title: Elevated cytosolic Na+ decreases mitochondrial Ca2+ uptake during excitation-contraction coupling and impairs energetic adaptation in cardiac myocytes.
abstract: Mitochondrial Ca2+ ([Ca2+]m) regulates oxidative phosphorylation and thus contributes to energy supply and demand matching in cardiac myocytes. Mitochondria take up Ca2+ via the Ca2+ uniporter (MCU) and extrude it through the mitochondrial Na+/Ca2+ exchanger (mNCE). It is controversial whether mitochondria take up Ca2+ rapidly, on a beat-to-beat basis, or slowly, by temporally integrating cytosolic Ca2+ ([Ca2+]c) transients. Furthermore, although mitochondrial Ca2+ efflux is governed by mNCE, it is unknown whether elevated intracellular Na+ ([Na+]i) affects mitochondrial Ca2+ uptake and bioenergetics. To monitor [Ca2+]m, mitochondria of guinea pig cardiac myocytes were loaded with rhod-2-acetoxymethyl ester (rhod-2 AM), and [Ca2+]c was monitored with indo-1 after dialyzing rhod-2 out of the cytoplasm. [Ca2+]c transients, elicited by voltage-clamp depolarizations, were accompanied by fast [Ca2+]m transients, whose amplitude (delta) correlated linearly with delta[Ca2+]c. Under beta-adrenergic stimulation, [Ca2+]m decay was approximately 2.5-fold slower than that of [Ca2+]c, leading to diastolic accumulation of [Ca2+]m when amplitude or frequency of delta[Ca2+]c increased. The MCU blocker Ru360 reduced delta[Ca2+]m and increased delta[Ca2+]c, whereas the mNCE inhibitor CGP-37157 potentiated diastolic [Ca2+]m accumulation. Elevating [Na+]i from 5 to 15 mmol/L accelerated mitochondrial Ca2+ decay, thus decreasing systolic and diastolic [Ca2+]m. In response to gradual or abrupt changes of workload, reduced nicotinamide-adenine dinucleotide (NADH) levels were maintained at 5 mmol/L [Na+]i, but at 15 mmol/L, the NADH pool was partially oxidized. The results indicate that (1) mitochondria take up Ca2+ rapidly and contribute to fast buffering during a [Ca2+]c transient; and (2) elevated [Na+]i impairs mitochondrial Ca2+ uptake, with consequent effects on energy supply and demand matching. The latter effect may have implications for cardiac diseases with elevated [Na+]i.
authors.affiliation: Johns Hopkins University, Institute of Molecular Cardiobiology, Division of Cardiology, 720 Rutland Ave, 1059 Ross Bldg, Baltimore, MD 21205-2195, USA.
authors.forename: Christoph
authors.initials: C
authors.lastname: Maack
authors.forename: Sonia
authors.initials: S
authors.lastname: Cortassa
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Aon
authors.forename: Anand N
authors.initials: AN
authors.lastname: Ganesan
authors.forename: Ting
authors.initials: T
authors.lastname: Liu
authors.forename: Brian
authors.initials: B
authors.lastname: O'Rourke
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D004734:Energy Metabolism / Q000187:drug effects*; D006168:Guinea Pigs; D006706:Homeostasis; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D009200:Myocardial Contraction / Q000187:drug effects*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D009243:NAD / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology*
chemical_list: D009243:NAD; D012964:Sodium; D002118:Calcium