Regulating and assessing risks of cholinesterase-inhibiting pesticides: divergent approaches and interpretations.

This document presents a revised framework for conducting worker and dietary risk assessments for less-than-lifetime exposures to organophosphate or carbamate pesticides based on red blood cell (RBC) or brain acetylcholinesterase (AChE) inhibition or the presence of clinical signs and symptoms. The proposals for appropriate uncertainty factors are based on the biological significance of the cholinesterase (ChE) inhibition noted at the lowest-observed-effect level (LOEL) and the degree of uncertainty in the extrapolation between human and animal data. An extensive evaluation of industry data, not previously summarized, and the available literature indicate that the following risk assessment principles are supportable and protective of human health: Plasma ChE inhibition is not an adverse effect, and therefore should not be utilized in risk assessments. Red blood cell AChE is not associated with the nervous system and inhibition is not per se an adverse (neurotoxic) effect. When available, cholinergic effects or brain AChE inhibition data should take precedence over RBC AChE for determining no-observed-effect levels (NOELs). When available, human RBC AChE inhibition or cholinergic effects data should take precedence over animal data for determining NOELs. Due to the lack of adversity associated with inhibition of RBC AChE, the use of a 10-fold (10x) uncertainty factor from the NOEL is adequate when RBC AChE inhibition data from either animal or human studies are used to assess human risk. Due to greater potential for adversity, NOELs for brain AChE inhibition and cholinergic effects identified in animal studies should receive a default uncertainty factor of 100x; lower uncertainty factors may be used on a case-by-case basis. NOELs based on cholinergic effects noted in human studies should only require a 10x uncertainty factor, since an interspecies extrapolation factor from animals to humans is unnecessary. For RBC and brain AChE activity the threshold for defining a NOEL should be less than or equal to 20% difference from control activity in all species. For risk assessment purposes, duration and route of the study should reflect the expected duration and route of exposure for humans (i.e., a 21-d or 28-d dermal study for subchronic occupational dermal exposure assessment). When dermal data are not available, a subchronic oral toxicity study and an appropriate dermal penetration factor should be used. A general default of 10% absorption should be used, analogous to the United Kingdom and German exposure models that are widely used in Europe. The recommendations in this document are generally consistent with current risk assessment procedures used by Canada, the European Community (EC), and the United Kingdom (UK).

Inhibition of ozone-induced nitric oxide synthase expression in the lung by endotoxin.

Inhalation of the pulmonary irritant ozone is associated with an accumulation of macrophages in the lung. These cells, along with type II epithelial cells, are activated to release increased quantities of hydrogen peroxide and nitric oxide, two reactive mediators that have been implicated in tissue injury. In the present studies we determined whether pretreatment of rats with bacterially derived endotoxin, which modulates oxidant levels in tissues, could abrogate the effects of ozone on lung injury and nitric oxide production. Acute exposure of rats to ozone (2 parts per million, 3 h) resulted in nitric oxide production in the lung as measured by electron paramagnetic resonance spin trapping. This was correlated with expression of inducible nitric oxide synthase (iNOS) mRNA in the lung as determined by in situ hybridization. Particularly high levels of iNOS were evident in alveolar macrophages and type II cells. Alveolar macrophages isolated from ozone-treated rats also expressed increased iNOS mRNA and protein as measured by Northern and Western blotting, respectively, and produced more nitric oxide compared with cells from air-exposed animals. Treatment of rats with endotoxin (5 mg/kg, intravenously), 30 min prior to ozone, was found to abrogate ozone-induced increases in iNOS mRNA and protein expression, as well as nitric oxide production by alveolar macrophages. This was associated with a reduction in ozone-induced tissue injury as determined by levels of lung lavage fluid protein. Ozone inhalation also resulted in a reduction in intracellular glutathione in alveolar macrophages, an effect that was blocked by endotoxin administration. Taken together, these data provide evidence that the protective effects of endotoxin against ozone-induced injury are mediated, at least in part, by alterations in levels of lung oxidants and antioxidants.

Stimulation of nitric oxide production in rat lung lavage cells by anti-Mac-1beta antibody: effects of ozone inhalation.

Acute inhalation of the pulmonary irritant ozone is associated with an inflammatory response characterized by increased numbers of macrophages in the lung that release elevated quantities of nitric oxide. The accumulation of phagocytes in the lung is dependent on expression of leukocyte adhesion molecules including Mac-1. In the present studies, we determined whether activation of the Mac-1 receptor is involved in regulating nitric oxide production by lung phagocytes, and whether this response is modified following acute ozone inhalation. Cells were isolated from the lung by bronchoalveolar lavage 48 h after exposure of female Sprague-Dawley rats to air or ozone (2 parts per million, for 3 h). Anti-Mac-1beta antibody, but not anti-Mac-1alpha antibody, stimulated nitric oxide production by cells from both air- and ozone-exposed animals. Cells from ozone-exposed rats produced more nitric oxide and expressed greater quantities of inducible nitric oxide synthase mRNA than did cells from air-exposed animals. Production of nitric oxide in response to anti-Mac-1beta was also found to be augmented by cross-linking of the Mac-1beta receptor. Pretreatment of lavage cells with granulocyte/macrophage colony-stimulating factor (GM-CSF), which activates phagocytes, enhanced the expression of Mac-1beta and increased anti-Mac-1beta-induced nitric oxide production by the cells. Lavage cells from ozone-exposed animals were more responsive to GM-CSF than were cells from control animals. Taken together, these data suggest that the Mac-1beta adhesion molecule may contribute to phagocyte activation and mediator release during ozone-induced inflammatory reactions in the lung.

Taurine protects rat bronchioles from acute ozone-induced lung inflammation and hyperplasia.

Ozone is a potent respiratory irritant known to induce lung injury in humans and experimental animals. The present studies determined if ozone-induced lung inflammation was modified by pretreatment of animals with taurine, a detoxifying antioxidant. Rats were pretreated for 10 days with 5% taurine in their drinking water (controls received water only) prior to exposure to 2 ppm ozone for 3 h. At 2, 6, 12, 24, 48, and 72 h after ozone exposure, rats were anesthetized and the lungs were perfusion-fixed for histopathologic evaluation. An additional group of rats was used to examine bronchoalveolar lavage cell counts and hydroxyproline levels. A count of bronchoalveolar lavage cells 48 h after ozone exposure showed significantly fewer total inflammatory cells and fewer polymorphonuclear leukocytes accompanied by a reduction in hydroxyproline in the lavage fluid of ozone-exposed rats pretreated with taurine compared to rats that did not receive taurine. Light microscopy revealed an inflammatory cell infiltrate in the lungs of rats exposed to ozone. This was followed by focal hyperplasia in the terminal and respiratory bronchioles. Rats pretreated with taurine and then exposed to ozone showed none of these alterations. In addition, although there was a significant reduction in cell proliferation as measured by DNA precursor incorporation in the lungs of rats pretreated with taurine prior to ozone exposure compared to unsupplemented rats, the distribution of labeled cells was the same in taurine supplemented and unsupplemented groups. Also, significantly higher levels of taurine were found in the plasma, whole blood, and lavage fluid of rats pretreated with dietary taurine compared to rats that received water only. The results suggest that supplemental taurine protects rat lung epithelium from acute ozone-induced lung inflammation and hyperplasia.

Inhibition of macrophages with gadolinium chloride abrogates ozone-induced pulmonary injury and inflammatory mediator production.

Acute inhalation of toxic doses of ozone (O3) induces macrophage accumulation in the lung and the release of cytotoxic and proinflammatory mediators. To evaluate the role of macrophages and their mediators in the pathophysiologic response of the lung to O3, we examined the effects of the macrophage inhibitor, gadolinium chloride (GdCl3), on O3-induced inflammation, mediator production, and lavage fluid protein levels. Rats were pretreated with GdCl3 (7 mg/kg, intravenously) or control 24 h prior to exposure to air or O3 (2 parts per million, 3 h). Animals were killed 48 h after exposure. GdCl3 pretreatment of rats was found to abrogate O3-induced increases in the number of cells, as well as the amount of protein recovered in bronchoalveolar lavage fluid. Following GdCl3 pretreatment of rats, lung lavage cells consisting of > 90% macrophages were found to produce significantly less nitric oxide and express less inducible nitric oxide synthase (iNOS) when compared to cells from rats exposed to O3. O3-induced alterations in superoxide anion production by alveolar macrophages, both in vitro and in situ, were also attenuated by GdCl3 pretreatment of rats. In addition, increases in tumor necrosis factor alpha (TNF-alpha) and fibronectin in lung tissue induced by O3 were reduced. Taken together, these data provide support for the hypothesis that macrophages contribute to the pathogenesis of O3-induced lung injury.

Macrophages and inflammatory mediators in tissue injury.

Tissue injury induced by a diverse group of xenobiotics appears to involve both direct and indirect damage to target cells. Thus, while chemicals may act directly on target cells resulting in toxicity, they may also act indirectly by recruiting and activating resident and inflammatory tissue macrophages. Macrophages are potent secretory cells that release an array of mediators, including proinflammatory and cytotoxic cytokines and growth factors, bioactive lipids, hydrolytic enzymes, reactive oxygen intermediates, and nitric oxide--each of which has been implicated in the pathogenesis of tissue injury. The potential role of macrophages and their mediators in tissue injury has been extensively investigated in the lung and the liver. In both of these tissues, xenobiotics induce localized macrophage accumulation and mediator release. Furthermore, when macrophage functioning is blocked, pulmonary and hepatic injury-induced agents such as ozone, bleomycin, acetaminophen, carbon tetrachloride, and galactosamine are reduced. These data provide direct support for the hypothesis that macrophages and the mediators they release contribute to xenobiotic-induced tissue injury.

Pulmonary and hepatic effects of inhaled ozone in rats.

Nitric oxide is a highly reactive molecule that has been implicated in host defense and in tissue injury. In the present studies we analyzed the effects of brief exposure of rats to inhaled ozone on production of this mediator by lung macrophages and type II epithelial cells. We found that ozone exposure (1-2 ppm, 3 hr) induced a marked increase in spontaneous nitric oxide production by alveolar (AM) and interstitial macrophages, as well as type II cells. These effects were apparently due to increased expression of inducible nitric oxide synthase protein and mRNA, which was evident in vitro in isolated cells and in situ in histologic sections. Macrophages and epithelial cells from ozone-treated rats were also sensitized to produce increased amounts of nitric oxide in response to inflammatory cytokines such as interferon-gamma, a response that was also mediated by inducible nitric oxide synthase. Unexpectedly, we also discovered that brief inhalation of ozone caused dramatic effects on the liver, including increased production of nitric oxide by hepatocytes and enhanced protein synthesis. These data suggest that this inhaled irritant induces an acute phase response. Additional studies indicate that AM from ozone-treated rats produced significantly more tumor necrosis factor-alpha and interleukin-1 than did cells from control animals. Elevated levels of tumor necrosis factor-alpha were also noted immunohistochemically in both lung and liver tissue. These results indicate that the extrapulmonary effects of ozone may be mediated by inflammatory cytokines released by activated lung macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced production of interleukin-1, tumor necrosis factor-alpha, and fibronectin by rat lung phagocytes following inhalation of a pulmonary irritant.

Interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-alpha), and fibronectin are macrophage-derived mediators thought to be important in the pathogenesis of lung injury, inflammation, and fibrosis. In the present studies, we examined the effects of acute exposure of rats to the pulmonary irritant, ozone (O3), on production of these mediators by lung phagocytes. Cells were isolated from lungs 48 h after exposure of rats to air or O3 (2 ppm, 3 h). We found that cells from O3-exposed rats released 2- to 3-fold more IL-1 and TNF-alpha into the culture medium than did cells from air-exposed rats. These effects were time dependent, reaching a maximum at 2 and 24 h for IL-1, and 2 to 4 h for TNF-alpha. We also found that alveolar macrophages from O3-treated rats produced increased amounts of fibronectin, both alone and in response to transforming growth factor-beta, lipopolysaccharide, and interferon-gamma when compared with cells from control rats. Examination of immunohistochemically stained tissue sections indicated increased IL-1, TNF-alpha, and fibronectin in lungs from O3-exposed animals when compared with control animals. IL-1 and TNF-alpha were localized in lung macrophages, whereas fibronectin was associated with blood vessel walls and the lung interstitium. These results demonstrate that lung phagocyte production of these inflammatory mediators is elevated following O3 exposure and suggest that they may play a role in oxidant-induced pulmonary inflammation and injury.

Production of nitric oxide by rat type II pneumocytes: increased expression of inducible nitric oxide synthase following inhalation of a pulmonary irritant.

Nitric oxide is a highly reactive molecule that has been implicated in host defense and tissue injury. In the present studies, we determined whether rat type II alveolar epithelial cells have the capacity to produce this mediator. We found that type II cells synthesize significant quantities of nitric oxide after treatment with the inflammatory cytokines, interferon-gamma (IFN-gamma) and/or interleukin-1 beta (IL-1 beta), or with the combination of IFN-gamma and tumor necrosis factor-alpha. In contrast to rat alveolar macrophages, type II cells were unresponsive to lipopolysaccharide. Production of nitric oxide by type II cells in response to IFN-gamma was dose dependent, reaching a maximum at 100 U/ml, and blocked by NG-monomethyl-L-arginine (L-NMA), a nitric oxide synthase inhibitor. Northern blot analysis demonstrated that nitric oxide production by type II cells was due to expression of mRNA for an inducible form of nitric oxide synthase (iNOS). Following brief exposure of rats to irritant-inducing doses of ozone (2 ppm, 3 h), type II cells were found to produce significantly more nitric oxide than were cells from control animals. This was due to increased expression of iNOS mRNA. Cells from ozone-treated rats were also sensitized to produce more nitric oxide in response to IFN-gamma and IL-1 beta. This was associated with a marked increase in expression of iNOS mRNA and enzyme protein in the cells. We also found that ozone inhalation caused enhanced production of hydrogen peroxide, as well as spontaneous and IFN-gamma-induced cytostasis of type II cells toward P815 mouse mastocytoma cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced production of nitric oxide by rat alveolar macrophages after inhalation of a pulmonary irritant is associated with increased expression of nitric oxide synthase.

Alveolar macrophages represent the first line of defense of the lung against inhaled environmental agents. These cells release a variety of inflammatory mediators including reactive oxygen and nitrogen intermediates that have been implicated in host defense and in tissue injury. In the present studies we characterized production of these mediators by lung phagocytes after exposure of rats to an inhaled pulmonary irritant. Freshly isolated alveolar macrophages from control rats were found to produce nitric oxide as well as hydrogen peroxide and superoxide anion in response to in vitro treatment with inflammatory mediators such as IFN-gamma or LPS and phorbol esters, respectively. Production of nitric oxide by lung phagocytes was enhanced in the presence of superoxide dismutase. Western blot analysis revealed that production of nitric oxide after treatment of the cells with IFN-gamma and LPS was a result of increased expression of inducible nitric oxide synthase. After brief exposure of rats to ozone (O3, 1 to 2 ppm, 3 h), a pulmonary irritant and inflammatory agent that is rapidly converted to molecular oxygen, lung phagocytes produced significantly increased amounts of nitric oxide when compared with control animals. These cells were also sensitized to produce more nitric oxide in response to in vitro treatment with IFN-gamma and LPS. This was due, at least in part, to increased expression of inducible nitric oxide synthase by the cells, which was evident in protein blots and in immunohistochemically stained sections of lung tissue. In further studies we found that O3 inhalation also caused enhanced production of hydrogen peroxide, but an apparent decrease in release of superoxide anion by lung phagocytes. Taken together, these data demonstrate that acute irritant exposure modifies production of reactive oxygen and nitrogen intermediates by lung phagocytes. These alterations may be important in the pathophysiologic response of the lungs to irritants.

Induction of functionally active platelet-activating factor receptors in rat alveolar macrophages.

Freshly isolated alveolar macrophages from control rats express low numbers of platelet-activating factor (PAF) receptors (4.6 x 10(3) binding sites/cell, Kd = 1.3 nM). These receptors appeared to be functionally inert in that PAF failed to induce intracellular calcium mobilization in the cells. Following brief exposure of rats to ozone (2 ppm, 3 h), a pulmonary irritant and inflammatory agent that is rapidly converted to molecular oxygen, expression of PAF receptors was markedly up-regulated on lung phagocytes (13.1 x 10(3) binding sites/cell), with no significant effect on receptor affinity (Kd = 2.0 nM). In these cells, PAF, but not lyso-PAF, an inactive analog, caused a rapid and transient rise in intracellular calcium demonstrating that the PAF receptors were functionally active. The calcium response to PAF, which was largely due to mobilization of calcium from intracellular stores, was found to be dose-dependent in the nanomolar concentration range and inhibitable by the PAF receptor antagonist triazolam. The results of these studies may be important in elucidating mechanisms of lung inflammation induced by this model irritant.

Differential eicosanoid synthesis by murine fetal thymic non-lymphoid cells.

The temporal patterns of synthesis of prostaglandin (PG)E2 and PGI2 by organ-cultured fetal thymic lobes and the cell population(s) responsible for synthesis of such products within the murine fetal thymus have been investigated. Embryonic day 14 thymic lobes were organ-cultured in defined media for 14 days and the media were collected every 24 h and replaced with fresh media. Collected media were processed for quantitation of either PGE2 or PGI2. Lobes were also cultured in 2'-deoxyguanosine (1.35 mmol/L) to produce an enriched non-lymphoid population. The per cent cyclooxygenase-positive cells within non-lymphoid cell-enriched lobes as well as the capacity of such lobes to synthesize either PGE2 or PGI2 were determined and compared with that of intact thymic lobes. Results demonstrate that fetal thymic lobes, in vitro, differentially synthesize PGI2 and synthesize PGE2 at a constant rate. Moreover, lobes enriched for non-lymphoid cells contain a greater percentage of cyclooxygenase-positive cells and synthesize increased amounts of eicosanoids per 10(4) cells compared with controls.

Thymic eicosanoid profiles and cyclooxygenase localization in prenatal and postnatal CBA/J mice.

Previous work in our laboratory has implicated arachidonic acid metabolites as modulators of fetal murine thymocyte development in vitro. Therefore, we determined whether distinct temporal patterns existed with regard to the capacity for eicosanoid synthesis by fetal and postnatal thymic lobes which might relate to early proliferative and differentiation events within the thymus. Thin layer chromatography was used to demonstrate differential synthesis of 15-HETE, 5-HETE, PGD2, 6-keto PGF1 alpha, and PGF2 alpha by both fetal and postnatal thymic lobes. In contrast, PGE2 and TXB2 were synthesized at a constant level. These findings were in part substantiated by radioimmunoassay of culture media for PGE2 and 6-keto PGF1 alpha following fetal thymic organ culture for 7 or 8 days. In addition, we were able to demonstrate immunoreactive cells for cyclooxygenase in early embryonic and postnatal thymic lobes. Taken together, the above findings permit the suggestion that metabolites of arachidonic acid modulate development of thymocytes and, in turn, their capacity for synthesis is modulated as the thymus develops.