title: Proton-sensing Ca2+ binding domains regulate the cardiac Na+/Ca2+ exchanger.
abstract: The cardiac Na(+)/Ca(2+) exchanger (NCX) regulates cellular [Ca(2+)](i) and plays a central role in health and disease, but its molecular regulation is poorly understood. Here we report on how protons affect this electrogenic transporter by modulating two critically important NCX C(2) regulatory domains, Ca(2+) binding domain-1 (CBD1) and CBD2. The NCX transport rate in intact cardiac ventricular myocytes was measured as a membrane current, I(NCX), whereas [H(+)](i) was varied using an ammonium chloride "rebound" method at constant extracellular pH 7.4. At pH(i) = 7.2 and [Ca(2+)](i) < 120 nM, I(NCX) was less than 4% that of its maximally Ca(2+)-activated value. I(NCX) increases steeply at [Ca(2+)](i) between 130-150 nM with a Hill coefficient (n(H)) of 8.0 ± 0.7 and K(0.5) = 310 ± 5 nM. At pH(i) = 6.87, the threshold of Ca(2+)-dependent activation of I(NCX) was shifted to much higher [Ca(2+)](i) (600-700 nM), and the relationship was similarly steep (n(H) = 8.0±0.8) with K(0.5) = 1042 ± 15 nM. The V(max) of Ca(2+)-dependent activation of I(NCX) was not significantly altered by low pH(i). The Ca(2+) affinities for CBD1 (0.39 ± 0.06 μM) and CBD2 (K(d) = 18.4 ± 6 μM) were exquisitely sensitive to [H(+)], decreasing 1.3-2.3-fold as pH(i) decreased from 7.2 to 6.9. This work reveals for the first time that NCX can be switched off by physiologically relevant intracellular acidification and that this depends on the competitive binding of protons to its C(2) regulatory domains CBD1 and CBD2.
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat-Aviv 69978, Israel and; Laboratory of Molecular Cardiology, Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Maryland 21201.
authors.forename: Liron
authors.initials: L
authors.lastname: Boyman
authors.affiliation: Laboratory of Molecular Cardiology, Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Maryland 21201.
authors.forename: Brian M
authors.initials: BM
authors.lastname: Hagen
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat-Aviv 69978, Israel and.
authors.forename: Moshe
authors.initials: M
authors.lastname: Giladi
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat-Aviv 69978, Israel and.
authors.forename: Reuben
authors.initials: R
authors.lastname: Hiller
authors.affiliation: Laboratory of Molecular Cardiology, Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Maryland 21201.
authors.forename: W Jonathan
authors.initials: WJ
authors.lastname: Lederer
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat-Aviv 69978, Israel and. Electronic address: dhanan@post.tau.ac.il.
authors.forename: Daniel
authors.initials: D
authors.lastname: Khananshvili
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D006352:Heart Ventricles / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D051381:Rats; D017207:Rats, Sprague-Dawley; D019831:Sodium-Calcium Exchanger / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D019831:Sodium-Calcium Exchanger; D002118:Calcium