In vivo longitudinal variations in protein synthesis in developing ovine intestines.
Changes in fractional rates of protein synthesis (Ks) were investigated at different small and large intestinal sites in 1-, 5-, and 8-wk-old milk-fed and 8-wk-old weaned lambs, a species with early intestinal maturation similar to most domestic animals and humans, with the use of a flooding dose of L-[3H]valine. Between 1 and 8 wk of age, Ks did not change significantly in the duodenum, the cecum, or the colon of milk-fed lambs, but was depressed by 30% in the jejunum and by 39% in the ileum. This was because of reduced ribosomal capacity, i.e., total RNA-to-protein ratio (Cs) in the jejunum, and also alterations in both Cs and protein synthetic efficiency, i.e., rate of synthesis relative to RNA (KRNA) in the ileum. Ks values throughout the small intestine were significantly higher (45-55%) in weaned lambs than in 8-wk-old milk-fed animals. This enhancement of protein synthesis was mainly related to an increase in KRNA (27-40%). Ks decreased by 43% from the duodenum to the ileum in both milk-fed and weaned 8-wk-old animals, but not in 1- and 5-wk-old milk-fed lambs, because of a marked reduction in KRNA. It was concluded that changes in nutrients at weaning, weaning itself, or both, enhanced protein synthesis without any specific effect on small intestinal site. By contrast, intrinsic developmental factors were responsible only for the regional differences in small intestinal Ks that occurred at 8 wk of age. Longitudinal variations in protein synthesis may contribute to the establishment of the well-recognized jejunoileal gradients of brush-border enzymes and villus height that characterize the mature mammalian small intestine.