Ca2+/Calmodulin-Dependent Protein Kinase II (CaMKII) Activity Modulates Sinoatrial Nodal Pacemaker Cell Energetics

Abstract

In rabbit sinoatrial node cells (SANC), high basal (i.e., without β-adrenergic receptor stimulation) Ca2+ activated adenylate cyclase (AC) via cAMP/PKA-CaMKII-dependent protein phosphorylation guarantees the occurrence of rhythmic intracellular Ca2+ release from sarcoplasmic-reticulum which drives spontaneous action potentials (APs). This high-throughput signaling consumes ATP. We have previously demonstrated that basal AC-cAMP/PKA signaling directly, and Ca2+ indirectly, not only consume ATP, but also regulate mitochondrial ATP production. A role for CaMKII in regulation of SANC ATP supply has not been explored. Based upon its involvement in the aforementioned phosphorylation signaling cascade we tested the hypotheses that, basal CaMKII activity not only regulates ATP consumption, but also ATP production. We superfused single, isolated rabbit SANC or SANC suspensions with CaMKII inhibitors (KN-93 or autocamtide-2 Related Inhibitory Peptide (AIP)) and measured cytosolic Ca2+, cAMP, energetic indices and spontaneous AP firing rate. A partial reduction in basal CaMKII activity by KN-93 (0.5 μmol/L) or AIP (2 μmol/L) markedly slowed the kinetics of intracellular Ca2+ cycling and decreased the spontaneous AP firing rate. Further suppression of CaMKII activity (KN-93 3 μmol/L; AIP 10 μmol/L) to a level that eliminates all spontaneous APs, decreased the cAMP level and reduced O2 consumption and flavoprotein fluorescence. ATP was depleted,even though the ATP demand decreased. The structural inactive analog of KN-93, KN-92, neither decreased ATP demand (no effect on spontaneous AP firing rate) nor ATP turnover. Therefore,CaMKII signaling is required not only to drive normal automaticity in rabbit SANC, but is also tightly linked to SANC bioenergetics. Future studies are required to determine whether this link between CaMKII signaling and mitochondrial energetics occurs indirectly, via changes in Ca2+ release that affects activation of Ca2+-AC-cAMP/PKA signaling, or directly, via phosphorylation of mitochondrial proteins.