title: Sarcoplasmic reticulum Ca2+ pumping kinetics regulates timing of local Ca2+ releases and spontaneous beating rate of rabbit sinoatrial node pacemaker cells.
abstract: BACKGROUND
  Sinoatrial node cells (SANCs) generate local, subsarcolemmal Ca(2+) releases (LCRs) from sarcoplasmic reticulum (SR) during late diastolic depolarization. LCRs activate an inward Na(+)-Ca(2+) exchange current (I(NCX)), which accelerates diastolic depolarization rate, prompting the next action potential (AP). The LCR period, ie, a delay between AP-induced Ca(2+) transient and LCR appearance, defines the time of late diastolic depolarization I(NCX) activation. Mechanisms that control the LCR period, however, are still unidentified.
  
  
  OBJECTIVE
  To determine dependence of the LCR period on SR Ca(2+) refilling kinetics and establish links between regulation of SR Ca(2+) replenishment, LCR period, and spontaneous cycle length.
  
  
  RESULTS
  Spontaneous APs and SR luminal or cytosolic Ca(2+) were recorded using perforated patch and confocal microscopy, respectively. Time to 90% replenishment of SR Ca(2+) following AP-induced Ca(2+) transient was highly correlated with the time to 90% decay of cytosolic Ca(2+) transient (T-90(C)). Local SR Ca(2+) depletions mirror their cytosolic counterparts, LCRs, and occur following SR Ca(2+) refilling. Inhibition of SR Ca(2+) pump by cyclopiazonic acid dose-dependently suppressed spontaneous SANCs firing up to ≈50%. Cyclopiazonic acid and graded changes in phospholamban phosphorylation produced by β-adrenergic receptor stimulation, phosphodiesterase or protein kinase A inhibition shifted T-90(C) and proportionally shifted the LCR period and spontaneous cycle length (R(2)=0.98).
  
  
  CONCLUSIONS
  The LCR period, a critical determinant of the spontaneous SANC cycle length, is defined by the rate of SR Ca(2+) replenishment, which is critically dependent on SR pumping rate, Ca(2+) available for pumping, supplied by L-type Ca(2+) channel, and ryanodine receptor Ca(2+) release flux, each of which is modulated by cAMP-mediated protein kinase A-dependent phosphorylation.
authors.affiliation: Laboratory of Cardiovascular Science, Gerontology Research Center, NIA, NIH, 5600 Nathan Shock Dr, Baltimore, MD 21224-6825, USA.
authors.forename: Tatiana M
authors.initials: TM
authors.lastname: Vinogradova
authors.forename: Didier X P
authors.initials: DX
authors.lastname: Brochet
authors.forename: Syevda
authors.initials: S
authors.lastname: Sirenko
authors.forename: Yue
authors.initials: Y
authors.lastname: Li
authors.forename: Harold
authors.initials: H
authors.lastname: Spurgeon
authors.forename: Edward G
authors.initials: EG
authors.lastname: Lakatta
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001683:Biological Clocks / Q000187:drug effects / Q000502:physiology*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology* / Q000378:metabolism / Q000502:physiology; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000502:physiology; D012849:Sinoatrial Node / Q000166:cytology / Q000201:enzymology* / Q000378:metabolism / Q000502:physiology; D013997:Time Factors
chemical_list: D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases