Electrochemical Na+ and Ca2+ gradients drive coupled-clock regulation of automaticity of isolated rabbit sinoatrial nodal pacemaker cells

Am J Physiol Heart Circ Physiol. 2016 Jul 1;311(1):H251-67. doi: 10.1152/ajpheart.00667.2015. Epub 2016 May 20.


Coupling of an intracellular Ca(2+) clock to surface membrane ion channels, i.e., a "membrane clock, " via coupling of electrochemical Na(+) and Ca(2+) gradients (ENa and ECa, respectively) has been theorized to regulate sinoatrial nodal cell (SANC) normal automaticity. To test this hypothesis, we measured responses of [Na(+)]i, [Ca(2+)]i, membrane potential, action potential cycle length (APCL), and rhythm in rabbit SANCs to Na(+)/K(+) pump inhibition by the digitalis glycoside, digoxigenin (DG, 10-20 μmol/l). Initial small but significant increases in [Na(+)]i and [Ca(2+)]i and reductions in ENa and ECa in response to DG led to a small reduction in maximum diastolic potential (MDP), significantly enhanced local diastolic Ca(2+) releases (LCRs), and reduced the average APCL. As [Na(+)]i and [Ca(2+)]i continued to increase at longer times following DG exposure, further significant reductions in MDP, ENa, and ECa occurred; LCRs became significantly reduced, and APCL became progressively and significantly prolonged. This was accompanied by increased APCL variability. We also employed a coupled-clock numerical model to simulate changes in ENa and ECa simultaneously with ion currents not measured experimentally. Numerical modeling predicted that, as the ENa and ECa monotonically reduced over time in response to DG, ion currents (ICaL, ICaT, If, IKr, and IbNa) monotonically decreased. In parallel with the biphasic APCL, diastolic INCX manifested biphasic changes; initial INCX increase attributable to enhanced LCR ensemble Ca(2+) signal was followed by INCX reduction as ENCX (ENCX = 3ENa - 2ECa) decreased. Thus SANC automaticity is tightly regulated by ENa, ECa, and ENCX via a complex interplay of numerous key clock components that regulate SANC clock coupling.

Keywords: calcium; cardiac automaticity; electrochemical driving forces; sodium.

Publication types

  • Research Support, N.I.H., Intramural

MeSH terms

  • Action Potentials
  • Animals
  • Biological Clocks* / drug effects
  • Calcium / metabolism*
  • Calcium Channels / metabolism
  • Calcium Signaling* / drug effects
  • Computer Simulation
  • Digoxigenin / pharmacology
  • Epithelial Sodium Channels / metabolism
  • Heart Rate* / drug effects
  • In Vitro Techniques
  • Male
  • Models, Cardiovascular
  • Numerical Analysis, Computer-Assisted
  • Periodicity*
  • Rabbits
  • Sinoatrial Node / cytology
  • Sinoatrial Node / drug effects
  • Sinoatrial Node / metabolism*
  • Sodium / metabolism*
  • Sodium-Calcium Exchanger / metabolism
  • Time Factors


  • Calcium Channels
  • Epithelial Sodium Channels
  • Sodium-Calcium Exchanger
  • Sodium
  • Digoxigenin
  • Calcium