The pulmonary responses of rats are extremely marked when compared to other large mammalian species such as non-human primates and humans.
In studies reported by Nikula et al (1997, 2001), it has been proposed that the intrapulmonary particle retention patterns and tissue reactions in rats may not be predictive of pulmonary retention patterns and tissue responses in either primates or humans. Male monkeys and rats were exposed for 7 hours/day, 5 days/week for 24 months to diesel exhaust (2 mg/m3), coal dust (2 mg/m3), or diesel exhaust and coal dust combined (1 mg/m3 each) and were subsequently examined histopathologically (Nikula et al, 1997). In all exposed groups, monkeys retained a similar amount or more particulate material in the lungs than did rats. Rats retained a greater proportion of the particulate material in the alveolar ducts and alveoli, whereas monkeys retained a greater proportion of particulate material in the interstitium. Rats, but not monkeys, had significant alveolar epithelial hyperplastic, inflammatory, and septal fibrotic responses to the retained particles. Similar to the findings in monkeys, up to 91% of the retained particulate material in the lungs of coal miners was located in the lung interstitium (Nikula et al, 2001). It was suggested by the authors that these differences in particulate tissue distribution in rats and humans might bring different lung cells into contact with retained particulates or particle-containing macrophages. This may account for the differences in species responses to inhaled particulates.
Studies also show that the rat’s response to retained particles is more serious and adverse than for other species. Nikula et al (1997) compared the anatomical patterns of particle retention and the lung tissue responses between rats and cynomolgus monkeys following chronic exposure to diesel exhaust and coal dust. Lung sections from the monkeys and rats exposed for 24 months to filtered ambient air, diesel exhaust (2 mg soot/m³), coal dust (2 mg respirable particulate material/m³) or diesel exhaust and coal dust combined (1 mg soot and 1 mg respirable coal dust/m³) were examined histopathologically. The relative volume density of particulate material and the volume percentage of the total particulate material in defined pulmonary compartments were determined morphometrically to assess the relative amount and the anatomic distribution of retained particulate material. With one exception (diesel exhaust), relatively more particulate material was retained in the monkey lungs relative to the rat lungs, indicating that the rats clear the dust at a faster rate relative to the monkeys. There was no significant difference between diesel exhaust-exposed monkeys and rats in the relative amount of retained particulate materials. In addition, rats retained a greater portion of the particulate material in lumens of alveolar ducts and alveoli than monkeys; and monkeys retained a greater portion of the particulate material in the interstitium than rats. Rats, but not monkeys, had significant alveolar epithelial hyperplastic, inflammatory and septal fibrotic responses to the retained particles. The authors concluded: “These results suggest that intrapulmonary particle retention patterns and tissue reactions in rats may not be predictive of retention patterns and tissue responses in primates exposed to poorly soluble particles at concentrations representing high occupational exposures. The pulmonary responses of the rats were severe compared to the primate, where the insult to the lungs was handled without adverse consequences.”
In a subsequent study, Nikula et al (2001) evaluated the influence of exposure concentration or dose on the distribution of particulate material within the lungs of rats and humans. In this study the investigators used morphometric methods to assess the influence of exposure concentration on particle retention by evaluating histologic lung sections from rats and humans. The rats had been exposed for 24 months to diesel exhaust at 0.35, 3.5, or 7.0 mg soot/m³. The human subject groups included 1) nonsmokers who did not work as miners; 2) nonsmoking coal miners who worked under the current standard of 2 mg dust/m³ for 10-20 years; and 3) nonsmoking coal miners who worked under the former standard of <10 mg dust/m³ for 33 to 50 years. The distribution of retained particles within the lung compartments was markedly different between species. In all three groups of rats, 82 to 85% of the retained particulate material was located in the alveolar and alveolar duct lumens, primarily in macrophages. In humans, 57, 68, and 91% of the retained particulate material, respectively, was located in the interstitium of the lung in the three aforementioned study groups. The authors concluded: “These results show that chronically inhaled diesel soot is retained predominantly in the airspaces of rats over a wide range of exposures, whereas in humans, chronically inhaled particulate material is retained primarily in the interstitium. In humans, the percentage of particles in the interstitium is increased with increasing dose (exposure concentration, years of exposure, and/or lung burden). This difference in distribution may bring different lung cells into contact with the retained particles or particle-containing macrophages in rats and humans and, therefore, may account for differences in species response to inhaled particles.”
The two studies by Nikula et al (1997, 2001) provide important insights, as they clearly delineate significant species differences in pulmonary responses to inhaled particulates between rats and primates, including humans. The studies by Nikula and colleagues comparing lung responses to inhaled particles in rats versus larger mammals demonstrate the following: 1) the disposition and dosimetry differ between rats and either monkeys or humans (rat = alveolar; monkey/human = interstitial sites); and 2) rats produce significantly augmented and sustained pulmonary inflammogenic, epithelial and fibro-proliferative responses when compared to either monkeys or humans. The available data also suggest that rats are significantly more sensitive in the development of adverse lung responses to inhaled particle exposures when compared to: 1) other rodent species; and 2) larger mammals such as monkeys and humans.
In a review paper, Nikula (2000) compared histopathological results from chronic bioassay studies with particulates in rats vs. nonhuman primates. This review summarised data from studies with exposure for 2 years or more using 5 poorly soluble nonfibrous materials – diesel exhaust, carbon black, titanium dioxide, talc, and coal dust. The following studies were reviewed: Chronic inhalation studies of rats and monkeys identically exposed to diesel exhaust (Lewis et al, 1989), coal dust (Lewis et al, 1989), petroleum coke dust (Klonne et al, 1987), raw or processed shale dusts (MacFarland et al, 1982), bertrandite (Wagner et al, 1969), and beryl ore (Wagner et al, 1969). The comparisons of rat and monkey responses to chronic inhalation of poorly soluble particles led to some important insights.
Lewis and colleagues (1989) exposed rats and monkeys to filtered air or to diesel exhaust or coal dust at 2 mg respirable particulate material/m³, 7 h/day, 5 days/wk for 24 months. Diesel exhaust- and coal dust-exposed rats and monkeys retained particulate material in their lungs. The anatomic distribution of the retained material differed between rats and monkeys (Nikula et al, 1997; Lewis et al, 1989). Rats retained particulate material predominantly within alveolar macrophages located in the lumens of alveoli and alveolar ducts. Monkeys retained more of the particulate material within macrophages in the interstitium and pleura than in the lumens of alveoli and alveolar ducts. Responses to the exposures and retained material also differed between rats and monkeys. Rats, but not monkeys, had significant alveolar epithelial hyperplastic and inflammatory responses to the retained particulate material (Nikula et al, 1997; Lewis et al, 1989).
Klonne and colleagues (1987) exposed Sprague-Dawley rats and cynomolgus monkeys to 0, 10.2, or 30.7 mg/m³ micronized delayed process petroleum coke for 6 h/day, 5 days/wk (weekly exposure rate up to 921 mg/m³) for 24 months. The rats exhibited retention of particulate material in macrophages, chronic inflammation, focal fibrosis alveolar-bronchiolar metaplasia, sclerosis, squamous metaplasia of alveolar epithelium, and keratin cysts. Two of 48 males and 10 or 44 (23%) females in the high concentration group had keratin cysts. Several rats had multiple cysts. In monkeys, the pulmonary histopathology was limited to accumulations of macrophages containing particulate material.
MacFarland and co-workers (1982) exposed F344 rats and cynomolgous monkeys to 0, 10, or 30 mg/m³ respirable raw or processed shale dust for 6 h/day, 5 days/wk (weekly exposure rate up to 900 mg/hour/m3) for 24 months. All of the rats developed proliferative bronchiolitis and alveolitis (i.e., inflammation with epithelial hyperplasia) and most developed chronic inflammation with nonprogressive fibrosis, cholesterol clefts, and micro-granulomas. The monkeys accumulated more pigment-laden macrophages in the bronchiolar and alveolar walls than in the alveolar lumens. The majority of monkeys had no reaction to the accumulated material. A few monkeys had occasional foci of sub acute inflammation.
Wagner and colleagues (1969) exposed Sprague-Dawley and Green-acres Controlled Flora (strain not otherwise identified) rats, Syrian golden hamsters, and squirrel monkeys to bertrandite or beryl ore dusts. The exposures were to 0 or to 15 mg/m³ of the ores for 6 h/day, 5 days/week (weekly exposure rate of 450 mg mg/hour/m3) for 17 months (rats and hamsters for 23 months (monkeys). Alveolar epithelial hyperplasia and chronic inflammation with granulomas were present in rats exposed to both materials. By 12 months, 5 of 11 rats had foci of squamous metaplasia or lesions referred to as tiny epidermoid tumours. By 17 months, 7 and 9 of 19 rats had adenomas and adenocarcinomas, respectively and 4 of 19 had epidermoid tumours. The ore-exposed hamsters had alveolar epithelial hyperplasia, bronchiolar-alveolar metaplasia, and a few granulomatous lesions. No lesions other than accumulations of particle-containing macrophages and mononuclear cells around respiratory bronchioles and blood vessels were present in monkeys.
The results of studies wherein rats and monkeys were exposed to identical aerosolised particle test substances indicate that although both rats and monkeys retain particulate material in their lungs when chronically exposed to high concentrations of dusts or soot, the retention pattern within anatomical compartments is different. Moreover, the pulmonary tissue response to the particulate material is different. Specifically rats retain a greater proportion of the material in intralumenal alveolar macrophages, and they respond with a greater degree of epithelial hyperplasia and active inflammation when compared to the response of monkeys.