[Protein metabolism in the newborn lamb. II. Influence of diet on protein synthesis and turnover].

Leucine turnover and protein fractional synthesis rates were measured in fed and unfed newborn lambs. The fed group (4 animals) was fed hourly with cow colostrum throughout the experiment which started at birth; the unfed group (5 animals) was given saline (NaCl 9 0/00). Two hours after birth, L-(1-14C) leucine was infused into the jugular vein at a constant rate for 8 h. Eight blood samples were taken during the infusion period; at the end of the experiment, the liver, skin and three muscles (supraspinatus, longissimus dorsi, tensor fasciae latae) were sampled. The specific activity of free and protein-bound leucine was measured in these samples and in the whole body (table 2). The specific activity of free leucine reached a quasi-steady state in the plasma within 2 h after the beginning of infusion (fig. 1). The irreversible loss rate of plasma leucine and the leucine catabolic rate were higher in fed than in unfed lambs. The highest fractional rates of protein synthesis occurred in the liver, skin and whole body (table 3). They were similar in both groups. In contrast, fractional rates of protein synthesis in muscles were higher in fed than in unfed animals. Thus, the proportion of whole body protein synthesis, accounted for by muscle protein synthesis, was higher in fed than in fasting newborn lambs (fig. 2). The leucine flux from whole body protein breakdown was unaffected by nutritional status (table 1). Feeding colostrum stimulated muscle protein synthesis, which was in keeping with body weight gain and protein accretion.