The use of multiple endpoints to define the mechanism of action of reproductive toxicants and germ cell mutagens.
Although a variety of endpoints are routinely assessed in reproductive toxicity studies, the inclusion of additional (and potentially more sensitive) endpoints may increase our ability to detect adverse effects on male or female reproduction and also provide information pertaining to the mechanism of action of the reproductive toxicant. Methyl chloride (MeCl) is a well characterized reproductive toxicant in the male rat, and can serve as a model to illustrate the importance of using multiple endpoints to determine the biological basis of chemically induced toxicity in the reproductive system. Exposure of male rats to MeCl results in bilateral testicular degeneration and epididymal inflammation and sperm granuloma formation. Females bred to these males in a dominant lethal assay exhibit elevated rates of postimplantation embryonic death during the first 2 weeks after treatment and increased preimplantation embryonic loss during weeks 2 to 8 post-exposure. Since the chemical is known to be a direct-acting mutagen in vitro and a kidney carcinogen in vivo, the increased embryo death rate observed might reasonably be considered good evidence that MeCl is a direct-acting germ cell mutagen. However, subsequent investigations revealed that MeCl-induced preimplantation loss was a result of cytotoxic rather than genotoxic effects on sperm, with a significant decrease in the count of motile sperm of normal morphology in exposed males during weeks 2 to 8 after treatment. In fact, examination of the fertilization rate during these weeks using a system of embryo recovery and culture revealed that the entire elevated rate of preimplantation loss detected in the dominant lethal assay was the result of failure of fertilization; it had no genetic component at all. Postimplantation death is considered a more reliable indicator of dominant lethality than is preimplantation loss. In the MeCl dominant lethal assay, such increased postimplantation loss was detected only when the fertilizing sperm had been present at the site of MeCl-induced acute inflammation in the cauda epididymis. Inflammatory cells, such as those in the MeCl-exposed epididymis, are known to produce a variety of genetic lesions in the DNA of neighboring cells. Therefore, male rats were concurrently exposed to MeCl and treated with an anti-inflammatory agent (BW755C) to inhibit the epididymal inflammation.(ABSTRACT TRUNCATED AT 400 WORDS)
Working
P K
PK
Department of Pharmacological Sciences, Genentech, San Francisco, CA 94080.
Chellman
G J
GJ
D000818:Animals; D005837:Genitalia, Male / Q000187:drug effects*; D006842:Hydrocarbons, Brominated / Q000633:toxicity; D008297:Male; D008737:Methyl Chloride / Q000633:toxicity*; D009153:Mutagens / Q000633:toxicity; D051381:Rats; D013094:Spermatozoa / Q000187:drug effects*
D006842:Hydrocarbons, Brominated; D009153:Mutagens; C005218:methyl bromide; D008737:Methyl Chloride