Equine enterocytes actively oxidize l-glutamine, but do not synthesize l-citrulline or l-arginine from l-glutamine or l-proline in vitro.
Abstract: In livestock species, the enterocytes of the small intestine are responsible for the synthesis of citrulline and arginine from glutamine and proline. At present, little is known about de novo synthesis of citrulline and arginine in horses. To test the hypothesis that horses of different age groups can utilize glutamine and proline for the de novo synthesis of citrulline and arginine, jejunal enterocytes from 19 horses of three different age groups: neonates (n = 4; 7.54 ± 2.36 d of age), adults (n = 9; 6.4 ± 0.35 yr), and aged (n = 6; 22.9 ± 1.0 yr) with healthy gastrointestinal tracts were used in the present study. Enterocytes were isolated from the jejunum and incubated at 37 °C for 30 min in oxygenated (95% O2/5% CO2) Krebs bicarbonate buffer (pH 7.4) containing 5 mM D-glucose and 0 mM, 2-mM L-[U-14C]glutamine, or 2 mM L-[U-14C]proline plus 2 mM L-glutamine. Concentrations of arginine, citrulline, and ornithine in cells plus medium were determined using high-performance liquid chromatography. Results indicate that the rate of oxidation of glutamine to CO2 was high in enterocytes from neonatal horses, but low in cells from adult and aged horses. Enterocytes from all age groups of horses did not degrade proline into CO2. Regardless of age, equine enterocytes formed ornithine from glutamine and proline, but failed to convert ornithine into citrulline and arginine. Because arginine is an essential substrate for the synthesis of not only proteins, but also nitrogenous metabolites (e.g., nitric oxide, polyamines, and creatine), our novel findings have important implications for the nutrition, performance, and health of horses. The amino acid arginine (Arg) is a precursor for the synthesis of multiple biological molecules including nitric oxide, polyamines, and creatine that are involved in cell proliferation, cellular remodeling, dilation of blood vessels, and phosphocreatine production for a readily available source of energy. Multipurpose capabilities of Arg have increased the interest in its effects in other species and must be evaluated in the horse. Levels of Arg are deficient in the milk of mammals such as humans, cows, sheep, and pigs, but their neonates are capable of synthesizing citrulline and Arg from glutamine and proline in the small intestine. High concentrations of Arg in milk have been observed in the horse, warranting investigation in case that the foal cannot synthesize Arg to support growth and thus rely on milk as the sole source of Arg. To date, no research has determined the endogenous production of Arg in horses to support metabolic and physiological processes; therefore, our experiment quantifies the synthesis of Arg in enterocytes of the small intestine of neonatal, adult, and aged horses. Data collected from this study serve as the necessary first step to determine the Arg requirement in the horse that has over-reaching implications to improve the growth, performance, reproductive efficiency, and to enhance longevity of the horse.
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Publication Date: 2022-03-12 PubMed ID: 35275603PubMed Central: PMC9030134DOI: 10.1093/jas/skac077Google Scholar: Lookup
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Summary
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This research investigates how equine intestine cells, also known as enterocytes, use glutamine, an amino acid, but do not create citrulline or arginine, two other types of amino acids, from glutamine or proline in a laboratory setting.
Study Purpose and Methodology
- The aim of this study was to examine the assumption that horses of varying ages can use the amino acids glutamine and proline to create the other two amino acids, citrulline and arginine. This is something that other livestock species have shown to do. Yet, it is not well-understood in relation to horses.
- The authors studied enterocytes sourced from the small intestines of 19 horses of various age groups. These included neonates (newborn horses), adults, and aged horses, all of which were verified to have healthy gastrointestinal tracts.
- The enterocytes were subjected to an oxygenated environment at 37 °C for half an hour. In this environment, they were exposed to glucose along with either a carbon-labelled version of glutamine or proline (plus glutamine).
- The researchers then used high-performance liquid chromatography to measure levels of arginine, citrulline, and another amino acid known as ornithine in both the cells and the medium.
Results and Implications
- The study found that neonatal horses’ enterocytes oxidize glutamine to CO2 at a high rate, whereas enterocytes from adult and aged horses showed lower rates. None of the enterocytes from any horse age group degraded proline into CO2.
- All age groups’ enterocytes produced ornithine from glutamine and proline, but failed to convert ornithine into the amino acids citrulline and arginine.
- These findings challenge the general understanding of arginine synthesis in the equine digestive system. Arginine is crucial for the synthesis of proteins and other nitrogenous metabolites such as nitric oxide, polyamines, and creatine, which have extensive roles in cellular activities.
- The study implies repercussions for the optimal nutrition, performance, and health of horses. Therefore, these results warrant further investigation of arginine in horses, especially due to its significant functions in cell proliferation, cellular remodeling, blood vessel dilation, and phosphocreatine production for immediate energy provision.
- Future efforts should focus on determining the required levels of arginine in horses to support metabolic and physiological processes. This could provide crucial insights on how to improve growth, performance, reproductive efficiency, and enhance the overall lifespan of horses.
Cite This Article
APA
Martinez RE, Leatherwood JL, Bradbery AN, Silvers BL, Fridley J, Arnold CE, Posey EA, He W, Bazer FW, Wu G.
(2022).
Equine enterocytes actively oxidize l-glutamine, but do not synthesize l-citrulline or l-arginine from l-glutamine or l-proline in vitro.
J Anim Sci, 100(4), skac077.
https://doi.org/10.1093/jas/skac077 Publication
Researcher Affiliations
- Department of Animal Science, Texas A&M University and Texas A&M AgriLife Research, College Station, TX 77843, USA.
- Department of Animal Science, Texas A&M University and Texas A&M AgriLife Research, College Station, TX 77843, USA.
- Department of Large Animal Clinical Sciences, Texas A&M University, College Station, TX 77843, USA.
- Department of Animal and Range Sciences, Montana State University, Bozeman, MT 59717, USA.
- Department of Animal Science, Texas A&M University and Texas A&M AgriLife Research, College Station, TX 77843, USA.
- Department of Large Animal Clinical Sciences, Texas A&M University, College Station, TX 77843, USA.
- Department of Large Animal Clinical Sciences, Texas A&M University, College Station, TX 77843, USA.
- Department of Animal Science, Texas A&M University and Texas A&M AgriLife Research, College Station, TX 77843, USA.
- Department of Animal Science, Texas A&M University and Texas A&M AgriLife Research, College Station, TX 77843, USA.
- Department of Animal Science, Texas A&M University and Texas A&M AgriLife Research, College Station, TX 77843, USA.
- Department of Animal Science, Texas A&M University and Texas A&M AgriLife Research, College Station, TX 77843, USA.
MeSH Terms
- Animals
- Arginine / metabolism
- Carbon Dioxide / metabolism
- Citrulline / metabolism
- Enterocytes
- Glutamine / metabolism
- Horses
- Ornithine / metabolism
- Proline / metabolism
Grant Funding
- American Quarter Horse Foundation
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Citations
This article has been cited 3 times.- Martinez RE, Leatherwood JL, Bradbery AN, Paris BL, Hammer CJ, Kelley D, Bazer FW, Wu G. Evaluation of dietary arginine supplementation to increase placental nutrient transporters in aged mares. Transl Anim Sci 2023 Jan;7(1):txad058.
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