Kinetics of cellular proliferation in regenerating mouse liver pretreated with the alkylating drug dipin.

The effect of prior exposure to the alkylating drug 1,4-Bis[N,N'-di(ethylene)-phosphamide] piperazine (Dipin) on the cell cycle progression in a regenerating mouse liver is reported, using cytological, autoradiographical and DNA-cytophotometrical methods. In Dipin-treated animals 3H-TdR pulse labelled nuclei appeared 22 h after the partial hepatectomy, followed by a rapid rise in labelling index to a very high level between 43-54 h, and then by a gradual decline to 72 h. Four injections of 3H-thymidine at 16 h intervals resulted in 89% labelling of hepatocytes at 72 h. The DNA-synthesis time was assessed by DNA photometry combined with 3H-TdR autoradiography on the same nucleus. The DNA-content in Dipin-pretreated nuclei was doubled after 26 h, approximately three times longer than in the controls. Mitoses were completely absent during the first 2-4 days. They were observed subsequently for up to 2 months but with varying frequency. The frequency of binucleated hepatocytes remained constant during liver regeneration. Hence, pretreatment of resting liver with Dipin altered the kinetics of cell proliferation following partial hepatectomy. Delay in division resulted primarily from an increase in the DNA-synthesis time and a prolonged block of the transition from G2 to mitosis. No changes in the duration of the prereplicative period and in the size of the proliferative pool were noticed. The increase of the mitotic cycle time led to synchronization of the cell population. About a half of the hepatocytes were seen in the S phase by the end of the second day and by the end of the third day about 80% of the cells had passed into the G2 phase.