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Vol. 289, Issue 2, 981-992, May 1999
Department of Pharmacology, College of Medicine, University of
California-Irvine, Irvine, California (F.J.E., R.B.); and
Department
of Chemistry, Chapman University, Orange, California (M.T.G.)
We describe a simple method for calculating the pharmacological
activity of an agonist (A) relative to a standard agonist (S) using
only the concentration-response curves of the two agonists. In most
situations, we show that the product of the ratios of maximal responses
(Emax
A/Emax
S) and potencies
(EC50
S/EC50
A) is
equivalent to the product of the affinity and intrinsic efficacy of A
expressed relative to that of S. We refer to this term as the IRA value
of A. In a cooperative system where the concentration-response curve of
the standard agonist is steep and that of the test agonist is flatter
with a lower maximal response, the simple calculation of IRA described
above underestimates agonist activity; however, we also describe a
means of correcting the IRA in this situation. We have validated our
analysis with modeling techniques and have shown experimentally that
the IRA values of muscarinic agonists for stimulating contractions in
the guinea pig ileum (M3 response) are in excellent
agreement with those measured in the phosphoinositide assay on Chinese
hamster ovary cells expressing the M3 muscarinic receptor.
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