Effect of citric acid-cycle intermediates on oxaloacetate utilization and succinate oxidation

One-year high fat diet affects muscle-but not brain mitochondria

It is well known that few weeks of high fat (HF) diet may induce metabolic disturbances and mitochondrial dysfunction in skeletal muscle. However, little is known about the effects of long-term HF exposure and effects on brain mitochondria are unknown. Wistar rats were fed either chow (13E% fat) or HF diet (60E% fat) for 1 year. The HF animals developed obesity, dyslipidemia, insulin resistance, and dysfunction of isolated skeletal muscle mitochondria: state 3 and state 4 were 30% to 50% increased (P<0.058) with palmitoyl carnitine (PC), while there was no effect with pyruvate as substrate. Adding also succinate in state 3 resulted in a higher substrate control ratio (SCR) with PC, but a lower SCR with pyruvate (P<0.05). The P/O2 ratio was lower with PC (P<0.004). However, similar tests on isolated brain mitochondria from the same animal showed no changes with the substrates relevant for brain (pyruvate and 3-hydroxybutyrate). Thus, long-term HF diet was associated with obesity, dyslipidemia, insulin resistance, and significantly altered mitochondrial function in skeletal muscle. Yet, brain mitochondria were unaffected. We suggest that the relative isolation of the brain due to the blood-brain barrier may play a role in this strikingly different phenotype of mitochondria from the two tissues of the same animal.

Substrate modulation of fatty acid effects on energization and respiration of kidney proximal tubules during hypoxia/reoxygenation

Kidney proximal tubules subjected to hypoxia/reoxygenation develop a nonesterified fatty acid-induced energetic deficit characterized by persistent partial mitochondrial deenergization that can be prevented and reversed by citric acid cycle substrates. To further assess the role of competition between fatty acids and substrates on inner membrane substrate carriers in the deenergization and the contribution to deenergization of fatty acid effects on respiratory function, digitonin-permeabilized rabbit and mouse tubules were studied using either addition of exogenous oleate after control normoxic incubation or increases of endogenous fatty acids produced by hypoxia/reoxygenation. The results demonstrated major effects of matrix oxaloacetate accumulation on succinate-supported energization and respiration and their modification by fatty acids. Improvements of energization in the presence of fatty acids by glutamate were shown to result predominantly from lowering matrix oxaloacetate rather than from amelioration of transmembrane cycling of fatty acids and uncoupling. Mouse tubules had 2.5 fold higher rates of succinate utilization, which resulted in stronger effects of oxaloacetate accumulation than rabbit tubules. Hypoxia/reoxygenation induced respiratory inhibition that was more severe for complex I-dependent substrates. Fatty acids themselves did not acutely contribute to this respiratory inhibition, but lowering them during 60 min. reoxygenation to allow recovery of ATP during that period alleviated it. These data clarify the basis for the nonesterified fatty acid-induced mitochondrial energetic deficit in kidney proximal tubules that impairs structural and functional recovery and provide insight into interactions that need to be considered in the design of substrate-based interventions to improve mitochondrial function.