive Ca uptake in Na-free conditions
o resistance to salt-induced hypertension
What are some Na+ source within the VSMs?
- Non-selective cation channels (TRPs) – membrane channels
- Voltage-dependent Sodium channels – membrane channels
- ENaC (epithelial sodium channels)
- Na / K ATPase – membrane potential is maintained by these as they restore the ionic balance after action potentials
What are Na+ / K+ ATPases blocked by?
o Ouabain
o cardiac glycosides (digoxin)
o cardiotonic steroids (endogenous oubain, bufalin, marinobufagenin)
o Cardiotonic Steroids = increased in salt-dependent hypertensive models
o These drugs produce contractions
Evidence that rho kinase plays a role in hypertension/rho kinase inhibitors are a treatment for hypertension?
• Rho kinase inhibitors (Y27632 and fasudil, HA1077)
o lower BP in 3 models of hypertension
o Produce greater cerebral vasoldiln in hypertensive vs normotensives
• Increased Rho Kinase activity in stroke prone hypertensive rats and salt-sensitive models
• Coronary vasospasm
o Increased mRNA for Rho K
o Spasm is reduced by Y27632
What are VSMs influenced by?
VSMs influenced by endothelium and chemicals released from fat surrounding the vessels
In obese people, the adipocytes become inflames and there’s different vasomodulatroy chemicals released
Why does endothelial regulation differs between conduit (left) and resistance arteries (right)?
Endothelial cells are complex and can differ
- NO and hydroperoxide in the conduit arteries
- Low resistance GAP junctions in the resistance arteries – electrical activity passing from endo cells to smooth muscle cells
- Ca/K channels in resistance arteries – produce endothelial hyperpolarisation that can affects VSM
How does nitrate reduce blood pressure?
Nitrate goes into bowels and absorbed by kidney, the excreted
Nitrate that gets to salivary glands gets converted by bacteria into nitrite, nitrite degrades and releases NO – relaxes blood vessels
