Mechanisms that match ATP supply to demand in cardiac pacemaker cells during high ATP demand

The spontaneous action potential (AP) firing rate of sinoatrial node cells (SANCs) involves high-throughput signaling via Ca2+-calmodulin activated adenylyl cyclases (AC), cAMP-mediated protein kinase A (PKA), and Ca2+/calmodulin-dependent protein kinase II (CaMKII)-dependent phosphorylation of SR Ca2+ cycling and surface membrane ion channel proteins. When the throughput of this signaling increases, e.g., in response to beta-adrenergic receptor activation, the resultant increase in spontaneous AP firing rate increases the demand for ATP. We hypothesized that an increase of ATP production to match the increased ATP demand is achieved via a direct effect of increased mitochondrial Ca2+ (Ca2+ m) and an indirect effect via enhanced Ca2+-cAMP/PKA-CaMKII signaling to mitochondria. To increase ATP demand, single isolated rabbit SANCs were superfused by physiological saline at 35 +/- 0.5 degrees C with isoproterenol, or by phosphodiesterase or protein phosphatase inhibition. We measured cytosolic and mitochondrial Ca2+ and flavoprotein fluorescence in single SANC, and we measured cAMP, ATP, and O-2 consumption in SANC suspensions. Although the increase in spontaneous AP firing rate was accompanied by an increase in O-2 consumption, the ATP level and flavoprotein fluorescence remained constant, indicating that ATP production had increased. Both Ca2+ m and cAMP increased concurrently with the increase in AP firing rate. When Ca2+ m was reduced by Ru360, the increase in spontaneous AP firing rate in response to isoproterenol was reduced by 25%. Thus, both an increase in Ca2+ m and an increase in Ca2+ activated cAMP-PKA-CaMKII signaling regulate the increase in ATP supply to meet ATP demand above the basal level.