Electron transport and protection of liver slices in the late stage of paracetamol injury

Cytoprotective effects of carvedilol against oxygen free radical generation in rat liver

The protective effects of carvedilol, an antihypertensive agent, against oxidative injury caused by acetaminophen were studied in rat liver. Male Wistar rats (250 +/- 30 g) were pre-treated with carvedilol (3.6 mg/kg, p.o.) for 10 days and on the 11th day received an overdose of acetaminophen (800 mg/kg, p.o.). Four hours after acetaminophen administration, blood was collected to determine serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT). After that, rats were killed and the livers were excised to determine reduced glutathione (GSH), thiobarbituric acid reactive substances (TBARS) and carbonyl protein contents, and the activity of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione S-transferase (GST), and also the DNA damage index. Acetaminophen significantly increased the levels of TBARS, the DNA damage and SOD, AST and ALT activities. Carvedilol was able to prevent lipid peroxidation, protein carbonilation and DNA fragmentation caused by acetaminophen. Moreover, this drug prevented increases in SOD, AST and ALT activities. These results show that carvedilol exerts cytoprotective effects against oxidative injury caused by acetaminophen in rat liver. These effects are probably related to the O2*- scavenging property of carvedilol or its metabolites.

Paracetamol (acetaminophen)-induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches

An overview is presented on the molecular aspects of toxicity due to paracetamol (acetaminophen) and structural analogues. The emphasis is on four main topics, that is, bioactivation, detoxication, chemoprevention, and chemoprotection. In addition, some pharmacological and clinical aspects are discussed briefly. A general introduction is presented on the biokinetics, biotransformation, and structural modification of paracetamol. Phase II biotransformation in relation to marked species differences and interorgan transport of metabolites are described in detail, as are bioactivation by cytochrome P450 and peroxidases, two important phase I enzyme families. Hepatotoxicity is described in depth, as it is the most frequent clinical observation after paracetamol-intoxication. In this context, covalent protein binding and oxidative stress are two important initial (Stage I) events highlighted. In addition, the more recently reported nuclear effects are discussed as well as secondary events (Stage II) that spread over the whole liver and may be relevant targets for clinical treatment. The second most frequent clinical observation, renal toxicity, is described with respect to the involvement of prostaglandin synthase, N-deacetylase, cytochrome P450 and glutathione S-transferase. Lastly, mechanism-based developments of chemoprotective agents and progress in the development of structural analogues with an improved therapeutic index are outlined.

Cell protection by fructose is independent of adenosine triphosphate (ATP) levels in paracetamol injury to rat liver slices

Fructose protects cells against several types of injury but the mechanism of protection is uncertain. We have used paracetamol injury in rat liver slices as a model system to investigate the role of ATP levels in protection by fructose. Fructose depletes ATP levels in a concentration-dependent fashion in liver slices obtained from non-induced rats. Liver slices recover their ATP levels in the presence of fructose concentrations up to 10 mM. However, in the presence of of 20mM fructose, ATP levels are depleted for the duration of 240 min incubation. Adenine at 100 microM reverses the ATP depletion induced by 20 mM fructose in slices over 240 min incubation. Liver slices obtained from phenobarbitone induced rats were exposed to 10 mM paracetamol for 120 min and, then, incubated without paracetamol, with or without fructose for another 240 min. Introduction of 10 mM or 20 mM fructose in the second stage of incubation prevents paracetamol-induced injury. Fructose at 20 mM induces a rapid and marked depletion in slice ATP levels and these remain low throughout the second 240 min incubation period. Fructose at 10 mM maintains high ATP levels, even in paracetamol-treated slices. There is a profound protective effect against paracetamol-induced injury by either concentration. This suggests that protection is not dependent on high or on low ATP levels. Incubation of paracetamol-treated slices in the presence of 20 mM fructose plus 100 microM adenine in the second 240 min incubation period still results in the same level of protection as with 20 or 10 mM fructose along while reversing the ATP depletion observed with 20 mM fructose.

Protection in the late stages of paracetamol-induced liver cell injury with fructose, cyslosporin A and trifluoperazine

A fixed combination of the three components, fructose, cyclosporin A and trifluoperazine (FCAT), was found to protect in the late stage of paracetamol-induced liver cell injury both in vivo and in an in vivo/in vitro system. Rats pre-induced with phenobarbitone were given a paracetamol dose of 1 g/kg i.p. The combination of FCAT was given orally 3 h or 3 and 8 h after paracetamol and was able to afford protection as seen by measurements of plasma alanine transaminase (ALT) levels at 24 h. In the in vivo/in vitro system, rats pre-induced with phenobarbitone were dosed with paracetamol 1 g/kg i.p. to initiate injury and liver slices were then taken 3, 4 and 5 h later. The liver slices were then incubated for up to 18 h with the protective agents (FCAT) and the progression of injury followed. Injury was assessed by lactate dehydrogenase (LDH) leakage into the medium and potassium content of the slices. FCAT significantly reduced the injury even as assessed after 5 h in vivo initiation and 18 h progression in vitro. Mitochondrial membrane potential was also maintained in the FCAT-treated liver slices from paracetamol-treated rats as seen by the ability to maintain a gradient of triphenyl methyl phosphonium (TPMP+) between the cell and external medium. All three compounds are required for protection, indicating that more than one event is critical to the survival of the cell and each target point needs to be protected for effective long-term cell survival. The in vivo/in vitro system has been found to give a better comparability to the in vivo situation than injury models that take 6 h or less.

High resolution NMR spectroscopic studies on the metabolism and futile deacetylation of 4-hydroxyacetanilide (paracetamol) in the rat

Paracetamol (4-hydroxyacetanilide, acetaminophen) was synthesized with the acetyl group labelled with C2H3 (paracetamol-C2H3), and dosed to rats i.p. at 25 mg/kg (N = 5) and 40 mg/kg (N = 3) body weight. Paracetamol, with a 13CH3 in the acetyl group (paracetamol-13CH3) was also synthesized and dosed to rats i.p. at 40 mg/kg (N = 3). The metabolism and excretion of the 2H-labelled compound was followed in the rat using 600 MHz 1H and 92.1 MHz 2H NMR spectroscopy of urine collected 0-8, 8-24, 24-32 and 32-48 hr post-dosing. The metabolism of paracetamol-13CH3 was also monitored using 600 MHz 1H NMR spectroscopy of urine collected 0-8, 8-24 and 24-48 hr post-dosing. For paracetamol-C2H3 the total recovery of the sulphate, glucuronide and N-acetyl cysteinyl metabolites via the urine accounted for 61.2 +/- 14.1% of the 25 mg/kg dose and 61.4 +/- 8.8% of the 40 mg/kg dose. For paracetamol-13CH3 the recovery was 102.7 +/- 3.7% indicating that the low % urinary recovery with the C2H3-labelled drug is the result of isotope effects on the disposition of paracetamol. In the case of the paracetamol-C2H3, quantitative 1H NMR analysis of urine showed that 13.3 +/- 0.5 and 10.0 +/- 1.2 mole % (25 and 40 mg/kg, respectively) of the urinary paracetamol sulphate recovered following dosing of the deuterium labelled drug had the C2H3 acetyl groups replaced by C1H3 acetyl groups from endogenous sources. In the case of the paracetamol-13CH3 8.9 +/- 0.7 mole % of the sulphate conjugate had also been transacetylated to paracetamol-12CH3. There was no significant difference between the level of futile deacetylation observed for the deuterated and 13C-labelled drug. Overall these data indicate a high level of deacetylation followed by reacetylation (i.e. futile deacetylation) prior to excretion of paracetamol via the nephrotoxic intermediate 4-aminophenol. The level of deacetylation is much higher than has previously been thought which may cast new light on the role of 4-aminophenol in the development of paracetamol induced nephrotoxicity.

Paracetamol (acetaminophen) poisoning

Paracetamol poisoning caused by intentional overdose remains a common cause of morbidity. In this article the mechanism of toxicity and the clinical effects and treatment of poisoning, including specific antidotal therapy, are reviewed. Areas for further research directed at reducing morbidity and mortality from paracetamol poisoning are considered.

Covalent binding of morphine to isolated rat hepatocytes

Incubation of [3H]morphine with isolated hepatocytes caused covalent binding of [3H]-morphine to hepatocellular proteins. Sulfhydryl compounds protected against morphine-induced toxicity and decreased covalent binding. Analysis of covalently bound proteins in the cytosol by electrophoresis indicated that covalently bound radiolabel was associated with macromolecules greater than 25 kDa and increased throughout the incubation. In contrast, covalent binding to the particulate fraction was highly selectively associated with three protein bands of 50-53 and 33 kDa. Covalent binding of morphine to particulate fraction proteins was observed in hepatocytes which exhibited cellular damage. We conclude that the covalent binding of morphine to protein is associated with morphine-induced cytotoxicity.

Effect of paracetamol on mitochondrial membrane function in rat liver slices

The effect of paracetamol on mitochondrial function was studied using rat liver slices. Changes in the potential of the mitochondrial and plasma membrane were monitored using [3H]-triphenylmethylphosphonium (TPMP+) and [14C]thiocyanate (SCN-) probes, respectively. Liver slices were exposed to 10 mM paracetamol for various time periods (0-360 min) after loading with TPMP+. The release of TPMP+ which correlates with a decrease in the mitochondrial membrane potential became significant after 30 min incubation with 10 mM paracetamol. The change in the mitochondrial membrane potential was shown to be independent of cytochrome P450 activity. No significant change in plasma membrane potential was observed, until the release of lactate dehydrogenase (LDH) had begun, 4 hr after exposure, reflecting the ultimate stages of cell injury by paracetamol. These results suggest that paracetamol elicits a direct effect on the mitochondrial function before cell injury develops and adds further evidence to the role of mitochondria in paracetamol toxicity.

Prevention of paracetamol-induced liver injury by fructose

Hepatic cell injury was studied in an in vitro system using rat liver slices incubated in two stages. During the first 2 hr slices were exposed to 10 mM paracetamol, this was absent during the subsequent 4 hr of incubation. Cell damage was quantified at the end by measuring leakage of lactic dehydrogenase, increase in water content and potassium loss. Treatment of slices with 20 mM fructose in the second period of incubation prevented paracetamol-induced damage. The effect of fructose was not modified by the continued presence of paracetamol in the second incubation period. The inhibition of glycolysis either with 1 mM NaF or 10 microM iodoacetate blocked the effect of fructose. The protective effect afforded by fructose was not duplicated by the addition of lactate. All these findings strongly suggest an increase in intracellular ATP levels as the most probable explanation for the protective effect of fructose, and point to fructose as a potentially useful therapeutic tool for protection of the liver late in paracetamol intoxication.