A new method for determining affinity constants on isolated organs when a threshold value and spare receptors are...

Area of Science:

• Pharmacology
• Biophysics
• Biochemistry

Background:

• Fixed ratio combinations of agonists and competitive antagonists mimic partial agonists, producing submaximal concentration-effect (E/[A]) curves.
• Previous methods involved linear plotting of relative curve height (H) against [antagonist]/[agonist] ratio (phiH).

Purpose of the Study:

• To develop a method for quantifying threshold values in agonist-effector systems.
• To adapt existing techniques for analyzing E/[A] curves in the presence of competitive antagonists.
• To estimate agonist apparent affinity (KA) and efficacy (eES) using a novel approach.

Main Methods:

• Utilizing fixed ratio agonist-competitive antagonist concentrations to create artificial partial agonists.
• Analyzing E/[A] curves, which may exhibit nonlinear, parabolic-like plots in systems with a threshold.
• Quantifying the threshold value to convert nonlinear plots into linear plots.

Main Results:

• Demonstrated that artificial partial agonists can induce E/[A] curves of varying heights, allowing threshold quantification.
• Successfully converted nonlinear plots to linear plots by accounting for the threshold value.
• Estimated apparent affinity (KA) and efficacy (eES) parameters for agonists like noradrenaline and acetylcholine.

Conclusions:

• The developed method effectively quantifies threshold values in agonist-effector systems.
• This technique provides a reliable way to estimate agonist affinity and efficacy parameters.
• Practical application shown using noradrenaline and acetylcholine on various effectors.