Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) due to gradual change of the membrane potential called diastolic depolarization (DD). Spontaneous, submembrane local Ca2+ releases (LCR) from ryanodine receptors (RyR) occur during late DD and activate an inward Na+/Ca2+ exchange current to boost the DD rate and fire an action potential (AP). Here we studied the extent of basal CaMKII activation and role of basal CaMKII-dependent protein phosphorylation in generation of LCRs and regulation of normal automaticity of intact rabbit SANC. The basal level of activated (autophosphorylated) CaMKII in rabbit SANC surpassed that in ventricular myocytes (VM) by~2-fold and this was accompanied by high basal level of protein phosphorylation. Specifically, phosphorylation of phospholamban (PLB) at CaMKII-dependent Thr17 site was ~3-fold greater in SANC compared to VM, and RyR phosphorylation at CaMKII-dependent Ser2815 site was ~10-fold greater in the SA node, compared to ventricle. CaMKII inhibition reduced phosphorylation of PLB and RyR, decreased LCR size, increased LCR periods (time from AP-induced Ca2+ transient to subsequent LCR) and suppressed spontaneous SANC firing. Graded changes in CaMKII-dependent phosphorylation (indexed by PLB phosphorylation at Thr17site) produced by CaMKII inhibition, β-AR stimulation or phosphodiesterase inhibition were highly correlated with changes in SR Ca2+ replenishment times, LCR periods and concomitant changes in spontaneous SANC cycle lengths (R2=0.96). Thus, high basal CaMKII activation modifies phosphorylation state of Ca2+ cycling proteins PLB, RyR, L-type Ca2+ channels (and likely others), adjusting LCR period and characteristics, and ultimately regulates both normal and reserve cardiac pacemaker function.
- Sinoatrial node cells
- CaMKII-dependent phosphorylation
- ryanodine receptors
- sarcoplasmic reticulum Ca2+-ATPase
- Copyright © 2015, American Journal of Physiology - Heart and Circulatory Physiology