Ca2+-Dependent Phosphorylation of Ca2+ Cycling Proteins Generates Robust Rhythmic Local Ca2+ Releases in Cardiac Pacemaker Cells

The spontaneous beating of the heart is governed by spontaneous firing of sinoatrial node cells, which generate action potentials due to spontaneous depolarization of the membrane potential, or diastolic depolarization. The spontaneous diastolic depolarization rate is determined by spontaneous local submembrane Ca2+ releases through ryanodine receptors (RyRs). We sought to identify specific mechanisms of intrinsic Ca2+ cycling by which sinoatrial node cells, but not ventricular myocytes, generate robust, rhythmic local Ca2+ releases. At similar physiological intracellular Ca2+ concentrations, local Ca2+ releases were large and rhythmic in permeabilized sinoatrial node cells but small and random in permeabilized ventricular myocytes. Furthermore, sinoatrial node cells spontaneously released more Ca2+ from the sarcoplasmic reticulum than did ventricular myocytes, despite comparable sarcoplasmic reticulum Ca2+ content in both cell types. This ability of sinoatrial node cells to generate larger and rhythmic local Ca2+ releases was associated with increased abundance of sarcoplasmic reticulum Ca2+-ATPase (SERCA), reduced abundance of the SERCA inhibitor phospholamban, and increased Ca2+-dependent phosphorylation of phospholamban and RyR. The increased phosphorylation of RyR in sinoatrial node cells may facilitate Ca2+ release from the sarcoplasmic reticulum, whereas Ca2+-dependent increase in phosphorylation of phospholamban relieves its inhibition of SERCA, augmenting the pumping rate of Ca2+ required to support robust, rhythmic local Ca2+ releases. The differences in Ca2+ cycling between sinoatrial node cells and ventricular myocytes provide insights into the regulation of intracellular Ca2+ cycling that drives the automaticity of sinoatrial node cells.