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Research ArticleArticle

The Relationship between Agonist Intrinsic Activity and the Rate of Endocytosis of Muscarinic Receptors in a Human Neuroblastoma Cell Line

Philip G. Szekeres, Jennifer A. Koenig and J. Michael Edwardson
Molecular Pharmacology April 1998, 53 (4) 759-765; DOI: https://doi.org/10.1124/mol.53.4.759
Philip G. Szekeres
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Jennifer A. Koenig
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J. Michael Edwardson
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Abstract

The molecular mechanisms underlying the internalization of G protein-coupled receptors are still poorly understood. Normally agonists but not antagonists cause internalization (defined here as a reduction in the number of receptors at the cell surface), suggesting a functional relationship between agonist activity and internalization. In this study we investigated the effects of eight muscarinic ligands on the rate constants for endocytosis and recycling of m3 muscarinic acetylcholine receptors in human SH-SY5Y neuroblastoma cells. We found that there was a linear correlation between the intrinsic activity of the ligand and its ability to increase the rate constant for endocytosis, suggesting that the same active conformation of the receptor is responsible for stimulating both second messenger generation and receptor endocytosis. In contrast, the rate constant for recycling did not depend on which agonist had triggered receptor endocytosis, suggesting that recycling is a purely constitutive process. Because receptor internalization depends on the rate constants for both endocytosis and recycling, the relationship between internalization and intrinsic activity is nonlinear. In particular, mathematical modeling of receptor trafficking revealed that under certain conditions very small (3% or less) increases in the rate constant for endocytosis are sufficient to cause substantial receptor internalization. An important implication of this analysis is that extremely weak partial agonists (which may in practice be indistinguishable from antagonists) may produce significant receptor internalization.

Footnotes

    • Received October 22, 1997.
    • Accepted December 11, 1997.
  • Send reprint requests to: Dr. J. M. Edwardson, Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QJ, UK. E-mail: jme1000{at}cus.cam.ac.uk

  • This work was supported by the Wellcome Trust.

  • The American Society for Pharmacology and Experimental Therapeutics
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Molecular Pharmacology: 53 (4)
Molecular Pharmacology
Vol. 53, Issue 4
1 Apr 1998
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Research ArticleArticle

The Relationship between Agonist Intrinsic Activity and the Rate of Endocytosis of Muscarinic Receptors in a Human Neuroblastoma Cell Line

Philip G. Szekeres, Jennifer A. Koenig and J. Michael Edwardson
Molecular Pharmacology April 1, 1998, 53 (4) 759-765; DOI: https://doi.org/10.1124/mol.53.4.759

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Research ArticleArticle

The Relationship between Agonist Intrinsic Activity and the Rate of Endocytosis of Muscarinic Receptors in a Human Neuroblastoma Cell Line

Philip G. Szekeres, Jennifer A. Koenig and J. Michael Edwardson
Molecular Pharmacology April 1, 1998, 53 (4) 759-765; DOI: https://doi.org/10.1124/mol.53.4.759
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