Citrulline Generation Test: What Does It Measure?

Review: Methods and biomarkers to investigate intestinal function and health in pigs

Society is becoming increasingly critical of animal husbandry due to its environmental impact and issues involving animal health and welfare including scientific experiments conducted on farm animals. This opens up two new fields of scientific research, the development of non- or minimally invasive (1) methods and techniques using faeces, urine, breath or saliva sampling to replace existing invasive models, and (2) biomarkers reflecting a disease or malfunction of an organ that may predict the future outcome of a pig's health, performance or sustainability. To date, there is a paucity of non- or minimally invasive methods and biomarkers investigating gastrointestinal function and health in pigs. This review describes recent literature pertaining to parameters that assess gastrointestinal functionality and health, tools currently used to investigate them, and the development or the potential to develop new non- and minimally invasive methods and/or biomarkers in pigs. Methods described within this review are those that characterise gastrointestinal mass such as the citrulline generation test, intestinal protein synthesis rate, first pass splanchnic nutrient uptake and techniques describing intestinal proliferation, barrier function and transit rate, and microbial composition and metabolism. An important consideration is gut health, and several molecules with the potential to act as biomarkers of compromised gut health in pigs are reported. Many of these methods to investigate gut functionality and health are considered 'gold standards' but are invasive. Thus, in pigs, there is a need to develop and validate non-invasive methods and biomarkers that meet the principles of the 3 R guidelines, which aim to reduce and refine animal experimentation and replace animals where possible.

State of the art: the role of citrulline as biomarker in patients with chemotherapy- or graft-versus-host-disease-induced mucositis

PURPOSE OF REVIEW

Serum or plasma citrulline levels are used as biomarker for a broad spectrum of intestinal functions. During high-dose chemotherapy, citrulline levels are decreased due to mucositis, a common side effect of chemotherapy. This may decrease intestinal function and result in diarrhea. In this review, most recent studies investigating citrulline as biomarker for intestinal function are discussed, with focus on patients with oncological diseases, specifically hematological malignancies with chemotherapy- or Graft-versus-Host-disease (GVHD)-induced mucositis.

RECENT FINDINGS

Citrulline has recently been widely studied in relation to intestinal function and various clinical conditions. It seems therefore a promising noninvasive biomarker in clinical practice for more than intestinal function alone. The association between citrulline levels and intestinal function in patients with hematological malignancies, with or without mucositis remains unclear, as no other parameters of intestinal function for this purpose were assessed.

SUMMARY

In conclusion, citrulline seems to be a promising noninvasive biomarker for various intestinal conditions in general, and potentially for intestinal function in patients with chemotherapy- or GVHD-induced mucositis. It is unclear from recent literature whether high fecal volume or diarrhea as side effect, results in impaired intestinal function and severe malabsorption and if citrulline biomarkers can be useful to detect this.

Developmental changes in the utilization of citrulline by neonatal pigs

Developmental changes in the renal expression and activity of argininosuccinate synthase (ASS1) and argininosuccinate lyase (ASL), enzymes that use citrulline for the production of arginine, have been reported. Thus, the ability of neonates, and especially premature neonates, to produce arginine may be compromised. To determine the utilization of citrulline in vivo, we measured renal expression of ASS1 and ASL and conducted citrulline compartmental and noncompartmental kinetics using [¹⁵N]citrulline in pigs of five different ages (from 10 days preterm to 5 wk of age). The tracer was given in substrate amounts to also test the ability of neonatal pigs to use exogenous citrulline. Preterm and term pigs at birth had lower ASS1 and ASL expression than older animals, which was reflected in the longer half-life of citrulline in the neonatal groups. The production and utilization of citrulline by 1-wk-old pigs was greater than in pigs of other ages, including 5-wk-old animals. Plasma citrulline concentration was not able to capture these differences in citrulline production and utilization. In conclusion, the developmental changes in renal ASS1 and ASL gene expression are reflected in the ability of the pigs to use citrulline. However, it seems that there is an excess capacity to use citrulline at all ages, including during prematurity, since the bolus dose of tracer did not result in an increase in endogenous citrulline. Our results support the idea that citrulline supplementation in neonatal, including premature, pigs is a viable option to increase arginine availability.