A new method for estimating dissociation constants of competitive and non-competitive antagonists with no prior knowledge of agonist concentrations.

1. A method is presented which enables the dissociation constant (KI) of a competitive, pseudo-irreversible or non-competitive antagonist-receptor complex to be estimated without knowledge of agonist concentrations. 2. The technique has been tested using sets of concentration-response data which simulated these various types of antagonism. 3. The points for each set of simulated data could be plotted both as agonist concentration-response curves at fixed antagonist concentrations and vice versa, producing paired data sets. 4. pKI-values were estimated from such paired data sets using appropriate graphical and computer curve-fitting methods. 5. For competitive antagonism, for each paired data set the computer curve-fitting techniques gave the same value for pKI, assuming drug-receptor interaction to be 1:1 and agonist concentrations to be known. 6. When agonist concentrations were assumed unknown, pKIS could not be estimated by the conventional method (using agonist dose-ratios) but could still be obtained (for competitive, pseudo-irreversible and non-competitive antagonism) by the new method. 7. This new method should be especially useful for measuring dissociation constants of antagonists against neuronally- or ionophoretically-released agonists. It may also be useful when agonist is applied exogenously, especially if suitable drugs are not available to block agonist uptake and/or metabolism.