No abstract

1. Protein fractional synthesis rate (FSR) was measured in some major tissues and in the whole body of six 1-week-old sucking lambs by a large injection of ~- 4. The relative percentage contribution of liver, skin and skeletal muscle to whole-body protein synthesis was 11.7, 13.1, and 29.0.5. We concluded that tissue protein FSR in lambs were in exactly the same decreasing order, from visceral tissues to skeletal muscles, as observed in rats. The ovine FSR estimates and the partitioning of protein synthesis between tissues were in the same range as values recently obtained by flooding-dose experiments in immature rats, piglets, and even in chicks. These findings suggest that inter-species differences are rather limited.

In vivo pancreatic protein synthesis rates were obtained from the uptake of L-[3,4(n)-3H]valine co-injected with a flooding dose of unlabeled valine into 1-, 5-, and 8-wk-old suckling lambs, and 8-wk-old weaned animals. Protein fractional synthesis rate was 184%/d at 1 wk of age and 153%/d in 5-wk-old animals (P greater than 0.05). This lack of developmental change resulted from constant (P greater than 0.05) ribosomal capacities (total RNA/protein ratios) and efficiencies of protein synthesis (synthetic rates relative to RNA). No further alteration for protein fractional synthesis rate (144%/d) occurred in 8-wk-old suckling animals (P greater than 0.05). In contrast, 8-wk-old ruminants exhibited higher protein fractional synthesis rate (244%/d) than 8-wk-old suckling animals, although ribosomal capacity was markedly higher in both 8-wk-old groups than in youngest animals (P less than 0.05). The present findings clearly indicate that in vivo protein synthesis in the developing ovine pancreas depends primarily on age. Potentialities for increased rates of pancreatic protein synthesis, i.e., increases in total RNA content and ribosomal capacity appear between 5 and 8 wk of age in this species. At 8 wk of age, however, when lambs are generally weaned, solid food ingestion resulted in a rise for both fractional and absolute rates of protein synthesis, essentially because ruminants maintained a higher efficiency of protein synthesis than milk-fed animals (P less than 0.005). Finally, there was a relationship between pancreatic protein synthesis and protein intake in only ruminant lambs.

Both the small intestine and liver account for 33 % to 49 % of total protein synthesis in the rat, from 3 to 105 weeks of life . Very little information is available, however, on the earliest stages of postnatal development. We describe here changes in protein synthesis before weaning in the small intestine and liver of one and five-week old lambs. The same large dose method was used for both ages to overcome problems that arise when measuring protein synthesis in these tissues (McNurlan et al., 1979