Effect of warfarin on the kinetics of the vitamin K-dependent clotting factors in rats.

The objectives of this study were to compare the time course of activities and rates of synthesis of activities for the separate clotting factors II, VII, IX, and X and to relate the rate of synthesis of activity of each factor to the plasma concentration of warfarin in individual rats after acute and chronic dosing with warfarin. Sequences of blood samples were obtained from each rat for 50 to 70 hours after an acute dose of warfarin or for 120 hours after a chronic loading dose plus 12-hour maintenance doses of warfarin and assayed for factor activities and warfarin concentration. The half-lives for degradation of factor activities ranged from 2.6 to 9.0 hours for the four factors. During periods of changing warfarin concentration (acute dosing) factor VII and X activities and rates of synthesis of activity showed large rapid changes, while factors II and IX responded more slowly. As the warfarin concentration diminished, the factor X rate of synthesis of activity appeared to exceed predrug values in all rats. During chronic dosing with warfarin the factor II activity and rate of synthesis of activity was depressed the most. The percent depression of the rate of synthesis of activity for each factor was related linearly to the logarithm of the plasma concentration of warfarin for the range 0 to 80% depression with acute dosing. However, this relationship was not suitable to explain the apparent overshoot in factor X rate of synthesis of activity.