Nephrotic proteinuria has no net effect on total body protein synthesis: Measurements with C-13 valine

In nephrotic syndrome, significant amounts of plasma proteins, mostly of he patic origin, are lost in urine. Total hepatic protein synthesis increases, suggesting that other protein pools must be conserved to maintain steady s tate. This can be accomplished either by decreased amino acid oxidation or decreased protein synthesis in other organs to replace lost liver-derived p roteins, To determine the effect of nephrotic syndrome on total-body protei n metabolism, we compared whole-body valine use in seven nephrotic patients and five controls using a primed continuous infusion of [1-C-13]-valine, w ith additional priming of (NaHCO3)-C-13. Plasma [C-13]-valine,C-13 alpha ke toisovaleric acid, and the expired (CO2)-C-13 enrichments were used to asse ss whole-body valine flux, valine oxidation, and nonoxidative valine dispos al (NOVD). The valine flux into the blood compartment (97.7 +/- 3.0 versus 95.3 +/- 3.3 mu mol/kg/h), oxidation of valine (19.4 +/- 1.9 versus 21.2 +/ - 2.8 mu mol/kg/h), and NOVD (78.3 +/- 2.5 versus 74.2 +/- 2.7 mu mol/kg/h) were not statistically different in patients compared with controls, Valin e oxidation correlated positively with urinary urea excretion (r = 0.70; P = 0.01) in all subjects. Compared with control subjects who have similar ur inary urea excretion, nephrotic subjects do not compensate for urinary loss of protein by decreased amino acid oxidation or decreased nonoxidative val ine disposal. Previous studies have shown that synthesis of several hepatic proteins increases when subjects are fed the same dietary regime, whereas the present study shows that total-body protein synthesis does not increase . This would imply reduced synthesis of nonhepatic protein pools. (C) 2000 by the National Kidney Foundation, Inc.