The role of growth hormone and amino acids on brain protein synthesis in aged rats given proteins of different quantity and quality

Abstract

The purpose of the present study was to determine whether the regulation of brain protein synthesis was mediated through changes in the plasma concentrations of insulin and growth hormone (GH), and whether the concentrations of amino acids in the brain and plasma regulate the brain protein synthesis when the quantity and quality of dietary protein is manipulated. Two experiments were done on three groups of aged rats given diets containing 20% casein, 5% casein or 0% casein (Experiment 1), and 20% casein, 20% gluten, or 20% gelatin (Experiment 2) for 1 d (only one 5-h period) after all rats were fed the 20% casein diet for 10 d (only 5-h feeding per day). The aggregation of brain ribosomes, the concentration in plasma GH, and the branched chain amino acids in the plasma and cerebral cortex declined with a decrease of quantity and quality of dietary protein. The concentration of plasma insulin did not differ among groups. The results suggest that the ingestion of a higher quantity and quality of dietary protein increases the concentrations of GH and several amino acids in aged rats, and that the concentrations of GH and amino acids are at least partly related to the mechanism by which the dietary protein affects brain protein synthesis in aged rats.

The Growth Hormone Affects the Brain Protein Synthesis Rate in Hypophysectomized Aged Rats

“…In the previous studies, we demonstrated that the protein synthesis in brain regions and plasma concentration of GH decreased with a decrease in the quantity and quality of dietary protein in aged rats (12)(13)(14). Recent studies have shown that GH may affect functions related to the central nervous system.…”

“…Hypophysectomy and treatment with GH may have controlled RNA activity and been one of the factors affecting brain protein synthesis in aged rats. Our previous results strongly indicated that the quantity and quality of dietary protein might regulate the concentration of GH and control the rates of protein synthesis in the brain regions of aged rats (12)(13)(14). Therefore, the concentration of GH may be at least partly related to the mechanism by which dietary protein affects brain protein synthesis in aged rats.…”

“…We demonstrated that the rate of protein synthesis in the brain decreased with age in rats after weaning ( 11 ) . In many investigations, the protein synthesis in the brain and the concentration of plasma growth hormone (GH) has been shown to depend on the quantity and quality of dietary protein in aged rats (12)(13)(14). GH has been known to increase tissue protein synthesis by stimulating translational activity ( 15 ).…”

The effects of dietary protein on the somatotropic axis: a comparison of soy, gelatin, α-lactalbumin and milk

“…These findings are in accordance with our finding that milk intake increases the GH response when compared with placebo. Furthermore a study by Ohsumi et al (2007) investigated the influence of gelatin compared with casein protein on GH secretion. In contrast to our findings, they found reduced GH concentrations after ingestion of a lower quality of dietary protein (that is, gelatine) compared with a higher quality dietary protein (that is, casein) in aged rats (Ohsumi et al, 2007).…”

“…Furthermore a study by Ohsumi et al (2007) investigated the influence of gelatin compared with casein protein on GH secretion. In contrast to our findings, they found reduced GH concentrations after ingestion of a lower quality of dietary protein (that is, gelatine) compared with a higher quality dietary protein (that is, casein) in aged rats (Ohsumi et al, 2007). In their study, GH concentrations were measured once after an ingestion period of 5 h, which is different from our protocol where we measured GH concentrations every 20 min after ingestion of a constant amount of protein.…”