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OBJECTIVE: To measure hepatic mitochondrial respiration as well as the ability of hepatic mitochondria to transport reducing equivalents by shuttle systems in rats fed an energy dense diet. DESIGN: Rats were fed a control (CD) or energy dense (ED) diet for 15 days. MEASUREMENTS: State 3 and State 4 oxygen consumption were measured in isolated mitochondria using glutamate+malate or pyruvate+malate as substrate. We also measured malate-aspartate shuttle activity and mitochondrial alpha-glycerophosphate dehydrogenase activity. RESULTS: ED rats, in comparison with CD rats, showed a significantly greater energy intake without a corresponding greater body weight gain and carcass lipid content. Compared to CD rats, ED rats also showed a significant increase in resting metabolic rate, which was abolished by propranolol. Hepatic mitochondrial state 3 respiration using glutamate+malate or pyruvate+malate as substrate as well as malate-aspartate shuttle activity significantly decreased, while mitochondrial alpha-glycerophosphate dehydrogenase significantly increased in ED rats compared to CD rats. CONCLUSIONS: Mitochondrial NADH oxidation is reduced and a greater fraction of cytoplasmic NADH can be transported to the mitochondria through the alpha-glycerophosphate shuttle and oxidized through the respiratory chain from complex II onwards. This mechanism could lead to a decrease in hepatic metabolic efficiency which is in line with the increased energy expenditure occurring in rats fed an energy dense diet.

We studied the relationship between membrane potential and respiration rate in isolated liver mitochondria from rats fed an energy dense diet. We conceptually divided the system into blocks of reactions that produced or consumed mitochondrial membrane potential and then measured the kinetic response of these blocks of reactions to this potential using NAD-linked and FAD-linked substrates. We show that decreased respiration rate with an NAD-linked substrate is accounted for by decreased kinetic response of the substrate oxidation pathway to the potential. No variation in the kinetic response of the above blocks of reactions to the potential was found using an FAD-linked substrate. These results indicate that FAD-linked and NAD-linked pathways are differently affected in rats fed an energy dense diet.