European Journal of Pharmacology: Environmental Toxicology and Pharmacology最新文献

The effects of low level exposure of rats to 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) on their immune system was investigated. Dietary administration to young adult male Leeds strain rats of a total dose of 3 μg/kg body weight of TCDD resulted in an exposure duration-dependent reduction of in vitro lipopolysaccharide-induced production of interleukin (IL)-1 in cultures of their splenic macrophages. A 30-day exposure produced approximately 30% suppression and 180-day exposure produced approximately 52% suppression. This reduction did not negatively influence lipopolysaccharide-induced proliferation of B cells, instead an enhancement of B cell proliferation was observed after 30 days exposure. A 180 day exposure significantly suppressed the generation of IL-2 by either concanavalin A or phorbol myristate acetate/calcium ionophore stimulation, and reduced the lectin-induced proliferation of splenic T cells. The 30-day TCDD exposure showed no such immunotoxicity. TCDD at both exposure durations suppressed the expression of the α chain of the IL-2 receptor in concanavalin A-activated T cells, without affecting the CD4⁺/CD8⁺ ratio. The results suggest that exposure to a low dietary dose of TCDD suppresses the functions of several T cell subsets, some of the immunotoxic effects being produced early, while others require a longer exposure period. The effect of TCDD on B cells appears to be of transient nature, with less potentially serious consequences. Such exposure also down-regulates the IL-1 production function of macrophages. A common mechanism of TCDD immunotoxicity may be on the multifunctional signal transduction pathways downstream to the activation of protein kinase C and Ca²⁺ flux.

Oxidative damage to the liver of lipopolysaccharide-treated rats was evaluated using four parameters: level of lipid peroxidation, changes in total GSH and GSSG concentrations and hepatic morphology. Bacterial lipopolysaccharide (10 mg/kg b.w.) was injected i.p. either at 6, 16 or 24 h before animals were killed. Lipopolysaccharide increased lipid peroxidation most dramatically when it was injected 6 h before killing. Hepatic total GSH increased after lipopolysaccharide in a time-dependent manner. The highest level of GSSG and largest GSSG/total GSH ratio were also observed in the group of animals injected with lipopolysaccharide 6 h before tissue collection. In a second study, lipopolysaccharide was injected 6 h before the animals were killed, with or without 1 mg/kg b.w. melatonin. Melatonin totally abolished lipopolysaccharide-induced increased in lipid peroxidation, exaggerated the rise in total GSH and reversed the increase in GSSG concentration. The liver showed obvious histological degenerative changes after lipopolysaccharide, effects that were counteracted by melatonin administration. The protection conferred by melatonin is presumably due to its antioxidant activity.

The porphyrinogenic action of hexafluorobenzene was investigated and compared to that of hexachlorobenzene. Metabolite patterns in the urine of exposed rats were determined to quantify the extent of metabolism through cytochrome P450 catalysed oxidation and glutathione conjugation. Results obtained demonstrate an almost similar extent of formation of phenolic metabolites. However, in the urine of hexachlorobenzene exposed rats significantly higher levels of the N-acetyl-S-(pentahalophenyl)cysteine were observed than in the urine of hexafluorobenzene exposed rats. Hexafluorobenzene exposure did not result in induction of porphyria, whereas exposure to hexachlorobenzene did result in significantly elevated levels of urinary as well as liver porphyrins. Together these results indicate that if the reactive intermediate is indeed formed in the cytochrome P450 catalysed initial oxidative dehalogenation, the extent of its formation as well as its subsequent reactivity and reaction pathways vary with the type of the halogen substituents. Furthermore, the results seem to indicate that the extent of metabolism of hexahalogenated benzenes into urinary metabolites resulting from glutathione conjugation is a better indication of their porphyrinogenic action than their extent of metabolism to phenolic metabolites. Two explanations for this observation are presented.

This study analyzed if the paracrine liver endothelin system participates in the pathogenesis of CCl₄-induced hepatotoxicity. Wistar Kyoto rats were divided into four groups: a bosentan (mixed endothelin ET_A and ET_B receptor antagonist) treated group with CCl₄ intoxication, a vehicle treated group with CCl₄ intoxication, a nontreated control group and a bosentan treated control group. Hepatotoxicity was assessed by determination of alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH) followed by histopathological examinations. Tissue endothelin-1 concentrations and expression of endothelin receptor subtypes were analyzed. The tissue levels of endothelin-1 in the liver of rats with CCl₄ intoxication were significantly higher than those in normal rats. Scatchard analysis revealed no differences in the density and binding constant of endothelin ET_A and ET_B receptor between rats with CCl₄ intoxication and controls. Bosentan treatment of rats undergoing CCl₄ inhalation resulted in a significant protection against elevation of ALT, AST, LDH and bilirubin. Histopathological examination of liver sections for necrotic, swollen and lipid-laden cells revealed findings that were in agreement with the serum enzyme data. In conclusion, this study showed that the paracrine endothelin system is involved in the pathogenesis of CCl₄-induced hepatotoxicity and that the blockade of the stimulated liver endothelin system reduces CCl₄-induced liver injury.

Risk evaluation of complex environmental mixtures of polychlorinated dibenzo-p-dioxins and related halogenated aromatic hydrocarbons (polychlorinated dibenzofurans, azo- and azoxybenzenes, naphthalenes and some of the biphenyls) is currently carried out by measuring the concentration of each congener in the mixture and then multiplying every figure by its specific constant, toxic equivalency factor (TEF). All congeners are thought to produce highly similar effects albeit at different doses, and the TEFs are believed to represent the potencies of the congeners relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), considered the most toxic derivative of this class of environmental contaminants. Here we compared the acute toxicities of TCDD, 1,2,3,7,8-penta-, 1,2,3,4,7,8-hexa- and 1,2,3,4,6,7,8- heptachloro-dibenzo-p-dioxin in the most TCDD-susceptible (Long-Evans Turku AB; L-E) and the most TCDD-resistent (Han/Wistar Kuopio; H/W) rat strain. While L-E rats exhibited the expected rank order of sensitivities to the four dioxins, the higher chlorinated dioxins were more toxic than TCDD (in terms of acute lethality) to H/W rats, with the hexachlorodioxin showing the greatest potency. Even if the doses were adjusted according to the LD₅₀ values, both biochemical and morphological effects elicited by the dioxins turned out to depend, often critically, on strain, congener or the interaction of these two determinants. These findings demonstrate that the dioxins have distinct profiles of acute toxicities and underscore the importance of response and test organism in defining the TEFs.

Conditions for the induction of rat liver Ca²⁺-independent nitric oxide synthase were determined with killed Propionibacterium avidum, and compared with lipopolysaccharide endotoxin. Similar maximal induction was obtained intraperitoneally with the two types of inducers but killed Propionibacterium avidum gave a long-lasting induction while lipopolysaccharide displayed a rapid and short response. Moreover, the induction resulting from an intravenous administration of killed Propionibacterium avidum reached 60 times that of the control whereas lipopolysaccharide treatment induced a 24-fold stimulation only. It is noteworthy that with the first inducer the nitric oxide activity was stable with time whereas with the second one it dropped after 8 h. Whatever the route of administration of killed Propionibacterium avidum, some huge vacuolated Kupffer cells were found in the liver whose parenchyma was almost normal. Numerous monocytes, and unaltered Kupffer cells, were observed. Kupffer cells were identified to be responsible for the uptake of killed Propionibacterium avidum.

Brain tryptophan hydroxylase activity decreases within 15 min after a single administration of 3,4-methylenedioxymethamphetamine. In the present study, the effect of body temperature on this acute decrease of tryptophan hydroxylase activity was examined. 2 h after a single dose of 3,4-methylenedioxymethamphetamine (20 mg/kg, s.c.), rats exhibited hyperthermia (38.7°C) or hypothermia (35.8°C) when maintained at 25°C or 6°C, respectively. The rectal temperature of control animals maintained at 6°C was not altered. Tryptophan hydroxylase activity measured in the hippocampus, striatum and frontal cortex of hyperthermic rats treated with 3,4-methylenedioxymethamphetamine was decreased to 61%, 65% and 71% of control levels, respectively, 2 h after drug treatment. However, in hypothermic rats, 3,4-methylenedioxymethamphetamine had no effect on tryptophan hydroxylase activity in the hippocampus, striatum or frontal cortex. Non-drug-induced hyperthermia or hypothermia did not affect tryptophan hydroxylase activity. Since hypothermia may prevent the 3,4-methylenedioxymethamphetamine-induced decrease in tryptophan hydroxylase activity by reducing the formation of free radicals, the effect of a free radical scavenging agent, N-tert-butyl-α-phenylnitrone, was examined. N-tert-butyl-α-phenylnitrone (200 mg/kg, i.p.) alone caused hypothermia but had no direct effect on tryptophan hydroxylase activity. Preadministration of N-tert-butyl-α-phenylnitrone prevented 3,4-methylenedioxymethamphetamine from raising the temperature above normal and attenuated the drug-induced decrease in tryptophan hydroxylase activity in hippocampus, striatum and frontal cortex. However, when the rats treated with a combination of N-tert-butyl-α-phenylnitrone and 3,4-methylenedioxymethamphetamine were maintained at hyperthermic conditions, N-tert-butyl-α-phenylnitrone had no protective effect. These results suggest that body temperature plays a prominent role in the 3,4-methylenedioxymethamphetamine-induced acute decrease in tryptophan hydroxylase activity.

The effect of a novel leukotriene B₄ receptor antagonist N-[5-[[8-(1-hydroxy-2-phenyl)ethyl]dibenzofuran-2-yl]-5-hydroxypentanoyl] pyrrolidine (PF 10042) has been evaluated in comparison with 2-[3-(1-hydroxyhexyl)phenoxymethyl]quinoline hydrochloride (PF 5901), a specific inhibitor of the 5-lipoxygenase pathway of arachidonic acid metabolism, against platelet activating factor (PAF) and allergen induced bronchial hyperresponsiveness and pulmonary eosinophil infiltration in the guinea pig. PF 10042 significantly displaced radiolabelled [³H]leukotriene B₄ from binding sites on human neutrophils with an EC50 of 3 μM. PF 10042 (100 mg/kg, i.p.) significantly inhibited PAF and allergen induced bronchial hyperresponsiveness without reducing the concomitant eosinophil infiltration, whereas PF 5901 (100 mg/kg, p.o.) significantly inhibited both PAF and allergen induced bronchial hyperresponsiveness and eosinophil infiltration. We suggest from these results that PAF and allergen induced bronchial hyperresponsiveness may be secondary to the release of leukotriene B₄, but this lipoxygenase metabolite does not contribute significantly to the observed eosinophil infiltration.

Contact hypersensitivity responsiveness to dinitrofluorobenzene is depressed in mice that are sensitized through skin sites exposed to ultraviolet (UV) radiation. Local impairment of contact hypersensitivity by UV has been associated with a reduction in antigen-presenting cell activity within UV-irradiated skin sites marked by a decrease in the density of Ia-positive epidermal Langerhans cells. Our recent studies have demonstrated that neurogenic mediators (e.g. calcitonin gene-related peptide (CGRP) and nitric oxide (NO) contribute to cutaneous inflammation following exposure of rats to high-dose UV radiation. Since CGRP and NO inhibit antigen presentation by dendritic cells in vitro, we have investigated the possible involvement of CGRP and NO in local immunosuppression in UV-irradiated rodents. Hindpaw skin of Sprague-Dawley rats and back skin of UV-susceptible C57BL/6 mice was exposed to acute UV radiation (2.0 J/cm² and 0.5 J/cm², respectively). Alterations in cutaneous CGRP content were analyzed by a specific radioimmunoassay (RIA). In separate experiments, the CGRP receptor antagonist CGRP-(8–37) (10⁻⁵ M) and the nitric oxide synthase inhibitor $\text{N}{}^{\mathrm{<mtext>G</mtext>}}\text{-}\text{nitro}\text{-}\text{L}\text{-}\text{arginine}$ methyl ester (L-NAME) (2 × 10⁻⁵ M) were topically applied to UV-exposed skin before induction of contact hypersensitivity with dinitrofluorobenzene. Finally, we examined the effects of UV irradiation and epicutaneous application of CGRP on Ia-positive Langerhans cells by immunohistochemical analysis of epidermal sheets. It was found that UV exposure lead to a decrease in skin CGRP levels starting already 2 h after irradiation and reaching a minimum (less than 40% of non-irradiated control skin) at 6–12 h. Contact hypersensitivity reactions were significantly suppressed by UV radiation in rat skin (by 51%) and murine skin (by 80%). Topical administration of both CGRP-(8–37) and L-NAME before sensitization restored the capacity to respond to haptens applied to UV-exposed skin. Both UV exposure and topical CGRP reduced the density of Ia-positive epidermal cells. Our data indicate that CGRP may be released from sensory neurons following cutaneous UV irradiation and that CGRP and NO contribute to UV-induced local immunosuppression. Moreover, topical administration of CGRP or its antagonist may be able to modulate epidermal Langerhans cell activity in vivo.