Abstract 15015: Spontaneous Beating of Cardiac Pacemaker Cell Requires Basal Epac1 Activation
Spontaneous firing of sinoatrial node cells (SANC) is regulated by sarcoplasmic reticulum (SR) generated local subsarcolemmal Ca2+ releases (LCRs), which appear during diastolic depolarization (DD) and activate an inward Na+-Ca2+exchange current to increase DD rate and spontaneous SANC beating rate. Recent studies implicated activation of cAMP-binding protein Epac1 in the regulation of Ca2+ handling in ventricular myocytes (VM) via Epac1-PLC-CaMKII-dependent pathway. Compared to VM rabbit SANC have high basal level of cAMP, suggesting possible activation of Epac1 in the basal state. However, a role of Epac1 activation in modulation of SR Ca2+ cycling and spontaneous beating of SANC has not been evaluated. Here we studied impact of Epac1 inhibition or activation on spontaneous beating of rabbit SANC and possible mechanisms of Epac1-dependent regulation of cardiac pacemaker function.
Although the expression of Epac1 (RNA-sequencing) in rabbit SANC was 2-fold less, than in VM, a selective Epac1 inhibitor CE3F4 markedly decreased spontaneous SANC beating rate. Specifically, CE3F4 decreased LCR size and number per each spontaneous cycle (confocal microscopy, Ca2+ indicator Fluo-3), prolonged the LCR period (the interval between AP-induced Ca2+ transient and subsequent LCR) and increased spontaneous cycle length, suggesting elevated basal Epac1 activation in SANC. In contrast, Epac activation by 8-pCPT-2’-O-Me-cAMP-AM markedly augmented LCR size and number per each cycle, decreased the LCR period and increased spontaneous SANC firing rate. High basal Epac1 activation would sequentially activate downstream targets, i.e. PLC and CaMKII. Indeed, basal level of activated (autophosphorylated) CaMKII in rabbit SANC surpassed that in VM by ~2-fold and this was accompanied by high basal level of CaMKII-dependent phosphorylation of both PLB at Thr17 site and RyR at Ser2815 site. Basal PLC activation was also elevated in SANC, since inhibition of PLC by U-73122 suppressed LCRs and stopped spontaneous SANC firing. Thus, basal Epac1 activation is required for basal cardiac pacemaker function, and likely employs, similar to VM, Epac1-PLC-CaMKII-dependent pathway to regulate SR Ca2+ cycling, LCR characteristics and normal automaticity of SANC.
Author Disclosures: T.M. Vinogradova: None. D.R. Riordon: None. K.V. Tarasov: None. E.G. Lakatta: None.
- © 2016 by American Heart Association, Inc.