Kazumi Tsutsui

Comparison of the Effects of Ornithine and Arginine on the Brain Protein Synthesis Rate in Young Rats.

Brain protein synthesis and the plasma concentration of growth hormone (GH) are sensitive to dietary ornithine. The purpose of this study was to determine whether dietary arginine, the metabolite of ornithine, affects the brain protein synthesis, and to that end, the effects of arginine on brain protein synthesis were compared with that of ornithine treatment in young rats. Two experiments were done on five or three groups of young rats (5-wk-old) given 0%, 0.25%, 0.5%, 0.7% arginine or 0.7% ornithine-HCl added to a 20% casein diet for 1 d (only one 3 h period) (Experiment 1), or given a diet containing 0% or 0.7% ornithine-HCl or 0.7% arginine added to a 20% casein diet (Experiment 2). The concentrations of plasma growth hormone (GH) and fractional rates of protein synthesis in the brains increased significantly with the 20% casein+0.7% arginine diet and still more with the 20% casein+0.7% ornithine diet compared with the 20% casein diet alone. In the cerebral cortex and cerebellum, the RNA activity [g protein synthesized/(g RNA•d)] significantly correlated with the fractional rate of protein synthesis. The RNA concentration (mg RNA/g protein) was also related to the fractional rate of protein synthesis in these organs. The results suggest that the treatment with arginine is likely to increase the concentrations of GH and the rate of brain protein synthesis in rats, and that the effects of arginine on brain protein synthesis and GH concentration were lower than that of ornithine. The RNA activity is at least partly related to the fractional rate of brain protein synthesis.

Changes in Thyroid Hormone Are Not Involved in Regulating Brain Protein Synthesis in Adults Rats Fed Ornithine

Brain protein synthesis and the plasma concentration of growth hormone (GH) are sensitive to dietary ornithine. However, dietary ornithine does not increase brain protein synthesis in hypophysectomized rats. Because hypophysectomy may decrease the secretion of thyroid stimulated hormone (TSH), we assessed whether the regulation of brain protein synthesis was mediated by changes in the plasma concentrations of thyroid hormone and ghrelin in the 6-propyl-2-thiouracil (PTU, thyroid inhibitor)-treated or control adult rats fed ornithine. The four experimental groups consisted of PTU-treated and control (24-wk-old) male rats given 0% or 0.7% ornithine-HCl added to a 20% casein diet. The plasma concentrations of GH and ghrelin, and the fractional rates of protein synthesis and RNA activity [g protein synthesized/(g RNA•d)] in the brains were significantly increased after treatment with the 20% casein + 0.7% ornithine compared with the 20% casein diet alone in both the PTU-treated and control groups. Ornithine supplementation to the basal diet did not affect the plasma concentration of T3. The RNA concentration (mg RNA/g protein) was not related to the fractional rate of protein synthesis in the brain regions. The results suggest that dietary ornithine likely increases the rate of brain protein synthesis in control and PTU-treated rats, and that the ornithine-induced increase in the GH concentration may stimulate mainly brain protein synthesis via ghrelin. RNA activity is at least partly related to the fractional rate of brain protein synthesis.