Antagonism of acetaminophen hepatotoxicity by phospholipase A2 inhibitors.

A 250 mg/kg subcutaneous injection of acetaminophen (APAP) given to fasted mice was hepatotoxic as indicated by elevated serum alanine aminotransferase activity and electron microscopic studies. When extramitochondrial Ca2+ levels were 200 nM or greater, the APAP caused loss of mitochondrial Ca2+ homeostasis in a two step process that represents a sequence of events. The first step occurs 0 to 3 hours after APAP administration and the second step occurs 6 to 12 hours after APAP administration. Loss of mitochondrial Ca2+ homeostasis 6 to 12 hours after APAP correlates chronologically with nuclear damage as indicated by loss of nuclear Ca2+ sequestration between 6 and 12 hours after APAP administration. Pretreatment of mice with the phospholipase A2 (PLA2) inhibitors chlorpromazine or diltiazem one hour prior to APAP administration inhibits loss of mitochondrial Ca2+ homeostasis, prevents nuclear damage, and inhibits APAP hepatotoxicity as indicated by serum alanine aminotransferase activity and electron microscopic studies. This protective effect of chlorpromazine and diltiazem does not interfer with initial actions of APAP that influence mitochondrial function, but interfers with the sequence of events initiated by APAP that leads to loss of mitochondrial Ca2+ homeostasis and hepatotoxicity.