The calorigenic nature of hepatic ketogenesis: an explanation for the stimulation of respiration induced by fatty acid substrates

The stimulation of tricarboxylic acid-cycle flux by alpha-adrenergic agonists in perfused rat liver

Output of 14CO2 from 1-14C-labelled glutamate, 2-oxoglutarate or octanoate and from 4-methyl-2-oxo[2-14C]pentanoate was increased by more than 100% after infusion of phenylephrine into perfused livers of fed rats. Infusion of ethanol or sorbitol raised 3-hydroxybutyrate/acetoacetate ratios and decreased the output of 14CO2. Increases in 14CO2 output induced by phenylephrine were observed in the presence or absence of ethanol or sorbitol and were accompanied by elevated 3-hydroxybutyrate/acetoacetate ratios under all conditions examined. Phenylephrine had no effect on total tissue ATP/ADP ratios in livers from fed or starved rats. The data suggest that phenylephrine-induced increases in tricarboxylic acid-cycle flux do not arise from lowered matrix NADH/NAD+ or ATP/ADP ratios.

Mechanism of the stimulation of respiration by fatty acids in rat liver

The mechanism of stimulation of hepatic respiration by fatty acids was studied in isolated rat hepatocytes. Stimulation of respiration by fatty acids varied from about 35% to about 105% depending on chain length. The stimulatory effect of octanoate (1 mM) or oleate (0.5 mM) was prevented by oligomycin (2 micrograms/ml). With carboxyatractyloside (100 microM) and ouabain (2 mM) the stimulation of respiration was partially inhibited (by 50-70 and 50-60%, respectively). From these results it can be concluded that the increased rate of respiration after addition of fatty acids is coupled to ATP synthesis. A large part (50-60%) of this ATP is utilized by the (Na+ + K+)-ATPase.

The effect of acute ethanol treatment on rates of oxygen uptake, ethanol oxidation and gluconeogenesis in isolated rat hepatocytes

In hepatocytes isolated from fed rats, acute ethanol pretreatment (at a dose of 5.0 g/kg body wt.) did not change rates of O2 uptake. In cells from starved animals, acute ethanol pretreatment increased O2 uptake by 17-29%. The increased O2 uptake in hepatocytes from starved rats was not accompanied by increased rates of ethanol oxidation, but was accompanied by increased rates of gluconeogenesis under some conditions. The provision of ethanol (10 mM) as a substrate to cells from fed or starved rats decreased O2 uptake in the absence of other substrates or in the presence of lactate, and increased it in the presence of pyruvate or lactate and pyruvate. The results of this study show that the acute effects of ethanol on liver O2 uptake are dependent on the physiological state of the liver. Previously reported large (2-fold) increases in O2 uptake after acute ethanol pretreatment may have been an artefact owing to low control uptake rates (approximately 1.8 micromol/min per g wet wt. of cells) in the liver preparation used. The ATP contents (2.4-2.6 micromol/g wet wt. of cells) and rates of O2 uptake (2.5-5.0 micromol/min per g wet wt. of cells) of cells used in the present study were the same as values reported under conditions close to those in vivo. Therefore the increase in O2 uptake in cells from starved rats after acute ethanol pretreatment is likely to be of physiological significance.

Reciprocal effects of energy utilization on palmitate oxidation and esterification in hepatocytes of fed rats

The effects of the energy-dependent process of urea synthesis from NH4Cl on the partition of [1-14C]palmitate between oxidation and esterification were examined in hepatocytes of fed rats. A high rate of urea formation from NH4Cl resulted in stimulation of total palmitate oxidation by 25 and 15% at 0.2 and 1 mM fatty acid, respectively. The stimulation of palmitate oxidation was reciprocally correlated with diminished palmitate incorporation into lipids, mainly triacylglycerols. This relationship was almost stoichiometric. NH4Cl increased the palmitate oxidation/esterification ratio from 0.72 to 1.13 and from 0.94 to 1.36 in the presence of 0.2 mM and 1 mM palmitate, respectively. The transaminase inhibitor, aminooxyacetate, strongly inhibited urea synthesis from NH4Cl, had little effect on the low beta-hydroxybutyrate/acetoacetate ratio in the presence of NH4Cl, completely reversed the changes in palmitate metabolism caused by NH4Cl and did not affect palmitate metabolism in the absence of NH4Cl. Therefore, the increased utilization of energy for urea synthesis was the causative factor by which NH4Cl stimulated total palmitate oxidation and led in consequence to its decreased esterification into lipids. Accordingly, these observations indicate that in liver cells the rate of ATP utilization is one of the determinants of triacylglycerol synthesis.

Effect of insulin on ketogenesis and fatty acid synthesis in rat hepatocytes incubated with dichloroacetate

In parenchymal liver cells isolated from fed rats, insulin increased the formation of 14CO2 from [1-14C]pyruvate (and presumably the flux through pyruvate dehydrogenase) by 14%. Dichloroacetate, an activator of the pyruvate dehydrogenase complex, stimulated this process by 133%. As judged from the conversion of [2-14C]pyruvate to 14CO2, the tricarboxylic acid cycle activity was not affected by insulin, but it was depressed by dichloroacetate. In hepatocytes from fed rats, incubated with glucose as the only carbon source, dichloroacetate caused a stimulation (31%) of fatty acid synthesis, measured as 3H incorporation from 3H2O into fatty acid, and an increased (134%) accumulation of ketone bodies (acetoacetate + D-3-hydroxybutyrate). Dichloroacetate did not affect ketone body formation from [14C]palmitate, suggesting that the increased accumulation of ketone bodies resulted from acetyl-CoA derived from pyruvate. Insulin stimulated fatty acid synthesis in hepatocytes from fed rats. In the combined presence of insulin plus dichloroacetate, fatty acid synthesis was more rapid than in the presence of either insulin or dichloroacetate, whereas the accumulation of ketone bodies was smaller than in the presence of dichloroacetate alone. Although pyruvate dehydrogenase activity, which is rate-limiting for fatty acid synthesis in hepatocytes from fed rats, is stimulated both by insulin and by dichloroacetate, the reciprocal changes in fatty acid synthesis and ketone body accumulation brought about by insulin in the presence of dichloroacetate suggest that insulin is also involved in the regulation of fatty acid synthesis at a mitochondrial site after pyruvate dehydrogenase, possibly at the partitioning of acetyl-CoA between citrate and ketone body formation.

The contribution of hepatic metabolism to diet-induced thermogenesis

In order to examine the possible contribution of the liver to diet-induced thermogenesis, we examined the metabolism of hepatocytes from rats that had been fed a varied choice of highly palatable human food items ("cafeteria feeding"). Liver cells derived from cafeteria-fed rats that had been fasted for 20 hours showed marked increases in rates of respiration and gluconeogenesis in the presence of glycerol or sorbitol. These cells were also much less sensitive to the inhibitory effects of rotenone than were hepatocytes of control animals. hepatocytes from fasted cafeteria-fed rats also demonstrated a substantially enhanced rate of fatty acid oxidation and ketogenesis, which did not appear to be correlated with cellular demands for adenosine triphosphate (ATP). This apparent fall in metabolic efficiency was confirmed by calorimetric studies, which indicated augmented cellular heat production. These changes in hepatic metabolism, associated with cafeteria-feeding, suggest that the liver may have a significant role in diet-induced thermogenesis.

Some unexplained features of hepatic ethanol oxidation

Ethanol is an excellent substrate for the liver, competing effectively for oxidation with other substrates such as fatty acids. Using isolated liver cells from fed and starved rats, we have found that ethanol strongly inhibits Krebs cycle oxidations, so that the combustion through the cycle of acetyl CoA, derived from fatty acids, is reduced more than 50%. In contrast, fatty acid beta-oxidation to acetyl CoA is inhibited only 20% in fed and fasted states. Ethanol was not antiketogenic. In the fed state, octanoate but not palmitate inhibited ethanol oxidation whereas in cells from fasted rats palmitate inhibited ethanol oxidation. Gluconeogenesis from lactate was reduced 50% in hepatocytes from fasted rats but oxygen consumption was unaffected. This paradoxical maintenance of oxygen consumption in a state where the only overt need for ATP synthesis is depressed, suggests that ethanol oxidation may not be exclusively coupled to ATP synthesis but also can be linked to other energy transducing processes.

Influence of fatty acids on energy metabolism. 1. Stimulation of oxygen consumption, ketogenesis and CO2 production following addition of octanoate and oleate in perfused rat liver

Changes in metabolic rates (oxygen consumption, ketogenesis, 14CO2 production from labelled fatty acids, glycolysis) following the addition of octanoate or oleate were studied in isolated livers from fed and starved rats perfused with Krebs-Henseleit bicarbonate buffer in a non-recirculating system. The following results were obtained. The infusion of fatty acids caused a large increase in the rate of oxygen consumption. The effect was greater with octanoate than with oleate and was half-maximal with fatty acid concentrations (free plus albumin bound) around 0.1 mM. The effects of oleate were only partially suppressed when the perfusate contained albumin concentrations near the physiological range. When fatty acids were oxidized at high rates, the glycolytic rate was diminished by 50%. The increase in oxygen consumption could not be explained fully by the increased ATP demand for fatty acid metabolism or by a compensation for the diminished extramitochondrial ATP generation. In the presence of phenylalkyl oxirane carboxylic acid, an inhibitor of the transport of long-chain acyl-CoA derivates into the mitochondria, ketogenesis and 14CO2 production from labelled oleate were strongly inhibited, whereas the increase in oxygen consumption was only slightly affected. In the presence of antimycin A, the increase in oxygen consumption due to fatty acids was totally abolished. Following pretreatment of rats with ciprofibrate (induction of enzymes for peroxisomal beta-oxidation of long-chain fatty acids), ketogenesis (but not 14CO2 production) from oleate was enhanced threefold. The increase in oxygen consumption, however, was not affected. In conclusion, the increase in hepatic oxygen consumption due to addition of fatty acids reflects a mitochondrial process; it is, in part, independent of the ATP demand of the cell. An uncoupling-like effect of fatty acids on the respiratory chain and its possible physiological significance in ketogenesis are discussed.

The effects of metformin compared to the effects of phenformin on the lactate production and the metabolism of isolated parenchymal rat liver cell

The metabolic effects of metformin were compared to the effects of phenformin on isolated parenchymal liver cells from fed and fasted rats with ethanol or glycerol as the only substrate. Both biguanides caused a fall in the hepatic oxygen-consumption, in the cellular ATP-content and in the ATP/ADP-ratio. The lactate production and the concentration-ratios of lactate/pyruvate and of beta-hydroxybutyrate/acetoacetate rose. The production rate of ketone bodies remained unchanged. This response was the same whether the hepatocytes were from fed or fasted rats and whether glycerol or ethanol was substrate. Only quantitative differences in the response on the biguanides were detected. The effects of the biguanides were dose dependent. Phenformin was ten times more potent than metformin. The same holds for their therapeutical potency. The findings indicate inhibition of the oxidative phosphorylation by both biguanides resulting in reduction of the cytoplasmic and the mitochondrial redox potentials causing enhanced lactate production. It is concluded that metformin excerts the same effects as phenformin on the hepatic metabolism, when the concentration ratio is about ten to one. Both biguanides give rise to elevated lactate production. This effect is highly increased when ethanol is present.

Metabolic effects of bacitracin in isolated rat hepatocytes

Bacitracin is a proteolytic inhibitor which interacts with the intracellular processing of insulin. Its effects on pyruvate, fatty acid and amino acid metabolism were examined in rat hepatocyte suspensions. Bacitracin (0.25-1.0 mM) increased the oxidation of [1-14C]pyruvate by 50-70% and presumably therefore increased the flux through pyruvate dehydrogenase. This was found both in the presence of extracellular Ca2+ and in its absence, but not in the presence of 2 mM-2-chloropropionate, which inhibits pyruvate dehydrogenase kinase. Insulin did not further stimulate [1-14C]pyruvate oxidation in the presence of 1 mM-bacitracin. Bacitracin decreased 14CO2 formation from [2-14C]pyruvate (20-40%) and [U-14C]palmitate (30-70%), suggesting a decreased flux through the tricarboxylic acid cycle. Fatty acid oxidation before acetyl-CoA formation was also decreased. Bacitracin decreased the incorporation of label from [3H]leucine into protein in the absence of insulin, but not in its presence. Bacitracin is commonly used in studies on insulin action. Our results suggest that in such studies the effects noted may be related not only to an interaction of bacitracin with the intracellular processing of insulin but also to direct metabolic effects of bacitracin independent of insulin.