Perturbations in cytosolic redox state impact muscle energy and pH balance during exercise: an in silico study

The use of computational modelling and simulation to capture and test hypotheses on metabolic control in muscle energetics has a rich history. However, the role of regulatory factors such as the dynamics of cytosolic redox state and pH in this subject matter has not yet been characterized. Here, we developed a physiologically based computational model to quantify the effects of perturbations in proton and redox balance on ATP energy balance of a fast-twitch (FT) myofiber fuelled by carbohydrate metabolism. The model was implemented using MATLAB (Mathworks Inc., MA, USA) and run on a standard PC. It describes the thermodynamically constrained kinetics of a biochemical reaction network by a set of differential-algebraic equations, predicting the changes in metabolite levels associated with energy balance in an open system. The model explicitly considers pH dynamics and its effects on enzyme kinetics and thermodynamics and captures redox coupling of glycolysis and oxidative phosphorylation. Adjustable model parameters were fitted based on dynamic in vivo 31 P-MR spectroscopic recordings of metabolite and pH levels in rodent FT muscle from the literature. The model was first-pass validated against independent datasets. The particular ATP synthesis control model of ADP and Pi feedback implemented in the model sufficed to balance a 100-fold dynamic range of ATP turnover. The model predicts a transient reduction of the cytosolic redox state followed by a reoxidation of NADH to approximately 50 % of initial [NADH] during 10 min of exercise for ATP turnover rates below and above the maximal velocity of mitochondrial ATP synthesis. Simulations of the impact of lactate dehydrogenase (LDH) knockout captured changes in metabolite levels in response to ischemic exercise reported in muscle biopsies of patients with LDH deficiency. Specifically, it reproduced increases in pyruvate, fructose 1,6-biphosphate and glycerol 3-phophate levels and showed that perturbations in cytoplasmic redox balance are the driving regulatory mechanism behind the decrease in ATP leading to muscle cell damage in these patients. The proposed model provides a novel simulation platform to test hypotheses in muscle physiology. Importantly, it informs on many metabolic variables, including cytosolic redox state, that have proven difficult to track experimentally. Such information may strengthen the rational basis for care and management of (often rare) muscular diseases. This work was supported in part by the Deutsche Forschungs Gesellschaft, the Dutch Spieren voor Spieren Foundation and the National Heart, Blood and Lung Institute of the USA. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.