Cholecystokinin-A Signaling Regulates Automaticity of Pacemaker Cardiomyocytes and Shortens Sinus Node Recovery Time

Aims The behavior of pacemaker cardiomyocytes (PCs) in the sinoatrial node (SAN) is modulated by neurohormonal and paracrine factors, many of which signal through G-protein coupled receptors (GPCRs). The aims of the present study are to catalog GPCRs that are differentially expressed in the mammalian SAN and to define the acute physiological consequences of activating the cholecystokinin-A signaling system in the SAN and in isolated PCs. Methods and Results Using bulk and single cell RNA sequencing datasets, we identify a set of GPCRs that are differentially expressed between SAN and right atrial tissue, including several whose roles in PCs and in the SAN have not been thoroughly characterized. Focusing on one such GPCR, Cholecystokinin-A receptor (CCKAR), we demonstrate expression of Cckar mRNA specifically in mammalian PCs, and further demonstrate that subsets of SAN fibroblasts and neurons within the cardiac intrinsic nervous system express cholecystokinin, the ligand for CCKAR. Using mouse models, we find that while baseline SAN function is not dramatically affected by loss of CCKAR, the firing rate of individual PCs is slowed by exposure to sulfated cholecystokinin-8 (sCCK-8), the high affinity ligand for CCKAR. The effect of sCCK-8 on firing rate is mediated by reduction in the rate of spontaneous phase 4 depolarization of PCs and is mitigated by activation of beta-adrenergic signaling. Finally, while perfusion of sCCK-8 onto the SAN ex-vivo reduces the beating rate modestly, exposure to sCCK-8 unexpectedly shortened the sinus node recovery time, a finding that has potential implications for treatment of SAN dysfunction. Conclusions (1) PCs express many GPCRs whose specific roles in SAN function have not been characterized, (2) the cholecystokinin-A pathway constitutes a novel signaling system that regulates SAN function and PC electrophysiology. Translational Perspective The time required for the sinoatrial node (SAN) to resume function after overdrive suppression is called the sinus node recovery time (SNRT). In the tachy-brady syndrome, pathological prolongation of the SNRT results in symptomatic sinus pauses that follow the termination of a tachyarrhythmia, leading to loss of consciousness. Tachy-brady syndrome is the most common presentation of sinus node dysfunction and can only be treated with a permanent pacemaker. In this study, we find that activation of the cholecystokinin-A signaling system can shorten the SNRT, raising the possibility of developing new pharmacological methods to treat tachy-brady syndrome. ### Competing Interest Statement V.V. received consulting fees from Merck and research funding from Amgen, neither of which are related to the research presented in this article.