Urea Synthesis After Protein Feeding Reflects Hepatic Mass in Rats

Authors

  • Thomas G. Brewer,

    Corresponding author
    1. Departments of Medical and Surgical Gastroenterology, Walter Reed Army Institute of Research and Gastroenterology Service, Walter Reed Army Medical Center, Washington, D.C. 20307; the Uniformed Services University School of Medicine, Bethesda, Maryland 20014; and the Department of Biometrics, University of Colorado School of Medicine, Denver, Colorado 80262
    • Thomas Brewer, M.D., Gastroenterology Service, Fitzsimons Army Medical Center, Aurora, Colorado 80045.
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  • William R. Berry,

    1. Departments of Medical and Surgical Gastroenterology, Walter Reed Army Institute of Research and Gastroenterology Service, Walter Reed Army Medical Center, Washington, D.C. 20307; the Uniformed Services University School of Medicine, Bethesda, Maryland 20014; and the Department of Biometrics, University of Colorado School of Medicine, Denver, Colorado 80262
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  • John W. Harmon,

    1. Departments of Medical and Surgical Gastroenterology, Walter Reed Army Institute of Research and Gastroenterology Service, Walter Reed Army Medical Center, Washington, D.C. 20307; the Uniformed Services University School of Medicine, Bethesda, Maryland 20014; and the Department of Biometrics, University of Colorado School of Medicine, Denver, Colorado 80262
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  • Strothers H. Walker,

    1. Departments of Medical and Surgical Gastroenterology, Walter Reed Army Institute of Research and Gastroenterology Service, Walter Reed Army Medical Center, Washington, D.C. 20307; the Uniformed Services University School of Medicine, Bethesda, Maryland 20014; and the Department of Biometrics, University of Colorado School of Medicine, Denver, Colorado 80262
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    • Deceased.

  • Michael A. Dunn

    1. Departments of Medical and Surgical Gastroenterology, Walter Reed Army Institute of Research and Gastroenterology Service, Walter Reed Army Medical Center, Washington, D.C. 20307; the Uniformed Services University School of Medicine, Bethesda, Maryland 20014; and the Department of Biometrics, University of Colorado School of Medicine, Denver, Colorado 80262
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  • This work was presented in part at the Annual Meeting of the American Association for the Study of Liver Disease, Chicago, Illinois, November 8, 1980, and published in abstract form (Gastroenterology 1980; 79:1007).

  • The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or reflecting the views of the Department of Defense.

Abstract

Urea synthesis is an exclusive biosynthetic function of the liver. Since the exact relationship between urea synthesis in vivo and functional liver mass remains unclear, we established an animal model using oral protein loading and measurement of resultant urea synthesis in rats. We studied rats subjected to sham operation, 40% hepatectomy, 66% hepatectomy, portacaval shunt and CC14-induced cirrhosis. Urea synthesis was calculated as the sum of urinary urea excretion and accumulation of urea in body water during the 6-hr period after oral administration of a casein protein load equivalent to 20 gm per kg body weight.

Peak urea synthesis rate in the sham-operated group of rats was 142 ± 11 μmoles per hr per gm wet liver weight (mean ± 1 S.D.), 473 ± 34 μmoles per hr per gm liver protein and 80 ± 5 μmoles per hr per mg liver DNA. This rate closely matched those of the other groups for each type of liver mass measurement. Marked reduction (p < 0.01) of urea synthesis on a DNA basis was noted only in the CCl4-cirrhotic livers, related to the significantly higher (p < 0.01) DNA content of the cirrhotic livers. Similarly, increased (p < 0.05) liver protein content of the sham-operated rats when compared with the other groups was reflected in slightly lower urea synthesis rates expressed on the basis of liver protein (p < 0.05) when compared to that of all other groups. The slopes of the trend lines which define the relationship of urea synthesis rates in each group to total liver mass (liver weight, protein content and DNA content) were not significantly different.

We conclude that urea synthesis rates after oral administration of a protein load reflect liver mass in terms of wet weight, liver protein content and liver DNA content. This relationship remains valid in the presence of changes in liver parenchymal mass, portacaval shunting and CC14-cirrhosis.

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