2023-06-222023-06-222018-09-182211-1247http://dx.doi.org/10.34944/dspace/8740http://hdl.handle.net/20.500.12613/8776Mitochondrial Ca2+ elevations enhance ATP production, but uptake must be balanced by efflux to avoid overload. Uptake is mediated by the mitochondrial Ca2+ uniporter channel complex (MCUC), and extrusion is controlled largely by the Na+/Ca2+ exchanger (NCLX), both driven electrogenically by the inner membrane potential (ΔΨm). MCUC forms hotspots at the cardiac mitochondria-junctional SR (jSR) association to locally receive Ca2+ signals; however, the distribution of NCLX is unknown. Our fractionation-based assays reveal that extensively jSR-associated mitochondrial segments contain a minor portion of NCLX and lack Na+-dependent Ca2+ extrusion. This pattern is retained upon in vivo NCLX overexpression, suggesting extensive targeting to non-jSR-associated submitochondrial domains and functional relevance. In cells with non-polarized MCUC distribution, upon NCLX overexpression the same given increase in matrix Ca2+ expends more ΔΨm. Thus, cardiac mitochondrial Ca2+ uptake and extrusion are reciprocally polarized, likely to optimize the energy efficiency of local calcium signaling in the beating heart.14 pagesengAttribution-NonCommercial-NoDerivs CC BY-NC-NDhttps://creativecommons.org/licenses/by-nc-nd/4.0/Calcium signalingNCLX distributionMitochondrial Ca2+ uniporter distributionCardiac excitation-energetics couplingMitochondria-sarcoplasmic reticulum contact sitesCa2+ mitochondriaCardiac muscleText