title: Sarcoplasmic reticulum Ca2+ refilling controls recovery from Ca2+-induced Ca2+ release refractoriness in heart muscle.
abstract: In cardiac muscle Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) is initiated by Ca2+ influx via L-type Ca2+ channels. At present, the mechanisms underlying termination of SR Ca2+ release, which are required to ensure stable excitation-contraction coupling cycles, are not precisely known. However, the same mechanism leading to refractoriness of SR Ca2+ release could also be responsible for the termination of CICR. To examine the refractoriness of SR Ca2+ release, we analyzed Na+-Ca2+ exchange currents reflecting cytosolic Ca2+ signals induced by UV-laser flash-photolysis of caged Ca2+. Pairs of UV flashes were applied at various intervals to examine the time course of recovery from CICR refractoriness. In cardiomyocytes isolated from guinea-pigs and mice, beta-adrenergic stimulation with isoproterenol-accelerated recovery from refractoriness by approximately 2-fold. Application of cyclopiazonic acid at moderate concentrations (<10 micromol/L) slowed down recovery from refractoriness in a dose-dependent manner. Compared with cells from wild-type littermates, those from phospholamban knockout (PLB-KO) mice exhibited almost 5-fold accelerated recovery from refractoriness. Our results suggest that SR Ca2+ refilling mediated by the SR Ca2+-pump corresponds to the rate-limiting step for recovery from CICR refractoriness. Thus, the Ca2+ sensitivity of CICR appears to be regulated by SR Ca2+ content, possibly resulting from a change in the steady-state Ca2+ sensitivity and in the gating kinetics of the SR Ca2+ release channels (ryanodine receptors). During Ca2+ release, the concomitant reduction in Ca2+ sensitivity of the ryanodine receptors might also underlie Ca2+ spark termination by deactivation.
authors.affiliation: Department of Physiology, University of Bern, Bern, Switzerland.
authors.forename: Peter
authors.initials: P
authors.lastname: Szentesi
authors.forename: Christophe
authors.initials: C
authors.lastname: Pignier
authors.forename: Marcel
authors.initials: M
authors.lastname: Egger
authors.forename: Evangelia G
authors.initials: EG
authors.lastname: Kranias
authors.forename: Ernst
authors.initials: E
authors.lastname: Niggli
mesh_terms: D000085:Acetates / Q000528:radiation effects; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D020013:Calcium Signaling / Q000502:physiology*; D002135:Calcium-Binding Proteins / Q000172:deficiency / Q000235:genetics; D000252:Calcium-Transporting ATPases / Q000502:physiology; D005029:Ethylenediamines / Q000528:radiation effects; D006168:Guinea Pigs; D007211:Indoles / Q000494:pharmacology; D015640:Ion Channel Gating; D007545:Isoproterenol / Q000494:pharmacology; D051379:Mice; D018345:Mice, Knockout; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D018408:Patch-Clamp Techniques; D010782:Photolysis; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019831:Sodium-Calcium Exchanger / Q000502:physiology; D014466:Ultraviolet Rays; D000099211:Phospholamban
chemical_list: D000085:Acetates; D002110:Caffeine; D002118:Calcium; D002135:Calcium-Binding Proteins; D000252:Calcium-Transporting ATPases; D005029:Ethylenediamines; D007211:Indoles; D007545:Isoproterenol; D019837:Ryanodine Receptor Calcium Release Channel; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019831:Sodium-Calcium Exchanger; D000099211:Phospholamban; C057014:DM-nitrophen; C000543:cyclopiazonic acid