Unit 4 Flashcards
What are adrenoceptors?
Membrane-bound receptors located throughout the body
- Mediates a diverse range of physiological responses to noradrenaline (norepinephrine) and adrenaline (epinephrine)
- Noradrenaline - transmitter released by sympathetic nerve terminals
- Adrenaline - a hormone secreted by the adrenal medulla
G protein-coupled receptors
Responsible for the “fight or flight” response
How are adrenoceptors classified?
Alpha and beta receptors
A total of 9 distinct subtypes
2 Main alpha-adrenoceptor subtypes, alpha1 and alpha2, each divided into 3 further subtypes (A, B and C)
3 Main beta-adrenoceptor subtypes, ß1, ß2, ß3
What are the second messengers of adrenoceptors?
Alpha 1:
- Activate phospholipase C, producing inositol triphosphate (IP3) and diacylglycerol (DAG) as second messengers to activate protein kinase C, resulting in phosphorylation of other enzymes
Alpha 2:
- Inhibit adenylyl cyclase, decreasing cAMP formation
ß-adrenoceptors (all subtypes):
- Stimulate adenylyl cyclase, increasing cAMP formation
What are the main effects of alpha-adrenoceptors?
Alpha 1 receptors:
- Vasoconstriction
- Relaxation of gastrointestinal smooth muscle
- Urethral smooth muscle contraction
- Salivary secretion
- Hepatic glycogenolysis
Alpha 2 receptors:
- Inhibition of transmitter release (inc. noradrenaline and acetylcholine release from autonomic nerves) [presynaptic receptors on nerve endings]
- Vascular smooth muscle contraction
- Platelet aggregation
- Insulin release
What are the main effects of ß-adrenoceptors?
ß1 Receptors:
- Found mainly in the heart
- Positive inotropic (increase heart rate) and chronotropic effects (increase contractility); positive lusitropic effect (speeds rate of relaxation)
ß2 Receptors:
- Smooth muscle relaxation
- Hepatic glycogenolysis
ß3 Receptors:
- Lipolysis and thermogenesis
- Bladder detrusor muscle relaxation
What do we know about drugs acting on adrenoceptors?
Overall activity to these drugs is governed by their affinity, efficacy and selectivity with respect to different types of receptor. In general:
- ß2-adrenoceptor selective agonists are useful as smooth muscle relaxants (especially in the airways - bronchodilator)
- ß1-adrenoceptor selective antagonists (often called ß blockers) are used mainly for their cardio-depressant effects
- Alpha1-adrenoceptor selective antagonists are used mainly for their vasodilator effects in cardiovascular indications
- Adrenaline (alpha/beta agonist) is uniquely important in cardiac arrest with its mixture of cardiac stimulant, vasodilator and vasoconstrictor actions
What are adrenoceptor agonists?
Noradrenaline and adrenaline show relatively little receptor selectivity
- Cardiac arrest
Selective alpha-adrenoceptor agonists have relatively few clinical uses
- Selective alpha1 agonists include phenylephrine and oxymetazoline
- Selective alpha2 agonists include clonidine, UK14304 (brimonidine), B-HT920 (Talipexole) and alpha-methylnoradrenaline
Selective ß1 agonists include dobutamine. Increased cardiac contractility may be useful clinically, but all ß1 agonists can cause cardiac dysrhythmias
Selective ß2 agonists include salbutamol and terbutaline; used mainly for their bronchodilator action in asthma
What are adrenoceptor antagonists?
Alpha-adrenoceptor antagonists:
- Non-selective: Phenoxybenzamine
- Alpha1-selective: Prazosin, Doxazosin
- Alpha2-selective: Yohimbine, Idazoxan (alpha2 antagonists are not used therapeutically)
Is Phenoxybenzamine a selective adrenoceptor antagonist?
Phenoxybenzamine is NOT selective for alpha receptors and also antagonises the actions of acetylcholine, histamine and serotonin
- Decreases blood pressure (blockade of alpha-receptor-mediated vasoconstriction)
- Reflex tachycardia (due to increase of noradrenaline release through blockage of alpha 2 receptors)
- Long lasting as it binds covalently to the receptor (irreversible antagonist)
- Not widely used clinically, except in the preparation for patients with phaeochromocytoma for surgery.
What do the alpha-adrenoceptor antagonists prazosin, doxazosin and tamsulosin do? What about yohimbine?
PRAZOSIN/DOXAZOSIN/ TAMULOSIN:
- Alpha1-antagonists
- Cause vasodilation and decrease arterial pressure
- Less reflex tachycardia compared to phenoxybenzamine
- Relaxation of the smooth muscle of the bladder neck and prostate capsule –> reduce urinary sphincter tone and inhibit prostatic hypertrophy in humans.
YOHIMBINE:
- Alpha2-antagonist
- Not used clinically
What are ß-adrenoceptor antagonists?
Important effects on the cardiovascular system and on bronchial smooth muscle
Non-selective between ß1 and ß2
- Propanolol, alprenolol, oxprenolol
ß1-antagonists (less effect on smooth muscle)
- Atenolol, metoprolol
CLINICAL USES:
- Angina, myocardial infarction, prevention of recurrent dysrhythmias
- Heart failure (only in well-compensated patients)
UNWANTED EFFECTS:
- Bronchoconstriction, hypoglycaemia etc
- Lower risk with ß1-selective antagonists
Most ß-adrenoceptor antagonists are inactive on ß3 receptors so do not affect lipolysis
What are metabolism and pharmacokinetic studies?
ISOLATED TISSUE (IN VITRO):
- Dose-response curve
- Evaluating drug metabolism, transporter function and drug-induced organ toxicity
- Optimise choice of doses for in vivo studies
PERFUSED ORGAN (EX VIVO):
- Greater control of the drug concentration applied to the organ - better prediction of drug metabolism and pharmacokinetics
- Mimic diseased models with altered metabolism (different expression of certain transporters)
WHOLE ANIMAL (IN VIVO):
- Bioavailability: Determine gastrointestinal absorption and elimination rates
- Multifactorial: metabolism, distribution, interaction of drugs
- Compare pharmacokinetics across species
- Determine the no-observed-adverse-effect-level (NOAEL)
What type of receptors are adrenoceptors?
All adrenoceptors are G protein-coupled receptors (GPCRs)
Why is an ion channel necessary?
Ions are charged molecules and unable to cross the plasma membrane due to the arrangement of the hydrophilic heads and hydrophobic tails in the lipid bilayer
To cross the plasma membrane, they must interact with membrane-spanning receptors = ion channels
What are 2 distinctive features of ion channels?
- The rate of ion transport through the channel is very high
- Ions pass through channels down their electrochemical gradient
What is diversity of ion channels?
Selectivity for a particular ion species:
- Determined by the size of the pore and the nature of the channel lining
Their gating properties:
- The mechanism that controls the transition between opened and closed states of the channel
Their molecular architecture/ structure:
- Physical make up sequence similarity
Their localisation
What is ion channel selectivity?
Ion channels are usually either cation (+) or anion (-) selective
CATION CHANNELS:
- May be selective for Na+, K+, Ca2+
- May be non-selective and permeable to all three ions
ANION CHANNELS:
- Mainly permeable to Cl- but other types do exist
What is selectivity filter?
The narrowest region of an ion channel pore
EXAMPLE:
The selectivity filter in KcsA is composed of a sequence of 5 amino acids: TVGYG. Four K+ binding sites inside the selectivity filter.
In each of these binding sites, the K+ ion is located at the centre of a cage formed by 8 oxygen atoms
The archetypal channel pore is just 1 or 2 atoms wide at its narrowest point and is selective for specific species of ion, such as sodium or potassium. However, some channels may be permeable to the passage of more than one type of ion, typically sharing a common charge. Ions often move through the segments of the channel pore in single file nearly as quickly as the ions move through free solution
What is ion channel gating?
Gating involves some type of conformational changes in the protein.
Conformational change occurs in a discrete area of the channel, leading to its opening
The entire channel changes conformation
Ball and chain type mechanism
Ion channel gating, what opens the channel?
- VOLTAGE-GATED CHANNELS (VOCs):
- Underlie the mechanism of membrane excitability
- Are opened when the cell is depolarised
- Most important ones are selective Na+, K+ and Ca2+ channels - LIGAND-GATED CHANNELS:
- Have a ligand binding site and are activated following the binding of a ligand to the channel protein (all are non-selective cation channels)
- Typically these are receptors on which fast neurotransmitters act:
–> Nicotinic acetylcholine receptors (nAChR), Glutamate, GABA - RECEPTOR-OPERATED CHANNELS (ROCs):
- Open in response to hormones (or drugs) acting on other cell membrane receptors. Second messenger
Where are ion channels found?
In ALL cells - focussed on excitable cells: neurons, muscle cells, and secretory cells
What are excitable cells?
Excitability is a property of cell membrane
Excitable cells can generate changes in their membrane potential, and produce an action potential. The electric current in neurons is used to rapidly transmit signals through the animal. In muscles, it is used to initiate contraction.
What happens in activation of excitable cells and ion channels?
Activation of excitable cells (depolarisation) may involve:
- An increase in membrane permeability to Na+
- An increase in intracellular Ca2+ due to:
–> Release from intracellular Ca2+ stores (endoplasmic reticulum/ sarcoplasmic reticulum (ER/SR))
–> Increased membrane permeability to Ca2+
What are ion gradients maintained by in nerves? And in which direction do ions move?
Ion Pumps - i.e. ATPases such as the Na+/K+ pump
The relative impermeability of the cell membrane
Ions always move down the concentration gradient from a high to low conc.
What happens in a typical cell during DEPOLARISATION
1) Stimulus
2) Membrane depolarisation
3) Change in membrane potential
4) Na+ enters via ion channel
5) Then Na+ is slowly removed from the cell and the cell returns to rest
What is an ion channel?
- Transmembrane pore forming proteins - allow the passage of ions (charged particles) in/out of a cell
- Distinguished based on ion selectivity, gating mechanism, and sequence similarity
- Can be voltage-gated, ligand-gated, pH-gated, or mechanically-gated
What is ion channel superfamily; diversity?
- More than 300 types of ion channels
- More than 500 genes encoding ion channels subunits
Can be classified by molecular structure ex. 2,4,6TM
How are ion channels classified?
- Classification by molecular structure (number of gates)
- Classification by the nature of gating
- Classification by type of ions passing through the gates
- Classification by cellular localisation
Other classification: Duration of response to stimuli
What is classification by gating?
In general terms, there are 3 main groups of gated ion channels:
- Voltage-gated ion channels (VOC)
- Ligand-gated ion channels (neurotransmitter)
- Receptor-operated ion channels (ROCs) or ion channels activated by second messengers
In what state do voltage-gated ion channels exist?
Exist in 3 different states:
- Resting - channel closed, this prevails at the normal resting potential
- Activated - open state favoured by a brief depolarisation
- Inactivated - blocked state resulting from a trap-door like occlusion of the channel by an intracellular “floppy” part of the channel protein
Open in response to changes in membrane potential
Provide rapid changes in the ion content of the cell
What are the types of voltage-gated ion channels?
Within this group there are several examples:
POTASSIUM (K+) CHANNELS (Ky):
- Primarily, stabilisation of negative membrane potential
- Depolarisation/ hyperpolarisation (at least 40 members)
SODIUM (NA+) CHANNELS:
- Critical for action potential initiation and propagation (at least 9 members)
CALCIUM (Ca2+) CHANNELS:
- Mediate calcium influx required for muscle contraction, secretion and neurotransmitter release (at least 10 members)
SOME TRANSIENT RECEPTOR POTENTIAL CHANNELS (TRP):
- The family is very diverse (at least 28 members)
What is the structure of Voltage gated Na+ Channels?
4 alpha subunits
The alpha subunit is repeated 4 times (I-V) to form the Na+ channel
Each alpha subunit contains 6 membrane-spanning regions
24 transmembrane segments
What are some examples of drugs acting on voltage-gated sodium channels?
CHANNEL BLOCK:
Neurotoxins:
- Tetrodotoxin
- Saxitoxin
- Conotoxins
Other:
- Local anaesthetics
BLOCK OF INACTIVATION:
- Veratridine
- Scorpio toxins
- DTT
Persistent activation of nerves
- Inhibit contraction of myocytes
- Treatment for some cardiac arrhythmias
- Treatment of pain
What is the structure of ligand-gated ion channels (inotropic receptors)?
Have a ligand binding site and are activated following binding of a ligand to the channel protein (no 2nd messengers required!)
E.g. Nicotinic acetylcholine receptors (nAChR)
- These gate Na+ and K+
Assembled from 4 different subunits alpha x2, ß, γ, δ
- Each subunit comprises four transmembrane domains
2ACh binding sites - Ash binding to both sites activates channel opening
What are some examples of drugs acting on nicotinic receptors?
Agonists:
- Nicotine, Lobeline
Antagonists:
- Non-depolarising neuromuscular blockers:
–> Curare, tubocurarine, pancuronium
- Depolarising neuromuscular blockers:
–> Suxamethonium
How are ion channels activated by second messengers?
Open in response to hormones (or drugs) acting on other cell membrane receptors. Are METABOTROPIC - use 2nd messengers
Examples of these types of ion channel are Transient Receptor Potential Channels (TRPs) which are a type of non-selective cation channels that gate both Na+ and Ca2+
EXAMPLES OF DRUGS:
- Agonist = capsaicin
- Antagonists = Capsazepine (novartis), NGX-4010 (Qutenza; Gruenthal Ltd)
What are some drugs acting at potassium channels (Agonists and antagonists)?
AGONISTS:
- Potassium channel openers (activate ATP-sensitive K+ channels in vascular smooth muscle). This leads to relaxation and vasodilation
- Antihypertensives = Pinacidil, minoxidil
- Anti-angina, antiarrhythmic
ANTAGONISTS/BLOCKERS:
- Sulphonylureas (anti-diabetic) block inwardly rectifying potassium channels, (depolarisation that opens voltage-gated calcium channels), this then stimulates insulin secretion
- Tetraethylammonium (TEA) and 4-aminopyridine (4-AP) (multiple sclerosis) block voltage-gated potassium channels (Kv channels). Prolong action potentials and increase neurotransmitter release.
- Dendrotoxins are a class of presynaptic neurotoxins produced by mamba snakes that block particular subtypes of voltage-gate potassium channels (enhance release of acetylcholine)
What are some diseases related to potassium channels?
Channelopathies
- 90 genes coding for the main subunits of potassium channels
- Mutations in genes coding for potassium channels lead to dysfunctions in:
–> Neuronal system (alzheimer’s, Parkinson’s)
–> Cardiac system (arrhythmias)
–> Neonatal diabetes mellitus (KATP)
What are some examples of calcium channels?
Functionally there are three types;
1) Voltage-operated calcium channels (VOCCs):
- Found in the membrane of excitable cells (neuron, muscle)
- For example, L-type calcium channel. But also N-type, P/Q and R-type and T-type (pacemaker potential)
2) Store-operated calcium channels (SOCs) (ligand gated):
- Found on the plasma membrane of both excitable and non-excitable cells
- The major Ca2+ entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+ stores activated Ca2+ influx (store-operated Ca2+ entry)
–> Major source of intracellular calcium in non-excitable cells (myocytes, endocrine cells)
- IP2R/RyR (Intracellular calcium channels):
–> Are located on the endoplasmic reticulum/ sarcoplasmic reticulum (ER/SR) and activated by second messengers (IP3, Ca)
What are calcium channel blockers?
Clinically used - calcium channel blocking drugs
- Calcium channel blockers prevent calcium from entering cells of the heart and blood vessel walls, resulting in lower blood pressure
- L-type calcium channel blockers are used to treat hypertension
- EXAMPLES OF DRUGS: Verapamil, Amlodipine
How are ion channels classified by location?
PLASMA MEMBRANE CHANNELS:
- Ex. voltage-gated potassium channels (Kv), Sodium channels (NaV), calcium channels (CaV) and chloride channels (ClC)
ENDOPLASMIC RETICULUM CHANNELS:
-Example: RyR
MITOCHONDRIAL CHANNELS:
- Example: KATP
What is nociception?
= Pain awareness
Is mediated by nerve endings receptors in peripheral tissues and transmitted to the CNS
Transmission can be disrupted by drugs acting on neurotransmitter receptors or by blocking the sodium channels
What are local anaesthetics?
Drugs which reversibly block the generation and propagation of electrical impulses in the excitable tissues
Have local analgesic effects. Reversible loss of sensation in a portion of the body.
How do local anaesthetics work?
Local anaesthetics disrupt voltage-dependent Na+ ion channel (Navs) function within the neuronal membrane preventing the transmission of the neuronal action potential (in sensory, motor and sympathetic nerve fibres)
It does this by physically plugging the pore of the channel
What are the structures of local anaesthetics?
Clinical local anaesthetics belong to 2 classes (related to cocaine)
1) Aminoamide (-NH-CO-) most commonly used clinically
2) Aminoester (-O-CO-) more likely to cause an allergic reaction
What are some examples of local anaesthetics?
Ester Agents:
- Cocaine, procaine, amethocaine and chloroprocaine
- Cocaine, derived from the leaves of the coca plant Erythroxylon coca, is historically important as the forerunner of all local anaesthetic compounds. Although its use in Vet Med was popularised by Sir Frederick Hobday, it is now rarely used.
- Procaine was the first synthetic agent and is now no longer used clinically. Short half-life, poor penetration
- Tetracaine (=amethocaine) and cinchocaine are older agents. Tetracaine is used for spinal anaesthesia.
Amides:
- Lignocaine, Buvivacaine
- Lidocaine (=lignocaine) and mepivacaine. Good penetration, medium half-life. Lidocaine is used by IV injection as an anti-dysrhythmic
- Bupivacaine, ropivacaine and levobupivacaine. Longest duration of action (200 mins) widely used.
What is pKa?
Acid dissociation constant.
HCl –> H+ + Cl-
The larger the value of pKa, the smaller the extent to which an acid dissociates.
pKa is a log of Ka
A weak acid has a pKa value in the approximate range 2-12 in water
A strong acid has a pKa value around 2 in water
pKa determines the proportion of molecules that exist in lipid-soluble rather than a water-soluble state:
- The time for onset of local anaesthesia is determined by the proportion of molecules that convert to the tertiary lipid structure when exposed to physiological pH.
What are properties of local anaesthetics?
Ester-containing compounds are inactivated in the plasma and tissue by esterase enzymes.
Amides are more stable and these have a longer plasma half life.
All LAs are weak bases with a pKa between 8 and 9
Thus, at physiological pH, they are mainly ionised but not completely
This is essential for the penetration of the nerve sheath and axon membrane since compounds that are totally ionised cannot penetrate
How are local anaesthetics applied?
Topical surface anaesthesia:
- Applied by spray or ointment. Usually applies to the nose, mouth, bronchial tree, cornea, and urinary tract
- Risks: systemic toxicity at high concentration
Infiltration:
- Applied subcutaneously directly into tissue or around the nerves
- Used in minor surgery
- Risks: Only suitable for small areas or risk of systemic toxicity
Intravenous regional anaesthesia:
- Injected intravenously distal to pressure cuff to arrest blood flow
- Used for limb surgery
- Risks: Systemic toxicity when cuff is removed
Nerve block:
- Injection close to nerve trunk to cause loss of sensations
- Used for dental surgery
- Risks: Accurate placement of needle is essential
Spinal anaesthesia:
- Applied into subcarotid space to act on spinal roots.
- Used when general anaesthesia can’t be used
- Risks: Bradycardia and hypotension due to sympathetic block
Epidural anaesthesia:
- Applied into epidural spaces blocking spinal roots
- Used when general anaesthesia cannot be used (childbirth)
- Risks: Similar to spinal but less probable
Why is local anaesthesia used?
- Acute pain (trauma, surgery, infection)
- Chronic pain
- Surgery and dentistry
- To enable painless venipuncture
- Surface anaesthesia for endoscopic procedures
Often minor procedures:
- Epidural anaesthesia, c-sections and orthopaedic procedures
- Local anaesthesia of the eye for examination
What are pharmacokinetic properties of local anaesthesia?
ABSORPTION (UPTAKE):
- Determined by dosage, site of injection, physical properties of the drug, local tissue blood flow
DISTRIBUTION (LIPID SOLUBILITY, PROTEIN BINDING):
- Amides are widely distributed after intravenous bolus administration
- Rapid distribution: Brain, liver, kidney, heart
- Slower distribution in less perfused tissues: muscle, GIT
- Sequestration in fat
METABOLISM AND EXCRETION:
- Converted into the liver (amide) or plasma (esters) to more water-soluble metabolites which are excreted in urine
WHAT IS BUPIVACAINE?
- Half time distribution (min) 28
- T1/2 elimination (h) 3.5
- Volume distribution at steady state (L) 72
- Clearance (L/min) 0.47
What is the duration of action of Local Anaesthetics?
Depends on their affinity for proteins:
- Plasma proteins
- Sodium channels
Depends on the time a local anaesthetic remains in close proximity to neural fibre
- Sequestration
- Constriction of neighbouring vasculature
What is sensitivity to local anaesthetics?
Small nerve fibres are more sensitive than large nerve fibres:
- Thus, motor axons being large in diameter are relatively resistant to the actions of local anaesthetics.
- Myelinated fibres are blocked before non-myelinated fibres of the same diameter
- Thus, the sequence of loss of nerve function proceeds as loss of pain, temperature, touch, proprioception, and then skeletal muscle tone
- This is why people may still feel touch but not pain when using local anaesthesia (nociceptive impulses are carried by A and C fibres)
Nociceptive and sympathetic transmission are blocked first!!!!
How do local anaesthetics interact with Sodium ion channels?
Local anaesthetics block the initiation and propagation of action potentials by preventing the voltage-dependent increase in Na+ conductance
This is achieved by physically plugging the pore of voltage-gated Na+ conductance
This is achieved by physically plugging the pore of voltage-gated Na+ channels (depends on the state)
Local anaesthetics must reach their site of action by penetrating the nerve sheath and axonal membrane as unionised species
So they have to be weak bases!!!
What are ionic/non-ionic forms of local anaesthetics?
Local anaesthetic activity is strongly pH dependent and increased at alkaline pH and vice versa
This is because at an alkaline pH the proportion of ionised molecules is low
Local anaesthetics must penetrate the nerve sheath and axon membrane (as their non-ionised form) to reach their site of action on the inner side of the Na+ channel
Because the ionised form is not membrane permeant - penetration is very poor at acidic pH
Clinical implication: inflamed tissue is acidic and resistant to Local anaesthetics
Once inside the axon, it is the ionised form of Local anaesthetics that interact with the Na” channel
How do local anaesthetics interact with Na+ channels?
Use-dependent block of Na+ channels
The more the channels are opened the greater the block becomes
This is typical of antidysrhythmic and anti epileptic drugs
Many local anaesthetics enter the channel more readily when it is open than when it is closed
The blocking site of the channel can be reached via the open channel on the inner surface of the membrane by the “ion” version of the local anaesthetic.
= HYDROPHILIC PATHWAY (USE-DEPENDENT)
The blocking site of the channel in the closed state can be reached through the outer pore of the membrane by the non ‘ion’ version of the local anaesthetic (more lipid soluble drugs)
= HYDROPHOBIC PATHWAY (NON-USE DEPENDENT)
What do membrane lipid-interacting local anaesthetics do?
Interact with membrane lipids to change fluidity, microviscosity and permeability of membranes.
Also influence the electric potential across lipid bilayers.
What are some unwanted side effects of local anaesthetics for ester agents and amides?
Ester agents can produce allergic reactions:
- Due to the sensitivity to their metabolite-produced para-aminobenzoic acid (PABA)
- They are metabolised by pseudocholine esterase
Amides are metabolised in the liver:
- Problem with patients with liver failure
- May induce liver injuries
What are some unwanted side effects of local anaesthetics involving the CNS and CVS?
Result mainly from leakage into circulation (but local anaesthetic plasma half-life is generally short)
Most local anaesthetics produce a mixture of both stimulant and depressive
CNS - agitation, confusion, tremors, convulsions and respiratory depression (main threat to life)
CVS - myocardial depression (bradycardia) and vasodilation leading to hypotension (rapid drop in BP can be life threatening
Hypersensitivity reactions (rare), usually in the form of allergic dermatitis
As local anaesthetics are absorbed from the injection site, their concentration rises in the bloodstream and the PNS and CNS are depressed in a dose-dependent manner
What do we know about new local anaesthetics?
Tens of millions of patients have operations requiring local anaesthesia each year. Current local anaesthetics act for less than 8hrs
Neosaxitoxin (neoSTX):
- Provides local anaesthesia for more than 24 hrs. It is a site 1 sodium-channel blocker, part of a larger class of emerging anaesthetics based on molecules derived from aquatic organisms (tetrodoxin)
Articaine (amide):
- Used in dentistry
- Amide type but contains a thiophene ring that increases lipid solubility
- Articaine is exceptional because it contains an additional ester group that is metabolised by esterases in blood and tissue
- Since articaine is hydrolised very quickly in the blood, the risk of systemic intoxication seems to be lower than with the other anaesthetics.
What is inflammation and how is inflammation initiated?
Protective process to mobilise defence mechanisms against infectious and non-infectious agents that cause tissue damage.
Skin and mucosal surfaces have cells beneath the epithelia/dermis which possess pattern-recognition receptors (e.g. Toll-like receptors):
- Pathogen associated molecular patterns (PAMPs)
- Damage associated molecular patterns (DAMPs) [released by damaged cells]
Dendritic cells, tissue macrophages, mast cells when activated by PAMPs/DAMPs
- Release cytokines (tumour necrosis factor-alpha and interleukin-1)
- Eicosanoids (lipid mediators - prostaglandins, leukotrienes) produced from damaged cells
- Mediators release from stored secretory granules (e.g. histamine)
What do acute phases of inflammation increase?
Blood flow to the area (vasodilation of arterioles) [redness and heat]
Post-capillary venule permeability - exudation of plasma into tissues [swelling]
- Activated complement, coagulation, fibrinolytic and kinin systems
Leucocyte recruitment into the tissues (chemotaxis) - primarily neutrophils first
Activation of primary afferent nerve endings; axon reflex (flare) [pain]
Redirection of tissue fluid flow towards lymph nodes (adaptive immune response)
Core body temperature [heat-fever]
What are the 4 cardinal signs of inflammation noted centuries ago?
- Rubor (redness)
- Calor (heat)
- Tumor (swelling)
- Dolor (pain)
Why inhibit an acute inflammatory response?
Swelling and fluid accumulation inhibits organ function/causes further damage
- E.g. lung function reduced with fulminant pneumonia due to tissue oedema
- Pressure on the brain with meningoencephalitis could lead to permanent damage
Pain becomes a welfare issue
Tissue irritation leads to the animal chewing/licking the lesion - further tissue damage
Drugs can be used to damp-down the acute inflammatory response
Complex nature of cell and mediator interactions - no one target will stop inflammatory response
Important inflammation allowed to resolve - leads to tissue healing
- Unless inflammatory stimulus persists, becomes chronic (e.g. auto-immune disease)
- Management of chronic inflammation is a whole other topic - need to suppress adaptive immune system
What are mediators of acute inflammation?
Understanding what mediates;
- Increased blood flow, swelling (plasma exudation), leucocyte chemotaxis, fever, pain. Helps to explain the properties of drugs targeting these mediators.
Lipid mediators - formed from cell membrane fatty acids under action of phospholipase A2 (PLA2)
- Activated by cell damage, C5a (complement) on neutrophils, Bradykinin on fibroblasts
- Acts on membrane phospholipids to release arachidonic acid (5,8,11,14-eicoatetraenoic acid)
- Arachidonic acid is a substrate for:
–> Cyclo-oxygenase enzymes (induced in the tissues by IL-1) generates prostaglandins
–> Lipoxygenase enzymes - generates leukotrienes and lipoxins - PLA2 also leads to the formation of platelet-activating factor (PAF)
What are Non-steroidal anti-inflammatory drugs (NSAIDs)?
Generally NSAIDs is to mean COX-inhibitors (aspirin-like drugs)
Aspirin (acetyl salicylic acid) has been in medical use since the 1890s
- Differs from all others - enters active site acetylates ser530, irreversibly inactivating COX
What are some Veterinary authorised NSAIDs?
Non-selective COX inhibitors: Flunixin, phenylbutazone, ketoprofen, tolfenamic acid, salicylate
COX-2 preferential: Carprofen, meloxicam
COX-2 selective: Robenacoxib, Firocoxib, cimicoxib, enflicoxib, mavacoxib
Dual COX and lipoxygenase inhibitor: Tepoxalin (no longer available)
EP4 receptor inhibitor: Grapiprant
Widely used in veterinary medicine to manage acute pain and inflammation in the peri-operative period and following musculoskeletal injury.
Chronic use is in the management of pain in osteoarthritis (not licensed for chronic use)
How selective do COX-2 inhibitors need to be?
Not straight forward as looking at relative potency vs. the two enzymes
- Need to be below IC20 for COX-1 and above IC80 for COX-2
- Need to look at drug kinetics
Drugs sequestered at sites of inflammation due to protein binding.
- Plasma kinetics do not determine duration of action.
Is inhibition of COX the only mechanism of anti-inflammatory action of NSAIDs?
Depends on the drug!
Some have free-radical scavenging activity (sulindac)
Aspirin inhibits NFkB - transcription factor for inflammatory mediator genes:
- Long-acting inhibition of platelet function - used to prevent thrombosis
What do we know about paracetamol action?
Paracetamol is an analgesic/antipyretic but much weaker anti-inflammatory
Selective for COX enzymes in the CNS - postulated different isoform (COX-3)
Analgesic/antipyretic of choice in infants (aspirin - risk of Reyes syndrome)
TOXIC TO CATS
What is GRAPIPRANT?
A selective antagonist at the EP4 receptor
EP4 receptor is a key receptor mediating the effects of PGE2
Primary receptor involved in PGE2 mediated effects on hyperalgesia (pain)
EP4 receptor also involved in vasodilatory and increase vascular permeability effects
When is the EP4 receptor Grapiprant used?
Authorised for the treatments of pain associated with mild to moderate osteoarthritis in dogs.
Very commonly causes vomiting in dogs (PGE2 has gastroprotective effects)
Shares the same potential to cause adverse effects on the kidney
Is a methylbenzenesulfonamide - not known whether sulphonamide hypersensitivity is a problem
