Control of myocardial energetics by Ca2+signal propagation to the mitochondrial matrixincludes local Ca2+ delivery from sarcoplasmicreticulum (SR) ryanodine receptors (RyR2) to theinner mitochondrial membrane (IMM) Ca2+uniporter (mtCU). mtCU activity in cardiacmitochondria is relatively low, while the IMMsurface is large, due to extensive cristae folding.Hence, stochastically distributed mtCU may notsuffice to support local Ca2+ transfer. Wehypothesized that mtCU concentrated atmitochondria-SR associations would promote theeffective Ca2+ transfer. mtCU distribution wasdetermined by tracking MCU and EMRE, theproteins essential for channel formation. Bothproteins were enriched in the IMM-outermitochondrial membrane (OMM) contact pointsub-mitochondrial fraction and, as super-resolutionmicroscopy revealed, located more to themitochondrial periphery (inner boundarymembrane) than inside the cristae, indicating highaccessibility to cytosol-derived Ca2+ inputs.Furthermore, MCU immunofluorescencedistribution was biased towards the mitochondria-SR interface (RyR2) and this bias was promoted byCa2+ signaling activity in intact cardiomyocytes.The SR fraction of heart homogenate containsmitochondria with extensive SR associations andthese mitochondria are highly enriched in EMRE.Size-exclusion chromatography suggested forEMRE- and MCU-containing complexes a widesize range, and also revealed MCU-containingcomplexes devoid of EMRE (thus disabled) in themitochondrial but not the SR fraction. Functionalmeasurements suggested more effective mtCUmediatedCa2+ uptake activity by the mitochondriaof the SR than of the mitochondrial fraction. Thus,mtCU ‘hotspots’ can be formed at the cardiacmuscle mitochondria-SR associations vialocalization and assembly bias, serving local Ca2+signaling and the excitation-energetics coupling.