EFFECT OF PHENOBARBITAL AND MIREX PRETREATMENTS ON CCL4, AUTOPROTECTION

Male Sprague-Dawley rats maintained on either normal diet (N) or on a diet containing phenobarbital (PB; 225 ppm) or mirex (M; 10 ppm) for 15 days received either corn oil or 1 single administration of a protective dose of CCl4 (0.3 ml/kg, po) on day 16. At 24, 48, 72, 96, or 144 hr after the protective dose, a high dose of CCl4 (5 ml/kg, po) was administered to rats of all the groups, and they were observed for 14-day lethality. In a second experiment, in rats maintained on N, PB, or M diet, liver microsomal cytochromes P-450, aminopyrine demethylase, and aniline hydroxylase were measured at various time points after the administration of the protective dose of CCl4. Serum aspartate transaminase, alanine transaminase, and sorbitol dehydrogenase elevations and histopathological changes observed under a light microscope were used as toxic end points to assess hepatotoxicity. Autoprotection was 100% when the high dose was given at 24 hr after the protective dose in N rats, whereas it was only 55% in PB- or M-pretreated rats. For later time points of 48, 72, and 96 hr, autoprotection was only around 50% in N rats, whereas it was almost 100% in PB- and M-pretreated rats. When the high dose was administered at 144 hr after the protective dose, autoprotection further declined to 25% in N rats and to 75% in M-treated rats, but it remained at 100% in PB-treated rats. The liver microsomal cytochromes P-450, aminopyrine demethylase, and aniline hydroxylase were induced in rats after the dietary treatment with PB or M when compared to the rats on N diet. However, after administration of the protective dose of CCl4 to these rats, these enzyme activities were decreased in all the groups at 24 hr after the protective dose, persisted at a low level even at the 72-hr time point, and then slowly recovered to normal by 120 hr. Liver injury was evident by serum enzyme elevations and histopathological changes in all the groups at 24 hr after the protective dose, but the injury was progressive in PB- and M-treated rats with maximum injury at 48 hr, injury in PB-treated rats being greater than that in M-treated rats. The livers recovered completely in all the groups by 120 hr as revealed by serum enzymes and liver histology. The levels of microsomal enzymes at various time points after the protective dose in N, PB, and M treatment groups correlate neither with liver toxicity nor with animal survival after the administration of the large dose of CCl4. Therefore, a postponement of the hepatocellular regeneration stimulated by the protective dose of CCl4 caused by prior exposure to PB and M, as reported earlier, appears to play a role in the correspondingly postponed maximal expression of CCl4 autoprotection. Furthermore, the prolongation of autoprotection by M and even greater effect by PB appears to be related to the greater stimulation of hepatocellular proliferation and augmented tissue repair processes attributable to the protective dose of CCl4 reported previously.