Basic science Flashcards
What are the features of cell membranes?
Phospholipid bilayer - hydrophilic heads on outside, hydrophobic tails on inside
Glycoproteins - specialized types which aid in cellular transport / messaging
What are the different ways of crossing the cell membrane?
Passive diffusion - movement down a concentration gradient. Requires no energy
Facilitated diffusion - molecules combine with membrane-bound carrier proteins to cross the cell membrane more quickly. Movement is still down a concentration gradient. An example is glucose, which as a highly ionized molecule would be transported relatively slowly by passive diffusion
Active transport - movement of a molecule against its concentration gradient, via a pump which requires energy to function. The most common example is the Na+/K+ ATPase which uses energy produced from the conversion of ATP > ADP to induce a conformational change in the pump. As a result 3 Na+ ions move out of the cell for every 2 K+ ions which enter the cell, producing a net negative charge within the cell
Pinocytosis - an area of the cell membrane invaginates around the target molecule to move it into the cell. The molecule may then be released into the cell or remain within a vacuole. This process usually occurs with larger molecules
What is the structure of the acetylcholine receptor?
Pentameric (5 subunits)
- 2x α subunits - where ACh binds
- β
- δ
- γ (foetal) or ε (adult)
Binding of ACh causes a conformational change to allow an influx of Na+ ions and subsequent membrane depolarization
What factors affect the rate of diffusion across the cell membrane?
Molecular size - small molecules diffuse more readily than larger ones. Graham’s law states that the rate of passive diffusion is inversely proportional to the square root of molecular size
Concentration gradient - Fick’s law states that the rate of transfer across a membrane is proportional to the concentration gradient across the membrane
Ionization - only uncharged molecules can diffuse through the lipophillic cell membrane. The degree to which a drug is ionized / unionized depends on the molecular structure of the drug and the pH of the solution in which it is dissolved
Lipid solubility - more lipid soluble molecules will be able to diffuse across the cell membrane more readily. Lipid solubility is given for the unionized form of molecule only, and therefore is considered separately from the pKa
Protein binding - only the unbound fraction of a drug in the plasma is free to cross the cell membrane. More clinically relevant in highly protein-bound drugs in which a small change in the bound fraction produces a significant change in the amount of unbound drug. This is important when conditions alter the concentrations of plasma proteins, e.g. in acute inflammation / infection, end stage liver disease and severe burns
Generally:
- Albumin - binds neutral or acidic drugs e.g. diazepam, warfarin, barbituates
- Globulins such as α1 acid glycoprotein bind basic drugs e.g. morphine
What is pKa?
pKa = the pH at which the molecules of a substance exist as 50% ionized / 50% unionized
For a weak acid - more ionized at a pH above the pKa. Therefore slower diffusion in more alkaline conditions.
For a weak base - more ionized at a pH below the pKa. Therefore slower diffusion in more acidic conditions. Remember local anaesthetics less effective in acidic conditions!
AIA - Acids ionized above
BIB - Bases ionized below
What is the Henderson-Hasselbach equation?
pH = pKa + log10 ([A-] / [HA])
[A-] = proton acceptor = conjugate base / weak base
[HA] = proton donor = weak acid / conjugate acid
How are orally administered drugs absorbed in the GI tract?
Only unionized drugs can readily cross the lipid membrane of the gut, except for drugs which have specific transport mechanisms
Acidic drugs (e.g. aspirin) are unionized in the acidic environment of the stomach, and therefore can be absorbed there and may have a relatively quicker speed of onset. However, acidic drugs are still mostly absorbed within the small bowel despite being in a relatively ionized state - because of the significantly greater surface area available for absorption
Basic drugs are relatively unionized within the small bowel, and therefore are mostly absorbed there
Salts of permanently charged drugs such as vecuronium and glycopyrrolate cannot be absorbed from the GI tract
What is bioavailability?
The proportion of a drug which reaches the systemic circulation after being given by a specific route, compared to the same dose administered intravenously (which has 100% bioavailability)
Can be calculated by comparing the respective areas under the curves on a time-plasma concentration graph
Oral bioavailability = AUCoral / AUCIV
How does distribution vary between different drugs in broad categories?
What is the volume of distribution?
Confined to plasma - Large molecules and highly protein-bound drugs (e.g. warfarin, phenytoin)
Limited distribution - Charged molecules, poorly lipid-soluble, relatively bulky e.g. non-depolarizing muscle relaxants. Can only leave plasma at capillaries with fenestrae i.e. muscle and work extracellularly
Extensive distribution - Highly lipid-soluble molecules. Initial distribution to tissues with highest blood flow (brain, lung, kidney, thyroid, adrenal) > moderate blood flow (muscle) > low blood flow (fat)
Volume of distribution - A representation of the distribution of a drug - the theoretical volume that would be necessary to contain the total amount of an administered drug at the same concentration that it is observed in the blood plasma.
Volume of Distribution (L) = Amount of drug in the body (mg) / Plasma concentration of drug (mg/L)
How does the blood-brain barrier control molecular transfer?
The BBB is the functional barrier between the CNS and the circulation
Simple diffusion - lipid soluble, low molecular weight drugs such as inhaled and intravenous anaesthetics
Active transport - glucose and hormones such as insulin
The BBB contains enzymes such as monoamine oxidase
Glycopyrrolate has a quaternary, charged nitrogen and therefore cannot pass the BBB. Atropine can pass and therefore have centrally mediated effects such as confusion or paradoxical bradycardia
The BBB can be disrupted:
- Intracranial injury / SAH may cause release of central neurotransmitters into the circulation causing circulatory disturbance
- Inflammation e.g. meningitis can increase permeability to penicillin, allowing a therapeutic benefit
How does the placenta control molecular transfer?
The placental membrane is composed of a phospholipid layer, and therefore is more readily crossed by lipid soluble, low molecular weight, unionized molecules. Drugs which have low protein binding will have a greater concentration gradient which will increase placental transfer
The placental membrane is much less selective than the blood-brain barrier and even molecules with moderate lipid solubility may cross
Foetal blood has a lower pH than maternal blood, and may further lower during foetal distress. This could become significant when a drug which is a weak base such as bupivacaine crosses the placenta. In the foetal circulation the drug will become relatively more ionized - this will reduce transfer back into the maternal circulation and may result in build-up to toxic levels
Lidocaine crosses the placenta more readily than bupivacaine due to its pKa which results in a higher proportion of the drug being in an unionized state
Anaesthetic agents generally cross the placenta
Opioids cross the placenta.
Remifentanil undergoes breakdown by widespreadester hydrolysis in the foetus, and therefore does not accumulate
Pethidine crosses the placenta and is metabolized to norpethidine - which is less lipid soluble and therefore may accumulate in the foetus
Non-depolarizing muscle relaxants do not cross the placenta as they are large, polar molecules. Very small amounts of suxamethonium cross the placenta which generally has no clinical effect
What is the Michaelis constant?
The concentration of substrate at which an enzyme is working at 50% of maximum rate. This represents hepatocellular enzyme metabolic capacity for a specific drug
Low = low metabolic capacity
High = high metabolic capacity
How does metabolism of a drug occur in the liver?
Phase 1
- Oxidation, reduction or hydrolysis
- Often catalysed by the cytochrome P450 enzyme system
- Other enzymes involved include monoamine oxidase, angiotensin converting enzyme, plasma esterases
Phase 2
- Methylation, glucoronidation, acetylation, sulphation
- Increases solubility to allow excretion in urine or bile
What are some examples of drugs which cause hepatic enzyme induction?
Reduce the concentration of drugs metabolized by the cytochrome P450 system
- Rifampicin
- Phenytoin
- Carbamazepine
- Phenobarbital
- Thiopental
- Alcohol
- Cigarette smoking
What are some examples of drugs which cause hepatic enzyme inhibition?
Increase the concentration of drugs which are normally metabolized by the cytochrome P450 system
- Amiodarone
- Metronidazole
- Cimetidine
- Isoniazid
- Phenelzine
How does a G-protein coupled receptor work?
Intermediate messenger production
- GPCRs are membrane-bround proteins with a serpentine structure consisting of seven helical regions that traverse the membrane
- G-proteins are a group of heterotrimeric proteins (3 subunits - α, β and γ) associated with the inner leaflet of the cell membrane that act as universal transducers involved in bringing about an intracellular change from an extracellular stimulus
- The GPCR binds a ligand on its extracellular side and the resultant conformational change increases the likelihood of coupling with a particular type of G-protein. Each GPCR can stimulate multiple G-proteins, resulting in signal amplification
- In the active form, GDP is bound to the α-subunit of the G-protein. As the G-protein binds to an activated GPCR GTP replaces GDP, and the resulting α-GTP subunit dissociates from the β-γ subunit. The α-GTP subunit then activates or inhibits a specific effector protein, such as an enzyme or an ion channel. In some systems, the β-γ subunit can also activate intermediary mechanisms. Each G-protein can activate several intermediate messengers, again resulting in signal amplification
- The α-subunit acts as a GTPase enzyme, which causes hydrolysis of GTP to GDP, and subsequent re-assication with the β-γ subunit to inactivate the signal transmission
There are 3 main classes of G-protein α-subunit:
* Gs - activate adenylyl cyclase > catalyses formation of cyclic AMP (cAMP) > stimulates protein kinase A > biochemical effects
* Gi - inhibit adenylyl cyclase
* Gq - activate phospholipase C > breaks down PIP₂ to form IP₃ and DAG
IP₃ - calcium release in endoplasmic reticulum
DAG - activates protein kinase C > biochemical effects
What is the structure and function of the GABAA receptor?
Pentameric
Ligand gated ion channel
Inhibitory
Binding of GABA causes a Cl- channel to open, causing influx of Cl- ions into the cell and subsequent membrane hyperpolarization - reducing likelihood of an action potential being triggered in the post-synaptic cell
Subunits - 2x α, 2x β, 1x γ
* GABA - binds at α/β interface to cause opening of Cl- ion channel
* General anaesthetics (propofol, barbiturates, etomidate, volatile agents) - bind at β subunit to prolong channel opening time
* Benzodiazepines - bind at α/γ interface and cause increased affinity of GABA for the GABAA receptor
What is the structure and function of the NMDA receptor
N-methyl-D-aspartate receptor
Excitatory
Comprised of two subunits - GluN1 and GluN2
* Glutamate binds to GluN2 subunit
* Glycine binds to GluN1 receptor
- Both glutamate and glycine must bind to activate the receptor.
- However at resting membrane potential the channel typically does not open as it is blocked by magnesium ions
- When the membrane is sufficiently depolarized, the block is release and the ion channel opens - allowing influx of calcium ions to trigger intracellular signalling pathways
Drugs which are primarily NMDA receptor antagonists include ketamine, nitrous oxide, xenon and memantine
