Alterations in protein metabolism and amino acid concentrations in rats fed by a high-protein (casein-enriched) diet - effect of starvation.

Rats were fed with a standard laboratory diet (SLD) or a high-protein diet (HPD). After three months changes in amino acid concentration and protein metabolism were examined in fed and 24h-fasted animals. In the blood of the HPD animals sacrificed in fed state were found higher concentrations of urea, aspartate, taurine, proline, valine, isoleucine, and leucine, and lower concentrations of glycine and cysteine. The main alterations in tissues were decreased concentrations of glycine and increased concentrations of valine, isoleucine, and leucine. Differences in weight, protein concentration, protein synthesis, and proteolysis in tissues were insignificant. The exception was soleus muscle in which higher values of protein synthesis and proteolysis were found in HPD animals. The response to starvation of HPD and SLD fed animals was different. In animals fed before starvation by HPD was found more pronounced decrease in a number of individual amino acids in plasma and tissues and more pronounced decrease in protein synthesis in muscle, spleen, jejunum, and colon. It is concluded that chronic intake of HPD has not positive effect on protein balance in any tissue, results in the imbalance in aminoacidemia in extracellular and intracellular fluid, and alters the response of the organism to starvation.