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

Abstract

EG. Mechanisms that match ATP supply to demand in cardiac pacemaker cells during high ATP demand. Am J Physiol Heart Circ Physiol 304: H1428 -H1438, 2013. First published April 19, 2013 doi:10.1152/ajpheart.00969.2012.-The spontaneous action potential (AP) firing rate of sinoatrial node cells (SANCs) involves high-throughput signaling via Ca 2ϩ -calmodulin activated adenylyl cyclases (AC), cAMP-mediated protein kinase A (PKA), and Ca 2ϩ / calmodulin-dependent protein kinase II (CaMKII)-dependent phosphorylation of SR Ca 2ϩ cycling and surface membrane ion channel proteins. When the throughput of this signaling increases, e.g., in response to ␤-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 Ca 2ϩ (Ca 2ϩ m) and an indirect effect via enhanced Ca 2ϩ -cAMP/PKA-CaMKII signaling to mitochondria. To increase ATP demand, single isolated rabbit SANCs were superfused by physiological saline at 35 Ϯ 0.5°C with isoproterenol, or by phosphodiesterase or protein phosphatase inhibition. We measured cytosolic and mitochondrial Ca 2ϩ and flavoprotein fluorescence in single SANC, and we measured cAMP, ATP, and O2 consumption in SANC suspensions. Although the increase in spontaneous AP firing rate was accompanied by an increase in O2 consumption, the ATP level and flavoprotein fluorescence remained constant, indicating that ATP production had increased. Both Ca 2ϩ m and cAMP increased concurrently with the increase in AP firing rate. When Ca 2ϩ 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 Ca 2ϩ m and an increase in Ca 2ϩ activated cAMP-PKA-CaMKII signaling regulate the increase in ATP supply to meet ATP demand above the basal level.calcium-activated adenylyl cyclase; bioenergetics; pacemaker automaticity REGULATION OF SINOATRIAL NODE cells (SANCs) pacemaker function involves a coupled-clock system (Fig. 1) (2,19). This high-throughput Ca 2ϩ -calmodulin-activated AC/PKA-CaMKII-Ca 2ϩ release signaling cascade in pacemaker cells is regulated in a feed-forward manner (27, 43) and requires physiological "brakes" to maintain the basal spontaneous action potential (AP) firing rate well below its maximum (Fig. 1). This regulation is accomplished by basal phosphodiesterase (PDE) (15, 27, 37) and protein phosphatase activity (1, 44) (Fig. 1). ATP is required to activate AC to produce cAMP, and a significant part of ATP is consumed to pump Ca 2ϩ into the SR, to maintain Na ϩ /K ϩ pump function and to maintain cell ionic homeostasis. Our prior studies indicate that the ATP supply required to support the basal SANC firing rate is also controlled by the Ca 2ϩ -AC/PKA-CaMKII cascade (39, 41) (Fig. 1). When the AP firing rate increases, e.g., in response to ␤-adrenergic receptor (␤-AR) stimulation (see Fig. 1), ATP production must increase to...

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