Richard A. Sloane

Reproductive endpoints in general toxicity studies : Are they predictive ?

The ability to use necropsy and in-life vaginal cytology data from 90-d general toxicity studies to predict the outcome of more definitive reproductive toxicity tests was evaluated using data from 18 compounds tested by the National Toxicology Program. Sperm motility and vaginal cytology evaluations (SMVCE) were performed at the end of 90-d toxicity studies. When these same compounds were tested in the definitive Reproductive Assessment by Continuous Breeding (RACB) design, 13 of the 18 compounds were classified the same way by both tests. The different conclusions for five compounds can be explained by differences in dose used or in endpoints evaluated. We conclude that reproductive-system necropsy data from general toxicity studies can provide a valuable preliminary indication of the likely reproductive toxicity of the compound under study.

Comparative toxicity of arsine gas in B6C3F1 mice, Fischer 344 rats, and Syrian Golden hamsters: System organ studies and comparison of clinical indices of exposure

In order to examine possible species differences in response to arsine exposure, multiple inhalation studies consisting of acute (1-day), subacute (14- and 28-day), and subchronic (90-day) exposures to this agent were conducted using three different species of rodents. Evaluations of hematopoietic organs and alterations in the heme biosynthetic pathway were the focus of these studies. Species used were B6C3F1 mice (exposed 1, 14, or 90 days), Fischer 344 rats (exposed 14, 28, or 90 days), and Syrian Golden hamsters (exposed 28 days). All arsine exposures were at concentrations of 0.5, 2.5, or 5.0 ppm except for 90-day studies, in which concentrations were lowered to 0.025, 0.5, or 2.5 ppm. No changes in body weight gain were observed in either sex of mice or hamsters. The only decrease in body weight gain occurred in male rats exposed to 5.0 ppm arsine for 28 days. Significant exposure-related increases in relative spleen weights occurred in both sexes of mice and rats in the 0.5 (except 14-day female rats), 2.5, and 5.0 ppm exposure groups from all studies and in hamsters in the 2.5 and 5.0 ppm exposure groups. Generally, increases in relative liver weight occurred in fewer exposure groups and were of a lesser magnitude than increases in spleen weight. Other parameters affected included decreased packed cell volumes (mice, rats, and hamsters), hematology profiles (rats), and an increase in delta-aminolevulinic acid dehydratase activity in all species. Arsenic content was measured in livers of rats after 90 days of exposure. Concentrations increased in relation to atmospheric concentrations of arsine. Histopathologic changes included increased hemosiderosis and extramedullary hematopoiesis in spleen and intracanalicular bile stasis (mice only) in liver. Additionally, bone marrow hyperplasia was observed in rats. Effects on other organs were not observed, suggesting that the hematopoietic system is the primary target for arsine. In conclusion, we have determined that the effects of arsine exposure upon mice, rats, and hamsters are similar. Most importantly, even though no effects on the hematopoietic system were observed following a single exposure to 0.5 ppm arsine which is 10 times the Threshold Limit Value (TLV) set by the American Conference of Governmental Industrial Hygienists, repeated exposure to 0.025 ppm (one-half the TLV) caused a significant anemia in rats.

Time‐varying concentration profile as a determinant of the inhalation toxicity of carbon tetrachloride

Certain limitations on the flexibility of small-animal inhalation exposure systems are overcome by the machine control and monitoring of the concentration of the gas or vapor of interest. Computer assistance of chamber operation allows the user to simulate time-varying concentration profiles accurately and repeatedly. We exposed rats to seven different profiles in which the maximum concentration of carbon tetrachloride (CCl4) was 1500 ppm and the product of concentration times time (C X T) was 4500 ppm X h. The purpose was to determine the effects of systematically varying the shape of the concentration profile on the expression of hepatotoxicity of a chemical about which much was already known. All of the exposures were conducted within a span of 6 h. Examination of the severity of vacuolation and pattern of necrosis could be used to distinguish some of the exposure profiles from others. For example, vacuolation was less severe when two equal pulses were presented with an interval of 60 min, rather than 180-240 min. The indexes of necrosis varied in a more complex way, and the differences among the profiles that accounted for the differences in the patterns of the histopathological changes were not immediately apparent. We concluded that the characteristic of a time-related variation in concentration is one of the determinants of the inhalation hepatotoxicity of CCl4 and that the simple, time-weighted, average concentration may not always fairly represent the best model for the study of problems in inhalation toxicology.

Failure of carbon disulfide and levothyroxine to modify the cardiovascular response of rabbits to a high-cholesterol diet

Exposure of rabbits for 12 weeks to 300 ppm carbon disulfide (CS2) for 6 h/day, 5 days/week, or to 25 mg/day of thiourea or 2% cholesterol in the diet, or to any combination thereof caused a significant reduction in the concentration of serum thyroxine (T4). The reduction of the concentration of serum T4 in rabbits by the treatments was completely offset by the inclusion of 0.1 mg/day of sodium levothyroxine in the diet. Ingestion of feed containing 2% cholesterol significantly increased the degree of atherosclerosis present in the aortic arch and significantly increased the oil red O positive lipid present in the heart and the aorta, with the aortic arch being the most severely affected. The response of the aorta and the heart to the 2% cholesterol diet was not significantly modified by concurrent exposure to CS2 by inhalation or by treatment with thiourea, a metabolite of CS2. We found no evidence that the development of cardiovascular lesions induced by a 2% cholesterol diet in rabbits was mediated by a mechanism involving a component of hypothyroidism.

Hepatic cholesterol metabolism following exposure to carbon disulfide in phenobarbital-treated rats

Male F344 rats were exposed either to 1.87 mg/L (600 ppm) carbon disulfide (CS2) for 6 hr/day by inhalation for 1, 2, or 3 consecutive days, or to 0.1% phenobarbital (PB) in the drinking water starting 5 days before exposure to CS2, or to both. Combined treatments (CS2 + PB) resulted in a decrease in hepatic cholesterol synthesis, increases in hepatic cholesterol concentration and relative liver weight, and histopathologic damage. Maximal inhibition of cholesterol synthesis was observed following 1 day of combined treatments, while the increases in hepatic cholesterol concentration were similar following 1, 2, or 3 days of combined treatments. Exposure to CS2 only produced a pattern of inhibition of cholesterol synthesis that was similar to, though less extensive than, that seen following combined treatments. All reported alterations caused by combined treatments of CS2 + PB were reversible; recovery was, in all cases, essentially complete by day 11 after a single exposure to 1.87 mg/L CS2. With the exception of cholesterol concentration where time to recovery was decreased by continuation of PB, the time required for recovery from the effects of combined treatments of CS2 + PB was not affected by whether or not PB was continued after CS2 exposure. The reported observations support the theory that metabolism of CS2 is involved in the expression of CS2-mediated alterations of hepatic cholesterol metabolism.

Hepatic Cholesterol Metabolism as a Function of Carbon Disulfide Concentration and Treatment with Phenobarbital

Male F344 rats were exposed to carbon disulfide (CS2) at 0, 30, 75, 150, 300, or 600 ppm for 6 hr by inhalation in the presence or absence of 0.1% phenobarbital (PB) in the drinking water starting 5 days before exposure to CS2. Exposure to 600 ppm CS2 only resulted in a decrease in hepatic cholesterol synthesis and an increase in the liver-to-body-weight ratio (relative liver weight); however, it caused no histopathological damage and had little or no consistent effect on the concentration of hepatic cholesterol or on hepatic water content. Treatment with PB alone resulted in increases in the concentration of hepatic cholesterol and relative liver weight. Exposure to 300 ppm CS2 + PB or to 600 ppm CS2 + PB resulted in a decrease in hepatic cholesterol synthesis and increases in the concentration of hepatic cholesterol, relative liver weight, hepatic water content, and histopathological damage. A concentration-response relationship was demonstrated between exposure to CS2 only and decreased hepatic cholesterol synthesis. A concentration-response relationship also was demonstrated between exposure to CS2 in rats that had been treated with PB and decreased hepatic cholesterol synthesis, increased hepatic cholesterol concentration, increased relative liver weight, increased hepatic water content, and histopathological damage. Treatment with PB lowered the concentration of CS2 required to alter hepatic cholesterol metabolism. The reported observations are consistent with the theory that oxidative metabolism is involved in the expression of CS2-mediated alterations of hepatic cholesterol metabolism.