Termination of cardiac Ca2+ sparks: role of intra-SR [Ca2+], release flux, and intra-SR Ca2+ diffusion.
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).
Zima
Aleksey V
AV
Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, IL 60612, USA.
Picht
Eckard
E
Bers
Donald M
DM
Blatter
Lothar A
LA
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
D002121:Calcium Channel Blockers; D019837:Ryanodine Receptor Calcium Release Channel; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases
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