title: Cytosolic Ca²⁺ buffering determines the intra-SR Ca²⁺ concentration at which cardiac Ca²⁺ sparks terminate.
abstract: Single ryanodine receptor (RyR) Ca(2+) flux amplitude (i(Ca-RyR)) decreases as intra-sarcoplasmic reticulum (SR) Ca(2+) levels fall during a cardiac Ca(2+) spark. Since i(Ca-RyR) drives the inter-RyR Ca(2+)-induced Ca(2+) release (CICR) that underlies the spark, decreasing i(Ca-RyR) may contribute to spark termination because RyRs that spontaneously close may stay closed. To test this possibility, we simultaneously measured local cytosolic and intra-SR ([Ca(2+)]cyto and [Ca(2+)]SR) during Ca(2+) sparks in permeabilized rabbit ventricular myocytes. Local cytosolic or intra-SR Ca(2+) dynamics were manipulated using Ca(2+) buffers. Buffer manipulations applied in cells had no effect on individual RyR channels reconstituted in planar lipid bilayers. Presence of a fast cytosolic Ca(2+) buffer (BAPTA) significantly suppressed Ca(2+) spark activity and sparks terminated earlier at a higher than usual [Ca(2+)]SR level (∼80% vs. ∼62%). When cytosolic Ca(2+) buffer power was reduced (i.e. cytosolic EGTA level decreased), sparks terminated later and at a lower than usual [Ca(2+)]SR level (∼45% vs. ∼62%). When intra-SR Ca(2+) buffer power was increased, sparks also terminated later and at a lower than usual [Ca(2+)]SR (∼48% vs. ∼62%). These results suggest that cytosolic local control of inter-RyR CICR by i(Ca-RyR) plays a substantial role during the spark termination process. Thus, alterations in local cytosolic Ca(2+) handling dynamics in the dyadic cleft (Ca(2+) buffering, extrusion, etc.) likely influence Ca(2+) spark termination.
authors.affiliation: Department of Cell and Molecular Physiology, Loyola University Chicago, 2160 South First Avenue, Maywood, IL 60153, United States.
authors.forename: Elisa
authors.initials: E
authors.lastname: Bovo
authors.affiliation: Department of Cell and Molecular Physiology, Loyola University Chicago, 2160 South First Avenue, Maywood, IL 60153, United States.
authors.forename: Stefan R
authors.initials: SR
authors.lastname: Mazurek
authors.affiliation: Department of Molecular Biophysics and Physiology, Rush University Medical Center, 1750 West Harrison Street, Chicago, IL 60612, United States.
authors.forename: Michael
authors.initials: M
authors.lastname: Fill
authors.affiliation: Department of Cell and Molecular Physiology, Loyola University Chicago, 2160 South First Avenue, Maywood, IL 60153, United States. Electronic address: azima@luc.edu.
authors.forename: Aleksey V
authors.initials: AV
authors.lastname: Zima
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D003600:Cytosol / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism* / Q000648:ultrastructure; D011817:Rabbits; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
chemical_list: D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium
keywords: Ca(2+) spark; Ca(2+)-induced Ca(2+) release; Confocal microscopy; Heart; Ryanodine receptor