EVALUATION OF THE MUTAGENICITY OF ACETOCHLOR TO MALE-RAT GERM-CELLS

Male rat dominant lethal (DL) assays conducted on the herbicide acetoc hlor are described. Single dose studies conducted at the maximum toler ated dose (MTD, less than or equal to 1000 mg/kg) produced no effects on any of the DL assay parameters at any of the ten weekly sampling pe riods. It is concluded that acetochlor is non-mutagenic to rat germ ce lls. Due to initial limited knowledge of the MTD of acetochlor it was also evaluated in the DL assay at a dose level of 2000 mg/kg. At this high dose level severe bodyweight loss and some deaths occurred among the treated animals. In addition, reduced implantations and reduced pr egnancy rates were observed at the third sampling period (18-25 days p ost dosing) in the absence of an increase in early post-implantation d eaths. These results indicated that the use of supra-MTD doses of acet ochlor had reduced the fertility of the treated males leading to the p roduction of a pseudo-DL assay response, as alerted to and defined by Ehling. Although several such pseudo-DL assay responses have been desc ribed, none have been explained mechanistically. It was therefore deci ded to pursue the effects seen in the DL assay when using supra-MTD do ses of acetochlor. Ova analysis of female rats mated with male rats ex posed to 2000 mg/kg acetochlor revealed unfertilized ova at the critic al third sampling time. Normal fertilization of ova was observed at th e first and fifth sampling period and, for a dose of 200 mg/kg acetoch lor, at the third sampling period. The magnitude and temporal nature o f these effects confirmed the induction of a pseudo-DL assay response, and studies were then undertaken to probe its genesis. Rats treated w ith 2000 mg/kg acetochlor had normal testicular and epididymal patholo gy and normal sperm numbers and sperm motility at the critical third s ampling period. Despite a small reduction in testicular and epididymal glutathione levels 12 h after exposure to 2000 mg/kg acetochlor, test icular LDH and LDH-X enzyme levels were unaffected. Further, no reduct ion in the level of free sulphydryl groups (-SH) were observed in epid idymal caput sperm heads isolated 0.5, 7 or 14 days after treatment of male rats with 2000 mg/kg acetochlor. The only sperm parameter affect ed by treatment with 2000 mg/kg acetochlor was an increase in epididym al cauda sperm with head abnormalities. The non-specific nature of thi s effect was considered inadequate to explain fully the high dose fert ility effects seen in the DL assays, which therefore remain unexplaine d, The present data establish that acetochlor is non-mutagenic to rat germ cells. They also confirm the importance of segregating mutagenic and fertility affects in the DL assay, and emphasize the need for appr opriate dose-setting studies prior to the conduct of rodent genetic to xicity assays. (C) 1997 Elsevier Science B.V.