Further studies of the dynamics of nitrogen metabolism in sheep

Abstract

1. A study of ammonia and urea metabolism in sheep was made using isotope dilution techniques with ( 15 NH 4 ) 2 SO 4 , [ 15 N]urea and [ 14 C]urea in order to determine quantitatively the movements of urea-N and NH 3 -N throughout the body of normal, feeding sheep. 2. Single injections of 15 N-labelled compounds were made into the rumen fluid NH 3 , caecal fluid NH 3 and the blood urea pools, in order to estimate the rates of flux through, and the transfer of N between, these and other nitrogenous pools in the body. 51 Cr EDTA was injected into the rumen and caecum with ( 15 NH 4 ) 2 SO 4 to allow estimation of fluid volumes and to provide an indication of mixing, and of times of transit of isotopes between different sampling sites in the digestive tract. 3. The sheep ate approximately 22 g lucerne chaff/h and the mean dietary N intake was 16.3 g/d. 4. The rate of flux of NH 3 through the rumen NH 3 pool was 15.0 g/d (i.e. 90% of the dietary N ingested; however, this amount also included N from plasma urea (1.1 g/d) and other endogenous sources including NH 3 derived from caecal NH 3 (0.4 g/d). 5. Only 40% of the N in isolated rumen bacteria was derived from NH 3 , indicating that a considerable proportion of their N requirements were obtained from compounds other than NH 3 (e.g. peptides and amino acids). 6. There was evidence of recycling of N between nitrogenous pools in the rumen, probably through rumen NH 3 → microbial N → NH 3 . 7. It was estimated that 5.3 g blood urea-N/d entered the digestive tract: 20% of this urea was degraded in the rumen, 25% in the caecum and the remainder was apparently degraded elsewhere; there was evidence of urea degradation in the large intestine posterior to the caecum and it is suggested that urea degradation and absorption of the resultant NH 3 may occur in the ileum. 8. Of the 4.8 g N/d entering the caecal NH 3 pool, 4.2 g N/d left and did not return and the difference (0.6 g N/d) was recycled, possibly through caecal NH 3 → microbial N → NH 3 . 9. A large proportion of the NH 3 entering the caecal NH 3 pool (70% or 3.2 g N/d) was apparently derived from degradation of nitrogenous products, other than urea, including rumen microbial N (1.0 g N/d) passing undigested from the small intestine. 10. Less than half the NH 3 -N of caecal origin entering the rumen passed through the blood urea pool; the remainder was apparently transported as other nitrogenous compounds in the blood or body fluids. 11. The results of the three experiments were combined in a general three-pool, opencompartment model which formally recognizes an unlimited number of other unspecified, interconnected pools together comprising the whole-animal system. Rates of flux through, and transfer of N between these and other nitrogenous pools in the body were calculated by solving this model and the information derived has been applied to whole-animal models with a view to subsequently using these models in computer simulation studies.