Role of the malate-aspartate shuttle on the metabolic response to myocardial ischemia

Abstract

The malate-aspartate (M-A) shuttle provides an important mechanism to regulate glycolysis and lactate metabolism in the heart by transferring reducing equivalents from cytosol into mitochondria. However, experimental characterization of the M-A shuttle has been incomplete because of limitations in quantifying cytosolic and mitochondrial metabolites. In this study, we developed a multi-compartment model of cardiac metabolism with detailed presentation of the M-A shuttle to quantitatively predict non-observable fluxes and metabolite concentrations under normal and ischemic conditions in vivo. Model simulations predicted that the M-A shuttle is functionally localized to a subdomain that spans the mitochondrial and cytosolic spaces. With the onset of ischemia, the M-A shuttle flux rapidly decreased to a new steady state in proportion to the reduction in blood flow. Simulation results suggest that the reduced M-A shuttle flux during ischemia was not due to changes in shuttle-associated enzymes and transporters. However, there was a redistribution of shuttle-associated metabolites in both cytosol and mitochondria. Therefore, the dramatic acceleration in glycolysis and the switch to lactate production that occur immediately after the onset of ischemia is mediated by reduced M-A shuttle flux through metabolite redistribution of shuttle associated species across the mitochondrial membrane.

Figure 1

Metabolic pathways in myocardium. This complex network incorporates most key biochemical reactions and pathways involved in cardiac metabolism including glycolysis, pyruvate oxidation, fatty acids oxidation, the tricarboxylic acid (TCA) cycle, oxidative phosphorylation and M-A shuttle. It also includes metabolite transport across cellular membrane and mitochondrial membrane. GLU, glucose; G6P, glucose-6-phosphate; GLY, glycogen; GAP, glyceraldyhe-3-phosphate; BPG, 1,3-bisphosphate-glycerate; PYR, pyruvate; LAC, lactate; TG, triglyceride; GLR, glycerol; FFA, free fatty acid; FAC, fatty acyl-CoA; PCr, phosphocreatine; Cr, creatine; CIT, citrate; α-KG, α-ketoglutarate; SCA, succinyl-CoA; SUC, succinate; MAL, malate; OAA, oxaloacetate; ACoA, acetyl-CoA; bADP, bound ADP; fADP, free ADP; ASP, aspartate; GLT, glutamate.

Figure 2

Comparison of experimental data with simulated metabolic responses to moderate ischemia (60% coronary blood flow reduction). a. changes in glycogen concentration; b. changes in lactate uptake. Experimental data are from in vivo swine study subjected to 60% blood flow reduction (Salem et al., 2004). Decrease in blood flow started at the 5^th minute of simulation.

Figure 3

Effects of M-A shuttle subdomain volume on cytosolic and mitochondrial response to moderate ischemia (60% reduction in flow). a. cytosolic NADH/NAD⁺ dynamics; b. mitochondrial NADH/NAD⁺ dynamics. V_cj/V_cell=V_mj/V_cell=f_j represents the effective volume of the M-A shuttle subdomain, j=species associated with M-A shuttle, and “w/o M-A shuttle subdomain” represents no distinct effective volume for shuttle-associated species.

Figure 4

Computer simulated responses of cytosolic (a) and mitochondrial (b) redox state during mild, moderate and severe ischemia (30%, 60% and 90% coronary blood flow reduction).

Figure 5

Response of M-A shuttle to ischemia. a. computer simulated changes of M-A shuttle flux in response to mild, moderate and severe ischemia (30%, 60% and 90% reduction in coronary blood flow, respectively); b-f. the dynamic responses of glutamate (GLT) (b), aspartate (ASP) (c), malate (MAL) (d), α-ketoglutarate (αKG) (e), and oxaloacetate (OAA) (f) to 60% reduction in flow. C_c and C_m represent cytosolic and mitochondrial concentrations, respectively.