title: Intracellular [Na(+)] modulates synergy between Na(+)/Ca          (2+) exchanger and L-type Ca          (2+) current in cardiac excitation-contraction coupling during action potentials.
abstract: Excitation-contraction coupling (ECC) in cardiac myocytes involves triggering of Ca(2+) release from the sarcoplasmic reticulum (SR) by L-type Ca channels, whose activity is strongly influenced by action potential (AP) profile. The contribution of Ca(2+) entry via the Na(+)/Ca(2+) exchanger (NCX) to trigger SR Ca(2+) release during ECC in response to an AP remains uncertain. To isolate the contribution of NCX to SR Ca(2+) release, independent of effects on SR Ca(2+) load, Ca(2+) release was determined by recording Ca(2+) spikes using confocal microscopy on patch-clamped rat ventricular myocytes with [Ca(2+)](i) fixed at 150 nmol/L. In response to AP clamps, normalized Ca(2+) spike amplitudes (ΔF/F                         (0)) increased sigmoidally and doubled as [Na(+)](i) was elevated from 0 to 20 mmol/L with an EC(50) of ~10 mmol/L. This [Na(+)](i)-dependence was independent of I                         (Na) as well as SR Ca(2+) load, which was unchanged under our experimental conditions. However, NCX inhibition using either KB-R7943 or XIP reduced ΔF/F                         (0) amplitude in myocytes with 20 mmol/L [Na(+)](i), but not with 5 mmol/L [Na(+)](i). SR Ca(2+) release was complete before the membrane repolarized to -15 mV, indicating Ca(2+) entry into the dyad (not reduced extrusion) underlies [Na(+)](i)-dependent enhancement of ECC. Because I                         (Ca,L) inhibition with 50 mmol/L Cd(2+) abolished Ca(2+) spikes, our results demonstrate that during cardiac APs, NCX enhances SR Ca(2+) release by synergistically increasing the efficiency of I                         (Ca,L)-mediated ECC.
authors.affiliation: Department of Physiology, University of Toronto, Heart and Stroke/Richard Lewar Centre of Excellence, Fitzgerald Building, 150 College Street, Room 68, Toronto, ON M5S 3E2, Canada.
authors.forename: Rafael J
authors.initials: RJ
authors.lastname: Ramirez
authors.forename: Rajan
authors.initials: R
authors.lastname: Sah
authors.forename: Jie
authors.initials: J
authors.lastname: Liu
authors.forename: Robert A
authors.initials: RA
authors.lastname: Rose
authors.forename: Peter H
authors.initials: PH
authors.lastname: Backx
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D003593:Cytoplasm / Q000737:chemistry / Q000378:metabolism; D056966:Excitation Contraction Coupling / Q000502:physiology*; D008297:Male; D018613:Microscopy, Confocal; D032383:Myocytes, Cardiac / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D019831:Sodium-Calcium Exchanger; C110374:sodium-calcium exchanger 1; D012964:Sodium; D002118:Calcium