Computational Modeling of Substrate‐Dependent Differential Regulation of Mitochondrial Bioenergetics in the Heart and Kidney Cortex and Outer Medulla
Authors: Zhang Xiao , Tomar Namrata , Sadri Shima , Audi Said , Cowley Allen W. , Dash Ranjan K. 
Published: May 2022

Rationale

It is recognized that the kinetics and efficiency of mitochondrial O2 consumption for ATP production depend on the choice of respiratory substrates. Various substrates differentially generate the reducing equivalents NADH and FADH2 via the tricarboxylic acid (TCA) cycle that feed electrons to the electron transport chain (ETC) driving oxidative phosphorylation (OxPhos). However, the contributions of these substrates and their combinations have not been systematically characterized in either the heart or the kidney, the two major energy consuming organs in the body.

Method

Bioenergetic responses (respiration and membrane potential) of mitochondria isolated from the heart and kidney cortex and outer medulla (OM) of adult Sprague‐Drawly rats were characterized with different substrate combinations and different ADP perturbations. To better understand these distinct substrate and tissue‐specific mitochondrial bioenergetics, thermodynamically‐constrained mechanistic computational models were developed by integrating the kinetics and regulation of mitochondrial substrate transport and oxidation, TCA cycle, and ETC/OxPhos, incorporating published data and the novel mitochondrial bioenergetics data from our laboratory. Intrinsic model parameters such as Michaelis‐Menten constants were assumed identical and fixed for the heart and kidney, while extrinsic model parameters such as maximal enzymatic rates were separately estimated based on experimental data for the heart and kidney mitochondrial bioenergetics.

Results

Consistent with the data, model simulations of the ADP‐induced state 3 responses of the heart and kidney cortex and OM mitochondria yielded dramatically different values, which for a given organ were also very distinct for different substrates. In addition, heart mitochondria energized with succinate without rotenone (blocker of complex I) and saturated [ADP] failed to produce a robust state 3 response. This was in contrast to kidney cortex and OM mitochondria for which responses to succinate±rotenone were similar, indicating that reverse electron transfer via complex I played less prominent regulatory role in the kidney than the heart. In addition, oxaloacetate, a TCA cycle intermediate, was shown to accumulate faster in the heart compared to the kidney, thereby potently inhibiting succinate oxidation more in the heart than the kidney. As such, the model was able to quantitatively characterize how various substrate combinations account for the differential contributions of different substrate oxidation pathways within the mitochondria towards OxPhos and ATP synthesis in the heart and kidney cortex and OM.

Conclusion

Modeling of these data provided a deeper quantitative understanding of key determinants of kinetic and molecular mechanisms involved in the differential regulation of substrate‐dependent mitochondrial bioenergetics in the heart and kidney cortex and OM.