Effect of dietary nitrogen content and intravenous urea infusion on ruminal and portal-drained visceral extraction of arterial urea in lactating Holstein cows

Urea extraction across ruminal and portal-drained visceral (PDV) tissues were investigated using 9 rumen-cannulated and multi-catheterized lactating dairy cows adapted to low-N (12.9% crude protein) and high-N (17.1% crude protein) diets in a crossover design. The interaction between adaptation to dietary treatments and blood plasma concentrations of urea was studied by dividing samplings into a 2.5-h period without urea infusion followed by a 2.5-h period with primed continuous intravenous infusion of urea (0.493 ± 0.012 mmol/kg of BW per h). Cows were sampled at 66 ± 14 and 68 ± 12 d in milk and produced 42 ± 1 and 36 ± 1 kg of milk/d with the high-N and low-N diets, respectively. The arterial blood urea concentration before urea infusion was 1.37 and 4.09 ± 0.18 mmol/L with low-N and high-N, respectively. Dietary treatment did not affect the urea infusion-induced increase in arterial urea concentration (1.91 ± 0.13 mmol/L). Arterial urea extraction across the PDV and rumen increased from 2.7 to 5.4 ± 0.5% and from 7.1 to 23.8 ± 2.1% when cows were changed from high-N to low-N, respectively. Urea infusion did not decrease urea extractions, implying that urea transport rates were proportional to arterial urea concentrations. Urea extraction increased more across the rumen wall than across the total PDV for low-N compared with high-N, which implies that a larger proportion of total PDV uptake of arterial urea is directed toward the rumen with decreasing N intake. The ruminal vein - arterial (RA) concentration difference for ammonia increased instantly (first sampling 15 min after initiation of infusion) to the primed intravenous infusion when cows were adapted to the low-N diet. The RA difference for ammonia correlated poorly to the ventral ruminal concentration of ammonia (r = 0.55). Relating the RA difference for ammonia to a function of both ruminal ammonia concentration and the RA difference for urea markedly improved the fit (r = 0.85), indicating that a large fraction of ammonia released to the ruminal vein is absorbed from an epithelial ammonia pool not in equilibrium with the ventral ruminal ammonia pool. Changing cows from high-N to low-N affected the relative blood urea clearance by kidneys and PDV. The clearance by the kidneys decreased from 41 to 27 ± 2 L/h and the clearance by the PDV increased from 52 to 105 ± 12 L/h when the diet was changed from high-N to low-N. In conclusion, urea transport across gut epithelia in cattle is adapting to N status and driven by mass action. Data are commensurable with a model for urea transport across gut epithelia based on regulated expression or activity of facilitative urea transporters.