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SIB2007 3

The Effects of Agonists And Their Concentration-Response Curves

What is an agonist? A drug which interacts with receptors (affinity).
What does the resulting drug-complex of an agonist generates? A response.
What is efficacy? The ability to alter the activity of a receptor. The relationship between receptor occupancy and the ability to initiate a response at the molecular, cellular, tissue ot system level after binding to a receptor.
What happens when there is a positive efficacy? Causes an increase in the receptor's activity.
What happens when there is a negative efficacy? Causes a decrease in the receptor's activity.
Why do the size of responses vary? They vary with the proportion of receptors occupied by the agonist, which varies with the concentration of the agonist.
What is occupancy? The proportion of receptors occupied.
What is the equation for occupancy? Number of receptors occupied / total number of receptors.
What happens to occupancy when the agonist concentration increases? Occupancy increases sigmoidally.
What does occupancy slow down? Most receptors become occupied.
What is K1? Forward reaction.
What is K2? Backward reaction.
What is the equation of a response reaction? K1 (Ntot - Na) [A] = K2 (Na)
What is Na? Number of receptors occupied by agonist.
What is Ntot? Total number of receptors.
What is Ntot - Na? Number of free receptors.
What is [A]? Concentration of agonist.
What is the equation which describes how occupancy varies with the concentration of agonist? Occupancy = [A] / [A] + Ka
What is Ka? Ka is an important constant and numerically equal to the concentration of drug which HALF of the receptors are occupied.
What is the equation for Ka? Ka = K2 / K1
What happens when Ka increases? Backward rate of reaction increases. Affinity deceases.
What happens when a drug with a higher Ka has higher backward reaction? Tend to form fewer complexes at a particular concentration.
What are the different types of agonists? Full agonists Partial agonist Inverse agonist Co-agonist Physiological agonist
What is a full agonist? They bind (have affinity for) and activate a receptor, displaying full efficacy at that receptor.
What are examples of full agonists? Isoproterenol which mimics the action of adrenaline at β adrenoreceptors Morphine, which mimics the actions of endorphins at µ-opioid receptors in the CNS.
What is a partial agonist? They have only partial efficacy at the receptor relative to a full agonist.
What are examples of partial agonist? Buspirone and aripiprazole.
What factors are determined by plotting the Dose-Response curves? Potency Efficacy Slope
What does EC50 stand for? Potency
What does Emax stand for? Efficacy
What is Potency? The measure of the concentration of a drug at which it is effective.
What is abbreviated A? The agonist, ligand, drug or hormone that binds to the receptor and initiates the response.
What is E? Response.
How is a maximum effect/response or Emax achieved? At higher concentrations it becomes appreciable and rises with increasing agonist concentration [A] until at sufficiently high concentrations it can no longer be increased by raising [A].
Why can the response of E be too low to measure? Because agonist is below a certain concentration.
What is the Emax? The maximum possible effect for the agonist. The concentration of A at which E is 50% of Emax is termed [A] 50.
What does the term "potency" refer to? Refers to the [A] 50 value.
What happens with lower [A] 50? The lower the concentration of a drug is required to produce 50% of a maximum effect and the higher the potency.
Why isn't potency important clincially as long as what? As long as the drug can be applied safely.
Why would someone want to elucidate an EC 50 value? To compare the potency of drugs with similar efficacies producing physiologically similar effects.
What value can efficacy take? 0-1. The higher the value the stronger the response.
What factors does efficacy have? The total number of receptors for that drug present in the tissue. The type of coupling exist between the receptor and response. The intrinsic efficacy of the drug at the receptor.
What is intrinsic efficacy? Determined by the ability of drug-receptor complex to activate the secondary mechanism (cellular transduction system) e.g. G protein
What value can instrinsic efficacy take? 0-infinity.
What does it mean when a drug has a high efficacy? with examples. It means that maximum obtained effect is high. Morphine used for severe pain but paracetamol for minor pain.
What is affinity? It is a measure of how strong a drug binds to its receptor.
What is affinity characterised by? By Kd or Ka (equilibrium dissociation constant).
What do drugs with higher affinity have? Lower Kd.
What factors affect size of response? Characteristics of tissue involved-total # of receptors, nature of receptors and response coupling. The agonist-receptor complex, agonists with higher intrinsic efficacy induce a bigger response although have similar affinity + act on the same receptor
What is the relationship between occupancy, agonist concentration and response? Response = f (∑ Ntot [A] / [A] + Ka)
Where did the idea of spare receptors come about? To generate a full response it is not necessary for an agonist to occupy ALL receptors: only a fraction of the total number of receptors needed to be occupied.
How can affinity be measured? Radioligand Binding.
What is radioligand binding? The use of radiolabelled drugs which bind to a particular receptor.
How do radiolabelled drugs bind? 1. Tissue of interest will be chosen 2. Preparation of membrane fraction by homogenisation and centrigation 3. Incubation with radiolabelled drug 4. Separation of free from bound drug by rapid filtration 5. Counting bound radioactivity
What are factors affecting the response to a drug in a patient? Genetic factor, weight, state of health and disease Age, gender Function of organs such as liver and kidney Pre-medication, placebo effect Tolerance/dependence/desensitisation Hypersentivity and Idiosyncratic reactions.
Why does drug tolerance occur? When a subject's reaction to a drug (such as a painkiller) decreases so that larger doses are required to acheieve the same effect.
What is Tachyphylaxis? The rapid development of drug tolerance.
What factors might be responsible for drug tolerance? Loss or change in receptors Physiological adaptation Increased metabolism or depletion of mediators.
What is drug dependence? The need (physical or psychological) to use a drug to pevent withdrawal symptoms.
What are hypersensitivity reactions? An application of a drug in some individuals may cause an immediate and severe allergic reaction due to development of antibodies, i.e. drug acts as an antigen.
What are idiosyncratic reactions also known as? Type B reactions.
What are idiosyncratic reactions? Drug reactions which occur rarely and unpredictably amongst the population. Not idiopathic which entails that cause is not known.
What is an adverse drug reaction also known as? Adverse drug event.
What is an adverse drug reaction? Describes the unwanted, negative consequences associated with the use of given medications.
What is the study of adverse drug reactions? Pharmacovigilance.
What does ADR stand for? Adverse Drug Reaction.
When does tachyphylaxis occurs? Occurs following continued administration.
Created by: Groberts00



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