AFFINITY & ANTAGONISM
RECEPTORS – INTRODUCTION:
- Binding sites of drug with functional correlation.
- Two important terms related to receptors are affinity & intrinsic activity (IA).
AFFINITY:
- Ability of drug to combine with receptor.
- After binding to receptor, ability to activate receptor is “intrinsic activity (IA)”.
- IA varies from –1 through zero to +1.
- Clinical implication of affinity:
- Drugs with high affinity used in low concentrations.
Types:
- Drugs divided into 4 types based on their intrinsic activities.
- Agonist.
- Partial agonist.
- Inverse agonist.
- Antagonist.
1. Agonist drugs:
- Binds to receptor & activates maximally.
- IA is +1.
2. Partial agonist:
- Activates receptor submaximally.
- IA between 0 & +1.
- Produces similar effect in absence of agonist yet decreases pure agonistic effect.
- Eg:
- Pindolol has partial agonistic activity at ß1-receptors.
- In presence of agonists (adrenaline & nor-adrenaline) –> Produces antagonistic effect (Decreased heart rate).
- Yet, even in high doses does not result in severe bradycardia (due to agonistic action).
3. Inverse agonist:
- Binds to receptor producing opposite effect.
- IA is negative.
- Eg: ß carboline – An inverse agonist at BZD receptors.
ANTAGONISM:
- Types: Physical, chemical, physiological or pharmacological.
1. Physical antagonist:
- Binds to drug & prevents its absorption.
2. Chemical antagonist:
- Combines with substance chemically.
- Eg: Chelating agents binding with metals.
3. Physiological antagonist:
- Produces an opposite action to substance by binding to different receptors.
- Eg: Adrenaline – Physiological antagonist of histamine.
- Adrenaline causes bronchodilation by binding to ß2-receptors.
- Opposite to action of histamine via H1 receptors ie., bronchoconstriction.
4. Pharmacological antagonists:
- Produces opposite actions by binding to same receptor.
- Eg: beta blockers.
Exam Important
- Affinity is ability of drug to combine with receptor.
- Drug’s ability to activate receptor is “intrinsic activity (IA)”.
- IA varies from –1 through zero to +1.
- Drugs with high affinity used in low concentrations.
- Drugs divided into 4 types based on their intrinsic activities which are agonist, partial agonist, inverse agonist & antagonist.
- Agonist drugs binds to receptor & activates maximally
- Partial agonist drugs activates receptor submaximally.
- Pindolol has partial agonistic activity at ß1-receptors.
- Inverse agonist drugs binds to receptor producing opposite effect.
- ß carboline is an inverse agonist at BZD receptors.
- Physical antagonist binds to drug & prevents its absorption.
- Chemical antagonist combines with substance chemically.
- Chelating agents binding with metals is an example of chemical antagonist.
- Physiological antagonist produces an opposite action to substance by binding to different receptors.
- Adrenaline is the physiological antagonist of histamine.
- Adrenaline causes bronchodilation by binding to ß2-receptors.
- Pharmacological antagonists produces opposite actions by binding to same receptor.
Don’t Forget to Solve all the previous Year Question asked on AFFINITY & ANTAGONISM
