Initial characterization of M2-muscarinic receptor overexpressing mouse heart

Abstract

There are five muscarinic receptor subtypes expressed in the human heart. The main subtype is the M 2 -muscarinic receptor. We hypothesized that overexpression of the M 2 -muscarinic receptor should augment any contractile effects that are M 2 -muscarinic receptor–mediated. Therefore, we generated a transgenic mouse with overexpression of the human M 2 -muscarinic receptor under the control of the heart-specific α-myosin heavy chain promoter (M 2 -TG). We performed contraction experiments with electrically stimulated (1 Hz) left atrial preparations (LA) and spontaneously beating right atrial preparations (RA) from adult M 2 -TG or from adult wild-type littermate mice (WT). We confirmed the expression of the human M 2 -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 M 2 -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 M 2 -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 M 2 -TG and WT. Overall, the number of RA that displayed absolute arrhythmias was higher in atria from M 2 -TG compared to atria from WT. No arrhythmias were noted in LA from M 2 -TG or WT. Stimulation of human M 2 -muscarinic receptors induced absolute atrial arrhythmias more often in RA from M 2 -TG than in RA from WT. Overexpressed M 2 -muscarinic receptors were silent to the force and beating rate.

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