Catecholamine-Independent Heart Rate Increases Require Ca2+/Calmodulin-Dependent Protein Kinase II

BACKGROUND—Catecholamines increase heart rate by augmenting the cAMP-responsive hyperpolarization-activated cyclic nucleotide-gated channel 4 pacemaker current (If) and by promoting inward Na/Ca exchanger current (INCX) by a “Ca clock” mechanism in sinoatrial nodal cells (SANCs). The importance, identity, and function of signals that connect If and Ca clock mechanisms are uncertain and controversial, but the multifunctional Ca/calmodulin-dependent protein kinase II (CaMKII) is required for physiological heart rate responses to β-adrenergic receptor (β-AR) stimulation. The aim of this study was to measure the contribution of the Ca clock and CaMKII to cardiac pacing independent of β-AR agonist stimulation. METHODS AND RESULTS—We used the L-type Ca channel agonist Bay K8644 (BayK) to activate the SANC Ca clock. BayK and isoproterenol were similarly effective in increasing rates in SANCs and Langendorff-perfused hearts from wild-type control mice. In contrast, SANCs and isolated hearts from mice with CaMKII inhibition by transgenic expression of an inhibitory peptide (AC3-I) were resistant to rate increases by BayK. BayK only activated CaMKII in control SANCs but increased L-type Ca current (ICa) equally in all SANCs, indicating that increasing ICa was insufficient and suggesting that CaMKII activation was required for heart rate increases by BayK. BayK did not increase If or protein kinase A-dependent phosphorylation of phospholamban (at Ser16), indicating that increased SANC Ca by BayK did not augment cAMP/protein kinase A signaling at these targets. Late-diastolic intracellular Ca release and INCX were significantly reduced in AC3-I SANCs, and the response to BayK was eliminated by ryanodine in all groups. CONCLUSIONS—The Ca clock is capable of supporting physiological fight-or-flight responses, independent of β-AR stimulation or If increases. Complete Ca clock and β-AR stimulation responses require CaMKII.