Elevated Cytosolic Na+ Decreases Mitochondrial Ca2+ Uptake During Excitation–Contraction Coupling and Impairs Energetic Adaptation in Cardiac Myocytes

Mitochondrial Ca ([Ca]m) regulates oxidative phosphorylation and thus contributes to energy supply and demand matching in cardiac myocytes. Mitochondria take up Ca via the Ca uniporter (MCU) and extrude it through the mitochondrial Na/Ca exchanger (mNCE). It is controversial whether mitochondria take up Ca rapidly, on a beat-to-beat basis, or slowly, by temporally integrating cytosolic Ca ([Ca]c) transients. Furthermore, although mitochondrial Ca efflux is governed by mNCE, it is unknown whether elevated intracellular Na ([Na]i) affects mitochondrial Ca uptake and bioenergetics. To monitor [Ca]m, mitochondria of guinea pig cardiac myocytes were loaded with rhod-2–acetoxymethyl ester (rhod-2 AM), and [Ca]c was monitored with indo-1 after dialyzing rhod-2 out of the cytoplasm. [Ca]c transients, elicited by voltage-clamp depolarizations, were accompanied by fast [Ca]m transients, whose amplitude (Δ) correlated linearly with Δ[Ca]c. Under β-adrenergic stimulation, [Ca]m decay was ≈2.5-fold slower than that of [Ca]c, leading to diastolic accumulation of [Ca]m when amplitude or frequency of Δ[Ca]c increased. The MCU blocker Ru360 reduced Δ[Ca]m and increased Δ[Ca]c, whereas the mNCE inhibitor CGP-37157 potentiated diastolic [Ca]m accumulation. Elevating [Na]i from 5 to 15 mmol/L accelerated mitochondrial Ca decay, thus decreasing systolic and diastolic [Ca]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 Ca rapidly and contribute to fast buffering during a [Ca]c transient; and (2) elevated [Na]i impairs mitochondrial Ca uptake, with consequent effects on energy supply and demand matching. The latter effect may have implications for cardiac diseases with elevated [Na]i.