The detection of the non-M2 muscarinic receptor subtype in the rat heart atria and ventricles

• Published in: Naunyn-Schmiedeberg's archives of pharmacology 2008-07, Vol.378 (1), p.103-116
• Main Authors: Myslivecek, Jaromir, Klein, Martin, Novakova, Martina, Ricny, Jan
• Format: Article
• Language: English
• Subjects: Animals; Binding, Competitive; Biomedical and Life Sciences; Biomedicine; Gene Expression; Heart Atria - drug effects; Heart Atria - metabolism; Heart Ventricles - drug effects; Heart Ventricles - metabolism; Male; Neurosciences; Original Article; Pharmacology/Toxicology; Rats; Rats, Wistar; Receptor, Muscarinic M1 - drug effects; Receptor, Muscarinic M1 - metabolism; Receptor, Muscarinic M2 - drug effects; Receptor, Muscarinic M2 - metabolism; Receptor, Muscarinic M3 - drug effects; Receptor, Muscarinic M3 - metabolism; Receptor, Muscarinic M5 - drug effects; Receptor, Muscarinic M5 - metabolism; Receptors, Muscarinic - drug effects; Receptors, Muscarinic - metabolism; Type C Phospholipases - metabolism
• ISSN: 0028-1298; 1432-1912
• DOI: 10.1007/s00210-008-0285-8

Mammal heart tissue has long been assumed to be the exclusive domain of the M 2 subtype of muscarinic receptor, but data supporting the presence of other subtypes also exist. We have tested the hypothesis that muscarinic receptors other than the M 2 subtype are present in the heart as minor populations. We used several approaches: a set of competition binding experiments with pirenzepine, AFDX-116, 4-DAMP, PD 102807, p-F-HHSiD, AQ-RA 741, DAU 5884, methoctramine and tripinamide, blockage of M 1 muscarinic receptors using MT7 toxin, subtype-specific immunoprecipitation experiments and determination of phospholipase C activity. We also attempted to block M 1 –M 4 receptors using co-treatment with MT7 and AQ-RA 741. Our results show that only the M 2 subtype is present in the atria. In the ventricles, however, we were able to determine that 20% (on average) of the muscarinic receptors were subtypes other than M 2 , with the majority of these belonging to the M 1 subtype. We were also able to detect a marginal fraction (6 ± 2%) of receptors that, based on other findings, belong mainly to the M 5 muscarinic receptors. Co-treatment with MT7 and AQ-RA 741 was not a suitable tool for blocking of M 1 –M 4 receptors and can not therefore be used as a method for M 5 muscarinic receptor detection in substitution to crude venom. These results provide further evidence of the expression of the M 1 muscarinic receptor subtype in the rat heart and also show that the heart contains at least one other, albeit minor, muscarinic receptor population, which most likely belongs to the M 5 muscarinic receptors but not to that of the M 3 receptors.