Urea metabolism in chronic renal failure.

Urea degradation was measured during 16 experiments in 13 chronic uremic patients being treated with essential amino acids or their analogues. [(14)C]Urea was injected i.v. and the clearance of labeled urea from its volume of distribution was compared with the simultaneous renal clearance of ordinary urea, which averaged 2.0 liters/day. The difference, extrarenal clearance of urea, averaged 3.1 liters/day as compared with a previously reported mean of 18 liters/day in normal subjects. Thus urea-splitting activity in the gut of uremic subjects expressed in these terms is far less than in normal individuals. Nevertheless, the amount of ammonia N formed from urea in these patients, 3.5 g/day, is not significantly different from normal, owing to their elevated plasma urea. In the same subjects, urea appearance rate was measured as the sum of urea excretion and the daily change in the urea pool. No negative correlation was noted between urea appearance and urea degradation, as might be expected if portal ammonia were being utilized for protein synthesis. However, urea production was positively correlated (r = 0.76) with urea degradation, suggesting that most of the resulting portal ammonia is converted back to urea. The results fail to support the view that degradation of urea in the gut promotes N conservation in uremic subjects maintained on low protein diets.