Short communication: Assessing urea transport from milk to blood in dairy cows

The concentration of urea in milk (MUC) has emerged as a potentially useful tool to predict urinary N excretion. Various factors may affect the relationship between MUC and urinary N excretion, including transport characteristics of urea from blood to milk and vice versa. The main objective of this study was to test whether substantial transport of urea from milk to blood exists in lactating dairy cattle. The subobjectives were (1) to assess the effects of various urea gradient levels between blood and milk on urea transport from milk to blood and (2) to test the occurrence of urea transport between different compartments of the mammary gland such as the cistern and the alveoli. Urea transport was studied in 2 multiparous lactating Holstein-Friesian cows (36.0 +/- 6.18 kg of milk/d; mean +/- SD). In 3 separate trials, boluses of [(NN)-N-15-N-15]urea were injected in the cisterns via the teat canals at 20, 60, and 120 min before the 1700-h milking at various levels of MUC and of blood plasma urea concentration (PUG). In trial 1, a primed continuous infusion of urea (105 g at the start, continuing with 20 g/h) into the jugular vein started at 0500 h and stopped at 0, 1, 2, and 3 h before the 1700-h milking on d 1, 2, 3, and 4, respectively. In trial 2, 5.5 g of urea was injected into the cisterns at 20, 60, and 120 min before the 1700-h milking on d 5, 6, and 7, respectively. In trial 3, urea fluxes were measured without an experimentally induced gradient between MUC and PUC on d 8, 9, and 10, respectively. During milking, successive milk samples were taken from first to last milk. Blood and milk were analyzed for N-15-urea enrichment. Levels of N-15-urea in blood increased after injection of a [(NN)-N-15-N-15]urea bolus in milk, indicating urea transport from milk to blood. Between 21.0 and 35.3% of injected [(NN)-N-15-N-15]urea in milk was recovered after 2 h. The fractional [(NN)-N-15-N-15]urea decline rate in milk varied between 0.0076 and 0.0096/min. The level of MUC, rather than the concentration gradient between MUC and PUG, appeared to affect this fractional rate of decline. Enrichment levels of N-15-urea in milk samples within a single milking showed that urea was transported from cistern milk to alveoli milk. In conclusion, the results indicate that transport of urea from milk to blood in lactating dairy cattle occurs and that urea is transported from cistern milk to alveoli milk.