Elsevier

Reproductive Toxicology

Volume 12, Issue 3, May–June 1998, Pages 333-346
Reproductive Toxicology

Original Articles
Male Reproductive Effects of Lead, Including Species Extrapolation for the Rabbit Model 1

https://doi.org/10.1016/S0890-6238(98)00010-0Get rights and content

Abstract

The effects of elevated blood lead on semen quality were evaluated in the rabbit model and compared to published effects in humans. Mature, male rabbits were given lead acetate by subcutaneous injection in the dose range of 0 to 3.85 mg/kg on a Monday-Wednesday-Friday basis. In each of eight treatment groups, a dosing regimen was developed to produce blood lead levels of 0, 20, 40, 50, 70, 80, 90, and 110 μg/dL. A 5-week pre-exposure period was followed by a 15-week exposure testing period allowing for response through six cycles of the seminiferous epithelium. Semen analyses revealed that increased blood lead levels were associated with adverse changes in the sperm count, ejaculate volume, percent motile sperm, swimming velocities, and morphology. Hormonal responses were minimal. Testicular pathology revealed a dose-dependent inhibition of spermiation. For six measures of semen quality, threshold estimates ranged from 16 to 24 μg/dL. Using the species extrapolation factor derived in this study, a rabbit dose would have to be divided by 1.56 to obtain the equivalent human dose for an equal percentage decrease in sperm concentration; however, rabbits are 3.75 more sensitive in terms of absolute decrease in sperm count for a given blood lead level.

Introduction

The scientific process for the evaluation of reproductive risk from drugs and chemicals includes extrapolation from data obtained in animal models [1]. The differences between species, however, may limit the appropriate and accurate application of toxic effects in human risk assessment. Important criteria for an animal model of reproduction in humans include the existence of documented, relatable physiology of the species and the ability to analyze ejaculated semen. These aspects permit studies in which toxic effects can be related to humans because the focus of medical evaluation of the male is on analysis of ejaculated whole semen, not just sperm cells.

The most common laboratory species, the rat, does have well-documented reproductive physiology. However, it does not readily permit analysis of ejaculated semen for longitudinal effects. A review of animal models by Amann [2] has emphasized that rabbits are the smallest and least expensive laboratory animals in which serial semen samples can easily be obtained for morphologic, biochemical, and fertility evaluation.

The purpose of this study was to evaluate the rabbit model for assessment of male reproductive responses to a model chemical, lead. A qualitative and quantitative comparison between the experimental responses in the rabbit and published epidemiologic responses in humans for lead would indicate the usefulness of the model for predicting human spermatotoxic responses to lead and other similarly acting chemical toxicants. Lead as a model reproductive toxicant offers a uniquely important perspective in the quantitative species comparison because human blood lead levels are generally reported along with the observed health effect. By focusing on differences as they relate to blood lead levels rather than environmental exposures, a far more efficient experimental approach can be used. The relationship between exposure and blood lead levels has been extensively studied. Therefore, the results of the present study are not route dependent.

Experimental evaluation of the effects of lead on reproductive function in rabbits has been reported for many years. In 1914, Cole and Bachuber [3] published data showing that offspring from Dutch male rabbits that were dosed with lead acetate showed increased mortality and decreased weights. In a more recently reported animal study using rabbits, Willems et al. [4] attempted to determine the effects of lead acetate administered by subcutaneous (s.c.) injection, but they failed to show effects on sperm morphology, sister chromatid exchanges, and on formation of micronuclei. The exposure was for 14 weeks; however, there were only five male rabbits in each treatment, and blood lead levels at the end of the Willems study were 6.6 μg/dL, 53.2 μg/dL, and 61 μg/dL. Sperm were obtained at sacrifice only. The design did not permit analysis of ejaculated semen, and the morphologic assessment reflected cells undergoing maturation at lower blood lead levels than those measured at the end of the study.

As a result of advantages offered by rabbits for reproductive toxicology, the National Toxicology Program agencies, National Institute of Environmental Health Sciences and National Institute for Occupational Safety and Health, have focused on research directed at statistical considerations of study designs [5] and a comparative evaluation of ethylene dibromide [6]. The comparative study demonstrated that although the rabbit was not as sensitive as humans, four of seven semen parameters that were altered in the human study [6] were significantly changed in rabbits. An important consideration was that the ethylene dibromide dosing in the rabbit study was only for 5 d. The conclusion was that the rabbit appears to be an important model for evaluating male reproductive toxicity in humans and warrants further evaluation and “validating any animal species as a general model for human response requires evaluation of many different classes of chemicals” [6].

Section snippets

Experimental Design and Schedule

The design described by Williams et al. [5] in which a 5-week pre-exposure (baseline testing) period is followed by a 10-week (exposure testing) period (Design 2) was used with an extension in the exposure testing period (15 weeks) to allow for the anticipated lead uptake (estimated to require 4 to 5 weeks to reach the target levels). This design allows for studying effects on semen quality through six cycles of the seminiferous epithelium [2]. Male rabbits were sampled weekly through the

Blood Lead Levels

Fig. 1 presents the blood lead levels over the 20 weeks of the study. The blood lead levels remained low at the limit of detection (1.1 μg/dL) for the first 5 weeks for all treatments and throughout the 20 weeks for the controls. All values were plotted, including those below the limit of detection. Excursions above the target levels occurred in the high dose treatments. The average blood lead levels during the last 5 weeks were 0.47 μg/dL for the controls, 24.8 μg/dL for the 20-μg/dL target

Discussion

The longitudinal design of this study was essential for studying spermatogenic endpoints; however, modeling of the longitudinal effects has presented many difficulties. The duration of spermatogenesis in the rabbit is 48 d; therefore, sperm produced before 48 d of the onset of exposure may not reflect the full effect of the toxicant. In fact, it is recommended that studies of male reproductive toxicants extend at least through six cycles of the seminiferous epithelium to allow for toxicant

Acknowledgements

The authors gratefully acknowledge the expert advice provided by Dr. Robt. H. Foote.

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    1

    This work was supported in part by IA # YO1-ES-40266, from NIEHS, National Toxicology Program.

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