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Insurmountable AT1 receptor antagonism: fitting models with experimental data
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responses are transient or self-limiting. Thus, we proposed that the mechanism of insurmountable antagonism by slowly dissociating AT1 receptor antagonists relates to their ability to reduce the rate of agonist association with the receptors, and thus to prevent the transient response peak. Because many different responses to AT1 receptor activation have transient time-courses, these models are probably well founded. We note that agonist response transience and two-stage antagonist binding are not mutually exclusive and so it is possible that both contribute to the insurmountability of AT1 receptor antagonism. However, a two-step antagonist binding process is not ‘needed’ to explain insurmountable AT1 receptor antagonism. Vauquelin et al. consider that response transience merely complicates ‘what is really occurring between, for example, the AT1 receptor antagonists and their receptor’ and claim that the use of the cumulated response index of inositol phosphate accumulation, can ‘marginalize’ such complications1. The lack of a clear transient in the accumulated inositol phosphate concentrations does not mean that response transience is not occurring. The transient nature of inositol phosphate production has been reported in other systems, and for the accumulated inositol phosphates to reach a steady level the underlying production rate must be transient. Furthermore, because a slow antagonist will always limit the onset of responses to a faster agonist, cumulated responses will always display insurmountable antagonism by slow antagonists even where the response is non-transient and the system reaches full equilibrium. Kenakin7 has discussed the shortcoming of cumulative response indices and their tendency to obscure response time-courses. Thus, far from protecting from unwanted complications, a cumulated response might introduce such complications. Finally, it has to be noted that because all of these models are inevitably much simpler than reality there is no point in attempting to compare the models’ goodness of fit to real data, even if more of the parameters had experimentally determined values. It is our view that the purpose of these types of models in pharmacology is to help inform experimental design and not to prove the validity of hypotheses. http://tips.trends.com
TRENDS in Pharmacological Sciences Vol.22 No.11 November 2001
Michael Lew* Arthur Christopoulos James Ziogas Dept of Pharmacology, University of Melbourne, Victoria 3010, Australia. *e-mail: [email protected] References 1 Vauquelin, G. et al. (2001) Insurmountable AT1 receptor antagonism: the need for different antagonist binding states of the receptor. Trends Pharmacol. Sci. 22, 343–344 2 Lew, M.J. et al. (2000) Dynamic mechanisms of non-classical antagonism by competitive AT1 receptor antagonists. Trends Pharmacol. Sci. 21, 376–381 3 Vauquelin, G. et al. (2001) A two-state receptor model for the interaction between angiotensin II type 1 receptors and non-peptide antagonists. Biochem. Pharmacol. 61, 277–284 4 Rang, H.P. (1966) The kinetics of action of acetylcholine antagonists in smooth muscle. Proc. R. Soc. London B Biol. Sci. 164, 488–510 5 Hall, D. and Parsons, S. (2001) Nonsurmountable antagonism: a general drawback of pre-steady-state measurement? Trends Pharmacol. Sci. 22, 63–65 6 Lew, M.J. et al. (2001) Non-surmountable antagonism: transcending steady state. Trends Pharmacol. Sci. 22, 65–66 7 Kenakin, T.P. (1997) Differences between natural and recombinant G protein-coupled receptor systems with varying receptor/G protein stoichiometry. Trends Pharmcol. Sci. 18, 456–464
Insurmountable AT1 receptor antagonism: fitting models with experimental data Response from Vauquelin et al.
It is of theoretical and potential clinical importance to determine whether antagonist-bound angiotensin AT1 receptors adopt only a single, slow dissociating state (Models 1 and 2)1 or whether they might adopt multiple binding states (two binding states as in Model 3)1. In this respect, Lew and colleagues show without dispute that single-state models also fit with several experimental data {association and dissociation rate of [3H]irbesartan and irbesartan versus angiotensin II inhibition curve in CHO-hAT1 cells (Chinese hamster ovary cells permanently transfected with the gene encoding the human AT1 receptor)} if they are appended by a non-linear (hyperbolic) stimulus–response function. These new findings illustrate the utility of kinetic
557
rather than equilibrium models and also highlight the potential contribution of a ‘receptor reserve’ to the ‘insurmountable behaviour’ of antagonists in general. However, published kinetic experiments comparing the maximal angiotensin-IImediated inositol phosphate production with the cell-surface binding of [3H]angiotensin II (Ref. 2) and the antagonist [3H]candesartan3 stipulate a linear stimulus–response function in CHO-hAT1 cells. These data support the two-state model but are not compatible with the modified single-state models because they rely on a hyperbolic stimulus–response function. A multiple binding state concept might therefore provide interesting guidance for studies aimed at unravelling the molecular mechanisms of antagonist–AT1-receptor interactions. Georges Vauquelin* Frederik Fierens Ilse Verheijen Patrick Vanderheyden Dept of Molecular and Biochemical Pharmacology, Institute for Molecular Biology and Biotechnology, Free University of Brussels (VUB), Paardenstraat 65, B-1640 Sint-Genesius Rode, Belgium. *e-mail: [email protected] References 1 Vauquelin, G. et al. (2001) Insurmountable AT1 receptor antagonism: the need for different antagonist binding states of the receptor. Trends Pharmacol. Sci. 22, 343–344 2 Vanderheyden, P.M.L. et al. (1999) Distinction between surmountable and insurmountable selective AT1 receptor antagonists by use of CHOK1 cells expressing human angiotensin II AT1 receptors. Br. J. Pharmacol. 126, 1057–1065 3 Fierens, F.L.P. et al. (1999) Binding of the antagonist [3H]candesartan to angiotensin II AT1 receptor-transfected Chinese hamster ovary cells. Eur. J. Pharmacol. 367, 413–422
Is St John’s wort a ‘Prozac-like’ herbal antidepressant? In their review on St John’s wort, Di Carlo et al.1 discussed the experimental and clinical pharmacology related to the antidepressant action of this plant. Their assertion that ‘the actions on neurotransmitter uptake by hyperforin [a biologically active constituent of St John’s wort] represent the main mechanism by which St John’s wort exerts
0165-6147/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
responses are transient or self-limiting. Thus, we proposed that the mechanism of insurmountable antagonism by slowly dissociating AT1 receptor antagonists relates to their ability to reduce the rate of agonist association with the receptors, and thus to prevent the transient response peak. Because many different responses to AT1 receptor activation have transient time-courses, these models are probably well founded. We note that agonist response transience and two-stage antagonist binding are not mutually exclusive and so it is possible that both contribute to the insurmountability of AT1 receptor antagonism. However, a two-step antagonist binding process is not ‘needed’ to explain insurmountable AT1 receptor antagonism. Vauquelin et al. consider that response transience merely complicates ‘what is really occurring between, for example, the AT1 receptor antagonists and their receptor’ and claim that the use of the cumulated response index of inositol phosphate accumulation, can ‘marginalize’ such complications1. The lack of a clear transient in the accumulated inositol phosphate concentrations does not mean that response transience is not occurring. The transient nature of inositol phosphate production has been reported in other systems, and for the accumulated inositol phosphates to reach a steady level the underlying production rate must be transient. Furthermore, because a slow antagonist will always limit the onset of responses to a faster agonist, cumulated responses will always display insurmountable antagonism by slow antagonists even where the response is non-transient and the system reaches full equilibrium. Kenakin7 has discussed the shortcoming of cumulative response indices and their tendency to obscure response time-courses. Thus, far from protecting from unwanted complications, a cumulated response might introduce such complications. Finally, it has to be noted that because all of these models are inevitably much simpler than reality there is no point in attempting to compare the models’ goodness of fit to real data, even if more of the parameters had experimentally determined values. It is our view that the purpose of these types of models in pharmacology is to help inform experimental design and not to prove the validity of hypotheses. http://tips.trends.com
TRENDS in Pharmacological Sciences Vol.22 No.11 November 2001
Michael Lew* Arthur Christopoulos James Ziogas Dept of Pharmacology, University of Melbourne, Victoria 3010, Australia. *e-mail: [email protected] References 1 Vauquelin, G. et al. (2001) Insurmountable AT1 receptor antagonism: the need for different antagonist binding states of the receptor. Trends Pharmacol. Sci. 22, 343–344 2 Lew, M.J. et al. (2000) Dynamic mechanisms of non-classical antagonism by competitive AT1 receptor antagonists. Trends Pharmacol. Sci. 21, 376–381 3 Vauquelin, G. et al. (2001) A two-state receptor model for the interaction between angiotensin II type 1 receptors and non-peptide antagonists. Biochem. Pharmacol. 61, 277–284 4 Rang, H.P. (1966) The kinetics of action of acetylcholine antagonists in smooth muscle. Proc. R. Soc. London B Biol. Sci. 164, 488–510 5 Hall, D. and Parsons, S. (2001) Nonsurmountable antagonism: a general drawback of pre-steady-state measurement? Trends Pharmacol. Sci. 22, 63–65 6 Lew, M.J. et al. (2001) Non-surmountable antagonism: transcending steady state. Trends Pharmacol. Sci. 22, 65–66 7 Kenakin, T.P. (1997) Differences between natural and recombinant G protein-coupled receptor systems with varying receptor/G protein stoichiometry. Trends Pharmcol. Sci. 18, 456–464
Insurmountable AT1 receptor antagonism: fitting models with experimental data Response from Vauquelin et al.
It is of theoretical and potential clinical importance to determine whether antagonist-bound angiotensin AT1 receptors adopt only a single, slow dissociating state (Models 1 and 2)1 or whether they might adopt multiple binding states (two binding states as in Model 3)1. In this respect, Lew and colleagues show without dispute that single-state models also fit with several experimental data {association and dissociation rate of [3H]irbesartan and irbesartan versus angiotensin II inhibition curve in CHO-hAT1 cells (Chinese hamster ovary cells permanently transfected with the gene encoding the human AT1 receptor)} if they are appended by a non-linear (hyperbolic) stimulus–response function. These new findings illustrate the utility of kinetic
557
rather than equilibrium models and also highlight the potential contribution of a ‘receptor reserve’ to the ‘insurmountable behaviour’ of antagonists in general. However, published kinetic experiments comparing the maximal angiotensin-IImediated inositol phosphate production with the cell-surface binding of [3H]angiotensin II (Ref. 2) and the antagonist [3H]candesartan3 stipulate a linear stimulus–response function in CHO-hAT1 cells. These data support the two-state model but are not compatible with the modified single-state models because they rely on a hyperbolic stimulus–response function. A multiple binding state concept might therefore provide interesting guidance for studies aimed at unravelling the molecular mechanisms of antagonist–AT1-receptor interactions. Georges Vauquelin* Frederik Fierens Ilse Verheijen Patrick Vanderheyden Dept of Molecular and Biochemical Pharmacology, Institute for Molecular Biology and Biotechnology, Free University of Brussels (VUB), Paardenstraat 65, B-1640 Sint-Genesius Rode, Belgium. *e-mail: [email protected] References 1 Vauquelin, G. et al. (2001) Insurmountable AT1 receptor antagonism: the need for different antagonist binding states of the receptor. Trends Pharmacol. Sci. 22, 343–344 2 Vanderheyden, P.M.L. et al. (1999) Distinction between surmountable and insurmountable selective AT1 receptor antagonists by use of CHOK1 cells expressing human angiotensin II AT1 receptors. Br. J. Pharmacol. 126, 1057–1065 3 Fierens, F.L.P. et al. (1999) Binding of the antagonist [3H]candesartan to angiotensin II AT1 receptor-transfected Chinese hamster ovary cells. Eur. J. Pharmacol. 367, 413–422
Is St John’s wort a ‘Prozac-like’ herbal antidepressant? In their review on St John’s wort, Di Carlo et al.1 discussed the experimental and clinical pharmacology related to the antidepressant action of this plant. Their assertion that ‘the actions on neurotransmitter uptake by hyperforin [a biologically active constituent of St John’s wort] represent the main mechanism by which St John’s wort exerts
0165-6147/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.