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Methacholine-induced decrease in the cholinergic muscarinic receptor content in the perfused working rat heart.

R R Reinhardt, R Roskoski

    The Journal of Pharmacology and Experimental Therapeutics
    |July 1, 1983
    PubMed
    Summary
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    Methacholine reduces muscarinic receptor sites in rat hearts, but this effect is reversible and requires receptor activation. This finding highlights the dynamic regulation of muscarinic receptors in a physiological setting.

    Area of Science:

    • Pharmacology
    • Cardiovascular Physiology
    • Receptor Biology

    Background:

    • Muscarinic receptors play a crucial role in regulating cardiac function.
    • Understanding receptor regulation is key to developing targeted therapies.
    • Agonist-induced changes in receptor number (downregulation) are a significant aspect of receptor biology.

    Purpose of the Study:

    • To investigate the effect of the muscarinic receptor agonist methacholine on receptor binding sites in a perfused working rat heart model.
    • To determine if methacholine causes a decrease in muscarinic receptor number and to characterize this change.
    • To explore the reversibility and requirements for agonist-induced muscarinic receptor regulation.

    Main Methods:

    • Perfusion of working rat hearts with methacholine (a muscarinic agonist) or vehicle control.

    Related Experiment Videos

  • Quantification of muscarinic receptor binding sites using (-)-[3H]-quinuclidinyl benzilate.
  • Scatchard analysis to assess changes in receptor affinity (dissociation constant).
  • Experiments involving co-perfusion with a muscarinic antagonist ((-)-scopolamine) and post-agonist washout periods.

    • Main Results:

    • Perfusion with methacholine significantly reduced muscarinic receptor binding sites (10-18%) in atrial and ventricular tissues.
    • Scatchard analysis showed no change in the apparent dissociation constant, indicating a reduction in receptor number, not affinity.
    • Co-perfusion with (-)-scopolamine prevented the methacholine-induced loss of receptors.
    • Receptor binding sites returned to control levels after a 2.5-hour washout period following methacholine perfusion.

    Conclusions:

    • Agonist-induced downregulation of muscarinic receptors requires active receptor stimulation, not just occupancy.
    • The observed decrease in muscarinic receptor number is a reversible phenomenon.
    • The perfused working rat heart is a suitable physiological model for studying muscarinic receptor regulation mechanisms.