Catecholamine-Independent Heart Rate Increases Require CaMKII

Background —Catecholamines increase heart rate by augmenting the cAMP responsive HCN4 9pacemaker current9 ( I f ) and/or by promoting inward Na + /Ca 2+ exchanger current ( I NCX ), by a 9Ca 2+ clock mechanism in sinoatrial nodal cells (SANCs). The importance, identity and function of signals that connect I f and Ca 2+ clock mechanisms are uncertain and controversial, but the multifunctional Ca 2+ and calmodulin-dependent protein kinase II (CaMKII) is required for physiological heart rate responses to β-adrenergic receptor (β-AR) stimulation. The aim of this stuy is to measure the contribution of the Ca 2+ clock and CaMKII to cardiac pacing independent of β-AR agonist stimulation. Methods and Results —We used the L-type Ca 2+ channel agonist BayK 8644 (BayK) to activate the SANC Ca 2+ clock. BayK and isoproterenol were similarly effective in increasing rates in SANCs and Langendorff-perfused hearts from WT 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 I Ca equally in all SANCs, indicating that increasing I Ca was insufficient and suggesting CaMKII activation was required for heart rate increases by BayK. BayK did not increase I f or protein kinase A (PKA)-dependent phosphorylation of phospholamban (at Ser16), indicating that increased SANC Ca 2+ by BayK did not augment cAMP/PKA signaling at these targets. Late diastolic intracellular Ca 2+ release and I NCX were significantly reduced in AC3-I SANCs and the response to BayK was eliminated by ryanodine in all groups. Conclusions —The Ca 2+ clock is capable of supporting physiological fight or flight responses, independent of β-AR stimulation or I f increases. Complete Ca 2+ clock and β-AR stimulation responses require CaMKII.