Abstract
Long-term feeding studies in rats have been carried out with a number of compounds known to produce liver enlargement in this species, and sequential measurements of hepatic enzymes have been correlated with histochemical, histological and ultrastructural changes in the liver. Administration of butylated hydroxytoluene (BHT) or safrole for 1 week in the diet at levels of 0. 4 and 0. 25% respectively produced liver enlargement and induction of the microsomal drug metabolising enzymes. During continued administration of BHT for up to 75 weeks this response was maintained unaltered and without production of liver injury. However, in the case of safrole, drug metabolising enzyme activity fell to around control levels by week 8 but the liver remained enlarged. Cytopathological changes, indicated histochemically by autophagy and depression of glucose-6-phosphatase, were observed at this stage. These became progressively more severe leading to focal necrosis and the development around week 60 of nodular hepatic lesions. A similar sequence of events was initiated after only 1 week in rats fed a diet containing 1. 0% Ponceau MX, a compound producing marked liver enlargement but only weak stimulation of drug metabolising enzyme activity. Thus, where liver enlargement is unaccompanied by drug metabolising enzyme induction, or where such enzyme induction is only transitory, gross pathological changes may develop in the enlarged liver. The nodular hepatic lesions produced by safrole and Ponceau MX have been compared with hyperplastic liver nodules induced by the hepatocarcinogen 2-acetylaminofluorene and found to differ in some aspects of their biochemical and biological characteristics. Finally, some in vitro studies were conducted to explore the unusual interaction of safrole with hepatic microsomal cytochrome P450. The stable binding to cytochrome P450 of a species apparently formed during the metabolism of safrole could be reversed by certain alternative substrates of cytochrome P450 with the production of novel spectral changes.