# Initial characterization of M2-muscarinic receptor overexpressing mouse heart

## TL;DR

This study shows that overexpressing M2-muscarinic receptors in mouse hearts does not enhance contractile effects but increases arrhythmias in certain atrial regions.

## Contribution

The novelty is the generation and characterization of a transgenic mouse model with heart-specific overexpression of the human M2-muscarinic receptor.

## Key findings

- Overexpression of M2-muscarinic receptors did not alter baseline contractile function or histology in transgenic mice.
- Carbachol-induced arrhythmias were more frequent in right atria from transgenic mice compared to wild-type controls.
- M2-muscarinic receptor overexpression had no effect on force or beating rate in left atria.

## Abstract

There are five muscarinic receptor subtypes expressed in the human heart. The main subtype is the M2-muscarinic receptor. We hypothesized that overexpression of the M2-muscarinic receptor should augment any contractile effects that are M2-muscarinic receptor–mediated. Therefore, we generated a transgenic mouse with overexpression of the human M2-muscarinic receptor under the control of the heart-specific α-myosin heavy chain promoter (M2-TG). We performed contraction experiments with electrically stimulated (1 Hz) left atrial preparations (LA) and spontaneously beating right atrial preparations (RA) from adult M2-TG or from adult wild-type littermate mice (WT). We confirmed the expression of the human M2-muscarinic receptor in the mouse heart by reverse transcription polymerase chain reaction (RT-PCR) and radioligand binding experiments at cardiac membranes and tissue sections. We did not detect differences in hematoxylin/eosin staining or Masson/Goldner staining between M2-TG and WT. We noticed that carbachol (10 nM–10 µM cumulatively applied) alone or in the presence of 1 µM isoprenaline reduced the force of contraction (FOC) to a similar extent in LA from M2-TG and WT. The beating rate in RA was similarly decreased by carbachol alone or by carbachol in the presence of 1 µM isoprenaline in M2-TG and WT. Overall, the number of RA that displayed absolute arrhythmias was higher in atria from M2-TG compared to atria from WT. No arrhythmias were noted in LA from M2-TG or WT. Stimulation of human M2-muscarinic receptors induced absolute atrial arrhythmias more often in RA from M2-TG than in RA from WT. Overexpressed M2-muscarinic receptors were silent to the force and beating rate.

The online version contains supplementary material available at 10.1007/s00210-025-04502-3.