Cardiac Pacemaker Cells Uniquely Match ATP Supply to Demand

Abstract

Sinoatrial node cells (SANC) within the heart's pacemaker govern the heart rhythm and contraction due to constitutively active, Ca2+-activated adenylyl-cyclases (AC) that generate a high level of cAMP/protein kinase A (PKA) dependent, localized, submembrane-compartmentalized Ca2+- and AC-cycling. This rhythmic Ca2+ cycling generates rhythmic action potentials (APs), which initiate Ca2+ release and SANC contraction. To drive the heart beat SANC must tightly regulate ATP production to supply sufficient ATP for cAMP production, Ca2+-cycling and contraction. To explore how ATP supply-demand is tightly balanced in isolated SANC we measured ATP levels: in control; during PKA-inhibition by specific inhibitor peptide PKI (15μM) or by H-89 (6μM); during intracellular Ca2+-buffering by BAPTA (25μM); during muscarinic-receptors (MR) activation by carbacol 1μM±MR-inhibition by atropine (10μM). PKA-inhibition by PKI or H-89, which blocked the major ATP-consumption processes, depleted ATP by 45±6% and 44±5%, respectively. Thus, complete inhibition of these major ATP consumption processes in SANC significantly reduces ATP levels even though ATP consumption is reduced. Buffering intracellular Ca2+ depleted ATP by 54±8%. Hence, Ca2+ not only has direct effects on the surface membrane potential to ignite APs, and to activate myofilament-displacement/force production, but also regulates ATP production. In contrast to SANC, in either stimulated (3Hz) or quiescent rabbit ventricular cells the same experimental perturbations had only minor (<6%) effects on ATP levels. These control mechanisms identified by pharmacological-perturbations are apparently utilized in nature, because MR-stimulation of SANC also depleted the ATP by 45±10% (by blocking the ATP-consumption processes). Notably, atropine, a MR-antagonist, substantially reversed the MR-stimulation effect, resulting in only 18±3% ATP depletion. These data suggest that in contrast to ventricular cells, the same signals that drive, and are derived from, the utilization of ATP in SANC also tightly couple the production of ATP to match energy demand.