Ammonia toxicity in cattle. V. Ammonia concentration of lymph and portal, carotid and jugular blood after the ingestion of urea.

Four rumen-fistulated Holstein steers were fitted with cannulas for the collection of portal, jugular and carotid blood. In addition, the thoracic duct of one steer was cannulated for the collection of lymph. Steers were given .125, .25 or .5 g urea/kg body weight 16 hr after a previous feeding. Within 5 min after the administration of the .5-g dose, rumen ammonia increased from 9.7 to 32.0 mg/100 ml, rumen pH from 6.47 to 7.87; portal blood ammonia from 1.02 to 8.01 mg/100 ml, carotid blood ammonia from .18 to 1.17 mg/100 ml and jugular blood ammonia from .13 to .36 mg/100 ml. Lymph ammonia increased from .22 to .32 mg/100 ml within 15 minutes. The .125- and .25-g doses or urea produced proportionate changes. In a second experiment, three Jersey cows were given .5 g urea/kg body weight, and the rates at which urea appeared in carotid and jugular blood were determined. Only small amounts of urea appeared in carotid and jugular blood during the first 5 min after dosing, but the concentrations then increased slowly but progressively. We concluded that because carotid blood ammonia concentration increased so rapidly after dosing with urea, ammonia must leak past the liver, and it is therefore unlikely, that there is a liver threshold for ammonia which must be exceeded before ammonia will reach the carotid artery. The marked difference in ammonia concentrations in carotid and jugular blood suggests that the brain takes up ammonia rapidly. While some ammonia is absorbed via the lymph, and thus bypasses the liver, the lymph does not appear to be a major contributor of ammonia to carotid blood.