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Summary: Mitochondrial 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. : Calcium signals control mitochondrial fuel generation. De La Fuente et al. report that in heart mitochondria, calcium uptake and extrusion are spatially separated; the most calcium-exposed area is an uptake hotspot, but it lacks extrusion to optimize signaling efficiency and minimize the energy expense of controlling mitochondrial function by calcium. Keywords: calcium signaling, NCLX distribution, mitochondrial Ca2+ uniporter distribution, cardiac excitation-energetics coupling, mitochondria-sarcoplasmic reticulum contact sites, Ca2+ mitochondria, cardiac muscle |
