Fertility and developmental neurotoxicity effects of inhaled hydrogen sulfide in Sprague-Dawley rats

Regulatory effects of hydrogen sulfide on the female reproductive system

Hydrogen sulfide (H2S), a colorless exhaust gas, has been traditionally considered an air pollutant. However, recent studies have revealed that H2S functions as a novel gas signaling molecule, exerting diverse biological effects on various systems, including the cardiovascular, digestive, and nervous systems. Thus, H2S is involved in various pathophysiological processes. As H2S affects reproductive function, it has potential therapeutic implications in reproductive system diseases. This review examined the role of H2S in various female reproductive organs, including the ovary, fallopian tube, vagina, uterus, and placenta. Additionally, the regulatory function of H2S in the female reproductive system has been discussed to provide useful insights for developing clinical therapeutic strategies for reproductive diseases.

A Paper-Based Ultrasensitive Optical Sensor for the Selective Detection of H2S Vapors

A selective and inexpensive chemical paper-based sensor for the detection of gaseous H2S is presented. The triggering of the sensing mechanism is based on an arene-derivative dye which undergoes specific reactions in the presence of H2S, allowing for colorimetric analysis. The dye is embedded into a porous cellulose matrix. We passively exposed the paper strips to H2S generated in situ, while the absorbance was monitored via an optic fiber connected to a spectrophotometer. The kinetics of the emerging absorbance at 534 nm constitute the sensor response and maintain a very stable calibration signal in both concentration and time dimensions for quantitative applications. The time and concentration dependence of the calibration function allows the extraction of unusual analytical information that expands the potential comparability with other sensors in the literature, as the limit of detection admissible within a given exposure time. The use of this specific reaction ensures a very high selectivity against saturated vapors of primary interferents and typical volatile compounds, including alkanethiols. The specific performance of the proposed sensor was explicitly compared with other colorimetric alternatives, including standard lead acetate strips. Additionally, the use of a smartphone camera to follow the color change in the sensing reaction was also tested. With this straightforward method, also affordable for miniature photodiode devices, a limit of detection below the ppm scale was reached in both colorimetric approaches.

Prenatal maternal stress induces visceral hypersensitivity of adult rat offspring through activation of cystathionine-β-synthase signaling in primary sensory neurons

Irritable bowel syndrome is a disorder of unknown etiology characterized by widespread, chronic abdominal pain associated with altered bowel movements. Increasing amounts of evidence indicate that stressors presented during gestational periods could have long-term effects on the offspring's tissue structure and function, which may predispose to gastrointestinal diseases. The aim of the present study is to determine whether prenatal maternal stressis a adverse factor affecting gastrointestinal sensitivity and to investigate possible mechanisms underlying prenatal maternal stress-induced visceral hypersensitivity in adult offspring. Prenatal maternal stress was induced in pregnant Sprague-Dawley rats by exposure to heterotypic intermitent stress from gestational day 7 to delivery. Prenatal maternal stress significantly increased visceromotor response to colorectal distention in adult offspring from the age of 6 weeks to 10 weeks. Prenatal maternal stress also enhanced neuronal excitability including depolarization of resting membrane potentials, reduction in rheobase, and an increase in the number of action potentials evoked by 2× and 3× rheobase current stimultion of colon-specific dorsal root ganglion neurons. Prenatal maternal stress remarkably enhanced expression of cystathionine-β-synthase and Nav1.7 in T13-L2 thoracolumbar dorsal root ganglions both at protein and mRNA levels. Intraperitoneal injection of aminooxyacetic acid, an inhibitor of cystathionine-β-synthase, attenuated prenatal maternal stress-induced visceral hypersensitivity in a dose-dependent manner. A consecutive seven-day administration of aminooxyacetic acid reversed the hyperexcitability of colon-specific dorsal root ganglion neurons and markedly reduced Nav1.7 expression. These results indicate that the presence of multiple psychophysical stressors during pregnancy is associated with visceral hypersensitivity in offspring, which is likely mediated by an upregualtion of cystathionine-β-synthase and Nav1.7 expression. Prenatal maternal stress might be a significant contributor to irritable bowel syndrome, and cystathionine-β-synthase might be a potential target for treatment for chronic visceral hypersensitivity in patients with irritable bowel syndrome.

Dexamethasone ameliorates H2S-induced acute lung injury by increasing claudin-5 expression via the PI3K pathway

Acute lung injury (ALI) is a major outcome of exposure to high levels of hydrogen sulfide (H₂S). Dexamethasone (DXM) has been used to treat ALI. However, the mechanisms involved in H₂S-induced ALI and the protective mechanisms of DXM in treating ALI are still nebulous. To explore the mechanisms involved, we evaluated the role of claudin-5 in the protective effect of DXM against H₂S-induced ALI. Sprague-Dawley rats were exposed to H₂S to establish the ALI model. In parallel with the animal model, a cell model was also established by incubating human umbilical vein endothelial cells (HUVECs) with NaHS. Lung hematoxylin-eosin staining, electron microscope assay, and wet/dry ratio were used to identify whether the ALI was successfully induced by H₂S, and changes in claudin-5 expression were detected in both rats and HUVECs. Our results revealed that claudin-5 was markedly decreased after H₂S exposure and that DXM significantly attenuated the H₂S-induced downregulation of claudin-5 in both rats and HUVECs. In the animal experiment, p-Akt and p-FoxO₁ presented a similar tendency as claudin-5, but their levels decreased 6 h prior to the levels of claudin-5. In a further investigation, the DXM-induced protective effect on ALI and rescue effect on downregulation of claudin-5 were both blocked by LY294002. The current study demonstrated that claudin-5 was involved in the development of H₂S-induced ALI and that DXM exerted protective effects through increasing claudin-5 expression by activating the phosphatidylinositol 3-kinase pathway. Therefore, claudin-5 might represent a novel pharmacological target for treating ALI induced by H₂S and other hazardous gases.

Involvement of water channel Aquaporin 5 in H2S-induced pulmonary edema

Acute exposure to hydrogen sulfide (H₂S) poses a significant threat to life, and the lung is one of the primary target organs of H₂S. However, the mechanisms involved in H₂S-induced acute pulmonary edema are poorly understood. This study aims to investigate the effects of H₂S on the expression of water channel aquaporin 5 (AQP5) and to elucidate the signaling pathways involved in AQP5 regulation. In an in vivo study, C57BL6 mice were exposed to sub-lethal concentrations of inhaled H₂S, and histological injury of the lungs and ultrastructure injury of the epithelial cells were evaluated. With real-time PCR and western blot assays, we found that H₂S exposure contributed to a significant decrease in AQP5 expression both in murine lung tissue and the A549 cell line, and the ERK1/2 and p38 MAPK signaling pathways were demonstrated to be implicated in AQP5 regulation. Therefore, adjusting AQP5 protein levels could be considered a therapeutic strategy for the treatment of APE induced by H₂S and other hazardous gases.

Direct Dosing of Preweaning Rodents in Toxicity Testing and Research: Deliberations of an ILSI RSI Expert Working Group

Laboratory animal studies designed to assess the effects of exposure of a test substance during postnatal development are commonly utilized in basic research and to evaluate potential hazard to children for chemical and pharmaceutical regulation. Direct dosing, defined here as the administration of a test substance directly to a preweaning mammal, has been identified as a useful tool that can be used in the conduct of such studies for regulatory purposes. The International Life Sciences Institute Risk Science Institute (ILSI RSI) convened an Expert Working Group to develop guidance on the design and implementation of direct dosing regulatory studies on preweaning mammals, which was published as an ILSI monograph in 2003 (Zoetis and Walls, Principles and Practices for Direct Dosing of Pre-Weaning Mammals in Toxicity Testing and Research, Washington, DC: ILSI Press, 2003). A summary of the Working Group conclusions regarding direct dosing studies with laboratory rodents are presented here, although the ILSI monograph also includes rabbits, canines, swine and nonhuman primates. Issues to be considered when designing the protocol include selection of the test species, the route of administration, dose levels, and the timing of dosing. Knowledge of the maturational status of the test species and information on critical windows of development are important in creating a valid study design. Most common routes of administration (e.g., oral, inhalation, injection) are possible with typical laboratory species; however, adjustments may be necessary due to practical considerations. Information on the pharmacokinetic profile in young animals versus adults and in the test species versus humans is very useful for determining dosing parameters. The conduct of the study and the interpretation of the data will be improved by an understanding of confounding factors as well as statistical and biological issues specific for postnatal studies. Ultimately, the success of the study will depend upon careful preparation, including thorough training of the technical staff.

Windows of sensitivity to toxic chemicals in the motor effects development

Many chemicals currently used are known to elicit nervous system effects. In addition, approximately 2000 new chemicals introduced annually have not yet undergone neurotoxicity testing. This review concentrated on motor development effects associated with exposure to environmental neurotoxicants to help identify critical windows of exposure and begin to assess data needs based on a subset of chemicals thoroughly reviewed by the Agency for Toxic Substances and Disease Registry (ATSDR) in Toxicological Profiles and Addenda. Multiple windows of sensitivity were identified that differed based on the maturity level of the neurological system at the time of exposure, as well as dose and exposure duration. Similar but distinct windows were found for both motor activity (GD 8-17 [rats], GD 12-14 and PND 3-10 [mice]) and motor function performance (insufficient data for rats, GD 12-17 [mice]). Identifying specific windows of sensitivity in animal studies was hampered by study designs oriented towards detection of neurotoxicity that occurred at any time throughout the developmental process. In conclusion, while this investigation identified some critical exposure windows for motor development effects, it demonstrates a need for more acute duration exposure studies based on neurodevelopmental windows, particularly during the exposure periods identified in this review.

Absence of Hydrogen Sulfide-Induced Hypometabolism in Pigs: A Mechanistic Explanation in Relation to Small Nonhibernating Mammals

Artificially induced hypometabolism in nonhibernating mammals may have considerable clinical implications. Numerous studies in small rodent models have demonstrated that hydrogen sulfide (H2S) induces hypometabolism, supposedly as a result of histotoxic hypoxia. However, the induction of hypometabolism is absent in large animals following H2S administration. To determine the cause of this animal size-dependent discrepancy in H2S pharmacodynamics, the effects of sodium H2S (NaSH; 5 mg/kg/h, 4-hour intravenous administration) on systemic, pneumocardial, hematological, biochemical, microvascular (sublingual), and histological parameters were investigated in pigs. After 4 h, no differences were observed between the NaSH and control group with respect to systemic, pneumocardial, hematological, biochemical, and histological parameters. However, NaSH triggered significant hyperperfusion in the sublingual microcirculation, as evidenced by an increased blood vessel diameter (154 ± 16 and 85 ± 25% vs. baseline for NaSH and NaCl, respectively), total vessel density (139 ± 18 and 98 ± 13%, respectively), and perfused vessel density (139 ± 18 and 99 ± 13%, respectively). These phenomena are consistent with microvascular changes that occur during a panting response, an important heat loss mechanism (i.e., thermoregulatory effector) in pigs that is controlled by the thermoneutral zone (Ztn). On the basis of our findings and the literature, a mechanistic explanation is provided for the differential manifestation of hypometabolism between small and large animals. In large animals, H2S does not act via histotoxic hypoxia but likely triggers carotid bodies to transmit a hypoxic signal, which subsequently lowers the Ztn and activates heat loss mechanisms (e.g., panting) to align ATP consumption with ATP production through hypothermia. Since large animals have a small surface:size ratio, the cooling rate is too inefficient to accommodate hypothermia and subsequent hypometabolism. This is why large animals do not exhibit hypometabolism, despite the activation of thermoregulatory effectors. This is also a reason for the poor translatability of artificial hypometabolism to the clinical setting.

Dexamethasone ameliorates H₂S-induced acute lung injury by alleviating matrix metalloproteinase-2 and -9 expression

Acute lung injury (ALI) is one of the fatal outcomes after exposure to high levels of hydrogen sulfide (H₂S), and the matrix metalloproteinases (MMPs) especially MMP-2 and MMP-9 are believed to be involved in the development of ALI by degrading the extracellular matrix (ECM) of blood-air barrier. However, the roles of MMP-2 and MMP-9 in H₂S-induced ALI and the mechanisms of dexamethasone (DXM) in treating ALI in clinical practice are still largely unknown. The present work was aimed to investigate the roles of MMP-2 and MMP-9 in H₂S-induced ALI and the protective effects of DXM. In our study, SD rats were exposed to H₂S to establish the ALI model and in parallel, A549 cells were incubated with NaHS (a H₂S donor) to establish cell model. The lung HE staining, immunohistochemisty, electron microscope assay and wet/dry ratio were used to identify the ALI induced by H₂S, then the MMP-2 and MMP-9 expression in both rats and A549 cells were detected. Our results revealed that MMP-2 and MMP-9 were obviously increased in both mRNA and protein level after H₂S exposure, and they could be inhibited by MMP inhibitor doxycycline (DOX) in rat model. Moreover, DXM significantly ameliorated the symptoms of H₂S-induced ALI including alveolar edema, infiltration of inflammatory cells and the protein leakage in BAFL via up-regulating glucocorticoid receptor(GR) to mediate the suppression of MMP-2 and MMP-9. Furthermore, the protective effects of DXM in vivo and vitro study could be partially blocked by co-treated with GR antagonist mifepristone (MIF). Our results, taken together, demonstrated that MMP-2 and MMP-9 were involved in the development of H₂S-induced ALI and DXM exerted protective effects by alleviating the expression of MMP-2 and MMP-9. Therefore, MMP-2 and MMP-9 might represent novel pharmacological targets for the treatment of H₂S and other hazard gases induced ALI.

Systemic hydrogen sulfide administration partially restores normal alveolarization in an experimental animal model of bronchopulmonary dysplasia

Arrested alveolarization is the pathological hallmark of bronchopulmonary dysplasia (BPD), a complication of premature birth. Here, the impact of systemic application of hydrogen sulfide (H2S) on postnatal alveolarization was assessed in a mouse BPD model. Exposure of newborn mice to 85% O2 for 10 days reduced the total lung alveoli number by 56% and increased alveolar septal wall thickness by 29%, as assessed by state-of-the-art stereological analysis. Systemic application of H2S via the slow-release H2S donor GYY4137 for 10 days resulted in pronounced improvement in lung alveolarization in pups breathing 85% O2, compared with vehicle-treated littermates. Although without impact on lung oxidative status, systemic H2S blunted leukocyte infiltration into alveolar air spaces provoked by hyperoxia, and restored normal lung interleukin 10 levels that were otherwise depressed by 85% O2. Treatment of primary mouse alveolar type II (ATII) cells with the rapid-release H2S donor NaHS had no impact on cell viability; however, NaHS promoted ATII cell migration. Although exposure of ATII cells to 85% O2 caused dramatic changes in mRNA expression, exposure to either GYY4137 or NaHS had no impact on ATII cell mRNA expression, as assessed by microarray, suggesting that the effects observed were independent of changes in gene expression. The impact of NaHS on ATII cell migration was attenuated by glibenclamide, implicating ion channels, and was accompanied by activation of Akt, hinting at two possible mechanisms of H2S action. These data support further investigation of H2S as a candidate interventional strategy to limit the arrested alveolarization associated with BPD.

Physiological implications of hydrogen sulfide: a whiff exploration that blossomed

The important life-supporting role of hydrogen sulfide (H(2)S) has evolved from bacteria to plants, invertebrates, vertebrates, and finally to mammals. Over the centuries, however, H(2)S had only been known for its toxicity and environmental hazard. Physiological importance of H(2)S has been appreciated for about a decade. It started by the discovery of endogenous H(2)S production in mammalian cells and gained momentum by typifying this gasotransmitter with a variety of physiological functions. The H(2)S-catalyzing enzymes are differentially expressed in cardiovascular, neuronal, immune, renal, respiratory, gastrointestinal, reproductive, liver, and endocrine systems and affect the functions of these systems through the production of H(2)S. The physiological functions of H(2)S are mediated by different molecular targets, such as different ion channels and signaling proteins. Alternations of H(2)S metabolism lead to an array of pathological disturbances in the form of hypertension, atherosclerosis, heart failure, diabetes, cirrhosis, inflammation, sepsis, neurodegenerative disease, erectile dysfunction, and asthma, to name a few. Many new technologies have been developed to detect endogenous H(2)S production, and novel H(2)S-delivery compounds have been invented to aid therapeutic intervention of diseases related to abnormal H(2)S metabolism. While acknowledging the challenges ahead, research on H(2)S physiology and medicine is entering an exponential exploration era.

Proceedings of the Hydrogen Sulfide Health Research and Risk Assessment Symposium October 31-November 2, 2000

The Hydrogen Sulfide Health Research and Risk Assessment Symposium came about for several reasons: (1) increased interest by the U.S. Environmental Protection Agency (EPA) and several state agencies in regulating hydrogen sulfide (H2S); (2) uncertainty about ambient exposure to H2S; (3) confusion and disagreement in the literature about possible health effects at low-level exposures; and (4) presentation of results of a series of recent animal bioassays. The American Petroleum Institute (API) proposed this symposium and the EPA became an early co-sponsor, with the Chemical Industry Institute of Toxicology (CIIT) and the American Forest & Paper Association (AF&PA) contributing expertise and funding assistance. The topics covered in this symposium included Animal Research, Human Research, Mode-of-Action and Dosimetry Issues, Environmental Exposure and Monitoring, Assessment and Regulatory Issues, and closed with a panel discussion. The overall goals of the symposium were to: gather together experts in H2S health effects research and individuals from governmental agencies charged with protecting the public health, provide a venue for reporting of recent research findings, identify gaps in the current information, and outline new research directions and promote research collaboration. During the course of the symposium, presenters provided comprehensive reviews of the state of knowledge for each topic. Several new research proposals discussed at the symposium have subsequently been initiated. This report provides a summary of the talks, poster presentations, and panel discussions that occurred at the Hydrogen Sulfide Health and Risk Assessment Symposium.

Development and use of a single-animal whole-body system for inhalation exposure

Inhalation exposure studies, in which test subjects are fully or partially immersed in an atmosphere containing a compound of interest, are usually carried out using one of two possible exposure systems: large whole-body chambers or systems that expose only the animal's nose or head. Whole-body chambers may require large quantities of test compound, which can pose a problem if the chemical is expensive or available in limited quantities. Nose- or head-only systems can help conserve test compound but may cause stress or injury to animals. To address these concerns, the authors developed an exposure system consisting of small single-animal whole-body chambers. They exposed 80 mice and 80 rats to five test compounds at various concentrations. Though the system was labor-intensive for animal care technicians, it effectively exposed animals to precise chemical doses without causing adverse effects, using less test compound than would have been required in a conventional whole-body chamber.

Hydrogen sulfide raises cytosolic calcium in neurons through activation of L-type Ca2+ channels

Hydrogen sulfide (H(2)S) concentration can be maintained in cell cultures within the range reported for rat brain by repetitive pulses of sodium hydrogen sulfide. Less than 2 h exposure to H(2)S concentrations within 50 and 120 microM (i.e., within the upper segment of the reported physiological range of H(2)S in rat brain), produces a large shift of the intracellular calcium homeostasis in cerebellar granule neurons (CGN) in culture, leading to a large and sustained increase of cytosolic calcium concentration. Only 1 h exposure to H(2)S concentrations within 100 and 300 microM raises intracellular calcium to the neurotoxic range, with nearly 50% cell death after 2 h. L-type Ca(2+) channels antagonists nimodipine and nifedipine block both the H(2)S-induced rise of cytosolic calcium and cell death. The N-methyl-D-aspartate receptor antagonists (+)-MK-801 and DL-2-amino-5-phosphonovaleric acid afforded a nearly complete protection against H(2)S-induced CGN death and largely attenuated the rise of cytosolic calcium. Thus, H(2)S-induced rise of cytosolic calcium eventually reaches the neurotoxic cytosolic calcium range, leading to glutamate-induced excitotoxic CGN death. The authors conclude that H(2)S is a major modulator of calcium homeostasis in neurons as it induces activation of Ca(2+) entry through L-type Ca(2+) channels, and thereby of neuronal activity.

Hydrogen sulphide-induced hypothermia attenuates stress-related ulceration in rats

1. Hydrogen sulphide (H(2)S) acts as a gaseous cellular messenger and has recently been reported to induce a suspended animation-like state in mice. The aim of the present study was to investigate the protective role of H(2)S exposure in stress gastric ulcer. 2. In the present study, we used a rat model of water immersion and restraint stress (WRS) to induce the typical stress disease, namely stress gastric ulcer. Rats were treated with WRS for 4 h, with or without pre-exposure to H(2)S (160 p.p.m. H(2)S for 2.5 h). 3. In H(2)S-exposed rats, body temperature was significantly reduced by 2.5C (P < 0.01) and oxygen consumption was reduced by 37.1% (P < 0.01) compared with control rats. Plasma levels of H(2)S were increased by 20.8% (P < 0.01) following pre-exposure. Pre-exposure to H(2)S significantly reduced the gastric ulcer index, from 24 +/- 9 to 9 +/- 2 (P < 0.01), in WRS rats. In addition, WRS increased plasma levels of adrenocorticotropin (ACTH) and corticosterone 4.7- and 4.8-fold, respectively (both P < 0.01). Pre-exposure to H(2)S markedly suppressed plasma ACTH and corticosterone level by 34.4 and 53.2%, respectively (both P < 0.01), and reduced WRS-elevated myeloperoxidase (MPO) activity by 19%. In the present study, WRS increased gastric malondialdehyde and conjugated diene content by 42 and 68%, respectively (both P < 0.01), and H(2)S exposure reduced lipid peroxide production. Finally, H(2)S exposure inhibited the WRS-elevated expression of glucose-regulated protein 78 and caspase 12, markers of endoplasmic reticulum stress. 4. In conclusion, a low concentration of H(2)S may be a new pharmacological tool for induced hypothermia to prevent severe stress-induced diseases and multifarious trauma in the clinical setting.

Changes in intracellular pH play a secondary role in hydrogen sulfide-induced nasal cytotoxicity

Hydrogen sulfide (H(2)S) is a naturally occurring gas that is also associated with several industries. The potential for widespread human inhalation exposure to this toxic gas is recognized as a public health concern. The nasal epithelium is particularly susceptible to H(2)S-induced pathology. Cytochrome oxidase inhibition is postulated as one mechanism of H(2)S toxicity. Another mechanism by which the weak acid H(2)S could cause nasal injury is intracellular acidification and cytotoxicity. To further understand the mechanism by which H(2)S damages the nasal epithelium, nasal respiratory and olfactory epithelial cell isolates and explants from naive rats were loaded with the pH-sensitive intracellular chromophore SNARF-1 and exposed to air or 10, 80, 200, or 400 ppm H(2)S for 90 min. Intracellular pH was measured using flow cytometry or confocal microscopy. Cell lysates were used to quantify total protein and cytochrome oxidase activity. A modest but statistically significant decrease in intracellular pH occurred following exposure of respiratory and olfactory epithelium to 400 ppm H(2)S. Decreased cytochrome oxidase activity was observed following exposure to >10 ppm H(2)S in both respiratory and olfactory epithelia. None of the treatments resulted in cytotoxicity. The intracellular acidification of nasal epithelial cells by high-dose H(2)S exposure and the inhibition of cytochrome oxidase at much lower H(2)S concentrations suggest that changes in intracellular pH play a secondary role in H(2)S-induced nasal injury.

Environmental exposure to natural sour gas containing sulfur compounds results in elevated depression and hopelessness scores

Some parts of Masjid-i-Sulaiman (MIS) (Khozestan provinces, southwest of Iran) are contaminated with subsurface leakage of natural sour gas containing H(2)S. It is reported that the incidence of suicide by self-burning in MIS is very high. High endogenous H2S levels have been found in the brain and it is involved in the brain functions. Because there is no report about the effect(s) of natural sour gas containing sulfur compounds on the function(s) of human brain, the present study was done. The study was performed on 128 individuals exposed or unexposed to natural sour gas. The exposed group consisted of 64 persons (39 males, 25 females). Unexposed subjects were matched by sex, age, and educational levels. Depression and hopelessness were determined using Beck's depression inventory (BDI) and Beck's hopelessness (BHS) questionnaires. Using multiple linear regression models, the averages of BDI (t=2.637, P=0.009) and BHS (t=3.344, P=0.001) were significantly higher among the exposed subjects than among the control group. In Lali 57.8% and 14.1% of subjects had no depressed mood and moderate to severe depressed mood, respectively, while in MIS 31.3% and 35.9% of subjects had no depressed mood and moderate to severe depressed mood, respectively; the difference was significant (chi2=12.88, df=3, P=0.005). In Lali 35.9% and 12.5% of subjects had no hopelessness at all and moderate to severe hopelessness, respectively, while in MIS 17.2% and 32.8% of subjects had no hopelessness at all and moderate to severe hopelessness, respectively, the difference was significant (chi2=11.49, df=3, P=0.009). Replication is necessary and health implications are discussed.

Respiratory tract toxicity of inhaled hydrogen sulfide in Fischer-344 rats, Sprague–Dawley rats, and B6C3F1 mice following subchronic (90-day) exposure

The goal of this study was to characterize the toxicity of hydrogen sulfide (H2S), including nasal and pulmonary effects, in adult male and female Fischer-344 and Sprague-Dawley rats and B6C3F1 mice. Animals underwent whole-body exposure to 0, 10, 30, or 80 ppm H2S for 6 h/day for at least 90 days. Exposure to 80 ppm H2S was associated with reduced feed consumption during either the first exposure week (rats) or throughout the 90-day exposure (mice). Male Fischer-344 rats, female Sprague-Dawley rats, and female B6C3F1 mice exposed to 80 ppm H2S had depressed terminal body weights when compared with air-exposed controls. Subchronic H2S inhalation did not result in toxicologically relevant alterations in hematological indices, serum chemistries, or gross pathology. Histologic evaluation of the nose showed an exposure-related increased incidence of olfactory neuronal loss (ONL) and rhinitis. ONL occurred following exposure to > or =30 ppm H2S in both sexes of all experimental groups, with one exception, male Sprague-Dawley rats demonstrated ONL following exposure to 80 ppm H2S only. A 100% incidence of rhinitis was found in the male and female B6C3F1 mice exposed to 80 ppm H2S. In the lung, exposure to H2S was associated with bronchiolar epithelial hypertrophy and hyperplasia in male and female Sprague-Dawley rats following exposure to > or =30 ppm H2S and in male Fischer-344 rats exposed to 80 ppm H2S. Our results confirm that the rodent nose, and less so the lung, are highly sensitive to H2S-induced toxicity, with 10 ppm representing the NOAEL for ONL following subchronic inhalation.

Utilizing data from multiple studies (meta-analysis) to determine effective dose-duration levels. Example: rats and mice exposed to hydrogen sulfide

The objective of this exercise was to incorporate as much data as possible from multiple studies, that may differ in exposure durations, to derive a chemical-specific dose-duration response curve from which to identify toxicity markers (e.g., ED01, benchmark dose, and LD50). This has the advantage of incorporating more information than single-study assessments to improve estimates and reduce confidence intervals, and determining toxicity markers as functions of exposure duration as well as dose. The example used mortality for rats and mice, analyzed separately, from acute exposure to hydrogen sulfide (dose refers to airborne concentration of H(2)S). Statistical methods were applied to determine when data from different studies could be pooled. EC01, EC10, and EC50 (doses with response rates of 1, 10, and 50%) were estimated, with 95% confidence intervals, at durations of 5, 10, and 30 min, and 1, 2, 4, and 6 h. A single dose-duration response curve for mortality was fit to the rat data for exposures of 5 min, 10 min, 30 min, and 1h, using a logistic curve additive in log(dose) and log(duration). Separate fits of that model were required, however, at 2, 4, and 6h, due to an increasing impact of duration relative to concentration as duration increased. The curves for rats fit the data exceedingly well and exhibited a threshold-like response followed by a steep incline as concentration increased. There were fewer data for mice but the response pattern for mortality clearly differed from rats. This example demonstrates the feasibility of extending the concept of single-study benchmark doses to multiple-study dose-duration benchmarks, using U.S. EPA's program CatReg. Similar applications to long-term animal studies could be considered.

US EPA's acute reference exposure methodology for acute inhalation exposures

The US Environmental Protection Agency (EPA) National Center for Environmental Assessment is engaged in the development of a methodology for Agency use to perform risk assessments for non-cancer effects due to acute inhalation exposures. The methodology will provide general guidance for deriving chemical-specific acute exposure benchmarks called acute reference exposures (AREs). Chemical-specific AREs are analogous to reference concentra tions (RfCs) for chronic non-cancer effects and will be incorporated in chemical-specific files in the US EPA's Integrated Risk Information System (IRIS) as they are developed and reviewed. AREs will have wide applicability in assessing the potential health risks of accidental and routine acute releases of chemicals to the environment. The proposed methodology for ARE development provides a framework for choosing an optimal derivation approach, depending on the type of data available, from the no-observed-adverse-effect level (NOAEL), benchmark concentration (BMC), or categorical regression approaches. Uncertainty factors are applied to the point of departure, determined by one of the recommended approaches, to derive the ARE. Due to the capability to use more exposure-response information than the NOAEL approach allows, exposure-response analyses such as BMC and categorical regression are favored as methods to develop the point of departure when the available database will support such analyses. The NOAEL approach is suitable when the data are insufficient to support exposure-response modeling. Applications of the proposed ARE methodology are illustrated by the derivation of example AREs for hydrogen sulfide and hexachlorocyclopentadiene, which showcase the categorical regression and NOAEL approaches, respectively. In addition, a recent review of the proposed ARE methodology by the US EPA Risk Assessment Forum is discussed.

Neurotoxicological effects associated with short-term exposure of Sprague-Dawley rats to hydrogen sulfide

Although hydrogen sulfide (H2S) is a known neurotoxic hazard, only a limited number of experimental animal studies have examined its neurochemical or behavioral effects. Our aim was to determine if short-term inhalation exposure of rats to H2S would result in altered brain catecholamnine levels or impaired learning and memory. Three groups of adult male CD rats were tested; two groups were exposed by nose-only inhalation (0, 30, 80, 200, or 400 ppm H2S) and one group was exposed by whole-body inhalation (0, 10, 30, or 80 ppm H2S) for 3 h per day forfive consecutive days. The first group (n = 10 rats per concentration) was tested immediately following each daily nose-only H2S exposure for spatial learning with a Morris water maze. Core body temperatures were also monitored in these animals during and after the last H2S exposure. The second group of rats (n = 10 rats per concentration) was tested for spontaneous motor activity immediately following the fifth exposure. These rats were then euthanized and striatal, hippocampal, and hindbrain catecholamnine levels determined. A third group of rats (n = 5-7 rats per concentration) was pretrained on a multiple fixed- interval (FI) schedule and exposed whole-body. Daily performance on the FI schedule was compared for the week pre-exposure, for the exposure week immediately following daily exposures, and for the week postexposure. We observed significant reductions in motor activity, water maze performance, and body temperature following exposure only to high concentrations (> or = 80 ppm) of H2S. Exposure to H2S did not affect regional brain catecholamine concentrations or performance on the FI schedule. Additional studies using other measures of behavior and longer-term exposure to H2S may be required to more definitively address conditions under which H2S exposure results in behavioral toxicity.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. III: pharmacokinetic and pharmacodynamic considerations

We review pharmacokinetic and pharmacodynamic factors that should be considered in the design and interpretation of developmental neurotoxicity studies. Toxicologic effects on the developing nervous system depend on the delivered dose, exposure duration, and developmental stage at which exposure occurred. Several pharmacokinetic processes (absorption, distribution, metabolism, and excretion) govern chemical disposition within the dam and the nervous system of the offspring. In addition, unique physical features such as the presence or absence of a placental barrier and the gradual development of the blood--brain barrier influence chemical disposition and thus modulate developmental neurotoxicity. Neonatal exposure may depend on maternal pharmacokinetic processes and transfer of the xenobiotic through the milk, although direct exposure may occur through other routes (e.g., inhalation). Measurement of the xenobiotic in milk and evaluation of biomarkers of exposure or effect following exposure can confirm or characterize neonatal exposure. Physiologically based pharmacokinetic and pharmacodynamic models that incorporate these and other determinants can estimate tissue dose and biologic response following in utero or neonatal exposure. These models can characterize dose--response relationships and improve extrapolation of results from animal studies to humans. In addition, pharmacologic data allow an experimenter to determine whether exposure to the test chemical is adequate, whether exposure occurs during critical periods of nervous system development, whether route and duration of exposure are appropriate, and whether developmental neurotoxicity can be differentiated from direct actions of the xenobiotic.

Rudimentary hemidesmosome formation in congenital generalized junctional epidermolysis bullosa in the Sprague-Dawley rat

Seven of 14 newborn pups in a litter of Sprague-Dawley rats were found to have generalized detachment of the epidermis, which was thin, wrinkled, and hung in loose folds over distal extremities. Histologic and ultrastructural examination of the skin showed noninflammatory separation of the epidermis from the dermis at the lamina lucida of the basement membrane zone. Ultrastructurally, hemidesmosomes were small and had a rudimentary appearance; keratin tonofilaments in basal keratinocytes were detached from the hemidesmosomes. The skin lesions were consistent with generalized junctional epidermolysis bullosa, which has not previously been reported in the rat. In humans, generalized junctional epidermolysis bullosa is most commonly caused by autosomal recessive inheritance of defective proteins of the hemidesmosomes or anchoring filaments. The specific protein defect involved in the rat lesion was not determined because fresh frozen tissue was not available.

Olfactory neuron loss in adult male CD rats following subchronic inhalation exposure to hydrogen sulfide

Dysosmia and anosmia are reported to occur following human exposure to hydrogen sulfide (H2S) gas. The clinical association between H2S exposure and olfactory dysfunction in humans necessitates evaluation of the nasal cavity and olfactory system in experimental animals used to study H2S toxicity. The purpose of this study was to subchronically expose 10-week-old male CD rats to relatively low concentrations of H2S and to histologically evaluate the nasal cavity for exposure-related lesions. Rats (n = 12/group) were exposed via inhalation to 0, 10, 30, or 80 ppm H2S 6 h/d and 7 d/wk for 10 weeks. Following exposure to 30 and 80 ppm H2S, a significant increase in nasal lesions limited to the olfactory mucosa was observed. The lesions, which consisted of olfactory neuron loss and basal cell hyperplasia, were multifocal, bilaterally symmetrical, and had a characteristic rostrocaudal distribution pattern. Regions of the nasal cavity affected included the dorsal medial meatus and the dorsal and medial portions of the ethmoid recess. The no observed adverse effect level for olfactory lesions in this study was 10 ppm. For perspective, the American Conference of Governmental Industrial Hygienists threshold limit value (TLV) recommendation for H2S is currently 10 ppm (proposed revision: 5 ppm), so the concentrations employed in the present study were 3 and 8 times the TLV. These findings suggest that subchronic inhalation exposure to a relatively low level of H2S (30 ppm) can result in olfactory toxicity in rats. However, because of differences in the breathing style and nasal anatomy of rats and humans, additional research is required to determine the significance of these results for human health risk assessment.