Pharmacokinetics and Pharmacodynamics exam revision notes for the NMP Independent and Supplementary Prescribing exam.
Includes adverse reactions, antagonists/agonists, half-life, steady state and more!
Drugs inhibiting transport processes:
ION CHANNELS
CA2+ channels (Ca Channel Blockers)
NA+ channels (local anaesthetics)
K ATP channels (oral antidiabetics)
ACTIVE TRANSPORT
NA+/K+-ATPase (cardiac glycosides)
Pharmacodynamics – the effects of the drug on the body
Pharmacokinetics – the way the body affects the drug with time (ADME)
- A few drugs act by virtue of their physicochemical properties and this is called non-
specific drug action.
- Some drugs act as false substrates or inhibitors for certain transport systems or
enzymes.
, - Most drugs produce their effects by acting on specific protein molecules, usually
located in the cell membrane.
- These proteins are called receptors and usually respond to endogenous chemicals in
the body.
- These chemicals are either synaptic transmitter substances or hormones.
- Chemicals or drugs that activate receptors and produce a response are called
agonists.
- Some drugs are antagonists and combine with receptors, although do not activate
them.
- Antagonises reduce the probability of the transmitter substance combining with the
receptor and so reduce or block its action.
- The activation of receptors by an agonist or hormone is coupled to the physiological
or biochemical responses by transduction mechanisms that often involve molecules
called second messengers.
- The interaction between a drug and the binding site of the receptor depends on the
complementarily of ‘fit’ of the two molecules.
- The closer the fit and the greater the number of bonds, the stronger will be the
attractive forces between them and the higher the affinity of the drug for the
receptor.
- The ability of a drug to combine with one particular type of receptor is called
specificity.
- No drug is truly specific, but many have a relatively selective action on one type of
receptor.
Receptors
- Protein molecules that are normally activated by transmitters or hormones.
- Many receptors have now been cloned and their amino acids sequences determined.
- The four types of receptor are:
1. Agonist (ligand)-gated ion channels are made up of protein sub units that form a
central pore
2. G-protein-coupled receptors form a family of receptors with seven membrane-
spanning helices. They are linked to physiological responses by second
messengers.
3. Nuclear receptors for steroid hormones and thyroid hormones are present in the
cell nucleus and regulate transcription and thus protein synthesis.
4. Kinase-linked receptors are surface receptors that possess intrinsic tyrosine
kinase activity. They include receptors for insulin, cytokines and growth factors.
- Transmitter substances are chemicals released from nerve terminals that diffuse
across the synaptic cleft and bind to the receptors.
- This binding activates the receptors by changing their conformation and triggers a
sequence of postsynaptic events resulting in, for example, muscle contraction or
glandular secretion.
- Following its release, the transmitter is inactivated by either enzymic degradation or
reuptake.
- Many drugs act by either reducing or enhancing synaptic transmission.
- Hormones are chemicals released into the bloodstream.
, - They produced their physiological effects on tissues possessing the necessary specific
hormone receptors.
- Drugs may interact with the endocrine system by inhibiting or increasing hormone
release.
- Other drugs interact with hormone receptors, which may be activated or blocked.
- Local hormones, such as histamine, serotonin, kinins and prostaglandins are released
in pathological processes.
- The effects of histamine can sometimes be blocked with antihistamines.
- Drugs that block prostaglandin synthesis are widely used as anti-inflammatory
agents.
Transport Systems
- The lipid cell membrane provides a barrier against the transport of hydrophilic
molecules into or out of the cell.
- Ion channels are selective pores in the membrane that allow the ready transfer of
ions down their electrochemical gradient.
- The open-closed state of these channels is controlled either by the membrane
potential or by transmitter substances.
- Some chemicals are both voltage and transmitter gated.
- Voltage gated channels for sodium, potassium and calcium have the same basic
structure and subtypes exist for each different channel.
- Active transport processes are used to transfer substances against their
concentration gradients.
- They utilise special carrier molecules in the membrane and require metabolic
energy.
Enzymes
- These are catalytic proteins that increase the rate of chemical reactions in the body.
- Drugs that act by inhibiting enzymes include: anticholinesterases, which enhance the
action of acetylcholine; carbonic anhydrase inhibitors, which are diuretics;
monoamine oxidase inhibitors, which are antidepressants; and inhibitors of cyclo-
oxygenase, such as aspirin.
Second Messengers
- These are chemicals whose intracellular concentration increases or decreases in
response to receptor activation by agonists and which trigger processes that
eventually result in a cellular response.
G-Proteins
- G-protein-coupled receptors are linked to their responses by a family of regulatory
guanosine triphosphate (GTP)-binding proteins (G-Proteins).
- The receptor-agonist complex induces a conformational change in the G-Protein,
causing its a-subunit to bind to GTP.
- The a-GTP complex dissociates from the G-Protein and activates (or inhibits) the
membrane enzyme or channel.
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