Excessive reversible phenobarbital induced nuclear DNA-polyploidization in the growing mouse liver

Phenobarbital was injected intraperitoneally into male white NMRI mice aged 0.5, 1. 1.5, 3, 6 and 12 months at a dose of 120 mg/kg body weight for 10 consecutive days. The 0.5 month-old mice did not tolerate the phenobarbital dose and died. The experimental animals and one of the controls were sacrificed 1, 3, 5, 10, 15 and 20 days after phenobarbital administration was started. Liver weights were recorded and liver cells were isolated. The number of nuclei per cell was determined and the DNA-content of each single nucleus was measured by Feulgen fluorescence cytophotometry. Liver weights showed an increase of 25--30% during phenobarbital treatment and returned slowly to lower values after cessation of drug application. The hepatic enlargement was accompanied by an excessive and likewise reversible nuclear and whole cell DNA-polyploidization, i.e. polyploidization beyond the physiological age-dependent ploidy level; for example, mean values of 7.7 c per nucleus (versus 4.2 c in the controls) and 14.3 c for whole liver cells (versus 7.5 c in the controls) were found in 3 months-old animals after 5 days of treatment. As with the induction of microsomal enzymes, the augmentation of smooth endoplasmic reticulum, and the increase of RNA- and protein-synthesis, excessive DNA-polyploidization of liver cell nuclei appears to be an expression of hepatocellular hypertrophy due to the functional or metabolic stress imposed upon the liver by large quantities of phenobarbital. After cessation of drug administration the abnormally high ploidy cells are eliminated - probably by necrobiosis - and the liver cells return to their normal age-dependent DNA-ploidy level. The liver cells of the one-month-old animals revealed the physiological polyploidization to be slightly inhibited. This is probably due to some toxic effect of phenobarbital. Phenobarbital did not alter the number of nuclei per liver cell.