We sequenced the VKORC1 gene from 275 rats living in the wild in Japan. We found several types of novel base substitutions, some of which conferred warfarin resistance.
There was no difference in coagulation times between warfarin-sensitive and resistant rats measured under physiological conditions. However, after warfarin administration, no effect was noted in warfarin-resistant rats, although a prolonged coagulation time was noted in warfarin-sensitive rats.
We also determined the kinetic differences in hepatic microsomal VKOR-dependent activity between warfarin-resistant and sensitive rats. Warfarin-resistant rats showed 2-3-fold lower Vmax/Km values than did sensitive rats. In addition, we report that resistant rats found in the Tokyo area had a VKOR activity which was poorly inhibited by warfarin.
Finally, we conclude that reduced VKOR activity and warfarin resistance in the Japanese black rat might be due to mutations in the VKORC1 gene. However, further study is needed to clarify how such rats can maintain adequate vitamin K-dependent clotting factor levels, while simultaneously exhibiting low VKOR activity and warfarin resistance.