The rate and pattern of urea infusion into the rumen of wethers alters nitrogen balance and plasma ammonia.

Changes in N balance, urinary excretion of purine derivative (PD), urea, creatinine and ammonia and plasma ammonia, glucose, urea, insulin and IGF-1 were examined in four wethers (37 ± 2.6 kg BW). The animals were fitted with permanent ruminal catheters, fed lucerne hay (9.4 MJ/day; 23 g N/day; 7 g soluble N/day, 6 equal meals/day) and treated with contrasting rates of urea infusion into the rumen: first, a continuous infusion (CT), at 3.2 mg urea-N/min for 10 days and then a discontinuous infusion (DT) at 156 mg urea-N/min for 4 min; in 6 daily doses with the meals for 7 days. N balance was calculated from pooled samples of faeces and urine. Jugular blood samples were collected before and 1.5 h after the morning meal (M1) on days CT10, DT2, DT4 and DT6. N retention decreased during DT (p = 0.01) due to a significant increase of N excretion in urine (4 g/day; p = 0.009) and faeces (1 g/day; p = 0.02). Dry matter (p < 0.001) and N digestibility in vivo (p = 0.01) decreased significantly during DT. Urinary urea and PD excretion were not altered by treatment. Significant linear (p = 0.004) and quadratic (p = 0.001) effects were observed for plasma ammonia in M1 (from 170 CT10 to 235 µm DT2 and returned to 120 µm DT6). No changes were observed in plasma glucose, urea, insulin and IGF-1. Results indicate that changes from CT to DT reduced N retention in sheep due to enhanced urinary N excretion, but it was not associated with changes in urinary urea or PD excretion; or plasma concentrations of insulin and IGF-1. As the dry matter (DM) an N digestibility could account a 0.23 of the decrease in N retention; the largest fraction of the reduction in N retention remained unexplained by the results.

Splanchnic net balance of oxygen and metabolites in response to a discontinuous mesenteric vein infusion of ammonium in sheep.

To simulate daily episodes of high absorption associated with the intake of diets with high N content, four wethers (42 ± 3.4 kg body weight), fitted with permanent catheters in the femoral artery and splanchnic vessels, were infused with 340 µmol into the mesenteric vein for 3 h, during the morning meal, over seven consecutive days. On the 7th day, mass transfers of , urea, glucose, lactate, ß-OH-butyrate and O2 were measured across portal-drained viscera (PDV), liver and splanchnic tissues during the last 90 min of the infusion. Measurements were repeated on the following day, at the same time, without the infusion. Plasma concentration in the portal vein (+332 µm; p = 0.006), portal absorption (+424 µmol/min; p < 0.001), liver uptake (+375 µmol/min; p = 0.003) and urea N production (+338 µmol/min; p = 0.059) were higher during infusion. Mass transfers of urea, glucose, lactate, ß-OH-butyrate and O2 across the PDV, and glucose, lactate, ß-OH-butyrate and O2 across the liver, were not altered by the infusion. Results suggest that a daily, discontinuous increase in portal flow during a meal stimulates liver removal and urea N production but does not significantly affect liver glucose production and O2 consumption in sheep.