INFLUENCE OF FORAGE LEVEL AND CORN GRAIN PROCESSING ON WHOLE-BODY UREA KINETICS, AND SEROSAL-TO-MUCOSAL UREA FLUX AND EXPRESSION OF UREA TRANSPORTERS AND AQUAPORINS IN THE OVINE RUMEN, DUODENUM, AND CECUM
Date
2018-06-11
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Masters
Abstract
The process of urea recycling to the gastro-intestinal tract (GIT) is an evolutionary adaptation that allows ruminants to maintain a positive nitrogen (N) balance, particularly when faced with a dietary protein deficit. Recycled urea-N can enter all GIT compartments; however, the rumen is where most of the anabolic usage occurs as it provides a N source for microbial protein synthesis, thus providing amino acids to the host animal when microbial protein is digested at the small intestine. The objective of this thesis research was to determine the effects of forage level and corn grain processing on N utilization, primarily focusing on dietary effects on whole-body urea kinetics and apparent N balance, and ex vivo serosal-to-mucosal urea flux (Jsm-urea) across ruminal, duodenal, and cecal epithelia. Additionally, to better understand the mechanisms associated with whole-body N utilization across the aforementioned GIT regions, dietary effects on regional fermentation characteristics and messenger ribonucleic acid (mRNA) expression of urea transporters (UT) and aquaporins (AQP) were also assessed. Sheep were used as an experimental model for ruminants. Thirty-two wether lambs (37.2 ± 1.7 kg initial body weight [BW]) were blocked by BW into groups of 4 and assigned to 1 of 4 diets (n = 8) in a 2 x 2 factorial design. Dietary factors were forage level (30% [LF] vs. 70% [HF]; DM basis) and corn grain processing (whole-shelled [WS] vs. steam-flaked [SF]). Provision of WS or SF corn was expected to alter the location of starch digestion in the ruminant GIT, with SF corn expected to shift starch digestion to the rumen, and WS corn expected to shift starch digestion to the intestine and hindgut. Four blocks of lambs (n = 4) were used for the in vivo metabolism trial to determine N balance and whole-body urea kinetics. Whole-body urea kinetics were determined using 4-d double-labelled urea ([¹⁵N¹⁵N]-urea) isotopic infusions, with concurrent total collections of urine and feces to determine isotopic enrichments and N balance. After 23 d of dietary adaptation, all lambs were killed (one per day for logistical reasons) on the morning of d 24 and ruminal, duodenal, and cecal epithelia were collected to determine Jsm-urea (using the Ussing chamber model) and mRNA expression for UT and AQP. Lambs fed LF had greater DM (1.20 vs. 0.86 kg/d; P < 0.01) and N (20.1 vs 15.0 g/d; P < 0.01) intakes than those fed HF. Lambs fed SF corn had greater DM (1.20 vs. 0.86 kg/d; P < 0.01) and N (20.6 vs. 14.5 g/d; P < 0.01) intakes than those fed WS. When expressed as a percent of N intake, total N excretion was greater in lambs fed HF diets compared to those fed LF diets (103 vs. 63.0%; P < 0.01). Also, total N excretion (as a percent of N intake) was greater in lambs fed WS corn compared to those fed SF corn (93.6 vs. 72.1%; P = 0.02). Apparent N balance, expressed as a percent of N intake, was greater in lambs fed LF diets compared to those fed HF diets (37.0 vs. -2.55%; P < 0.01). Similarly, lambs fed SF corn also exhibited a greater apparent N balance (28.0 vs. 6.50; P = 0.02) compared to lambs fed WS corn, when expressed as a percent of N intake. Endogenous urea production (UER) tended to be greater in lambs fed HF diets (17.6 vs. 14.3 g/d; P < 0.10) compared to those fed LF diets. Moreover, lambs fed HF diets also had greater urinary urea-N loss (as a proportion of UER) (0.38 vs. 0.22; P < 0.01), reduced amounts of urea-N returning to the GIT (as a proportion of UER) (0.62 vs. 0.78; P < 0.01), and overall, a reduced amount of urea-N allocated towards anabolism (as a proportion of urea-N GIT entry; GER) (0.12 vs. 0.26; P < 0.01) compared to those fed LF diets. Ruminal pH was similar in lambs fed SF and WS with HF diets, but it was lower in lambs fed SF compared to WS with LF diets (interaction, P = 0.01). Lambs fed LF had a lower cecal pH compared to those fed HF (P < 0.01). Ruminal Jsm-urea was unaffected by diet. Duodenal Jsm-urea was greater in lambs fed HF compared to LF (77.5 vs. 57.2 nmol/(cm² × h); P < 0.01). There were positive correlations between Jsm-urea and serosal-to-mucosal mannitol flux (Jsm-mannitol) in duodenal (r = 0.88; P < 0.01) and cecal (r = 0.93; P < 0.01) epithelia. Lambs fed LF diets had increased mRNA expression of AQP-3 (1.21 vs. 0.90; P = 0.03) in ruminal epithelia and tended to have greater mRNA expression of AQP-3 (1.27 vs. 0.99; P < 0.10) in duodenal epithelia compared to lambs fed HF diets. Expression of UT-B mRNA in ruminal, duodenal, and cecal epithelia was largely unaffected by dietary treatment, except that cecal UT-B expression tended to be greater in lambs fed HF diets (0.95 vs. 0.71; P < 0.10) compared to lambs fed LF diets. The results presented in this thesis suggest that the provision of highly digestible diets improves N retention for anabolic usage and shifts urea-N excretion from the urine to the feces by enhancing urea recycling to the GIT. This shift in urea excretion can result in more stable forms of N losses, yielding an environmental benefit. Provision of LF diets also increased expression of AQP-3 in the ruminal and duodenal epithelia, providing insight into the molecular mechanisms associated with Jsm-urea in both ruminal and post-ruminal regions.
Description
Keywords
Ruminants, Urea recycling, Urea flux, Nitrogen metabolism, Forage level, Corn grain processing, Ussing chamber, Gastrointestinal tract, Urea transporters, Aquaporins, Whole-body urea kinetics
Citation
Degree
Master of Science (M.Sc.)
Department
Animal and Poultry Science
Program
Animal Science