Use of the cytokinesis-block micronucleus method in mouse splenocytes.

Mouse splenocytes have been used in the cytokinesis-block method for the evaluation of micronuclei induced by mutagenic agents in vitro as well as in vivo. Stimulation with concanavalin A for 48 h followed by 16-24-h treatment with 5 micrograms/ml cytochalasin B was found to be an optimum condition to obtain micronuclei in the binucleated splenocytes after the cells were cultured in vitro. Under the above conditions splenocytes from mice pretreated with a single i.p. injection of cyclophosphamide gave a significant increase in micronucleus production. This increase was dependent on the dose of cyclophosphamide (r = 0.99). A dose of 50 mg/kg resulted in 22% of the binucleated cells producing micronuclei, more than 20 times the level in the untreated control. The increase was also dependent on the time of cyclophosphamide injection before removal of the spleen. A duration of 4-8 h after cyclophosphamide injection gave rather sharp optimum values for the production of micronuclei. When splenocytes from non-treated mice were treated with mitomycin C together with cytochalasin B in the above in vitro condition, there was a significant increase in micronucleus production in the binucleated cells. It was also dependent on the dose of mitomycin C (r = 0.975) and a dose of 0.5 micrograms/ml resulted in a more than 20-fold increase over the untreated control. Thus, the use of mouse splenocytes in the cytokinesis-block micronucleus assay was shown to be sensitive enough for testing mutagenic agents in vivo as well as in vitro.