Anatomical and physiological parameters affecting gastrointestinal absorption in humans and rats.

Anatomical and physiological parameters of the gastrointestinal (GI) tract dramatically affect the rate and extent of absorption of ingested compounds. These parameters must be considered by nutritionists, pharmacologists and toxicologists when describing or modeling absorption. Likewise, interspecies extrapolation (e.g. from rat to human) requires species-to-species comparison of these parameters. The present paper (1) describes the alimentary canal and the barrier to absorption; (2) relates the major sites of absorption; (3) compares the dimensions and surface areas of human and rat intestinal tracts; (4) discusses motility of the gut and transit times through regions of the alimentary canal; (5) explains how luminal contents are altered by physical, chemical and metabolic processes; and (6) describes the flow of blood and lymph from the GI tract to the systemic circulation, including the enterohepatic circulation. Despite strong morphological similarities between humans and rats at the microscopic level, gross anatomical differences in the relative absorptive surface areas provide a basis for concluding that the human GI tract is capable of absorbing materials faster and to a greater extent than that of the rat. Differences in the environment of the GI lumen of the two species make it possible to infer which substances are more likely to be present in a dissolved/non-ionized state for each species. Taken together, these differences may be of sufficient magnitude to alter the assessment of risks/benefits for a given compound when those risks/benefits are based on interspecies extrapolations.

Hydroxylamine moiety of developmental toxicants is associated with early cell death: a structure-activity analysis.

BACKGROUND: Cellular debris, an indicator of cell death, appears in limb buds of gestational day 12 rabbit embryos 4 hr after either a subcutaneous injection of hydroxyurea to pregnant rabbits or an injection of hydroxyurea into the exocoelomic cavities of the embryos. This episode of early cell death appears to be central to the teratogenic action of hydroxyurea. Several chemicals that are structurally related to hydroxyurea, and that possess a terminal hydroxylamine moiety (-NHOH), also produce limb abnormalities. METHODS: To investigate whether the hydroxylamine moiety is responsible for early cell death and, therefore, is likely to be associated with teratogenesis, five structurally related hydroxylamine-bearing chemicals (hydroxylamine hydrochloride, N-methylhydroxylamine hydrochloride, hydroxyurea, acetohydroxamic acid, and hydroxyurethane) were administered at equimolar doses to rabbits either by subcutaneous (8.55 mmol/kg) or intracoelomic (2.66 micromol/embryo) injection on gestational day 12. Five additional chemicals, structurally similar to the hydroxylamine-bearing compounds, but possessing a terminal amino group (-NH(2)) (ammonium hydroxide, methylamine, urea, acetamide, and urethane), were tested at equimolar or higher doses by an identical protocol. In a subsequent experiment, the antioxidant propyl gallate (3.0 mmol/kg or 1.30 micromol/embryo) was co-administered with the hydroxylamine-bearing compounds to determine its effect on early cell death. Embryos were harvested 4 or 8 hr after treatment and analyzed by light microscopy. RESULTS: Cellular debris was obvious in forelimb buds from embryos treated with the hydroxylamine-bearing compounds; however, none of the amino compounds produced an early episode of embryonic cell death. In all cases, the antioxidant propyl gallate prevented or delayed the early episode of cell death observed after treatment with the hydroxylamine-bearing compounds. CONCLUSIONS: These results are consistent with the concept that the rapidly occurring embryonic cytotoxicity induced by hydroxylamine-bearing compounds involves a free radical mechanism that requires the presence of a terminal hydroxylamine group for initiation.

Appropriate use of animal models in the assessment of risk during prenatal development: an illustration using inorganic arsenic.

BACKGROUND: Assessing risks to human development from chemical exposure typically requires integrating findings from laboratory animal and human studies. METHODS: Using a case study approach, we present a program designed to assess the risk of the occurrence of malformations from inorganic arsenic exposure. We discuss how epidemiological data should be evaluated for quality and criteria for determining whether an association is causal. In this case study, adequate epidemiological data were not available for evaluating the potential effect of arsenic on development. Consequently, results from appropriately designed, conducted, and interpreted developmental toxicity studies, which have been shown to be predictive of human risk under numerous scenarios, were used. In our case study, the existing animal data were not designed appropriately to assess risk from environmental exposures, although such studies may be useful for hazard identification. Because the human and animal databases were deficient, a research program comprising modern guideline toxicological studies was designed and conducted. RESULTS: The results of those studies in rats, mice, and rabbits indicate that oral and inhalational exposures to inorganic arsenic do not cause structural malformations, and inhalational exposures produced no developmental effects at all. The new study results are discussed in conjunction with considerations of metabolism, toxicokinetics, and maternal toxicity. CONCLUSIONS: Based on the available experimental data, and absent contrary findings from adequately conducted epidemiological studies, we conclude that exposure to inorganic arsenic by environmentally relevant routes poses no risk of the occurrence of malformations and little risk of other prenatal developmental toxicity in developing humans without concomitant and near-lethal toxicological effects in mothers.

An assessment of the developmental toxicity of inorganic arsenic.

A critical analysis of the literature base regarding the reproductive and developmental toxicity of arsenic compounds, with emphasis on inorganic arsenicals, was conducted. The analysis was stimulated by the great number of papers that have purported to have shown an association between exposure of pregnant laboratory animals to arsenic compounds and the occurrence of offspring with cranial neural tube defects, particularly exencephaly. For the most part, the literature reports of arsenic developmental toxicity in experimental animals are inadequate for human risk assessment purposes. Despite the shortcomings of the experimental database, several conclusions are readily apparent when the animal studies are viewed collectively. First, cranial neural tube defects are induced in rodents only when arsenic exposure has occurred early in gestation (on Days 7 [hamster, mouse], 8 [mouse], or 9 [rat]). Second, arsenic exposures that cause cranial neural tube defects are single doses that are so high as to be lethal (or nearly so) to the pregnant animal. Third, the effective routes of exposure are by injection directly into the venous system or the peritoneal cavity; even massive oral exposures do not cause increases in the incidence of total gross malformations. Fourth, repetition of similar study designs employing exaggerated parenteral doses is the source of the large number of papers reporting neural tube defects associated with prenatal arsenic exposure. Fifth, in five repeated dose studies carried out following EPA Guidelines for assessing developmental toxicity, arsenic was not teratogenic in rats (AsIII, 101 micromol/kg/d, oral gavage; 101 micromol/m3, inhalation), mice (AsV, 338 micromol/kg/d, oral gavage; est. 402 micromol/kg/d, diet), or rabbits (AsV, 21 micromol/kg/d, oral gavage). Data regarding arsenic exposure and adverse outcomes of pregnancy in humans are limited to several ecologic epidemiology studies of drinking water, airborne dusts, and smelter environs. These studies failed to (1) obtain accurate measurements of maternal exposure during the critical period of organogenesis and (2) control for recognized confounders. The lone study that examined maternal arsenic exposure during pregnancy and the presence of neural tube defects in progeny failed to confirm a relationship between the two. It is concluded that under environmentally relevant exposure scenarios (e.g., 100 ppm in soil), inorganic arsenic is unlikely to pose a risk to pregnant women and their offspring.

Species difference in 1,3-dinitrobenzene testicular toxicity: in vitro correlation with glutathione status.

It has been previously shown that 1,3-dinitrobenzene (DNB) causes testicular damage to the rat but not the hamster. The present study of DNB's mechanism of toxic action has exploited this species difference in susceptibility. Seminiferous tubules were isolated from Golden Syrian hamsters and incubated with 100 microM DNB or vehicle for 22 h. (A similar study with rat tubules has been published.) Formation of DNB metabolites were monitored over time; hamster tubules had a greater capacity than rat tubules for reductively metabolizing (activating) DNB. However, hamster tubules did not show the marked DNB-induced ATP depletion seen in rat tubules. Levels of mitochondrial glutathione and activities of enzymes that protect against oxidative stress were measured in both rat and hamster tubules. The observed differences in the capacity for detoxification of oxidants may underlie the difference in susceptibility to DNB-induced testicular toxicity between these species.

1,3-Dinitrobenzene metabolism and toxicity in seminiferous tubules isolated from rats of different ages.

Previous in vivo studies in rats have shown that susceptibility to 1,3-dinitrobenzene (DNB)-induced testicular damage increases with age. The present study has used an in vitro approach to investigate the possibility that differences in testicular metabolism contribute to the age-related differences in toxicity. Seminiferous tubules were isolated from Sprague-Dawley rats (30, 75 and 120 days old) and incubated with 100 microM DNB for 22 h. Formation of metabolites and tubular levels of ATP and glutathione (GSH) were monitored over time. There was no difference in seminiferous tubule metabolic capacity among the three ages of rats examined. After 22 h of incubation with DNB, ATP levels were 20-30% of control and GSH levels were 70-90% of control, but neither parameter showed an age-related difference in decline. Based on these biochemical indicators of cell health, this study would suggest that the lack of testicular toxicity in young animals in vivo may be due to the previously described shorter half-life with consequent reduced exposure of the testis to DNB and that the age-related increase in severity of lesion between 75 and 120 days of age cannot be explained by differences in tubular metabolism of DNB or whole-animal toxicokinetics.

Metabolism and testicular toxicity of 1,3-dinitrobenzene in the rat: evaluation of the stage-synchrony model.

Because many testicular toxicants cause damage to specific stages of spermatogenesis, the present study has investigated the utility of a model in which the testis is synchronized to contain only a few closely related spermatogenic stages. The susceptibility of different stages to 1,3-dinitrobenzene (1,3-DNB) toxicity was investigated in rats, the testes of which had been stage synchronized by a vitamin A depletion/repletion (VADR) procedure. 1,3-DNB (25 mg/kg, IP) or vehicle was injected 58, 61, or 78 d after vitamin A readministration, and testicular histopathology was evaluated 48 h later. At the time of sacrifice, testes in the three groups were synchronized to stages I-VI, VII-IX, or X-XIV+I. The data indicated that tubules in all stages of spermatogenesis, in both synchronized and unsynchronized animals, demonstrated histopathologic changes in response to 1,3-DNB. However, the lesion seen in synchronized animals was more severe and less stage specific than that seen in weight-matched, unsynchronized animals. This increase in degree of susceptibility could be partially explained by differences in toxicokinetics. Stage-synchronized testes could provide unique insights into stage-specific cellular and molecular events, especially for in vitro studies where the stage enrichment could be maximally exploited. However, results obtained from in vivo toxicity studies using animals subjected to VADR should be interpreted carefully in light of the confounding physiologic/metabolic perturbations potentially induced by the VADR procedure.

Metabolism and testicular toxicity of 1,3-dinitrobenzene in the rat: effect of route of administration.

Studies investigating the testicular toxicity of 1,3-dinitrobenzene (1,3-DNB) have utilized both the oral (po) and intraperitoneal (ip) routes of administration. These two administration routes could be expected to produce different pharmacokinetic profiles and, potentially, different degrees of toxicity. In the present work, the effect of route of administration upon 1,3-DNB disposition and susceptibility to testicular damage has been investigated. Male Sprague-Dawley rats were given 25 mg/kg 1,3-DNB either ip or po. Metabolites were quantitated in blood, urine, and feces, and methemoglobin levels were determined. Peak blood levels of 1,3-DNB and its major metabolite were three times higher in ip-dosed rats than in po-dosed rats. While the lower blood levels seen after po administration were maintained for greater than 6 hr, blood levels fell rapidly after ip dosing, reaching po levels at 6 hr postadministration. Peak methemoglobin levels in ip-dosed animals were twice that of po-dosed animals. Route of administration had a minor effect on the levels of urinary metabolites, while there was a significantly higher excretion of metabolites in the feces of po-dosed animals. Despite the markedly higher 1,3-DNB blood levels after ip administration, there were only subtle differences in testicular damage. The data raise the possibility that above a threshold level of 1,3-DNB in the blood, only the duration of testicular exposure to the toxicant may govern susceptibility to testicular toxicity.

Holographic technique for determining the phase of vibrating objects.

A means is demonstrated for determining the phase of vibrating objects by using either time average holography or real time holographic interferometry. The technique consists of vibrating a mirror in either the object or reference leg at the same frequency as the object and observing the effects on the fringe pattern when either the phase or amplitude of the mirror vibration is altered. Proper adjustment of these parameters places an interferometric node (fringe-contrast maximum) at any object point. The phase is then known from the phase of the vibrating mirror.