Utilization of amino acids by pre-ruminant lambs. III.* The influence of age and the dietary proportions of essential and non-essential amino acids on the levels of urea, ammonia and free amino acids in the blood plasma of milk-fed lambs

Abstract

Jugular blood samples were obtained from 10.5 kg and 28 kg lambs receiving a diet of reconstituted cows' whole milk. The lambs were then given diets in which the proportion of essential amino acids (BAA) in the dietary crude protein was altered over a wide range. A second blood sample was taken after lambs had received such diets for 12 days. Plasma obtained from these samples was analysed for free amino acids, urea and ammonia. The pattern of plasma free amino acids (PFAA) in lambs given reconstituted cows' whole milk is described. In both the pre-treatment and post-treatment samples, the heavier lambs appeared to have lower plasma levels of all EAA, and high plasma levels of glycine, serine, urea and ammonia. In the lighter lambs, there were pronounced responses of PFAA levels to changes in the dietary proportion of EAA. At low proportions, the levels of most EAA in plasma were low. Lysine and phenylalanine were exceptions. In addition, levels of many non-essential amino acids (non-EAA), particularly serine and glycine, were high. At high proportions of EAA, plasma levels of all EAA, especially methionine, rose markedly. Within the non-EAA, serine, proline and glycine were reduced, while taurine and cystathionine increased. In the plasma of the heavier lambs, the response of some amino acids to a given dietary change differed from the response in the lighter lambs. This was especially true of methionine, tyrosine, phenylalanine and arginine. There was also marked between-animal variation in plasma levels. When expressed as molar proportions of total PFAA, results were similar to those of the lighter lambs. There was a pronounced similarity between the response of the PFAA to diets with a low proportion of EAA, and the PFAA pattern characteristic of developing kwashiorkor. __________________ *Part II, Aust. J. Agric. Res., 28, 933 (1977).