M3 Muscarinic Receptors Mediate Positive Inotropic Responses in Mouse Atria: a Study with Muscarinic Receptor Knockout Mice

The potential functional roles of M(3) muscarinic receptors in mouse atria were examined by pharmacological and molecular biological techniques, using wild-type mice, muscarinic M(2) or M(3) receptor single knockout (M(2)KO, M(3)KO), and M(2) and M(3) muscarinic receptor double knockout mice (M(2)/M(3)KO). Real-time quantitative reverse transcriptase-polymerase chain reaction analysis showed that the M(2) receptor mRNA was expressed predominantly in mouse atria but that the M(1), M(3), M(4), and M(5) receptor subtypes were also expressed at low levels. Carbachol (10 nM-30 microM) decreased the spontaneous beating frequency of right atria isolated from wild-type mice. Studies with subtype-preferring antagonists and atria from M(2)KO mice confirmed that this activity is mediated by the M(2) receptor subtype. In left atria from wild-type mice, carbachol decreased the amplitude of electrical field stimulation-evoked contractions (negative inotropic action), but this inhibition was transient and was followed by a gradual increase in contraction amplitude (positive inotropic response). In atria from M(3)KO mice, the transient negative inotropic action of carbachol changed to a sustained negative inotropic action. In contrast, in atria from M(2)KO mice, carbachol showed only positive inotropic activity. In atria from M(2)/M(3) double KO mice, carbachol was devoid of any inotropic activity. These observations, complemented by functional studies with subtype-preferring antagonists, convincingly demonstrate that atrial M(3) muscarinic receptors mediate positive inotropic effects in mouse atria. Physiologically, this activity may serve to dampen the inhibitory effects of M(2) receptor activation on atrial contractility.