Bivascular liver perfusion in the anterograde and retrograde modes: zonation of the response to inhibitors of oxidative phosphorylation

The acute effects of citrus flavanones on the metabolism of glycogen and monosaccharides in the isolated perfused rat liver

Citrus flavanones are often linked to their antihyperglycemic properties. This effect may be in part due to the inhibition of hepatic gluconeogenesis through different mechanisms. One of the possible mechanisms appears to be impairment of oxidative phosphorylation, which may also interfere with glycogen metabolism. Based on these facts, the purpose of the present study was to investigate the effects of three citrus flavanones on glycogenolysis in the isolated perfused rat liver. Hesperidin, hesperetin, and naringenin stimulated glycogenolysis and glycolysis from glycogen with concomitant changes in oxygen uptake. At higher concentrations (300 μM), hesperetin and naringenin clearly altered fructose and glucose metabolism, whereas hesperidin exerted little to no effects. In subcellular fractions hesperetin and naringenin inhibited the activity of glucose 6-phosphatase and glucokinase and the mitochondrial respiration linked to ADP phosphorylation. Hesperetin and naringenin also inhibited the transport of glucose into the cell. At a concentration of 300 μM, the glucose influx rate inhibition was 83% and 43% for hesperetin and naringenin, respectively. Hesperidin was the less active among the assayed citrus flavanones, indicating that the rutinoside moiety noticeably decrease the activity of these compounds. The effects on glycogenolysis and fructolysis were mainly consequence of an impairment on mitochondrial energy metabolism. The increased glucose release, due to the higher glycogenolysis, together with glucose transport inhibition is the opposite of what is expected for antihyperglycemic agents.

Influence of tamoxifen on gluconeogenesis and glycolysis in the perfused rat liver

The actions of tamoxifen, a selective estrogen receptor modulator used in chemotherapy and chemo-prevention of breast cancer, on glycolysis and gluconeogenesis were investigated in the isolated perfused rat liver. Tamoxifen inhibited gluconeogenesis from both lactate and fructose at very low concentrations (e.g., 5μM). The opposite, i.e., stimulation, was found for glycolysis from both endogenous glycogen and fructose. Oxygen uptake was unaffected, inhibited or stimulated, depending on the conditions. Stimulation occurred in both microsomes and mitochondria. Tamoxifen did not affect the most important key-enzymes of gluconeogenesis, namely, phosphoenolpyruvate carboxykinase, pyruvate carboxylase, fructose 1,6-bisphosphatase and glucose 6-phosphatase. Confirming previous observations, however, tamoxifen inhibited very strongly NADH- and succinate-oxidase of freeze-thawing disrupted mitochondria. Tamoxifen promoted the release of both lactate dehydrogenase (mainly cytosolic) and fumarase (mainly mitochondrial) into the perfusate. Tamoxifen (200μM) clearly diminished the ATP content and increased the ADP content of livers in the presence of lactate with a diminution of the ATP/ADP ratio from 1.67 to 0.79. The main causes for gluconeogenesis inhibition are probably: (a) inhibition of energy metabolism; (b) deviation of intermediates (malate and glucose 6-phosphate) for the production of NADPH required in hydroxylation and demethylation reactions; (c) deviation of glucosyl units toward glucuronidation reactions; (d) secondary inhibitory action of nitric oxide, whose production is stimulated by tamoxifen; (e) impairment of the cellular structure, especially the membrane structure. Stimulation of glycolysis is probably a compensatory phenomenon for the diminished mitochondrial ATP production. The multiple actions of tamoxifen at relatively low concentrations can represent a continuous burden to the overall hepatic functions during long treatment periods.

Effects of metformin on glucose metabolism of perfused rat livers

Although metformin has been used to treat type 2 diabetes for several decades, the mechanism of its action on glucose metabolism remains controversial. To further assess the effect of metformin on glucose metabolism this work was undertaken to investigate the acute actions of metformin on glycogenolysis, glycolysis, gluconeogenesis, and ureogenesis in perfused rat livers. Metformin (5 mM) inhibited oxygen consumption and increased glycolysis and glycogenolysis in livers from fed rats. In perfused livers of fasted rats, the drug (concentrations higher than 1.0 mM) inhibited oxygen consumption and glucose production from lactate and pyruvate. Gluconeogenesis and ureogenesis from alanine were also inhibited. The cellular levels of ATP were decreased by metformin whereas the AMP levels of livers from fasted rats were increased. Taken together our results indicate that the energy status of the cell is probably compromised by metformin. The antihyperglycemic effect of metformin seems to be the result of a reduced oxidative phosphorylation without direct inhibition of key enzymatic activities of the gluconeogenic pathway. The AMP-activated protein kinase cascade could also be a probable target for metformin, which switches on catabolic pathways such as glycogenolysis and glycolysis, while switches off ATP consuming processes.

Metabolic effects of carbenoxolone in rat liver

The action of carbenoxolone on hepatic energy metabolism was investigated in the perfused rat liver and isolated mitochondria. In perfused livers, carbenoxolone (200-300 microM) increased oxygen consumption, glucose production and glycolysis from endogenous glycogen. Gluconeogenesis from lactate or fructose, an energy-dependent process, was inhibited. This effect was already evident at a concentration of 25 microM. The cellular ATP levels and the adenine nucleotide content were decreased by carbenoxolone, whereas the AMP levels were increased. In isolated mitochondria, carbenoxolone stimulated state IV respiration and decreased the respiratory coefficient with the substrates beta-hydroxybutyrate and succinate. The ATPase of intact mitochondria was stimulated, the ATPase of uncoupled mitochondria was inhibited, and the ATPase of disrupted mitochondria was not altered by carbenoxolone. These results indicate that carbenoxolone acts as an uncoupler of oxidative phosphorylation and, possibly, as an inhibitor of the ATP/ADP exchange system. The inhibitory action of carbenoxolone on mitochondrial energy metabolism could be contributing to induce the mitochondrial permeability transition (MPT), a key phenomenon in apoptosis. The results of the present study can explain, partly at least, the in vivo hepatotoxic actions of carbenoxolone that were found in a previous clinical evaluation.

Effects of the Kielmeyera coriacea extract on energy metabolism in the rat liver

Kielmeyera coriacea Mart is a medicinal plant of the Clusiacea (Guttiferae) family used by the native population of Brazil in the treatment of several tropical diseases such as malaria, schistosomiasis, leishmaniasis, and fungal or bacterial infections. Kielmeyera coriacea is also effective as an antidepressant drug. Extracts of the plant are rich in xanthones. Compounds of this class have been reported to inhibit mitochondrial energy metabolism. For this reason the action of the Kielmeyera coriacea extract on hepatic energy metabolism was investigated in the present work, using isolated rat liver mitochondria and the perfused rat liver. In perfused livers the extract (20-80 microg/ml) caused stimulation of oxygen consumption, inhibition of gluconeogenesis and stimulation of glycogenolysis and glycolysis. In isolated mitochondria the Kielmeyera coriacea extract (5-20 microg/ml) stimulated state IV respiration, reduced the ADP/O ratio and decreased the respiratory coefficient. The activities of succinate-oxidase, NADH-oxidase, NADH dehydrogenase and succinate dehydrogenase were inhibited. The ATPase of intact mitochondria was stimulated and the ATPase of uncoupled mitochondria was inhibited. The results of this investigation suggest that the Kielmeyera coriacea extract impairs the hepatic energy metabolism by acting as mitochondrial uncoupler and inhibitor of enzymatic activities linked to the respiratory chain. The impairment of mitochondrial energy metabolism could lead to adverse metabolic effects by the use of the crude extract, but it could equally be the basis of its antiprotozoan and antifungal effects.

Metabolic effects of propofol in the isolated perfused rat liver

Inhibitory effects of the intravenous anaesthetic propofol on mitochondrial energy metabolism have been reported by several authors. Impairment of energy metabolism is usually coupled to reduction in ATP production, which in turn is expected to lead to several alterations in cell metabolism such as stimulation of glycolysis and inhibition of gluconeogenesis. The present work aimed at finding an answer to the question of how propofol affects energy metabolism-linked parameters in the isolated perfused rat liver. In the fed state, propofol increased glycogenolysis (glucose release), glycolysis (lactate and pyruvate production) and oxygen uptake in the range between 10 and 500 microM. In the liver of fasted rats, propofol up to 100 microM increased oxygen uptake but decreased gluconeogenesis from three different substrates: lactate, alanine and glycerol. When lactate was the substrate 50% inhibition occurred at a propofol concentration of 50 microM. Propofol (100 microM) decreased the ATP content of the liver (-33.3%), increased the AMP content (+25%) and decreased the ATP/ADP and ATP/AMP ratios (49 and 45%, respectively). Most effects of propofol are probably due to impairment of oxidative phosphorylation. Particularly, the combined differential action on oxygen uptake (stimulation) and gluconeogenesis (inhibition) is strongly suggestive of an uncoupling action also under the conditions of the intact cell. This effect, in turn, is consistent with the reported high affinity of the cellular hepatic structure, especially membranes, for propofol.

The action of oxybutynin on haemodynamics and metabolism in the perfused rat liver

The present study was planned to investigate the possible action of oxybutynin on liver haemodynamics and its influence on metabolic variables. The isolated liver perfused either bivascularly or monovascularly in the non-recirculating system was used for the experiments and Krebs/Henseleit-bicarbonate buffer (pH 7.4) as a perfusion fluid. Oxybutynin (25-200 microM) was infused, the infusion time for each concentration being 14 min. Portally infused oxybutynin increased the perfusion pressure starting at 100 microM. Oxygen uptake was diminished, also starting at 100 microM. Arterially infused oxybutynin also increased the perfusion pressure in the hepatic artery. Lactate and pyruvate releases were considerably diminished by oxybutynin. Glucose release showed a small initial stimulation, then returned to values slightly below the basal ones. Cessation of oxybutynin infusion resulted in progressive stimulation of glucose release. When Ca2+ was omitted all effects of oxybutynin vanished. The hepatic contents of glucose, glucose 6-phosphate and lactate in the presence of 200 microM oxybutynin increased 7.8-, 4.6- and 5.1 times, respectively. The pyruvate content was not changed. The ATP content was diminished by 26.6% in the presence of 200 microM oxybutynin, but the AMP content was increased by 64.3%. The ADP content was not changed. Apparently, upon administration of oxybutynin, a considerable fraction of the liver parenchyma ceased to be irrigated or almost so, which is apparent from the concomitant inhibition of oxygen uptake, pressure increase and inhibition of glucose, lactate and pyruvate release together with the simultaneous intracellular accumulation of glucose, lactate and glucose 6-phosphate.

Effect of Stryphnodendron adstringens (barbatimão) on energy metabolism in the rat liver

The action of a barbatimão extract on hepatic energy metabolism was investigated using isolated mitochondria and the perfused rat liver. In mitochondria the barbatimão extract inhibited respiration in the presence of ADP and succinate. Stimulation occurred, however, after ADP phosphorylation (state IV respiration). The ADP/O and respiratory control ratios were reduced. The activities of succinate-oxidase, NADH-oxidase and the oxidation of ascorbate were inhibited. The ATPase of intact mitochondria was stimulated, but the ATPases of uncoupled and disrupted mitochondria were inhibited. In perfused livers the extract caused stimulation of oxygen consumption, inhibition of gluconeogenesis and stimulation of glycolysis. Glucose release due to glycogenolysis was stimulated shortly after the introduction of the extract, but inhibition gradually developed as the infusion was continued. Apparently the barbatimão extract impairs the hepatic energy metabolism by three mechanisms: (1) uncoupling of oxidative phosphorylation, (2) inhibition of mitochondrial electron transport, and (3) inhibition of ATP-synthase.

Metabolic effects of trifluoperazine in the liver and the influence of calcium

The effects of trifluoperazine on hepatic cell metabolism were investigated using isolated perfused rat liver. The following effects of trifluoperazine were found: (1) trifluoperazine inhibited oxygen uptake, the site of action being the mitochondria. Half-maximal inhibition occurred at concentrations around 50 microM; with 100 microM trifluoperazine the effect was already maximal. When Ca2+ was withdrawn from the perfusion medium and the intracellular Ca2+ pools were exhausted, the inhibitory action on respiration was no longer observable. The reintroduction of Ca2+ restored inhibition. (2) Glycogenolysis and glycolysis were not significantly affected during the infusion of trifluoperazine. After stopping trifluoperazine infusion, however, glycogenolysis (glucose release) experienced a transitory stimulation. (3) Gluconeogenesis from lactate as the carbon source was inhibited by trifluoperazine. This inhibition was approximately proportional to the inhibition of oxygen uptake. Withdrawal of Ca2+ diminished, but it did not eliminate, inhibition of gluconeogenesis. (4) Ketogenesis was also inhibited in parallel with the inhibition of oxygen uptake. Withdrawal of Ca2+ from the perfusion fluid also abolished this action. (5) The effects of trifluoperazine were reverted very slowly when its infusion was stopped. The recovery of oxygen uptake at 50 min after cessation of the infusion was only 30%. Uptake of the substance was very fast. Absence of Ca2+ did not affect uptake. It was concluded that inhibition of mitochondrial energy metabolism is one of the most prominent effects of trifluoperazine in the liver. The fact that this inhibition depends on Ca2+ is unique.