The ability of direct and pro-carcinogens to induce unscheduled DNA synthesis (UDS) in primary culture of hepatocytes was examined. Hepatocytes were isolated from adult male rats with collagenase perfusion technique and maintained in short-term monolayer culture on collagen-coated plates in serum free modified Waymouth's medium. Incorporation of [$^3H$] thymidire into DNA in the presence of hydroxyurea was used to measure UDS.
N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) which elicit a 'short-patch' type of DNA repair and benzo(a) pyrene (B (a) P) which result in a 'long-patch' type of DNA repair were used as standard carcinogens to show repair patterns.
In continuous labelling experiments, UDS induced by B(a)P was increased linearly for 12hr in cultures, but that of MNNG was increased sharply then remained within 3 hr and then remained constant until 12 hr in cultures.
In pulse labelling experiments, the highest level of UDS induced by B(a)P was obtained in 3 to 6 hr in cultures, while in case of MNNG the highest level was appeared in 0 to 3 hr.
Maximum degree of UDS by MNNG was obtained when the experiment was done at 36 hr after cell plating, while in case of B(a)P optimal time of experiment was at 18 hr after cell plating.
In dose-response study. UDS induced by MNNG was increased upto $10^{-4}M$ MNNG. In case of B(a)P, highest level of UDS was obtained at $10^{-5}M$.
These results indicate that primary culture of rat hepatocytes system has potentiality as a screening system for environmental carcinogens.
