| Co-Investigator(Kenkyū-buntansha) |
NAGASE Sohji INSTITUTE OF CLINICAL MEDICINE, INTRNAL MEDICINE, UNIVERSITY OF TSUKUBA, ASSOCIATE PROFESSOR, 臨床医学系, 助教授 (10189128)
FUJIMORI Ken INSTITUTE OF CHEMISTRY, UNIVERSITY OF TSUKUBA, ASSOCIATE PROFESSOR, 化学系, 助教授 (90015983)
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| Research Abstract |
Introduction.We have reported that
1) synthesis of GSA, a uremic toxin, increases depending on urea concentration,
2) GSA is formed from argininosuccinic acid (ASA) and the hydroxyl radical or SIN-1 which generates superoxide and NO simultaneously. However, excesses of NO wich scavenges hydroxyl radical inhibited GSA synthesis, We also reported that arginine, citrulline or ammonia plus ornithine all of which increase arginine inhibit GSA synthesis in the presence of urea.
To elucidate the mechanism for increased GSA synthesis by urea, we investigated the effect of urea on ASA and the NO precursor, arginine.
Methods. Urea synthesis was investigated by the incorporation of 14C carbon dioxide into urea in isolated rat hepatocytes. The effect of high concentrations of urea on intermediates of the urea cycle were investigated in rat hepatocytes in vitro and rat organs in vivo.
Results. Urea at the higher concentrations than 50mg/dl inhibited urea synthesis. ASA was not detected without urea and 200 mgN/dl urea, but decreased arginine from 36 to 33 nmol/g wet cells. Ornithine which inhibits GSA synthesis, increased ASA markedly in a dose dependent manner and increased arginine. At 2h after the urea injection (n=5), rat serum arginine level decreased by 42%, and ornithine and citrulline level increased, significantly.
Discussion. Urea does not increase ASA, but decreases arginine. Any condition that increased arginine increased ASA and inhibited GSA syn- thesis. It is apparent that the ASA level did not determine GSA synthesis. Therefore, we propose that limited arginine in .hepatocytes, where the arginine level is extremely low to begin with, decreases NO, which in turn, increases the hydroxyl radical generation from superoxide and NO.
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