John B. Morris

First-pass metabolism of inspired ethyl acetate in the upper respiratory tracts of the F344 rat and Syrian hamster

Nasal tissues contain large amounts of carboxylesterase but its precise role in metabolism of inspired ester vapors is not known. The current study was aimed at quantitating the extent of metabolism of inspired ethyl acetate in the upper respiratory tract (URT) of the F344 rat and Syrian hamster. Ethyl acetate deposition was measured in the surgically isolated URT of these species under constant velocity unidirectional flow conditions. The degree of metabolism was estimated by mathematic modeling based on a simple venous-equilibration approach and by direct comparison of deposition efficiencies in naive and carboxylesterase-inhibited animals. Ethyl acetate deposition efficiencies averaged between 10 and 35% in the rat URT and 36 and 72% in the hamster. Carboxylesterase inhibition decreased deposition in both species. Both the modeling efforts and the direct comparisons between naive and inhibited animals indicated that significant amounts of the deposited ethyl acetate were metabolized in the URT of both species with the extent of metabolism being more pronounced in the hamster. Specifically, 40-65% of the deposited ethyl acetate was metabolized in the URT of the rat compared to 63-90% in the hamster. This first-pass metabolism (i) increased URT deposition efficiencies; (ii) led to production of high metabolite levels in URT tissues; and (iii) decreased the amount of parent ethyl acetate available for absorption into the bloodstream in the URT.

Dosimetry, toxicity and carcinogenicity of inspired acetaldehyde in the rat

Acetaldehyde is a ubiquitous air pollutant. It is an important industrial chemical and is also produced during the combustion of wood or tobacco. In smoky indoor atmospheres concentrations of the aldehyde may reach 100 ppb. Acetaldehyde is metabolized to acetate (releasing hydrogen ion) by aldehyde dehydrogenase a process which, in most tissues, represents a detoxification pathway. In vitro, acetaldehyde forms DNA-DNA and DNA-protein crosslinks. It is a clastogen, and inducer of sister chromatid exchanges, and is, perhaps, a weak mutagen. Inhalation exposure to 1000 ppm may induce DNA-protein crosslink formation in nasal tissues in the rat in vivo. Inhalation toxicity studies have shown acetaldehyde vapor causes chronic tissue injury and tumor formation in nasal tissues at exposure concentrations of 750 ppm or higher, with nasal olfactory mucosa being more sensitive than respiratory mucosa. Dosimetric estimates suggest that marked tissue injury and carcinogenicity occurs only at inspired concentrations which are sufficiently high to overwhelm nasal aldehyde dehydrogenase detoxification capacity. The induction of squamous cell carcinomas in the respiratory mucosa by acetaldehyde displays many analogies to the induction of squamous cell carcinomas by formaldehyde. For both vapors, non-linear concentration response relationships are observed for DNA-protein crosslink formation, tissue injury, and carcinogenicity, suggesting these responses are associated. For both vapors it is possible to document an exposure concentration that produces nasal respiratory epithelial injury without increasing tumor incidence, suggesting that for respiratory mucosa-derived tumors, exposure to non-cytotoxic concentrations may pose limited carcinogenic risk. In addition to squamous cell carcinomas of the respiratory epithelium, acetaldehyde exposure also results in nasal olfactory injury and tumors (adenocarcinomas) in the rat. The studies performed to date have not demonstrated a no observable effect level for these responses, therefore, the precise role of cytotoxicity and regenerative cell proliferation in the carcinogenic process in olfactory tissues can not be evaluated. Acetaldehyde metabolism via aldehyde dehydrogenase results in the formation of two hydrogen ions. The olfactory mucosa is quite sensitive to acid and dosimetric estimates suggest that the intracellular acid production rates that may occur in olfactory mucosa during acetaldehyde exposure may be sufficiently high to cause tissue damage. Such acid-induced tissue damage may enhance the genotoxic and tumorigenic potential of acetaldehyde in olfactory mucosa, and may, therefore, represent an important process in the production of tumors in this tissue.

Deposition of Ethanol and Acetone Vapors in the Upper Respiratory Tract of the Rat

The deposition of nonreactive gases in the lungs is well studied and is dependent on the ventilation-perfusion relationships of that organ. The current investigation was undertaken to determine if pulmonary ventilation-perfusion models could be successfully applied to quantitatively describe upper respiratory tract (URT) deposition of acetone and ethanol vapors. Toward this end, the deposition of these vapors was measured in the surgically isolated URT of the anesthetized rat at selected inspiratory flow rates (ranging from 70 to 500 ml/min) and exposure times (ranging from 3.3 to 13.3 min). The deposition of acetone in the URT followed apparent quasi-steady-state conditions, suggestive of a ventilation-perfusion relationship. Pulmonary ventilation-perfusion models predict the ratio of the amount deposited to the amount not deposited should be linearly related to the inverse of the inspiratory flow rate. Such a relationship was observed for URT acetone (r = 0.988, N = 16,p less than 0.0001) and ethanol (r = 0.963, N = 20, p less than 0.0001) deposition. From the deposition-flow relationships estimates of 0.31 and 0.33 ml/min were obtained for nasal blood flow in the ethanol and acetone studies, respectively. The 95% confidence limits for these estimates overlapped. The successful application of a ventilation-perfusion model to describe URT deposition suggests that acetone and ethanol vapors equilibrate between the inspired airstream and the capillary blood of the URT and that their deposition is dependent on both the URT ventilation and perfusion rates.

Uptake of Acrolein in the Upper Respiratory Tract of the F344 Rat

AbstractTo provide inhalation dosimetric information for acrolein vapor, the uptake of this irritant was measured during a 40-min exposure period in the surgically isolated upper respiratory tract (URT) of the anesthetized male F344 rat at selected unidirectional inspiratory (50–300 ml/min) or pseudo-cyclic (1 ml volume, 100 cycles/min with 7 ml/min being continuously bled off for analysis) flow conditions. In addition, the effect of acrolein on URT uptake of acetone vapor was studied via this approach to determine if this irritant was capable of altering the uptake of other vapors. In contrast to numerous other vapors, URT uptake of acrolein did not attain or maintain a steady state during the exposure, but rather slowly decreased throughout the 40-min exposure period. Average URT acrolein deposition efficiency (between 20 and 40 min of exposure) was significantly dependent (p < .001) on the inspired concentrations, with average deposition efficiencies of 62, 38, and 28% being observed at inspired concen...

Upper respiratory tract deposition of inspired acetaldehyde.

Inhalation exposure of rodents to high concentrations of acetaldehyde produces lesions in the upper respiratory tract (URT, all regions of the respiratory tract anterior to and including the larynx). Information on the inhalation dosimetric relationships for this vapor are needed for a comprehensive understanding of its inhalation toxicity. Toward this end, uptake of acetaldehyde was measured in the surgically isolated URT of the urethane-anesthetized male F344 rat under unidirectional (50, 100, 200, or 300 ml/min) and cyclic (100 ml/min) flow conditions at inspired concentrations of 1, 10, 100, or 1000 ppm. Under all flow conditions URT deposition efficiency was strongly dependent on inspired concentration. URT deposition efficiency (under cyclic flow) averaged 76, 48, 41, and 26% at 1, 10, 100, and 1000 ppm, respectively. Nasal acetaldehyde dehydrogenase activity averaged 1.2 micrograms/min. Absolute acetaldehyde deposition rates (micrograms/min) at 100 and 1000 ppm exceeded this activity by 5- to 100-fold, suggesting a possible mechanism for the reduced deposition efficiency at high concentrations. URT deposition under unidirectional flow was strongly dependent on the inspiratory flow rate. The effect of flow rate on deposition was reasonably predicted by the mass-transfer model of Aharonson et al. (J. Appl. Physiol. 37, 654-657, 1974). The uptake coefficients determined from the unidirectional flow studies were used to predict uptake under cyclic flow by integration of the model. The predicted cyclic deposition efficiencies differed from the observed efficiencies by 2.3 +/- 4.3% (mean +/- SEM), suggesting this model might provide a reasonable first approximation for acetaldehyde uptake under cyclic breathing conditions.

Upper Respiratory Tract Uptake of Acrylate Ester and Acid Vapors

AbstractInhalation exposure of the rodent to either of the esters ethyl acrylate (EA) or methyl methacrylate (MMA) results in nasal olfactory injury. The current study was designed to provide inhalation dosimetric data for these ester vapors as well as for their carboxyl-esterase metabolites, acrylic acid and methacrylic acid. Toward this end, uptake of these vapors was measured in the surgically isolated upper respiratory tract (URT) of the male F344 rat under constant-velocity unidirectional inspiratory (200 mllmin) or cyclic (207 mllmin mean inspiratory flow rate) flow conditions over a wide range of inspired concentrations. To examine the potential influences of carboxylesterase metabolism, uptake of the ester vapors was measured in naive (non-pretreated) rats and in rats pretreated with the carboxylesterase inhibitor bis-nitrophenylphosphate (BNPP). The URT uptake of EA averaged 24, 25, and 18% under cyclic flow at inspired concentrations of approximately 5, 25, and 100 ppm, respectively. Overall, up...

Metabolism and Deposition of Propanol and Acetone Vapors in the Upper Respiratory Tract of the Hamster

While it is thought that local metabolism may enhance nasal deposition of inspired vapors, there have been no studies designed to quantitate this effect. In the current study, deposition of 1-propanol (a metabolized vapor) and acetone (a nonmetabolized vapor) was studied in the surgically isolated upper respiratory tract (URT) of the anesthetized Syrian hamster. The effect of inspiratory flow rate on URT deposition of acetone could be described by a ventilation-perfusion (V-P) model with a nasal perfusion rate of 0.046 ml/min. Theoretical considerations predict that at enzyme saturation, deposition efficiency will be dependent upon the inspired concentration and the URT metabolism rates can be estimated from the concentration-dependence data. A concentration dependence on propanol deposition efficiency was observed at a flow rate of 200 ml/min, but not at 38 or 71 ml/min. At a flow rate of 200 ml/min, an apparent URT metabolism rate of 3.8 micrograms/min (95% confidence limits 0.5-7.1 micrograms/min) was estimated. In vitro studies on nasal tissue homogenates provided metabolism rates of approximately 2 micrograms/min, values which did not differ significantly from, and therefore, were in reasonable agreement with, the estimates from the deposition studies. After correction for the effect of metabolism, URT propanol deposition could be described by the V-P model with a nasal perfusion rate of 0.050 ml/min. This value did not differ significantly from that observed in the acetone studies. Thus, deposition of both propanol and acetone in the URT of the Syrian hamster could be described by the V-P model.(ABSTRACT TRUNCATED AT 250 WORDS)

A METHOD FOR MEASURING UPPER RESPIRATORY TRACT VAPOR UPTAKE AND ITS APPLICABILITY TO QUANTITATIVE INHALATION RISK ASSESSMENT

A thorough understanding of the toxicity of any substance requires knowledge of the relationships between exposure concentration and dose delivered to the critical target site. This is particularly true for inhalation exposures because inspired particles and vapors do not deposit uniformly in the respiratory tract. The current report describes in detail a methodology for measuring upper respiratory tract (URT) uptake of vapors in the rat. A urethane-anesthetized animal model is utilized in which two endotracheal tubes are inserted: one leading toward the lung to facilitate respiration, and the other toward the nose to allow air sampling through the nasal passages. The animal is placed in a nose-only exposure chamber and test vapor is drawn through the nose for periods up to 1 h. Uptake efficiency is calculated from the difference in vapor concentration between the inspired (chamber) air and air exiting the URT. Uptake data are provided for acetaldehyde and nicotine vapors, and a suggested experimental design that includes multiple air flow regimes and inspired concentrations is described. The data obtained by this methodology are not necessarily reflective of uptake efficiencies in normally breathing animals due to the nonphysiologic airflow regimes and the invasiveness of the procedure. The data so obtained are best utilized to support and validate state-of-the-art mathematical simulation models of regional vapor uptake. These models increase scientific rigor and reduce uncertainty in quantitative risk assessments for inhaled materials.

Seasonal Allergic Rhinitic and Normal Subjects Respond Differentially to Nasal Provocation with Acetic Acid Vapor

Individuals with seasonal allergic rhinitis (SAR) showa more marked nasal obstructive response (increases in nasal airways resistance or NAR) after provocation with chlorine gas (Cl2) than do nonrhinitic (NR) controls. We were interested in learning whether similar differential respon-siveness was apparent after provocation with acetic acid vapor. Sixteen nonsmoking, nonasth-matic subjects, aged 21–63 yr, equally divided by gender and nasal allergy status, were enrolled in a single-blinded crossover study involving exposure to acetic acid (AA) vapor (15 ppm) or air for 15 min on separate days 1 wk apart. NAR was measured in triplicate before, immedi-ately post-, and 15 min postexposure, was normalized to baseline on a given exposure day, and was expressed as Net [NAR/baseline] after acetic acid versus control (air) exposure. After log transformation to achieve normality, the mean loge of Net [NAR/baseline] was 0.22 for SAR subjects and -0.11 for NR subjects immediately postexposure (p < .05); the corresponding values were 0.24 and -0.08, respectively, at 15 min postexposure (p < .05). Inhalation of acetic acid at the (NIOSH-recommended) short-term exposure limit of 15 ppm for 15 min produces differential nasal airflow obstruction among SAR versus NR subjects, with the former showing greater physiologic reactivity to this stimulus. This differential responsiveness is consistent with our previous findings with Cl2, indicating that there may be a generalized susceptibility factor associated with allergic rhinitis. The response occurs with slight subjective nasal irritation.

Deposition of dibasic esters in the upper respiratory tract of the male and female Sprague-Dawley rat

Inhalation exposure of the male and female rat to high concentrations of a mixture of the dibasic esters dimethyl succinate (DMS), dimethyl glutarate (DMG), and dimethyl adipate (DMA) results in mild olfactory toxicity. This response is thought to be due to the in situ formation of acidic metabolites via nasal carboxylesterases. The current study was designed to provide inhalation dosimetric information for these vapors. Deposition of DMS, DMG, and DMA was measured in the surgically isolated upper respiratory tracts (URT) of ketamine-xylazine-anesthetized male and female rats under constant velocity flow conditions at a flow rate of 100 ml/min. Deposition of acetone was measured in both genders for comparative purposes. URT deposition efficiencies in excess of 98.3% were observed for DMS, DMG, and DMA in animals exposed to each vapor individually. No gender differences in deposition efficiency were observed for these vapors or for acetone. Deposition of DMS, DMG, and DMA was also measured in animals exposed to all three vapors simultaneously. Deposition efficiency under simultaneous exposure conditions ranged between 97.3 and 98.5%. These values were slightly lower (about 1%) than those obtained under individual exposure conditions (p less than 0.0001). The reduced deposition efficiency may have resulted from competitive inhibition of nasal metabolism due to the simultaneous presence of all three carboxylesterase substrate vapors in nasal tissues. If so, inhalation of dibasic ester vapors would be expected to inhibit the uptake of other carboxylesterase substrate vapors without influencing uptake of vapors which are not substrates for this enzyme. Such was observed in studies using DMS, ethyl acetate (the substrate vapor), and isoamyl alcohol (the nonsubstrate vapor). Specifically, simultaneous exposure to DMS markedly inhibited uptake of ethyl acetate without altering uptake of isoamyl alcohol. Gender differences were not observed in URT deposition of any of the six vapors used in the current study, DMS, DMG, DMA, ethyl acetate, isoamyl alcohol, or acetone, suggesting that gender differences in URT deposition may not be widespread among vapors. The high URT deposition efficiencies of the dibasic esters are consistent with the olfactory toxicity resulting from inhalation exposure to these vapors.