title: Optical mapping of sarcoplasmic reticulum Ca2+ in the intact heart: ryanodine receptor refractoriness during alternans and fibrillation.
abstract: BACKGROUND
  Sarcoplasmic reticulum (SR) Ca(2+) cycling is key to normal excitation-contraction coupling but may also contribute to pathological cardiac alternans and arrhythmia.
  
  
  OBJECTIVE
  To measure intra-SR free [Ca(2+)] ([Ca(2+)]SR) changes in intact hearts during alternans and ventricular fibrillation (VF).
  
  
  RESULTS
  Simultaneous optical mapping of Vm (with RH237) and [Ca(2+)]SR (with Fluo-5N AM) was performed in Langendorff-perfused rabbit hearts. Alternans and VF were induced by rapid pacing. SR Ca(2+) and action potential duration (APD) alternans occurred in-phase, but SR Ca(2+) alternans emerged first as cycle length was progressively reduced (217±10 versus 190±13 ms; P<0.05). Ryanodine receptor (RyR) refractoriness played a key role in the onset of SR Ca(2+) alternans, with SR Ca(2+) release alternans routinely occurring without changes in diastolic [Ca(2+)]SR. Sensitizing RyR with caffeine (200 μmol/L) significantly reduced the pacing threshold for both SR Ca(2+) and APD alternans (188±15 and 173±12 ms; P<0.05 versus baseline). Caffeine also reduced the magnitude of spatially discordant SR Ca(2+) alternans, but not APD alternans, the pacing threshold for discordance, or threshold for VF. During VF, [Ca(2+)]SR was high, but RyR remained nearly continuously refractory, resulting in minimal SR Ca(2+) release throughout VF.
  
  
  CONCLUSIONS
  In intact hearts, RyR refractoriness initiates SR Ca(2+) release alternans that can be amplified by diastolic [Ca(2+)]SR alternans and lead to APD alternans. Sensitizing RyR suppresses spatially concordant but not discordant SR Ca(2+) and APD alternans. Despite increased [Ca(2+)]SR during VF, SR Ca(2+) release was nearly continuously refractory. This novel method provides insight into SR Ca(2+) handling during cardiac alternans and arrhythmia.
authors.affiliation: From the Department of Pharmacology, School of Medicine, University of California, Davis (L.W., N.M.D.J., A.K.P.O., D.M.B., C.M.R.); and Institute of Cardiovascular and Medical Sciences, University of Glasgow, Glasgow, United Kingdom (R.C.M.).
authors.forename: Lianguo
authors.initials: L
authors.lastname: Wang
authors.forename: Rachel C
authors.initials: RC
authors.lastname: Myles
authors.forename: Nicole M
authors.initials: NM
authors.lastname: De Jesus
authors.forename: Alex K P
authors.initials: AK
authors.lastname: Ohlendorf
authors.forename: Donald M
authors.initials: DM
authors.lastname: Bers
authors.forename: Crystal M
authors.initials: CM
authors.lastname: Ripplinger
mesh_terms: D000200:Action Potentials; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects; D002304:Cardiac Pacing, Artificial; D056966:Excitation Contraction Coupling; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D010477:Perfusion; D011817:Rabbits; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000378:metabolism; D012032:Refractory Period, Electrophysiological; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism; D013997:Time Factors; D014693:Ventricular Fibrillation / Q000378:metabolism* / Q000503:physiopathology; D056969:Voltage-Sensitive Dye Imaging
chemical_list: D000318:Adrenergic beta-Agonists; D011943:Receptors, Adrenergic, beta; D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D007545:Isoproterenol; D002118:Calcium
keywords: arrhythmias, cardiac; ventricular fibrillation