Metabolic responses to nephrosis: effect of a low-protein diet.

To determine whether dietary protein restriction (LPD) causes protein catabolism in adriamycin nephrosis, nephrotic and control rats were paired by weight and gavage fed an 8.5% protein diet for 3 days (protocol 1) or 12 days (protocol 2). Fasting whole body protein turnover was then measured using a constant infusion of L-[1-14C]leucine. After 3 days of LPD, proteinuria decreased slightly and body weight did not change in either group. In contrast, leucine oxidation and urinary urea nitrogen excretion in nephrotic rats decreased by 18% and 37%, respectively (P < or = 0.05). After 12 days of LPD, weight loss did not differ between groups. In contrast to protocol 1, proteinuria decreased by 45% in nephrotic rats fed LPD for 12 days, and leucine oxidation rats increased to the level of control rats. Rates of whole body protein synthesis (PS) and degradation (PD) did not differ between nephrotic and control rats receiving LPD for 3 or 12 days, but were significantly lower than rates measured in rats fed 22% protein. We conclude that 1) proteinuria stimulates protein conservation even when dietary protein intake is restricted; 2) the decrease in amino acid oxidation was dependent on moderate proteinuria, since prolonged LPD ameliorated nephrosis and leucine oxidation rates increased to control levels; and 3) since weight loss and rates of whole body PS and PD in nephrotic and control animals were indistinguishable, moderate proteinuria did not increase protein catabolism.