Serum Citrulline Levels Exhibit Circadian Variation and Fluctuations in Relation to Food Intake in Mice.

Background: Plasma citrulline is a nonessential amino acid synthesized almost exclusively by enterocytes of the small intestine. Correlation between post-operative plasma citrulline levels and remnant small bowel mass and function has led to its use as a simple biomarker of functional intestinal mass in adults and children, with a proposed prognostic value for achieving enteral autonomy in the setting of intestinal failure. While it is standard to measure fasting levels in humans, the optimal timing of blood draws in relation to food intake and time of day has not been standardized and is poorly studied in animal models. We hypothesize that serum citrulline levels vary both throughout the day and with relation to food intake in mice.

Methods: Serum citrulline levels were measured from 12 C57BL/6 mice (6 - 9 weeks old) in the mornings and evenings, either after food intake or after 8 - 12 h of access to water only. Blood draws for each experimental set-up were performed 1 week apart according to our Institutional Animal Care and Use Committee (IACUC) guidelines for acceptable blood collection volumes for survival procedures. At the end of the experiment, mice were euthanized and small intestine was harvested for morphometric measurements. Group means were compared using Student's -test with significance assumed for P < 0.05.

Results: After withholding chow for 8 - 12 h, serum citrulline levels were significantly greater in the morning compared to levels drawn in the evening. When mice were allowed chow, there was no significant difference in serum citrulline levels drawn in the morning compared to the evening. There was no difference detected in villus height across the different experimental conditions.

Conclusions: Serum citrulline levels exhibit circadian variation and fluctuations in relation to food intake in mice, without apparent concurrent changes in enterocyte mass. There was no diurnal variation of serum citrulline levels in fed mice whereas fasted mice had significantly higher levels of serum citrulline in the morning compared to the evening. These findings underscore the paramount importance of consistent sample collection strategies in the setting of translational research.