Anti-blood coagulation
rodenticides, such as
warfarin, have been used all over the world. They inhibit
vitamin K epoxide reductase (VKOR), which is necessary for producing several blood clotting factors. This inhibition by
rodenticides results in lethal
hemorrhage in rodents. However, heavy usage of these agents has led to the appearance of
rodenticide-resistant rats. There are two major mechanisms underlying this resistance, i.e., mutation of the target
enzyme of
warfarin, VKOR, and enhanced metabolism of
warfarin. However, there have been few studies regarding the hepatic metabolism of
warfarin, which should be related to resistance. To investigate
warfarin metabolism in resistant rats, in situ liver perfusion of
warfarin was performed with resistant black rats (Rattus rattus) from Tokyo, Japan. Liver perfusion is an in situ methodology that can reveal hepatic function specifically with natural composition of the liver. The results indicated enhanced hepatic
warfarin hydroxylation activity compared with sensitive black rats. On the other hand, in an in vitro microsomal
warfarin metabolism assay to investigate kinetic parameters of
cytochrome P450, which plays a major role in
warfarin hydroxylation, the Vmax of resistant rats was slightly but significantly higher compared to the results obtained in the in situ study. These results indicated that another factor like electron donators may also contribute to the enhanced metabolism in addition to high expression of
cytochrome P450.