title: Termination of cardiac Ca2+ sparks: role of intra-SR [Ca2+], release flux, and intra-SR Ca2+ diffusion.
abstract: Ca(2+) release from cardiac sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) is regulated by dyadic cleft [Ca(2+)] and intra-SR free [Ca(2+)] ([Ca(2+)](SR)). Robust SR Ca(2+) release termination is important for stable excitation-contraction coupling, and partial [Ca(2+)](SR) depletion may contribute to release termination. Here, we investigated the regulation of SR Ca(2+) release termination of spontaneous local SR Ca(2+) release events (Ca(2+) sparks) by [Ca(2+)](SR), release flux, and intra-SR Ca(2+) diffusion. We simultaneously measured Ca(2+) sparks and Ca(2+) blinks (localized elementary [Ca(2+)](SR) depletions) in permeabilized ventricular cardiomyocytes over a wide range of SR Ca(2+) loads and release fluxes. Sparks terminated via a [Ca(2+)](SR)-dependent mechanism at a fixed [Ca(2+)](SR) depletion threshold independent of the initial [Ca(2+)](SR) and release flux. Ca(2+) blink recovery depended mainly on intra-SR Ca(2+) diffusion rather than SR Ca(2+) uptake. Therefore, the large variation in Ca(2+) blink recovery rates at different release sites occurred because of differences in the degree of release site interconnection within the SR network. When SR release flux was greatly reduced, long-lasting release events occurred from well-connected junctions. These junctions could sustain release because local SR Ca(2+) release and [Ca(2+)](SR) refilling reached a balance, preventing [Ca(2+)](SR) from depleting to the termination threshold. Prolonged release events eventually terminated at a steady [Ca(2+)](SR), indicative of a slower, [Ca(2+)](SR)-independent termination mechanism. These results demonstrate that there is high variability in local SR connectivity but that SR Ca(2+) release terminates at a fixed [Ca(2+)](SR) termination threshold. Thus, reliable SR Ca(2+) release termination depends on tight RyR regulation by [Ca(2+)](SR).
authors.affiliation: Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, IL 60612, USA.
authors.forename: Aleksey V
authors.initials: AV
authors.lastname: Zima
authors.forename: Eckard
authors.initials: E
authors.lastname: Picht
authors.forename: Donald M
authors.initials: DM
authors.lastname: Bers
authors.forename: Lothar A
authors.initials: LA
authors.lastname: Blatter
mesh_terms: D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects; D004058:Diffusion; D006352:Heart Ventricles / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D008856:Microscopy, Fluorescence; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D010539:Permeability; D011817:Rabbits; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism
chemical_list: D002121:Calcium Channel Blockers; D019837:Ryanodine Receptor Calcium Release Channel; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases
references.citation: Science. 1995 May 19;268(5213):1042-5
references.pmid: 7754383
references.citation: Am J Physiol. 1989 Feb;256(2 Pt 2):H328-33
references.pmid: 2537030
references.citation: J Physiol. 1998 Mar 15;507 ( Pt 3):667-77
references.pmid: 9508828
references.citation: J Physiol. 1999 Dec 15;521 Pt 3:575-85
references.pmid: 10601490
references.citation: Biophys J. 1992 Aug;63(2):497-517
references.pmid: 1330031
references.citation: Am J Physiol Cell Physiol. 2007 Sep;293(3):C1073-81
references.pmid: 17376815
references.citation: Biophys J. 2002 Dec;83(6):3134-51
references.pmid: 12496084
references.citation: Nat Cell Biol. 1999 Oct;1(6):323-9
references.pmid: 10559957
references.citation: J Gen Physiol. 2008 Apr;131(4):325-34
references.pmid: 18347081
references.citation: Cardiovasc Res. 2008 Jan 15;77(2):302-14
references.pmid: 18006486
references.citation: J Gen Physiol. 1985 Feb;85(2):189-246
references.pmid: 3981128
references.citation: Circ Res. 2006 Sep 29;99(7):740-8
references.pmid: 16946134
references.citation: J Gen Physiol. 1985 Feb;85(2):247-89
references.pmid: 2580043
references.citation: J Physiol. 1997 Apr 15;500 ( Pt 2):297-309
references.pmid: 9147318
references.citation: Biophys J. 2007 May 15;92(10):3541-55
references.pmid: 17351009
references.citation: Circ Res. 2006 Aug 4;99(3):283-91
references.pmid: 16794184
references.citation: Biophys J. 2000 Sep;79(3):1273-84
references.pmid: 10968991
references.citation: Science. 1993 Oct 29;262(5134):740-4
references.pmid: 8235594
references.citation: J Physiol. 2001 Jul 1;534(Pt 1):37-47
references.pmid: 11432990
references.citation: Biophys J. 2004 Mar;86(3):1293-307
references.pmid: 14990462
references.citation: Proc Natl Acad Sci U S A. 2000 Jul 5;97(14):7823-8
references.pmid: 10884414
references.citation: J Physiol. 2000 Jan 15;522 Pt 2:259-70
references.pmid: 10639102
references.citation: Biophys J. 2008 Mar 1;94(5):1867-79
references.pmid: 18024505
references.citation: Biophys J. 2008 Aug;95(4):2037-48
references.pmid: 18469084
references.citation: Science. 1995 Mar 31;267(5206):1997-2000
references.pmid: 7701323
references.citation: Physiol Rev. 2002 Oct;82(4):893-922
references.pmid: 12270947
references.citation: Proc Natl Acad Sci U S A. 1998 Dec 8;95(25):15096-101
references.pmid: 9844021
references.citation: Biophys J. 2002 Jul;83(1):59-78
references.pmid: 12080100
references.citation: J Physiol. 1999 Jul 15;518 ( Pt 2):469-78
references.pmid: 10381593
references.citation: Proc Natl Acad Sci U S A. 1997 Apr 15;94(8):4176-81
references.pmid: 9108125
references.citation: Circ Res. 2002 Sep 6;91(5):414-20
references.pmid: 12215490
references.citation: Proc Natl Acad Sci U S A. 2005 Sep 27;102(39):14104-9
references.pmid: 16172392
references.citation: Proc Natl Acad Sci U S A. 2005 Feb 22;102(8):3099-104
references.pmid: 15710901
references.citation: Cell Calcium. 2004 Jun;35(6):621-8
references.pmid: 15110152
references.citation: J Membr Biol. 1994 Feb;137(3):215-26
references.pmid: 8182731
references.citation: Biophys J. 1998 Dec;75(6):2801-10
references.pmid: 9826602
references.citation: Biophys J. 2008 Aug;95(3):1412-27
references.pmid: 18390622
references.citation: Biophys J. 1998 Nov;75(5):2302-12
references.pmid: 9788925
references.citation: Proc Natl Acad Sci U S A. 2003 Sep 30;100(20):11759-64
references.pmid: 13130076
references.citation: J Physiol. 1991 Apr;435:605-30
references.pmid: 1770453
references.citation: Circ Res. 2003 Jul 11;93(1):40-5
references.pmid: 12791706
references.citation: Science. 1993 May 7;260(5109):807-9
references.pmid: 8387229
references.citation: Circ Res. 1993 Nov;73(5):820-8
references.pmid: 8403253
references.citation: J Physiol. 1998 Nov 1;512 ( Pt 3):677-91
references.pmid: 9769413
references.citation: J Physiol. 1996 Oct 15;496 ( Pt 2):575-81
references.pmid: 8910239
references.citation: Cardiovasc Res. 2008 Jan 15;77(2):293-301
references.pmid: 18006488
references.citation: Ann N Y Acad Sci. 1998 Sep 16;853:130-48
references.pmid: 10603942