Vol. 300, Issue 1, 273-281, January 2002

Mechanisms of Circadian Rhythmicity of Carbon Tetrachloride Hepatotoxicity

James V. Bruckner, Raghupathy Ramanathan¹, K. Monica Lee² and Srinivasa Muralidhara

Department of Pharmaceutical and Biomedical Sciences, College of Pharmacy, the University of Georgia, Athens, Georgia

The toxicity of carbon tetrachloride (CCl₄) and certain other chemicals varies over a 24-h period. Because the metabolism of some drugs follows a diurnal rhythm, it was decided to investigate whether the hepatic metabolic activation of CCl₄ was rhythmic and coincided in time with maximum susceptibility to CCl₄ hepatotoxicity. A related objective was to test the hypothesis that abstinence from food during the sleep cycle results in lipolysis and formation of acetone, which participates in induction of liver microsomal cytochrome P450IIE1 (CYP2E1), resulting in a diurnal increase in CCl₄ metabolic activation and acute liver injury. Groups of fed and fasted male Sprague-Dawley rats were given a single oral dose of 800 mg of CCl₄/kg at 2- to 4-h intervals over a 24-h period. Serum enzyme activities, measured 24 h post dosing as indices of acute liver injury, exhibited distinct maxima in both fed and fasted animals dosed with CCl₄ near the beginning of their dark/active cycle. Blood acetone, hepatic CYP2E1 activity, and covalent binding of ¹⁴CCl₄/metabolites to hepatic microsomal proteins in untreated rats fed ad libitum followed circadian rhythms similar to that of susceptibility to CCl₄. Parallel fluctuations of greater amplitude were seen in rats fasted for 24 h. Hepatic glutathione levels were lowest at the time of greatest susceptibility to CCl₄. Acetone dose-response experiments showed high correlations between blood acetone levels, CYP2E1 induction, and CCl₄-induced liver injury. Pretreatment with diallyl sulfide suppressed CYP2E1 and abolished the circadian rhythmicity of susceptibility to CCl₄. These findings provide additional support for acetone's physiological role in CYP2E1 induction and for CYP2E1's role in modulating CCl₄ chronotoxicity in rats.