Cholinergic regulation of heart rate: Functional importance of l-type cav1.3 channels

Abstract

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 IKACh), while αi-subunits inhibit adenylate cyclase (AC) activity. AC inhibition reduces the intracellular concentration of cAMP, decreasing the activity of several ion channels involved in pacemaking, including “funny” f-(HCN4) and L-type Cav1.3 calcium channels. To determine the importance of Cav1.3, GIRK1/4 and HCN4 in cholinergic regulation of HR. We recorded the HR of isolated Langendorff perfused hearts and the pacemaker leading site in ex vivo sinus nodes of mice in control or during ACh perfusion. We used control wild-type (WT) mice, and five genetically modified mouse models: Cav1.3KO (no Cav1.3-mediated L-type current), Girk4KO (no IKACh), HCN4-CNBD (no cAMP-dependent regulation of HCN4), Girk4KO/HCN4-CNBD and Girk4KO/Cav1.3KO. Perfusion of 1 μM ACh significantly reduced HR in hearts from control (18%), Girk4KO (12%) and HCN4-CNBD (23%) animals, but not in hearts from in Cav1.3KO, Girk4KO/HCN4-CNBD and Girk4KO/Cav1.3 mice. 3 μM ACh significantly reduced HR in all the strains tested except for Girk4KO/Cav1.3KO animals. Preliminary results obtained from optical mapping experiments show ACh-induced shift of pacemaker leading site in WT and HCN4-CNBD mice. Girk4KO/Cav1.3KO mice show a larger shift of the leading pacemaker site than WT and HCN4-CNBD mice, to regions outside the sinus node area. Our data indicate that Cav1.3 channels play an important role in the cholinergic regulation of heart rate. Indeed, concomitant ablation of GIRK1/4 and Cav1.3 channels nearly abolished regulation in isolated murine hearts. Preliminary optical mapping results may indicate that this concomitant ablation increases pacemaker shift in cholinergic stimulation.