Investigation of hepatic warfarin metabolism activity in rodenticide-resistant black rats (Rattus rattus) in Tokyo by in situ liver perfusion

Abstract

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.