Effects of Octanoate and Oleate on Energy Metabolism in the Perfused Rat Liver

Abstract

The effects of octanoate and oleate were studied in the isolated fasted rat liver perfused without substrate and were compared with the effects of lactate. Measurements were made of hepatic adenine nucleotide content, formation of ketone bodies, urea and glucose and uptake of fatty acids, lactate and oxygen. Calculations of the Krebs cycle activity and the flux through the oxidation at C-3 were carried out. Comparing the extra oxygen consumption with extra ATP needs after addition of the fatty acids or lactate, ADP : 0 ratios were estimated for fatty acid oxidation and lactate gluconeogenesis.1. Octanoate as well as oleate induced a net decrease in 4 T P and an increase in AMP content of the liver. Total adenine nucleotides were unaltered. ATP : ADP ratios were lowered while the adenylate kinase mass-action ratio increased.2. The Krebs cycle activity was suppressed by both fatty acids and enhanced by lactate. Oxidation at C-3 was strongly stimulated by the fatty acids and was unaffected by lactate.3. An apparent ADP : 0 ratio of 2.7 was obtained after lactate addition, indicating a tightly coupled oxidative phosphorylation for the liver preparations used. Octanoate and oleate gave extremely low ratios of near one and near zero respectively.4. Perfusion with oligomycin caused a severe drop in oxygen uptake by the liver, which was unaltered after addition of fatty acids. Oligomycin added after the fatty acids caused an immediate fall in oxygen uptake to the level observed with oligomycin alone. 2,CDinitrophenol was able to stimulate the oligomycin-depressed respiration, both in the absence and in the presence of fatty acids. These results indicate that under our experimental conditions the fatty acids had no uncoupling effect and that microsomal fatty acid oxidation had to be minimal.5. Incorporation of [3H]leucine into proteins in liver and medium remained unchanged after addition of fatty acids, indicating that the low apparent ADP : 0 ratios are not the result of an enhanced synthesis of proteins.6. The possible relationship between fatty acid transport, energy-wasting futile cycles and the low apparent ADP : 0 ratios observed during fatty acid oxidation is discussed.Enzynies. 3-Hydroxy-3-methylglutaryl-CoA reductase or mevalonate : NADPf oxidoreductase (CoA-acylating) (EC

Effects of insulin and cytosolic redox state on glucose production pathways in the isolated perfused mouse liver measured by integrated 2H and 13C NMR

“…Decreased GNG during substrate deficiency is expected, but less obvious are the reasons why removal of both pyruvate and lactate dramatically impairs tricarboxylic-acid-cycle flux in both the presence and absence of insulin (Table 2). This observation, however, agrees with results reported by DeBeer et al [48], who used an independent method to calculate tricarboxylic-acid-cycle flux in rat livers perfused with octanoate in the absence of gluconeogenic precursors and found that tricarboxylic-acid-cycle flux was suppressed by more than 2-fold. The cause of this is unknown, but may reflect a decreased energy requirement when the liver is not synthesizing glucose.…”

“…We also found that mouse livers perfused with octanoate released ketones at a high rate compared with tricarboxylic-acid-cycle flux (Figure 3). Under these conditions it has been shown that ketone metabolism is responsible most of the oxygen consumption [48]. Taken together, these observations suggest a tight interaction between GNG and tricarboxylic-acid-cycle flux, whereas other pathways of energy production, such as β-oxidation, are, per se, less influential.…”

“…Approx. 25 % of livers were rejected because they did not maintain a pO 2 48.6 ng/ ml insulin plus 1.5 mM lactate and 0.15 mM pyruvate. Livers were perfused for 1 h, and effluent was collected from every 15 min to measure glucose production and O 2 consumption and to determine 2 H and 13 C enrichment patterns.…”

On the mechanism of the so-called uncoupling effect of medium- and short-chain fatty acids

“…9), it seemed plausible to speculate that the increased respiration is due to uncoupling of mitochondrial energy-conserving machinery. This was, however, incompatible with the observation [3] that the stimulation by octanoate and oleate of the respiration of perfused liver was abolished by oligomycin, a known inhibitor of ATP synthesis in mitochondria. It has been therefore proposed [3] that the increased respiration is not evoked by directly dissipating the mitochondrial energy charge but rather by some processes utilizing ATP.…”

“…It has long been recognized that fatty acids increase the rate of respiration of perfused liver [1][2][3][4][5] and isolated hepatocytes [6][7][8]. This effect is Abbreviations: DDA +, dimethyldibenzylammonium cation; Tricine, N-tris(hydroxymethyl)methylglycine.…”