Abstract 21546: Protein Phosphatase 1 Regulates Normal Automaticity of the Heart's Pacemaker Node Cells by Site-specific Modulation of Phospholamban Phosphorylation that Regulates Spontaneous Subsarcolemmal Local Ca2+ Releases

The heart's rhythm is driven by spontaneous action potential (AP) firing of sinoatrial node cells (SANC), in which cAMP-mediated, PKA-dependent local subsarcolemmal Ca2+ releases (LCRs) play a crucial role. SANC sarcomplasmic reticulum (SR) has been referred to as an intracellular ”Ca2+ clock” because it generates spontaneous periodic submembrane Ca2+ oscillations controlled by phosphorylation of phospholamban (PLB). Protein kinase A (PKA) and the Ca2+/calmodulin-dependent protein kinase II (CAMKII) regulate PLB by phosphorylation at Ser16 and Thr17 sites. We aimed to determine how protein phosphatase (PP) activity is involved in regulation of PLB phosphorylation, LCR generation and spontaneous rhythmic AP firing of SANC. In freshly isolated rabbit SANC, the PP1/2A inhibitor calyculin A (CyA), 100–500 nM, increased the spontaneous SANC beating rate, recorded using perforated patch, by 24–30%, from 179.4±13.1 to 230.6±7.45 beat/min (n=21). Confocal Ca2+ imaging of isolated SANC demonstrated that PP inhibit...