Whole-body urea kinetics and functional roles of urea transporters and aquaporins in urea secretion into the rumen in sheep fed diets varying in crude protein content and corn grain processing method.

ABSTRACT

The objectives were to determine the effects of dietary crude protein (CP) content and corn grain processing on whole-body urea kinetics and the functional roles of urea transporter-B (UT-B) and aquaporins (AQP) in serosal-to-mucosal urea flux (Jsm-urea) in ovine ruminal epithelia. Thirty-two Rideau-Arcott ram lambs were blocked by bodyweight into groups of 4 and then randomly allocated within blocks to 1 of 4 diets (n = 8) in a 2 × 2 factorial design. Dietary factors were CP content (11% [LP] vs. 16% [HP]) and corn grain processing (whole-shelled [WSC] vs. steam-flaked [SFC] corn). Whole-body urea kinetics and N balance were determined using 4-d continuous intrajugular infusions of [15N15N]-urea with concurrent collections of urine and feces with four blocks of lambs (n = 4). After 23 d on diets, lambs were killed to collect ruminal epithelia for mounting in Ussing chambers to determine Jsm-urea and the measurement of mRNA abundance of UT-B and AQP. Serosal and mucosal additions of phloretin and NiCl2 were used to inhibit UT-B- and AQP-mediated urea transport, respectively. Lambs fed HP had a greater (P < 0.01) N intake (29.4 vs. 19.1 g/d) than those fed LP; however, retained N (g/d or % of N intake) was not different. As a % of N intake, lambs fed SFC tended (P = 0.09) to have a lower N excretion (72.2 vs. 83.5%) and a greater N retention (27.8 vs. 16.6%) compared to those fed WSC. Endogenous urea-N production (UER) was greater in lambs fed HP compared to those fed LP (29.9 vs. 20.6 g/d; P = 0.02), whereas urea-N secreted into the gut (GER; g/d) and urea-N used for anabolic purposes (UUA; g/d) were similar. Lambs fed LP tended (P = 0.05) to have greater GERUER (0.78 vs. 0.66) and UUAGER (0.23 vs. 0.13) ratios, and a greater Jsm-urea (144.7 vs. 116.1 nmol/[cm2 × h]; P = 0.07) compared to those fed HP. Lambs fed SFC tended to have a lower NiCl2-insensitive Jsm-urea (117.4 vs. 178.4 nmol/[cm2 × h]; P = 0.09) and had a lower phloretin-insensitive Jsm-urea (87.1 vs. 143.1 nmol/[cm2 × h]; P = 0.02) compared to those fed WSC. The mRNA abundance of UT-B (0.89 vs. 1.07; P = 0.08) and AQP-3 (0.90 vs. 1.05; P = 0.07) tended to be lower in lambs fed SFC compared to those fed WSC. Overall, reducing CP content tended to increase the GERUER ratio with no changes in the expression or function of UT-B and AQP. Although corn grain processing had no effects on GER, feeding SFC increased the portion of urea secretion into the rumen that was mediated via UT-B and AQP.

In ruminants, urea produced in the liver as a nitrogenous waste can be secreted into the rumen where it can be used by rumen microorganisms as a source of nitrogen (N) for their growth. Therefore, urea secretion into the rumen is nutritionally important for ruminants particularly when dietary N intake is deficient. Urea secretion into the rumen occurs via transporter proteins in rumen tissue referred to as urea transporters (UT-B) and aquaporins (AQP). The purpose of this research was to investigate the effects of dietary crude protein (CP) content and corn grain processing on urea secretion into the rumen and the function of UT-B and AQP. Thirty-two Rideau-Arcott lambs were assigned to 1 of 4 diets in a 2 × 2 factorial design. Dietary factors were CP content (11% [LP] vs. 16% [HP]) and corn processing (whole-shelled [WSC] vs. steam-flaked [SFC] corn). When compared to feeding HP, feeding LP tended to increase urea secretion into the rumen, but there were no corresponding changes in UT-B and AQP function. Corn processing did not influence urea secretion into the rumen; however, the portion of urea secretion that was facilitated via UT-B and AQP was greater in lambs fed SFC compared to those fed WSC.