Résumé
Introduction : The cholinergic regulation of heart rate (HR) is mediated by acetylcholine (ACh)-dependent activation of M2-receptors (M2R). Activated M2R promote release of the βγ-subunit of G-proteins to directly gate GIRK1/4 channels (underlying the cardiac IKACh current), while αi-subunits inhibit adenylate cyclase (AC) activity. AC inhibition reduces the intracellular concentration of cAMP, decreasing the activity of ion channels involved in pacemaking, including “funny” f-(HCN4) and L-type Cav1.3 calcium channels. Purpose : To determine the importance of L-type Cav1.3 channels in the cholinergic regulation of heart rate. Methods : We recorded the frequency and the position of the pacemaker leading site in ex vivo sinus nodes and the HR of isolated Langendorff perfused hearts of mice in control or during ACh perfusion. We used control wild type (WT) mice, and five genetically modified mouse models: Cav1.3 knockout (KO, ablated Cav1.3-mediated L-type current), GIRK4KO (ablated IKACh current), HCN4-CNBD (selective deletion of cAMP-dependent regulation of HCN4), GIRK4KO/HCN4-CNBD and GIRK4KO/Cav1.3KO. Results : Data from optical mapping experiments showed that, under basal conditions, perfusion of 3 μM ACh significantly reduced the frequency of action potentials in WT (44%), HCN4-CNBD (38%), Cav1.3KO (65%) and GIRK4KO (8%) isolated mouse sinus node tissues. ACh application did not significantly affect the frequency of action potentials recorded in tissue from GIRK4KO/HCN4-CNBD and GIRK4KO/Cav1.3KO animals. Furthermore, in all the sinus node tissues tested, regardless of the genotypes, ACh shifted the pacemaker leading site from its normal position by at least 0.7 mm. Upon stimulation of the β-adrenergic pathway by Isoproterenol, to reproduce conditions of accentuated antagonism, 3μM ACh reduced HR in isolated hearts from WT (43.8%), HCN4-CNBD (38.7%), Cav1.3KO (25,4%), GIRK4KO (16.9%) and GIRK4KO/HCN4-CNBD (16.4%) mice. No significant HR reduction was recorded in hearts from GIRK4KO/Cav1.3KO animals. Conclusion : Our data indicate that L-type Cav1.3 channels are involved in cholinergic regulation of heart rate in mice. In addition, when the intracellular concentration of cAMP is elevated (i.e. under conditions of accentuated antagonism), the cholinergic regulation of sinus node pacemaking is predominantly ensured by Cav1.3 and KACh channels.