The missing link in the mystery of normal automaticity of cardiac pacemaker cells

Earlier studies of the initiating event of normal automaticity of the heart's pacemaker cells, inspired by classical quantitative membrane theory, focused upon ion currents (I_K, I_f) that determine the maximum diastolic potential and the early phase of the spontaneous diastolic depolarization (DD). These early DD events are caused by the prior action potential (AP) and essentially reflect a membrane recovery process. Events following the recovery process that ignite APs have not been recognized and remained a mystery until recently. These critical events are linked to rhythmic intracellular signals initiated by Ca²⁺ clock (i.e., sarcoplasmic reticulum [SR] cycling Ca²⁺). Sinoatrial cells, regardless of size, exhibit intense ryanodine receptor (RyR), Na⁺/Ca²⁺ exchange (NCX)-1, and SR Ca²⁺ ATPase-2 immunolabeling and dense submembrane NCX/RyR colocalization; Ca²⁺ clocks generate spontaneous stochastic but roughly periodic local subsarcolemmal Ca²⁺ releases (LCR). LCRs generate inward currents via NCX that exponentially accelerate the late DD. The timing and amplitude of LCR/I_NCX-coupled events control the timing and amplitude of the nonlinear terminal DD and therefore ultimately control the chronotropic state by determining the timing of the I_CaL activation that initiates the next AP. LCR period is precisely controlled by the kinetics of SR Ca²⁺ cycling, which, in turn, are regulated by 1) the status of protein kinase A-dependent phosphorylation of SR Ca²⁺ cycling proteins; and 2) membrane ion channels ensuring the Ca²⁺ homeostasis and therefore the Ca²⁺ available to Ca²⁺ clock. Thus, the link between early DD and next AP, missed in earlier studies, is ensured by a precisely physiologically regulated Ca²⁺ clock within pacemaker cells that integrates multiple Ca²⁺-dependent functions and rhythmically ignites APs during late DD via LCRs-I_NCX coupling.