Stephen M. Roberts

Principles of toxicology

Principles of toxicology : , Principles of toxicology : , کتابخانه دیجیتال جندی شاپور اهواز

The pharmacokinetics of 2,2′,5,5′-tetrachlorobiphenyl and 3,3′,4,4′-tetrachlorobiphenyl and its relationship to toxicity

The pharmacokinetics of two toxicologically diverse tetrachlorobiphenyls (TCBs) were measured in mice. After dosing to apparent steady-state conditions, 2,2,5,5-TCB was found to have a tissue elimination half-life of between 1.64 and 2.90 days. The half-life of 3,3,4,4-TCB was similar, ranging from 1.07 to 2.60 days. Systemic clearance and volume of distribution estimates were also similar for the two TCB isomers. The 3,3,4,4-isomer had a substantially greater partitioning from serum into adipose, liver, and thymic tissues. With dosing regimens developed using these measured pharmacokinetic parameters, experiments were undertaken to compare toxic potency of these two TCBs when similar tissue concentrations of the two isomers were achieved in target and storage tissues. These studies demonstrated that thymic atrophy occurs at lower 3,3,4,4-TCB doses and tissue concentrations than those required to produce hepatotoxicity. These two organ toxicities were produced only by 3,3,4,4-TCB despite the fact that equivalent or higher tissue concentrations of 2,2,5,5-TCB were achieved in vivo in all tissues. We conclude that the in vivo difference in the toxic potency of these two TCB isomers does not result from the significant differences in their tissue disposition, elimination, and ultimate bioaccumulation.

Procainamide hydroxylamine lymphocyte toxicity—I. Evidence for participation by hemoglobin

A number of lines of evidence suggest that the lupus-like symptoms associated with procainamide therapy may be caused by products of metabolic N-oxidation. In the present study, the perfusion of the isolated rat liver with a hemoglobin-free solution containing procainamide (100 microM) resulted in the rapid appearance of the N-oxidation metabolite procainamide hydroxylamine in the perfusate. Addition of procainamide hydroxylamine in vitro to whole rat blood (1-40 microM) resulted in a concentration-dependent loss of proliferative response among mononuclear cells isolated from the treated blood and cultured with mitogens (phytohemagglutinin, PHA-P: concanavalin A, Con A; and pokeweed mitogen, PWM), as well as a loss of viability. Similar effects on lymphocyte mitogen responsiveness were observed when procainamide hydroxylamine (1-40 microM) was added to rat whole splenic cell populations. Carbon monoxide or ascorbic acid pretreatment inhibited the toxicity of procainamide hydroxylamine to lymphocytes in whole blood, but only carbon monoxide pretreatment inhibited procainamide hydroxylamine-induced methemoglobin formation. These observations are consistent with the participation of hemoglobin in a redox cycle with procainamide hydroxylamine, generating products which are primarily responsible for its cytotoxicity in blood.

Centrally mediated opioid induced depression of hepatic glutathione: Effects of intracerebroventricular administration of mu kappa, sigma and delta agonists

It has recently been demonstrated that morphine produces a loss of hepatocellular glutathione in mice by virtue of its action within the central nervous system. The ability of opioid receptor antagonists to abolish morphines effect on hepatic glutathione suggests that this action is opioid-receptor mediated. The involvement of opioid receptors in this phenomenon is confirmed in the present study in mice by the ability of naltrexone, 100 micrograms administered intracerebroventricularly (i.c.v.), to completely block the decrease in hepatic glutathione induced by an i.c.v. injection of 100 micrograms of morphine. Intracerebroventricular administration of the selective mu (mu) opioid receptor agonist, (D-Ala2,N-MePhe4,Gly-ol5)enkephalin (DAGO; 25-50 micrograms), or the selective delta (delta) opioid agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE; 3-50 micrograms), like morphine, produced significant decreases in hepatic glutathione 3 h after administration. The selective kappa (kappa) opioid receptor agonists, ethylketocyclazocine (1-30 micrograms) and trans-(+/-)3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide-methane sulfonate (U50 488; 10-300 micrograms), as well as the selective sigma (sigma) opioid agonists, phencyclidine (PCP; 50-300 micrograms) and N-allylnormetazocine (SKF 10,047; 1-30 micrograms), had no effect on the concentrations of glutathione in the liver. It appears from these data that stimulation of mu- or delta-, but not kappa- or sigma-opioid receptors within the central nervous system results in a loss of hepatocellular glutathione.

An assay for phentolamine using high performance liquid chromatography with electrochemical detection

A new method is presented for the detection of phentolamine by high performance liquid chromatography with electrochemical detection. The electrochemical detector was used in the oxidative mode at +900 mV potential versus Ag/AgCl reference. The on-column detection limit for phentolamine using this method was 3 ng, and detector response was linear for 3-1000 ng injected on column. The coefficient of variation for replicate injections was 2.4%. The measurement of phentolamine in biological samples was accomplished using yohimbime as the internal standard; retention time for yohimbine was 3.0 min while phentolamine eluted at 4.75 min. Biological samples were buffered to pH 9.2 and extracted with diethyl ether, followed by back extraction into 0.1 N HCl. The extraction efficiency for this method was 99.4% for phentolamine in serum and 59.3% in liver tissue. The detection limit for phentolamine was 5 ng/ml for 1.0-ml serum samples, and was 10 ng/ml for 1.0-ml liver homogenate samples. The disappearance of phentolamine from serum and liver after administration of a single ip dose of phentolamine to mice was determined using this method. Absorption from the ip route was rapid, with peak phentolamine concentrations achieved in 15 min or less. The elimination half-life of phentolamine in serum was approximately 50 min and was paralleled by disappearance of phentolamine in the liver.

Antagonism of bromobenzene-induced hepatotoxicity by the α-adrenergic blocking agents, phentolamine and idazoxan

The coadministration of phentolamine, an alpha-adrenoreceptor antagonist, was found to be effective in antagonizing the hepatotoxicity produced by bromobenzene in B6C3F1 mice. Multiple doses of phentolamine, administered in dosages of 10 mg/kg, attenuated almost completely the acute lethality resulting from a 0.5 ml/kg dosage of bromobenzene. Consistent with this decline in lethality, the coadministration of phentolamine significantly altered the magnitude of hepatocellular necrosis, the elevation of serum alanine aminotransferase activity, and the glutathione depression normally produced by this dose of bromobenzene. These protective effects were not limited to phentolamine. Idazoxan, an adrenergic antagonist more specific for alpha 2-receptors, was equally effective in antagonizing the bromobenzene-induced hepatotoxicity. Measurements of serum catecholamine levels revealed that the administration of hepatotoxic doses of bromobenzene elevates serum epinephrine levels. Furthermore, the phentolamine antagonism of the bromobenzene hepatotoxicity could be correlated to elevated serum epinephrine levels in both a temporal and dose-dependent manner. Although the mechanism of the phentolamine antagonism remains to be established, one promising hypothesis involves its prevention of an epinephrine-mediated compromise in the glutathione-dependent detoxification of bromobenzene.

Perturbation of glutathione by a central action of morphine.

A series of studies were conducted in order to further characterize the previously reported effect of morphine to diminish hepatocellular concentrations of glutathione (GSH) in mice. Naive ICR mice administered morphine (i.p.) in doses up to 1000 mg/kg had diminished hepatic GSH concentrations, with a maximum depletion of approximately 50% occurring at doses of 250 mg/kg or greater. No such effect from an acute challenge with morphine was observed in morphine-tolerant mice. The intracerebro-ventricular administration of the opioid receptor antagonist naltrexone (250 micrograms) completely blocked the hepatic GSH depression resulting from the systemic (i.p.) administration of morphine (100 mg/kg). When morphine (100 micrograms) was administered by the i.c.v. route, GSH concentrations in liver and plasma were significantly altered while heart and kidney were unchanged. Variable responses to i.c.v. morphine were obtained in spleen, stomach and lung. The depression of hepatic GSH was found not to be a consequence of morphine-induced hypoxia or hypothermia, and could not be attributed to intracellular oxidation of GSH.

Antagonism of bromobenzene-induced hepatotoxicity by phentolamine: Evidence for a metabolism-independent intervention

A previous study has revealed that phentolamine markedly antagonizes the bromobenzene-induced hepatotoxicity and lethality in B6C3F1 mice. One potential mechanism by which phentolamine may diminish the bromobenzene-induced hepatotoxicity is by a direct or indirect interference with the metabolism of bromobenzene to toxic metabolites. In the present study, phentolamine cotreatment failed to alter the elimination of bromobenzene from serum or the distribution of bromobenzene to liver. This suggests that phentolamine cotreatment does not indirectly interfere with bromobenzene bioactivation secondary to changes in bromobenzene absorption, distribution, or elimination. Further, a phentolamine concentration 10- to 20-fold greater than those measured in vivo failed to alter the in vitro metabolism of bromobenzene to its ortho- and para-phenolic metabolites. It is believed that para-bromophenol represents the rearrangement product of the hepatotoxic 3,4-epoxide and that ortho-bromophenol is a product of the nonhepatotoxic 2,3-epoxide pathway. Thus, it appears that phentolamine does not antagonize bromobenzene-induced hepatotoxicity by inhibiting the formation of hepatotoxic intermediates, nor by enhancing metabolism via the nonhepatotoxic pathway. On the basis of these studies, we conclude that phentolamine antagonism of bromobenzene-induced hepatotoxicity occurs through a mechanism independent of bromobenzene bioactivation.

Antagonism of bromobenzene-induced hepatotoxicity by the α-adrenoreceptor blocking agents phentolamine and idazoxan: Role of hypothermia

A recent study from our laboratory revealed that cotreating mice with the alpha-adrenoreceptor antagonists phentolamine and idazoxan markedly diminished bromobenzene-induced hepatotoxicity. Subsequent studies also revealed that such cotreatment does not alter the pharmacokinetic disposition of bromobenzene in mice nor its bioactivation to reactive metabolites. In the present study, the possible role of hypothermia in the phentolamine antagonism of bromobenzene-induced hepatotoxicity was investigated. Bromobenzene alone caused a significant, dose-related hypothermia. The high dosage regimen (10 mg/kg per dose) of phentolamine or idazoxan that had been found to be hepatoprotective in earlier studies potentiated this hypothermia and more than doubled the net decrease in core body temperature experienced by the animals. Placing mice receiving bromobenzene in an environment with an ambient temperature of 10 degrees C likewise increased the hypothermia experienced by animals receiving bromobenzene. The magnitude of the net change in core body temperature elicited by exposure to cold was similar to but slightly less than the net change produced by cotreatment with either alpha-adrenoreceptor antagonist and the magnitude of the hepatoprotection this procedure provided against bromobenzene hepatotoxicity was equivalent to that observed with phentolamine cotreatment. In contrast, a lower dosage regimen of either adrenoreceptor antagonist (2.5 mg/kg per dose) resulted in no additional hypothermia yet still produced a near maximal antagonism of bromobenzene-induced hepatotoxicity. Further, increasing the ambient temperature to 30 degrees C completely reversed the phentolamine-induced (10 mg/kg per dose) increase in hypothermia, but did not affect phentolamines antagonism of the bromobenzene-induced changes in hepatic glutathione levels, serum alanine aminotransferase activity, or 24-hr mortality. Therefore, we conclude that while the hepatoprotective intervention of phentolamine can be mimicked by an exposure to cold that results in hypothermia, it is clear that alpha-adrenergic antagonists diminish the hepatotoxicity induced by bromobenzene by a mechanism that is independent of hypothermia.

Effects of piperonyl butoxide on halothane hepatotoxicity and metabolism in the hyperthyroid rat

A series of experiments were conducted to examine the potential role of phase I metabolism in halothane-induced liver injury in the hyperthyroid rat. The metabolism of halothane was determined in both hyperthyroid (triiodothyronine, 3 mg/kg per day, for 6 days) and euthyroid rats and in animals pre-treated with the cytochrome P-450 inhibitor piperonyl butoxide (75-100 mg/kg, i.p.). It was found that the hyperthyroid state, which is associated with a substantial increase in sensitivity to the hepatotoxic effects of halothane, decreases both oxidative and reductive routes of halothane metabolism in the rat. The production of trifluoroacetic acid (TFA), an oxidative metabolite, as well as that of chlorodifluoroethylene (CDF) and chlorotrifluoroethane (CTF), 2 reductive metabolites, was significantly reduced in hyperthyroid animals. Consistent with these findings serum and urinary bromide levels resulting from the formation of TFA, CDF or CTF were significantly reduced. The only route of halothane metabolism significantly increased by the hyperthyroid condition was the defluorination of halothane. Piperonyl butoxide administration did not render euthyroid animals sensitive to the halothane-induced hepatotoxicity and had no effect on the defluorination of halothane in euthyroid animals. However, piperonyl butoxide markedly increased the hepatotoxicity of halothane in hyperthyroid rats and, except for a modest increase in debromination reactions, decreased all measured indices of halothane metabolism including the defluorination of halothane. Thus, none of the observed changes in halothane metabolism produced by triiodothyronine or piperonyl butoxide treatment could be consistently correlated to the increases in hepatotoxicity linked to these 2 treatments. Based on these studies we suggest that the halothane hepatotoxicity induced in the hyperthyroid rat results from effects produced by either the parent compound or an as yet unidentified metabolite. In addition, these studies further demonstrate that considerable mechanistic differences exist for halothane-induced hepatotoxicity when comparing euthyroid and hyperthyroid animal models.