Effects of variation of necropsy time and fasting on liver weights and liver components in rats.

In rats, percent liver weight loss is greater than percent body weight loss within the 8 A.M.-4 P.M. period of the working day. The liver weight loss is principally the result of decreased water content, either carbohydrate (glycogen) bound water in rats with access to feed, or protein bound water in rats fasted overnight. During this period, percent kidney weight loss is approximately equal to percent body weight loss. To optimize the sensitivity of kidney and liver weight evaluation, it is recommended that rats be fasted overnight and that relative liver and kidney weights be expressed based on body weights taken immediately prior to necropsy. Since these procedures will not entirely eliminate necropsy time-related organ weight differences, the animal necropsy sequence must be randomized to distribute the remaining differences across all treatment groups.