Initial characterization of M₂-muscarinic receptor overexpressing mouse heart

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.