Receptor - Theories of Receptor ||Pharma Easy Notes || @Pharmacy

Theories of Receptor

Most Therapeutically useful drugs bind only transiently of their intended receptor
O (i) Occupation theory
R (ii) Rate theory
T (iii) The induced-fit theory of enzyme-substrate
M (iv) Macromolecular perturbation theory
A (v) Activation - Aggregation theory

1. Occupation theory

  • Proposed by Gaddum and Clark states
  • The intensity of the pharmacological effect is directly proportional to the number of receptors occupied by the drug.
  • The Pharmacological response of a drug molecule is a
  • Function of dose
  • Number of receptors available &
  • Its intrinsic activity

The rate of combination drug and receptor can be therefore expressed as
K1 [R] [A]
K= Association constant
R= Concentration of the receptor not occupied by the drug
A = Concentration of drug molecule or dose

Similarly, the rate of dissolution of the drug-receptor complex is given by the expression
K2 [RA]
K2 = Dissociation constant
[RA]= Concentration of receptor occupied by the drug

The term [RA]/r = fraction of the total number of receptors occupied by the drug

Relative response = [RA]x/[r] = x/1+KA/[A] -------------------- 1

The response is directly proportional to the intrinsic activity x

2. Rate theory

  • Patan and Rang in 1965 proposed that the most important factor determining drug action is the rate at which drug-receptor combination takes place.
  • The Pharmacological activity is a function of the rate of association and dissociation of the drug with the receptor and not the number of occupied receptors.
  • When the response is proportional to the no. of receptors occupied equation 1 is important and when the response is proportional to the rate of receptor occupation rather than to the proportion of receptor occupied equation 2 of rate theory is important.

3. The Induced -fit theory of enzyme-substrate interaction
  • This theory of Koshland was originally proposed for the action of substrates and enzymes. The drug approaches the receptor a conformation change occurs in the receptor to allow for effective binding.
  • e.g. Acetylcholine may interact with the regulating protein and alters the normal force, which stabilizes the structure of a protein, thereby producing a transient rearrangement in the membrane structure and consequent change in its ion-regulating property.
  • The receptor was suggested to be elastic and it could return to its original confirmation after the drug was released.
  • The drug may also undergo conformational changes.

4. Macromolecular peturbation theory

  • Combining the induced fit and rate theories yield the macromolecular peturbation theory.
  • This theory suggests that two types of confirmation changes exist and that the rate of their existence determines the observed biological response.
  • Agonist produce the specific perturbation required for a biological response while
  • Antagonist produces a non-specific peturbation that fails to yield a biological response.
  • This theory can partially account for the activity of partial agonists.

5. Activation- Aggregation theory

  • It was proposed by changeux and karlion.
  • According to this theory, even in the absence of a drug the receptors are in dynamic equilibrium between the active form (RO), which is responsible for biological and inactive form (TO).
  • Agonist shifts the equilibrium to an active form and antagonist shift to an inactive form.
  • This theory can account for the activity of inverse agonist which neither a typical agonist nor an antagonist but rather produce a response opposite to those of the agonist.

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