A full range of mouse sinoatrial node AP firing rates requires protein kinase A-dependent calcium signaling

Recent perspectives on sinoatrial nodal cell (SANC) ^* function indicate that spontaneous sarcoplasmic reticulum (SR) Ca ²⁺ cycling, i.e. an intracellular "Ca ²⁺ clock," driven by cAMP-mediated, PKA-dependent phosphorylation, interacts with an ensemble of surface membrane electrogenic molecules ("surface membrane clock") to drive SANC normal automaticity. The role of AC-cAMP-PKA-Ca ²⁺ signaling cascade in mouse, the species most often utilized for genetic manipulations, however, has not been systematically tested. Here we show that Ca ²⁺ cycling proteins (e.g. RyR2, NCX1, and SERCA2) are abundantly expressed in mouse SAN and that spontaneous, rhythmic SR generated local Ca ²⁺ releases (LCRs) occur in skinned mouse SANC, clamped at constant physiologic [Ca ²⁺]. Mouse SANC also exhibits a high basal level of phospholamban (PLB) phosphorylation at the PKA-dependent site, Serine16. Inhibition of intrinsic PKA activity or inhibition of PDE in SANC, respectively: reduces or increases PLB phosphorylation, and markedly prolongs or reduces the LCR period; and markedly reduces or accelerates SAN spontaneous firing rate. Additionally, the increase in AP firing rate by PKA-dependent phosphorylation by β-adrenergic receptor (β-AR) stimulation requires normal intracellular Ca ²⁺ cycling, because the β-AR chronotropic effect is markedly blunted when SR Ca ²⁺ cycling is disrupted. Thus, AC-cAMP-PKA-Ca ²⁺ signaling cascade is a major mechanism of normal automaticity in mouse SANC.