Defining the ionic basis of the fight or flight response of heart rate – IFOR

The increase in heart rate induced by catecholamines on sinoatrial node (SAN) pacemaker activity underlies the “fight-or-flight” response (FFR) of heart rate, which occurs in response to increase in animal’s physical activity or under a stressful situation. The capability of the SAN to generate the FFR of heart rate declines with ageing and may degenerate into bradycardia necessitating the implantation of an electronic pacemaker. SAN pacemaking is generated by an interplay between the activity of ion channels and intracellular diastolic calcium release from the sarcoplasmic reticulum, mediated by type 2 ryanodine receptors (RyR2). During FFR, catecholamines bind to beta-adrenergic receptors activating the cAMP/protein kinase A (PKA) signalling pathway, leading to phosphorylation of ion channels and proteins underlying calcium handling to accelerate pacemaker activity. However, attempts to abolish the SAN FFR by manipulation of ion channels or RyR2 have so far failed. Thus, within the “state of the heart” of the field, we still do not know which ion channel(s) isoforms constitute critical downstream effectors of cAMP/PKA signalling pathway to generate the SAN FFR. This gap of knowledge hampers the development of new pharmacologic tools to reduce the need of pacemaker implantation in the ageing population. Among ion channels potentially involved in SAN FFR, besides hyperpolarization-activated channels “funny” f-(HCN4) channels, SAN pacemaker cells (SANCs) differs from that of working cardiomyocytes by expression of two different voltage gated L-type Ca2+ channels. Indeed, while adult working myocardium expresses only L-type Cav1.2 channels, SANCs express also L-type Cav1.3 channels. Recently, consortium partners have collected ground-breaking data showing that concomitant loss of function of cAMP-dependent regulation of f-(HCN4) and of L-type Cav1.3 channels prevents SAN FFR, suggesting that RyR2 act downstream to Cav1.3 in the generation of SAN FFR. In this research programme we will employ a unique set of genetically-modified mice and rabbits in which the contribution of Cav1.3 (mouse), HCN4 (mouse) and RyR2 (mouse and rabbit) to the SAN FFR has been modified or abolished. The SAN FFR of these lines will be functionally explored in vivo and in vitro by employing telemetric recording of ECG, optical mapping of the SAN impulse, electrophysiology of SAN cells and imaging of RyR2 mediated Ca2+ release. We will pursue the following highly-collaborative aims: (i) To elucidate the mechanisms underlying the roles of Cav1.3 and HCN4 in the generation of SAN FFR. (ii) To understand the role of PKA-dependent regulation of L-type Cav1.3 channel activity in the SAN FFR. (iii) To understand how PKA-dependent phosphorylation of RyR2 contributes to the SAN FFR. (iv) To investigate functional regulation by the cAMP/PKA pathway of RyR2-dependent Ca2+ release by Cav1.3 during SAN FFR. (v) To transfer our findings on the role of HCN4 and Cav1.3 to a larger animal model of SAN FFR using a rabbit model of RyR2 mediated primary SAN bradycardia and sudden death. This project will elucidate the critical steps acting downstream to activation of beta-adrenergic receptors involving ion channels and RyR2 to the generation of the SAN FFR.