An optimal, generalized sampling time of 30 +/- 6 h after double dosing in the mouse peripheral blood micronucleus test.

Double dosing and single sampling seems to be the simplest and most reliable method for detecting clastogens in the mouse peripheral blood micronucleus test. Optimal sampling times after double dosing are studied here. Eleven clastogens [water soluble: colchicine, cyclophosphamide, cytosine arabinoside, 5-fluoro-2'-deoxyuridine, 5-fluorouracil (5-FU), 6-mercaptopurine, methotrexate (MTX), mitomycin C; water insoluble: N-2-acetylaminofluorene, diethyl sulfate, 7,12-dimethyl-benz[a]anthracene (DMBA)] were given i.p. twice to MS/Ae mice and peripheral blood samples were collected at 6 h intervals starting 24 h after the second treatment. Samples collected at 30 +/- 6 h were nearly optimal for all 11 chemicals. When DMBA, 5-FU and MTX were given, elevated micronucleus frequencies tended to last longer relative to those induced by direct-acting chemicals. Since samples collected 30 +/- 6 h after the second treatment with these three chemicals showed almost the same micronucleus frequencies observed in later samples, the time 30 +/- 6 h could be applied for these long-acting chemicals. Micronucleated reticulocytes persist in the peripheral blood (PB) longer than micronucleated polychromatic erythrocytes are retained in the bone marrow, which thus provides a simple, effective and generalized protocol for the mouse short-term PB micronucleus test, namely double dosing and sampling 30 +/- 6 h after the second dose. When one sample time has to be selected, 30 h after the second treatment is recommended.