Autoprotection in acetaminophen intoxication in rats: the role of liver regeneration

Autoprotection by acetaminophen, i.e. increased resistance to toxic effects caused by pretreatment, is a well-known phenomenon. The purpose of the present work was to identify mechanisms for increased acetaminophen tolerance induced by pretreatment of rats. One group of female Wistar rats (pretreated rats) received acetaminophen orally in increasing doses (1 to 4.3 g/kg) twice a week for 3 weeks, one group (naïve rats) received the vehicle. At time zero pretreated rats received a toxic dose of 7.5 g/kg (100% lethal in naïve rats), and naïve rats received a toxic dose of 4.3 g/kg. Blood and liver tissue were collected before and 12, 24, 36, and 48 hr after the toxic dose and were analysed for hepatic glutathione and cysteine contents, hepatic glutathione-S-transferase and blood alanine aminotransferase activity, as well as acetaminophen concentration in plasma. Steady-state mRNA levels of proteins involved in acetaminophen detoxification, cell division and acute phase response were measured, liver tissue was examined for proliferating cell nuclear antigen and degree of hepatocyte necrosis. Six naïve rats not receiving acetaminophen served as controls. The mortality was the same in pre-treated and naïve rats (33 percent). Thus, pretreatment increased the tolerance twice. Before the toxic dose pretreated rats compared to control rats had higher activity of glutathione-S-transferase (liver) and alanine aminotransferase (serum), higher hepatic mRNA level of glutathione-S-transferase and gamma-glutamylcysteine synthetase heavy and light chain subunits, and lower hepatic concentration of glutathione, cysteine and mRNA of CYP1A2 than control rats. After the toxic dose, the mRNA levels of glutathione-S-transferase, gamma-glutamylcysteine synthetase heavy and light chain subunits, and CYP1A2 in naïve rats rose, approaching those of pretreated rats. Proliferating cell nuclear antigen labelling was high in pretreated rats, while only slightly increased in a few of the naïve rats. Necrotic hepatocytes were found at all time intervals in pretreated rats, and in naïve rats they appeared after 12 hr, peaking after 36 hr. Pretreatment increased the tolerance to acetaminophen toxicity twice, as estimated by mortality. The data indicate that pretreatment may reduce the relative production of toxic metabolites, but it primarily enhances the protection against these metabolites by regenerating hepatocytes.