Catecholamine-Independent Heart Rate Increases Require Ca ²⁺ /Calmodulin-Dependent Protein Kinase II

Abstract

Background Catecholamines increase heart rate by augmenting the cAMP responsive HCN4 ‘pacemaker current’ (If) and/or by promoting inward Na+/Ca2+ exchanger current (INCX), by a ‘Ca2+ clock’ mechanism in sinoatrial nodal cells (SANCs). The importance, identity and function of signals that connect If and Ca2+ clock mechanisms are uncertain and controversial, but the multifunctional Ca2+ 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 Ca2+ clock and CaMKII to cardiac pacing independent of β-AR agonist stimulation. Methods and Results We used the L-type Ca2+ channel agonist BayK 8644 (BayK) to activate the SANC Ca2+ 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 ICa equally in all SANCs, indicating that increasing ICa was insufficient and suggesting CaMKII activation was required for heart rate increases by BayK. BayK did not increase If or protein kinase A (PKA)-dependent phosphorylation of phospholamban (at Ser16), indicating that increased SANC Ca2+ by BayK did not augment cAMP/PKA signaling at these targets. Late diastolic intracellular Ca2+ release and INCX were significantly reduced in AC3-I SANCs and the response to BayK was eliminated by ryanodine in all groups. Conclusions The Ca2+ clock is capable of supporting physiological fight or flight responses, independent of β-AR stimulation or If increases. Complete Ca2+ clock and β-AR stimulation responses require CaMKII.

Calmodulin-Dependent Protein Kinase II: Linking Heart Failure and Arrhythmias

“…BayK 8644 increased heart rate in a dose-dependent fashion over a similar dynamic range as isoproterenol, but without increasing I f , suggesting that Ca 2+ -dependent mechanisms were fully capable of generating physiological fight or flight heart rate responses. SAN cells and isolated hearts from mice with genetic CaMKII inhibition were refractory to heart rate increases and had reduced late diastolic intracellular calcium release and I NCX when treated with BayK 8644 compared to WT controls, even though Bay K increased I Ca equally in all SAN cells 34 . These findings suggested that CaMKII contributes to heart rate by a series of events that promote SR Ca 2+ release and I NCX in SAN cells.…”

“…Excess Ca 2+ in diastole increases the likelihood for DADs due to the electrogenic action of NCX1. Interestingly, the pathological actions of CaMKII to favor DADs in atrial 114 and ventricular 74 myocardium mirror the physiological role of CaMKII to promote RyR2 Ca 2+ leak and increased automaticity in SA nodal pacemaker cells 33,34 . While the role of CaMKII in RYR dysfunction in heart failure is controversial and in early studies mice with genetically altered RyR, resistant to CaMKII phosphorylation, are not protected from heart failure 32 , pathological effects of CaMKII were recently shown in knock-in mice with RyR genetically modified to mimic RyR phosphorylation by CaMKII (Ser 2814 Asp) 115 .…”

“…The presence of CaMKII at Ca V 1.2 (the predominant voltage-gated Ca 2+ channel in ventricular muscle) and at RyR2 may serve to coordinate and promote physiological activation of these ECC proteins 31 to increase the force-frequency relationship (i.e. the tendency of myocardium to generate more tension at faster stimulation rates) 32 and/or to engage ‘fight or flight’ acceleration of heart rate 33,34 . CaMKII is now recognized to bind to multiple voltage-gated ion channels (Ca 2+ , Na + and K + ) 35 , some of which will be discussed later in the context of disease.…”

“…Mode 2 gating appears to underlie I Ca facilitation, a dynamic pattern of increased peak I Ca and slowed I Ca inactivation, based on modeling studies 33,34,69 and because mutations that prevent mode 2 gating also reduce I Ca facilitation 35,39 . Mode 2 gating 7,70 and I Ca facilitation are hypothesized to be proarrhythmic by favoring early afterdepolarizations (EADs) 29,29,36,39,71 and SR Ca 2+ leak 37,72 , inward Na + /Ca 2+ exchanger current 38,73 and delayed afterdepolarizations (DADs) 39,74 .…”

Inositol 1,4,5-trisphosphate receptors and pacemaker rhythms

“…For example, IP 3 -activated IP 3 R1 may directly or indirectly interact with canonical transient receptor potential channel (TRPC) channels in smooth muscle [73]. (v) Ca 2+ release from IP 3 R could activate other Ca 2+ dependent cellular mechanism or signaling pathways such as Ca 2+ -activated isoforms of adenylyl cyclase [74], Ca 2+ /calmodulin-dependent protein kinase II, both of which have been shown to play an important role in regulation of heart rate, and they are highly dependent on intracellular Ca 2+ [75].…”