Cardiac cholinergic muscarinic receptors: changes in multiple affinity forms with down-regulation

Area of Science:

• Cardiovascular Pharmacology
• Neuropharmacology
• Receptor Dynamics

Background:

• Cholinergic muscarinic receptors play a key role in regulating heart function.
• Methacholine, an acetylcholine congener, is used to study muscarinic receptor behavior.
• Muscarinic receptors exhibit complex affinity states for agonists, complicating analysis.

Purpose of the Study:

• To investigate dynamic changes in rat heart muscarinic receptors upon methacholine perfusion.
• To develop and apply a graphical and computational method for analyzing multi-state receptor binding.
• To characterize alterations in receptor affinity and proportion in different cardiac regions.

Main Methods:

• Isolated working rat heart perfusion model.
• Methacholine (4 microM) perfusion for varying durations (1-2.5 hr).
• Agonist-antagonist competition binding assays analyzed by graphical methods and nonlinear least-squares analysis.

Main Results:

• Methacholine perfusion decreased muscarinic receptor content by 10-15% in atria and ventricles.
• Receptor affinity (KD) and proportions changed in atria, with right atrium converting to a single low-affinity state.
• Left ventricle showed decreased receptor content but no change in affinity states; all changes were reversible.

Conclusions:

• Rat heart muscarinic receptors undergo dynamic, reversible changes in content and affinity in response to methacholine.
• A novel analytical method accurately quantifies multi-state receptor parameters.
• Cardiac muscarinic receptor regulation is region-specific and sensitive to cholinergic stimulation.