Enhancing Mitochondrial Ca ²⁺ Uptake in Myocytes From Failing Hearts Restores Energy Supply and Demand Matching

Abstract

Abstract-Mitochondrial ATP production is continually adjusted to energy demand through coordinated increases in oxidative phosphorylation and NADH production mediated by mitochondrial Ca 2ϩ ( Key Words: energy metabolism Ⅲ excitation-contraction coupling Ⅲ heart failure Ⅲ ion transport Ⅲ Na ϩ /Ca 2ϩ exchanger Ⅲ oxidative phosphorylation C ardiac muscle contraction requires continuous matching of ATP supply with a constantly varying workload, yet the mechanism of mitochondrial bioenergetic control is still incompletely understood. The rate of oxidative phosphorylation depends on the protonmotive force across the inner membrane, which is influenced by the balance between the rate of production of reducing equivalents (NADH and FADH 2 ) by the tricarboxylic acid (TCA) cycle and the rate of electron transfer to O 2 by the respiratory chain. When energy demand increases, NADH oxidation is accelerated, requiring a concomitant increase in dehydrogenase activity to maintain NADH/NAD ϩ redox potential and ATP production. Two main lines of evidence support the idea that mitochondrial Ca 2ϩ ([Ca 2ϩ ] m ) homeostasis plays a central role in energy supply and demand matching. First, matrix-free Ca 2ϩ activates several enzymes in the TCA cycle, including pyruvate dehydrogenase, 2-oxoglutarate dehydrogenase, and NAD ϩ -linked isocitrate dehydrogenase, 1 thereby increasing NADH production. Second, increases in [Ca 2ϩ ] m have been recorded during excitation-contraction coupling and are correlated with changes in metabolism, indicating that mitochondria take up Ca 2ϩ in response to cytosolic Ca 2ϩ on a beat-to-beat basis (reviewed in 2,3 Materials and MethodsAn expanded methods section is included in the online data supplement available at http://circres.ahajournals.org. Briefly, isolated guinea pig cardiomyocytes were subjected to rapid change of workload: from resting state to 4-Hz stimulation and then back to resting state in the presence of 100 nmol/L isoproterenol. NADH autofluorescence was recorded. [Ca 2ϩ ] m was monitored with rhod-2 and [Na ϩ ] i was measured with SBFI. The heart failure model was produced with ascending aortic constriction. Figure 1A-B Figure 1B). Diastolic normalized rhod-2 fluorescence (F/F0) was 1.21Ϯ0.04l in 15 mmol/L [Na ϩ ] i at the end of stimulation and 1.07Ϯ0.04 after 100-second recovery at rest ( Figure 1B), whereas with 5 mmol/L [Na ϩ ] i , it was 1.35Ϯ0.05 at the end of stimulation and 1.16Ϯ0.04 after 100-second recovery at rest ( Figure 1A). The decrease of [Ca 2ϩ ] m accumulation in the cells with 15 mmol/L Na ϩ was associated with net oxidation of NADH. On a rapid increase of workload (4 Hz stimulation for 100 seconds) from rest with 15 mmol/L Na ϩ , NADH gradually decreased from 82.1Ϯ4.1% before stimulation to 63.7Ϯ7.4% at the end of stimulation ( Figure 1D). In contrast, with 5 mmol/L Na ϩ , the NADH levels were maintained during stimulation: NADH was 67.7Ϯ5.8% before stimulation and 66.3Ϯ6.3% at the end of stimulation ( Figure 1C). Figure 1G). Diastolic rhod-2 F/F0 in c...

Mechanisms that match ATP supply to demand in cardiac pacemaker cells during high ATP demand

“…In VM an increased cytosolic Ca 2ϩ is known to increase mitochondrial Ca 2ϩ (22). Figure 4 shows that perturbations that increase cytosolic Ca 2ϩ in SANC in the present study also lead to an increase in Ca (Fig.…”

“…3). Similarly, in guinea pig VM, the NAD(P)H level [flavoprotein is reciprocally related to mitochondrial NAD(P)H content] remains constant in response to an increase in AP firing rate (22) or to ISO (25), indicating that the ATP supply meets the demand.…”

“…There are several reports that indicate that mitochondrial metabolism is altered in heart failure (14,22). Specifically, during heart failure alterations in Ca 2ϩ m (22) and cAMP signaling (for review see Ref.…”

NCLX Protein, but Not LETM1, Mediates Mitochondrial Ca2+ Extrusion, Thereby Limiting Ca2+-induced NAD(P)H Production and Modulating Matrix Redox State

“…A rise in [Ca 2ϩ ] mt stimulates several matrix dehydrogenases, which in the presence of substrate are able to increase the NAD(P)H/NAD(P) ratio and, as a consequence, cause a net reduction of the matrix redox state. At the same time as respiration is accelerated, more reactive oxygen species are formed, which should shift redox couples in the direction of oxidation (43)(44)(45)(46)(47)(48)(49). Following stimulation with histamine, we observed a net reduction of the redox-sensitive thiol groups of roGFP1 expressed in the mitochondria of HeLa cells (Fig.…”

“…Two mitochondrial inner membrane proteins, namely NCLX (32) (sodium/calcium exchanger protein, mitochondrial; or sodium/ potassium/calcium exchanger 6, mitochondrial; or solute carrier family 24 member 6) and LETM1 (33) (25). As a result, the ratio of the redox couple NAD(P)H/ NAD(P) will increase (43)(44)(45)(46)(47). On the other hand, [Ca 2ϩ ] mt elevations accelerate respiration.…”