Use of an antagonist for estimating the degree of agonist stimulation during physiological release.

Antagonist activity can be measured either as the percentage reduction in the effect of an agonist or as a dose ratio. In this article, Dick Barlow explains how these are connected and how the relationship between the concentration of antagonist producing 50% inhibition (IC50) and the antagonist equilibrium constant (Ki) involves the degree of agonist stimulation ([A]/[A50]), which is the concentration of agonist expressed as a multiple of the concentration producing a half-maximal response. If Ki is known it is possible to use the IC50 value of the antagonist to estimate the degree of agonist stimulation in physiological experiments where there is neuronal or hormonal release of the agonist.