Elevated Cytosolic Na + Decreases Mitochondrial Ca 2+ Uptake During Excitation-Contraction Coupling and Impairs Energetic Adaptation in Cardiac Myocytes

Abstract

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