Catecholamines increase heart rate by augmenting the cAMP-responsive hyperpolarization-activated cyclic nucleotide-gated channel 4 pacemaker current (I_f) and by promoting inward Na⁺/Ca²⁺ exchanger current (I_NCX) by a “Ca²⁺ clock” mechanism in sinoatrial nodal cells (SANCs). The importance, identity, and function of signals that connect I_f and Ca²⁺ clock mechanisms are uncertain and controversial, but the multifunctional Ca²⁺/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 Ca²⁺ clock and CaMKII to cardiac pacing independent of β-AR agonist stimulation.

We used the L-type Ca²⁺ channel agonist Bay K8644 (BayK) to activate the SANC Ca²⁺ 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 Ca²⁺ current (I_Ca) equally in all SANCs, indicating that increasing I_Ca was insufficient and suggesting that CaMKII activation was required for heart rate increases by BayK. BayK did not increase I_f or protein kinase A-dependent phosphorylation of phospholamban (at Ser16), indicating that increased SANC Ca²⁺ by BayK did not augment cAMP/protein kinase A signaling at these targets. Late-diastolic intracellular Ca²⁺ release and I_NCX were significantly reduced in AC3-I SANCs, and the response to BayK was eliminated by ryanodine in all groups.

The Ca²⁺ clock is capable of supporting physiological fight-or-flight responses, independent of β-AR stimulation or I_f increases. Complete Ca²⁺ clock and β-AR stimulation responses require CaMKII.