title: Intracellular calcium transients underlying interval-force relationship in whole rat hearts: effects of calcium antagonists.
abstract: OBJECTIVE
  Much of the understanding about the cardiac interval-force relationship of the whole heart, including mechanical restitution and postextrasystolic potentiation (PESP), has been inferred from isolated muscle studies. We tested whether results from isolated muscles about intracellular Ca2+([Ca2+]i) transients underlying the interval-force relationship can be substantiated in whole hearts. Additionally, we investigated whether Ca2+ antagonists could alter [Ca2+]i transients underlying mechanical restitution and postextrasystolic potentiation.
  
  
  METHODS
  [Ca2+]i transients were studied in isolated perfused rat hearts by surface fluorometry and Indo-1. Using computer-controlled pacing protocols, we performed restitution curves for left ventricular developed pressure and [Ca2+]i (developed pressure and [Ca2+]i plotted as a function of extrasystolic intervals). To quantify restitution curves, we fitted monoexponential functions to plots and analyzed their shift and slope. Then, we used Ca2+ antagonists, low extracellular Ca2+([Ca2+]o) and PESP to modify restitution curves. [Ca2+]i transients in isolated rat hearts were interpreted as Ca2+ released from the sarcoplasmic reticulum.
  
  
  RESULTS
  Interval-dependent changes in developed pressure were strongly correlated to interval-dependent changes in the amplitude of [Ca2+]i transients in isolated whole rat hearts. Additionally, nifedipine and low [Ca2+]o led to similar downward shifts but not to a changed slope of restitution curves for [Ca2+]i. On the other hand, PESP increased the slope of restitution curves for [Ca2+]i. Furthermore, the effect of PESP on developed pressure was blunted by high concentrations of Ca2+ antagonists.
  
  
  CONCLUSIONS
  The results from isolated muscles about [Ca2+]i transients underlying the interval-force relationship could be substantiated in whole hearts. Additionally, low [Ca2+]i (induced by nifedipine or low [Ca2+]o) decreased the maximal Ca2+ release of the sarcoplasmic reticulum but did not change the release kinetics. On the other hand, PESP presumably accelerated Ca2+ release kinetics of the sarcoplasmic reticulum.
authors.affiliation: Department of Medicine, University of California San Francisco 94143, USA.
authors.forename: C E
authors.initials: CE
authors.lastname: Zaugg
authors.forename: S
authors.initials: S
authors.lastname: Kojima
authors.forename: S T
authors.initials: ST
authors.lastname: Wu
authors.forename: J
authors.initials: J
authors.lastname: Wikman-Coffelt
authors.forename: W W
authors.initials: WW
authors.lastname: Parmley
authors.forename: P T
authors.initials: PT
authors.lastname: Buser
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002304:Cardiac Pacing, Artificial; D005470:Fluorometry; D007211:Indoles; D007424:Intracellular Fluid / Q000378:metabolism*; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D010477:Perfusion; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
chemical_list: D002121:Calcium Channel Blockers; D007211:Indoles; C059587:Indo-1 pentaacetoxymethyl ester; D009543:Nifedipine; D002118:Calcium