title: The role of dyadic organization in regulation of sarcoplasmic reticulum Ca(2+) handling during rest in rabbit ventricular myocytes.
abstract: The dyadic organization of ventricular myocytes ensures synchronized activation of sarcoplasmic reticulum (SR) Ca(2+) release during systole. However, it remains obscure how the dyadic organization affects SR Ca(2+) handling during diastole. By measuring intraluminal SR Ca(2+) ([Ca(2+)]SR) decline during rest in rabbit ventricular myocytes, we found that ∼76% of leaked SR Ca(2+) is extruded from the cytosol and only ∼24% is pumped back into the SR. Thus, the majority of Ca(2+) that leaks from the SR is removed from the cytosol before it can be sequestered back into the SR by the SR Ca(2+)-ATPase (SERCA). Detubulation decreased [Ca(2+)]SR decline during rest, thus making the leaked SR Ca(2+) more accessible for SERCA. These results suggest that Ca(2+) extrusion systems are localized in T-tubules. Inhibition of Na(+)-Ca(2+) exchanger (NCX) slowed [Ca(2+)]SR decline during rest by threefold, however did not prevent it. Depolarization of mitochondrial membrane potential during NCX inhibition completely prevented the rest-dependent [Ca(2+)]SR decline. Despite a significant SR Ca(2+) leak, Ca(2+) sparks were very rare events in control conditions. NCX inhibition or detubulation increased Ca(2+) spark activity independent of SR Ca(2+) load. Overall, these results indicate that during rest NCX effectively competes with SERCA for cytosolic Ca(2+) that leaks from the SR. This can be explained if the majority of SR Ca(2+) leak occurs through ryanodine receptors in the junctional SR that are located closely to NCX in the dyadic cleft. Such control of the dyadic [Ca(2+)] by NCX play a critical role in suppressing Ca(2+) sparks during rest.
authors.affiliation: Department of Cell and Molecular Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, Illinois.
authors.forename: Elisa
authors.initials: E
authors.lastname: Bovo
authors.affiliation: Department of Cell and Molecular Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, Illinois.
authors.forename: Pieter P
authors.initials: PP
authors.lastname: de Tombe
authors.affiliation: Department of Cell and Molecular Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, Illinois. 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 / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006352:Heart Ventricles / Q000166:cytology*; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000166:cytology*; D011817:Rabbits; D012146:Rest / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors; D019284:Thapsigargin / Q000494:pharmacology
chemical_list: D004791:Enzyme Inhibitors; D019831:Sodium-Calcium Exchanger; D019284:Thapsigargin; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D002118:Calcium