Altered metabolite exchange between subcellular compartments in intact postischemic rabbit hearts

To examine metabolic regulation in postischemic hearts, we examined oxidative recycling of C-13 within the glutamate pool (GLU) of intact rabbit hearts. Isolated hearts oxidized 2.5 mmol/L [2-C-13]acetate during normal conditions (n=6) or during reperfusion after 10 minutes of ischemia (n=5). C-13-Nuclear magnetic resonance spectra were acquired every 1 minute. Kinetic analysis of C-13 incorporation into GLU provided both tricarboxylic acid (TCA) cycle flux and the interconversion rate (F-1) between the TCA cycle intermediate, alpha-ketoglutarate (alpha-KG), and the largely cytosolic GLU. The rate-pressure product in postischemic hearts was 46% of normal (P<.05). No difference in substrate utilization occurred between groups, with acetate accounting for 92% of the carbon units entering the TCA cycle at the citrate synthase step. TCA cycle flux in postischemic hearts was normal (normal hearts, 10.7 mu mol.min(-1).g(-1); postischemic hearts, 9.4 mu mol.min(-1).g(-1)), whereas F-1 was 72% lower at 2.9+/-0.4 versus 10.2+/-2.5 mu mol.min(-1).g(-1) (mean+/-SE) in normal hearts (P<.05). From additional hearts perfused with 2.5 mmol/L [2-C-13]acetate plus supplemental 5 mmol/L glucose, any potential differences in endogenous carbohydrate availability were proved not to account for the reduced rate alpha-KG and GLU exchange, which remained depressed in postischemic hearts; However, specific activities of the transaminase enzyme, catalyzing chemical exchange of ol-KG and GLU, were the same, and transaminase flux was 100 mu mol.min(-1).g(-1) in postischemic hearts versus 68 mu mol.min(-1).g(-1) in normal hearts. Normal transaminase activity and the increased flux in postischemic hearts are contrary to the reduced F-1. The findings indicate reduced metabolite transport rates across the mitochondrial membranes of stunned myocardium, particularly through the reversible alpha-KG-malate carrier.