Definition of RECEPTOR

macromolecular component of the organism that binds the drug d initiates its effect.
Most receptors are proteins that have undergone various post-translational modifications such as covalent attachments of carbohydrate, lipid and phosphate.
Types of bonds that hold the drug to its receptor:
Covalent bonding: usually with irreversible agonism or antagonism.
Ionic bonding: still the bond is strong.
Hydrogen bonding; the most popular.
Dipole-dipole interaction bonding.
Hydrophilic and hydrophobic bonding (sequestration).
Vander Waals forces.
Electrostatic force bonding.
Types of Receptors according to functionality:
Active Receptor: Functional and physiologically active
Spare Receptors: Functional but not physiologically active
Receptor of loss (silent): Neither functional nor physiologically active.
Albumin for acidic drugs and globulins for alkaline drugs and fatty tissues.





Theories of drug action:
Clarks theory:
The magnitude and intensity of drug effect is directly proportional to the no. of occupied receptors by that specified drug.
Rate theory:
The magnitude and intensity of drug effect is directly proportional to rate of drug-receptor complex formation (mostly specific to explain enzymes behavior as targeted molecules i.e. receptor.
Perturbation theory:
When a drug dissolved in cell plasma membrane it will induce and influence its fluidity and arrangement of its active components that will lead subsequently to alteration in its ionic permeability (mostly specific for high lipid soluble drugs as anesthesia).
Criteria of Receptors:
Cell Surface Receptors;
Regulation and transmission of cell membrane signaling.
Ionic current transduction.(e.g. Ach)
G-protein coupling. (Ad.& Nor Ad.)
Phosphorelation (JAK-STAT): Cytokines
Voltage channels: Ach in nerve impulse conduction
Tyrosine kinases: Insulin (phospho-tyrosine)




Intracellular receptors;
Bind lipid soluble drugs.
Bind steroids.
Regulate gene transcription

Effect of a drug:
Agonist drug: has both maximal affinity and efficacy.
Antagonist drug: has maximal affinity but no efficacy.
Partial Agonist or antagonist drug: has either or both sub-maximal affinity and efficacy.
What is efficacy:
It is the ability of the drug to induce maximal conformational changes at the receptor to produce its effect. It is a measure of Emax.
What is affinity:
It is the ability of the drug to bind to its receptor. It is a measure of potency (related to dose).
Types of Antagonism:
Pharmacological Antagonism:
Both drugs competed at same receptor.
2- Physiological Antagonism:
Both drugs competed at different receptors.
3- Chemical Antagonism: Usual Acid Base neutralization.
4- Biochemical Antagonism: Competition at same enzyme as site of metabolism.
5- Dispositional Antagonism; Receptor of loss.