Role of Cellular Compartmentation in the Metabolic Response to Stress Mechanistic Insights from Computational Models LUFANG ZHOU, a,d XIN YU, a,d MARCO E. CABRERA, a,b,c,d AND WILLIAM C. STANLEY c,d a Department of Biomedical Engineering, b Department of Pediatrics, c Departments of Physiology and Biophysics, and d Center for Modeling Integrated Metabolic Systems, Case Western Reserve University, Cleveland, Ohio, 44106-4970 USA ABSTRACT: The mechanisms controlling ATP generation in the transi- tion from normal resting conditions to either high work states or ischemia are poorly understood. ATP generation depends upon compartmenta- tion between the mitochondria and cytosol of metabolic pathways and key energy transfer species that cannot be easily assessed experimen- tally. We developed a multicompartment mathematical model of car- diac metabolism to simulate the metabolic responses to ischemia and in- creased workload. The model is based on mass balances, transport, and metabolic processes in cardiac tissue, and has three distinct compart- ments (blood, cytosol, and mitochondria). In addition to distinguishing between cytosol and mitochondria, the model includes a cytosolic sub- compartment for glycolytic metabolic channeling. The model simulations predict the rapid activation of glycogenolysis and lactate production at the onset of ischemia, and support the concept of localization of glycolysis to a cytosolic subcompartment. In addition, simulations show that mito- chondrial NADH/NAD + is primarily determined by oxygen consumption during ischemia, while cytosolic NADH/NAD + and lactate production are largely a function of glycolytic flux during the initial phase, and is con- trolled by mitochondrial NADH/NAD + and the malate–aspartate shuttle during the steady state. Finally, the model predicts that metabolic activa- tion with an abrupt increase in workload requires parallel activation of ATP hydrolysis, glycolysis, mitochondrial dehydrogenases, the electron transport chain, and ADP phosphorylation. Taken together, these studies demonstrate the importance of metabolic compartmentation in the regu- lation of cardiac energetics in response to acute stress, and they highlight the usefulness of computational models in this line of investigation. Address for correspondence: W.C. Stanley, Department of Physiology and Biophysics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-4970. e-mail: wcs4@case.edu Ann. N.Y. Acad. Sci. 1080: 120–139 (2006). C  2006 New York Academy of Sciences. doi: 10.1196/annals.1380.012 120