Computer simulation of metabolism of glucose-perfused rat heart in a work-jump

A computer model of glycolysis, the tricarboxylic acid cycle, and related amino acid metabolism, is described for a glucose-perfused experimental rat heart preparation suddenly switched from low work load (Langendorff perfusion) to high work load (left atrial perfusion). Glycolytic intermediate measurements suggest activation of phosphofructokinase within a few seconds. This activation, and also that of other glycolytic enzymes, is calculated as due to a sharp increase in cytoplasmic Mg2+ level, which overcomes the inhibitory effects of a rapid fall in cytoplasmic pH to 6.77 (calculated from a rapid fall in creatine phosphate). Increased glycolytic substrate is initially supplied by glycogenolysis mediated by phosphorylase b (activated by an early rise in cytoplasmic AMP), followed by increased glucose uptake from the perfusate. Testable predictions are made by the model, especially that lactate production rate should peak early. Additional experiments are described that verify these predictions and fill gaps in the original measurements. The role of modeling in interpreting such experiments is discussed.