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

摘要

The potential functional roles of M3 muscarinic receptors in mouse atria were examined by pharmacological and molecular biological techniques, using wild-type mice, muscarinic M2 or M3 receptor single knockout (M2KO, M3KO), and M2 and M3 muscarinic receptor double knockout mice (M2/M3KO). Real-time quantitative reverse transcriptase-polymerase chain reaction analysis showed that the M2 receptor mRNA was expressed predominantly in mouse atria but that the M1, M3, M4, and M5 receptor subtypes were also expressed at low levels. Carbachol (10 nM–30 μM) decreased the spontaneous beating frequency of right atria isolated from wild-type mice. Studies with subtype-preferring antagonists and atria from M2KO mice confirmed that this activity is mediated by the M2 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 M₃KO mice, the transient negative inotropic action of carbachol changed to a sustained negative inotropic action. In contrast, in atria from M₂KO mice, carbachol showed only positive inotropic activity. In atria from M₂/M₃ double KO mice, carbachol was devoid of any inotropic activity. These observations, complemented by functional studies with subtype-preferring antagonists, convincingly demonstrate that atrial M₃ muscarinic receptors mediate positive inotropic effects in mouse atria. Physiologically, this activity may serve to dampen the inhibitory effects of M₂ receptor activation on atrial contractility.