Temporal variation in the effects of warfarin on the vitamin K cycle.

In this in vivo study, the time-dependent effect of oral sodium warfarin was studied in male rats synchronized under a 12-hr light-dark cycle (light 0600-1800). Groups of 5 animals received an oral dose of 500 micrograms/kg of warfarin or saline at 0600 or 1800 and 1 mg/kg of vitamin K 8 hr later and the rats were sacrificed 240 min after vitamin K administration. The activities of the vitamin K reductase and vitamin K epoxide reductase were measured indirectly by determining the content of vitamin K1 and vitamin K epoxide reductase in the plasma and liver. The data obtained in control rats indicated that vitamin K and vitamin K 2,3 epoxide concentrations in plasma and liver were higher (P less than 0.05) at 1800 than at 0600. Warfarin had a greater (P less than 0.05) inhibitory effect on the vitamin K and vitamin K-epoxide reductases at 0600 compared to 1800; plasma levels of S- and R-warfarin did not vary with time of administration. The findings suggest that the activity of both reductases under control conditions, and the warfarin-induced inhibition of these enzymes varied depending on the time of drug administration.