Dose-response curves can be constructed to describe response or ligand-receptor complex formation as a function of the ligand concentration.
Antagonists make it harder to form these complexes by inhibiting interactions of the ligand with its receptor.
This is seen as a change in the dose response curve: typically a rightward shift or a lowered maximum.
present divided by the agonist required for half maximal response without antagonist ("control").
[citation needed] The fitting of the Schild plot to observed data points can be done with regression analysis.
is the total number of binding sites, and when the equation is plotted it is the horizontal asymptote to which the plot tends; more binding sites will be occupied as the ligand concentration increases, but there will never be 100% occupancy.
The binding of the ligand to the receptor at equilibrium follows the same kinetics as an enzyme at steady-state (Michaelis–Menten equation) without the conversion of the bound substrate to product.
A surmountable drug changes the binding affinity: A nonsurmountable drug changes the maximum binding: The Schild regression also can reveal if there are more than one type of receptor and it can show if the experiment was done wrong as the system has not reached equilibrium.
The first radio-receptor assay (RRA) was done in 1970 by Lefkowitz et al.,[dubious – discuss] using a radiolabeled hormone to determine the binding affinity for its receptor.
[1] A radio-receptor assay requires the separation of the bound from the free ligand.
[2] A method that does not require separation is the scintillation proximity assay that relies on the fact that β-rays from 3H travel extremely short distances.
Today, the fluorescence method is preferred to radioactive materials due to a much lower cost, lower hazard, and the possibility of multiplexing the reactions in a high-throughput manner.
Other detection methods such as surface plasmon resonance do not even require fluorophores.