Pharmacologic methods for identification of receptors

Area of Science:

• Pharmacology
• Biochemistry
• Drug Discovery

Background:

• Drug-receptor interactions are quantified using dissociation constants (Kd).
• Functional studies and radioligand binding assays are common methods for Kd determination.
• Agonist Kd determination via functional studies can be complex due to receptor inactivation.

Purpose of the Study:

• To compare Kd values for agonists and antagonists derived from functional and radioligand binding studies.
• To investigate discrepancies in Kd values between the two methods for agonists.
• To propose a model explaining the observed differences in Kd measurements.

Main Methods:

• Determined apparent equilibrium dissociation constants using functional and radioligand binding studies.
• Utilized prototype agonists and antagonists in rabbit aorta and guinea-pig ileum preparations.
• Administered chronic treatment to rabbits to assess changes in drug-receptor interactions.

Main Results:

• Dissociation constants agreed for antagonists but differed significantly for agonists between methods.
• Binding assay Kd values were higher for agonists than antagonists.
• Chronic treatment altered functional Kd for norepinephrine without affecting binding Kd or antagonist pA2 values.

Conclusions:

• A two-state receptor model explains why binding assays may miss functional changes in agonist Kd.
• Agonists exhibit differential affinity for receptor states, while antagonists do not.
• Employing both functional and radioligand binding assays is crucial for comprehensive drug receptor characterization.