Abstract
We describe a modification of receptor theory that enables the estimation of relative affinity constants for the inactive state of a G protein-coupled receptor. Our approach includes the traditional parameters of observed affinity (Kobs) and efficacy (fraction of ligand-receptor complex in the active state, ε) and introduces the concept of the fraction of the ligand-receptor complex in the inactive state (intrinsic inactivity, εi). The relationship between receptor activation and the ligand concentration is known as the stimulus, and the operational model expresses the response as a logistic function of the stimulus. The latter function includes Kobs and the parameter τ, which is proportional to ε. We introduce the parameter τi, which is proportional to εi. We have previously shown that the product, Kobsτ, of one agonist, expressed relative to that of another (intrinsic relative activity, RAi), is a relative measure of the affinity constant for the active state of the receptor. In this report, we show that the product, Kobsτi, of one agonist, expressed relative to that of another (intrinsic relative inactivity, RIi), is a relative measure of the affinity constant for the inactive state of the receptor. We use computer simulation techniques to verify our analysis and apply our method to the analysis of published data on agonist activity at the M3 muscarinic receptor. Our method should have widespread application in the analysis of agonist bias in drug discovery programs and in the estimation of a more fundamental relative measure of efficacy (RAi/RIi).
Footnotes
This work was supported by the National Institutes of Health National Institute of General Medical Sciences [Grant GM69829].
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
doi:10.1124/jpet.111.179291.
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ABBREVIATIONS:
- McN-A-343
- 4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium
- RSS
- residual sum of squares
- GTPγS
- guanosine 5′-O-(3-thio)triphosphate
- CHO
- Chinese hamster ovary.
- Received January 11, 2011.
- Accepted May 13, 2011.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics
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