Ellen J. O'Flaherty

Delineation of the role of metabolism in the hepatotoxicity of trichloroethylene and perchloroethylene: A dose-effect study

The relationship among dose, metabolism and hepatotoxicity in mice which resulted from subchronic exposure to the chlorinated solvents trichloroethylene (TRI) and perchloroethylene (PER) were examined. Male Swiss-Cox mice received either TRI (0 to 3200 mg/kg/day) of PER (0 to 2000 mg/kg/day) in corn oil by gavage for 6 weeks. Urinary metabolites from individual mice were quantified to estimate the extent to which each compound was metabolized. Four parameters of hepatotoxicity were assessed: liver weight, triglycerides, glucose-6-phosphatase (G6P) activity, and SGPT activity. TRI significantly affected liver weight and G6P activity; PER affected all four parameters. The metabolism of TRI was linearly related to dose through 1600 mg/kg, but then became saturated. The metabolism of PER was saturable. The dose-effect curves of the affected hepatotoxicity parameters of both compounds were nonlinear and resembled the dose-metabolism graph of the corresponding solvent. Plots of the hepatotoxicity data of each compound against total urinary metabolites were linear in all cases, suggesting that the hepatotoxicity of both PER and TRI in mice is directly related to the extent of their metabolism. This pattern is consistent with formation of the toxic intermediate in the primary metabolic pathway of each compound.

Physiologically based models for bone-seeking elements

A notching machine including a first cutting stage which makes four vertical cuts in the stringer, the outer two cuts defining the sides of the notch. The second cutting stage of the machine makes a horizontal cut which first separates a portion of the stringer between the inner two cuts thereby allowing the drive shaft of the second-stage cutting blade to pass through the partially formed notch and cut away the remaining portions of the stringer between the outer two cuts to define the notch.

Physiologically Based Models of Metal Kinetics

The issues confronting the modeler of metals kinetics are somewhat different from those with which the modeler of organic chemical behavior is faced. Particularly important features of metals kinetics include metal-protein binding and metal-metal interactions. Reduction, and for some metals oxidation, is frequently an intrinsic part of metal metabolism. Alkylation/dealkylation reactions may or may not render the metal less active, and the behavior of alkylated or dealkylated metabolites must often be included in a complete kinetic model. Despite these complexities, the kinetics of metals are as amenable to the techniques of physiologically based modeling as are the kinetics of organic chemicals. Like all models, those for metals kinetics have the potential to organize a variety of observations, sometimes including apparently inconsistent observations, into a coherent framework of behavior, to identify needs for more complete experimental information, and to assist the risk assessor in making judgments concerning dose-response relationships. Development of physiologically based models of the kinetic behavior of metals is in its very early stages. The kinetics of only four metals, arsenic, chromium, mercury, and lead, have been modeled with any degree of completeness. Of these, the lead model is the most fully realized at the present time. The chromium and mercury models are still in the process of development, and experimental data are being gathered to support further development and refinement of the arsenic model. We may expect to see continued progress made on these models and their practical applications, as well as the development of new models for other toxicologically significant metals such as cadmium, manganese, nickel, and aluminum.

Physiologically based models for bone-seeking elements. III: Human skeletal and bone growth

A model of skeletal and bone growth for the human from birth to maturity has been developed. Dry and hydrated bone, bone marrow, ash, and calcium are included in the model. Growth of the skeleton and its fractions is expressed as a set of allometric equations relating fraction volume or weight to body weight. Blood flow rates to mature bone and bone marrow are scaled from experimentally determined values in smaller animals, but bone and marrow volumes and growth patterns cannot be scaled directly from measurements or models in small animals. The growth model compares well with measured bone weights, ash weights, and bone and skeletal densities in humans. Its form is adaptable to physiologically based description of the kinetics of bone-seeking elements.

Use of Sequentially Administered Stable Lead Isotopes to Investigate Changes in Blood Lead during Pregnancy in a Nonhuman Primate (Macaca fascicularis)

The effects of pregnancy on the flux of lead from maternal bone were investigated in five females from a unique colony of cynomolgus monkeys (Macaca fascicularis) which had been dosed orally with lead (approximately 1100-1300 microg Pb/kg body wt) throughout their lives (about 14 years). Through the use of stable lead isotopes 204Pb, 206Pb, and 207Pb, it was possible to differentiate between the lead contributed to blood lead from the skeleton and the lead contributed from the current oral dose. Blood samples and bone biopsy samples taken before, during, and after pregnancy were analyzed for lead (total and stable isotope ratios) by thermal ionization mass spectrometry. Through the use of end-member unmixing equations, the contribution to blood of lead from maternal bone during pregnancy was estimated and compared to the contribution of lead from maternal bone before pregnancy. A 29 to 56% decrease in bone lead mobilization in the first trimester was followed by an increase in the second and third trimesters, up to 44% over baseline levels. In one monkey, the third-trimester increase did not reach baseline levels. In a single low-lead monkey, a similar decrease in the first trimester was followed by a 60% increase in the third trimester, indicating that a similar pattern of flux is seen over a wide range of lead concentrations. Analysis of maternal bone and fetal bone, brain, liver, and kidneys confirmed a substantial transplacental transfer of endogenous lead. Lead concentrations in fetal bone often exceeded maternal bone lead concentrations. From 7 to 39% of the lead in the fetal skeleton originated from the maternal skeleton.

Influence of Lead on Mineralization during Bone Growth

Lead will inhibit skeletal development and localize in areas of bone formation and resorption, but the mechanisms of lead toxicity in bone are largely unknown. This study used an ectopic bone (plaque) induction method to investigate the effect of lead on mineralization of cartilage in growing bone. Demineralized bone matrix was subcutaneously implanted in male Long-Evans rats to induce plaque formation. Of 64 rats which were provided deionized water, 32 were implanted with control matrix (control group). The remaining 32 rats were implanted with matrix containing a target concentration of 200 micrograms lead/g of plaque tissue as ectopic bone (lead-added group). Another group of 32 rats was continuously exposed to 1000 ppm lead in drinking water and subcutaneously implanted with control matrix (drinking water-lead group). Plaques were taken for analysis on Days 8 and 12 postimplantation. Alkaline phosphatase activity and cartilage mineralization were obliterated in lead-added plaques. However, calcium deposition was markedly enhanced in the lead-added plaques. Decreased alkaline phosphatase in Day 8 drinking water-lead plaques followed increased Day 12 drinking water lead plaque calcification. Enhanced cartilage calcification and reduced alkaline phosphatase activity in the drinking water-lead plaques was consistent with effects observed in the metaphyseal regions of bone in lead-exposed rats and pigs. The results of this study suggest that lead adversely influences bone development through disruption of mineralization during growth.

Effects of Lead Exposure on Skeletal Development in Rats

The effects of lead on growth in female rats and on growth and skeletal development in their offspring were investigated. No alteration in growth rate, compared to the growth rate in pair-fed controls, was observed in 48 weanling females continuously exposed to 250 or 1000 ppm lead in drinking water and fed a replete diet. After 49 days of exposure, all rats (24 pair-fed controls, 12 exposed to 250 ppm lead, and 12 exposed to 1000 ppm lead) were mated with control males. At parturition, six lactating dams each from the 250 and 1000 ppm lead groups were removed from lead exposure and given control drinking water, and six lactating dams each from the control group were given either 250 or 1000 ppm lead in drinking water. Exposure conditions for the remaining dams in the control, 250, and 1000 ppm groups were not changed. Maternal blood lead in the continuously lead-exposed groups was higher at the end of lactation than prior to mating. Lead exposure prior to parturition caused greater maternal tibial lead accumulation than lead exposure after parturition. In contrast, lead exposure prior to parturition had a lesser impact on offspring tibial lead accumulation than lead exposure after parturition. Decreases in tibial calcium and phosphorus were observed in dams exposed continuously to 250 or 1000 ppm lead; however, there was no apparent effect of lead on maternal growth-plate morphology or on growth-plate width. Offspring body weight was depressed relative to controls during suckling (Day 11) and after weaning (Day 24) in high-dose and continuously lead-exposed groups. Continuous lead exposure caused a greater decrease in offspring body weight than lead exposure only prior to or after parturition. Decreased tail length growth suggested possible effects of lead on tail vertebral bone growth. While tibial calcium and phosphorus levels were not changed in the weanlings, increased weanling growth-plate width, with disruption of chondrocyte organization, and wider metaphyseal trabeculae were observed. Although the mechanisms of these effects are not known, the results suggest that local lead-related effects on growth-plate chondrogenesis and metaphyseal mineralization may be involved.

Dependence of apparent blood lead half-life on the length of previous lead exposure in humans

The expected relationship between the apparent half-life of blood lead (PbB) and the length of previous exposure is compared with the observed relationship in a group of workers on strike from a primary smelter where they had been employed for from a few months to ten years. Within this group, the apparent PbB half-life was positively correlated both with pre-strike PbB and with length of exposure to lead. It is likely that the dependence of apparent half-life on pre-strike PbB is primarily a reflection of interindividual differences in lead kinetics in these men. The direct dependence of apparent half-life on duration of lead exposure is consistent with published data from former lead workers and from human subjects experimentally exposed to lead.

Chemical mixtures from a public health perspective: the importance of research for informed decision making.

When considered from a public health perspective, the central question regarding chemical mixtures is deceptively simple: Are current approaches to risk assessment for chemical mixtures affording effective (adequate) and efficient (cost-effective) protection for members of our society? Answering this question realistically depends on an understanding of the hierarchical goals of public health (i.e. prevention, intervention, treatment) and an accurate evaluation of the extent to which these goals are being achieved. To allow decision makers to make informed judgments about the health risks of chemical mixtures, adequate scientific knowledge and understanding must be available to support risk assessment activities, which are an integral part of the regulatory decision making process. Designing and implementing relevant research depends on the existence of a feedback loop between researchers and regulators, where the information needs of regulators influence the nature and direction of research and the information and understanding generated by researchers improves the scientific basis for public health decisions. A clear, consistent, commonly accepted taxonomy for describing important mixture-related phenomena is a key factor in creating and maintaining the necessary feedback loop. Ultimately, both researchers and regulators share a common goal with regard to chemical mixtures; improving the state-of-the-science so that we can make informed decisions about protecting public health. A survey of research issues and needs that are crucial to attaining this goal is presented.

Measurement of the flux of lead from bone to blood in a nonhuman primate (Macaca fascicularis) by sequential administration of stable lead isotopes.

To better understand the kinetics of the transfer of lead from bone to blood, we have developed and tested a method in which sequential doses of lead, each enriched with a different stable isotope, were administered in a nonhuman primate Macaca fascicularis whose skeleton had been previously labeled with lead of known isotopic composition. Lead isotopic ratios of blood and bone samples, analyzed by thermal ionization mass spectrometry (TIMS), were unmixed by isotope dilution techniques. The first label administered allows the contribution from historical bone stores to be measured. Subsequent labels allow measurement of both the historical bone stores and the previous labels that have become recently incorporated into bone. The method may be extended to studies of bone lead mobilization in pregnancy, lactation, menopause, or in disease states such as postmenopausal osteoporosis.