Synthesis of urea after stimulation with amino acids: relation to liver function.

Abstract

Hepatic urea synthesis is the organism''s main channel for the disposal of N and it may be an essential liver function. In 6 control subjects and 5 patients with cirrhosis of the liver, urea synthesis was studied during continuous infusion for 6-24 h of about 3 mmol .alpha.-amino N/h .times. kg body weight. The urea synthesis rate was calculated in intervals of 2 h as urinary excretion with correction for accumulation in the total body water, and for hydrolysis of urea in the gut. The peripheral venous plasma .alpha.-amino N concentration increased from 3 to about 14 mmol/l, and the urea N synthesis rate from 25 to about 215 mmol/h. In all cases the urea synthesis rate rose linearly with the .alpha.-amino concentration throughout the examined range. The slope of this linear relationship is an expression of the hepatic conversion of .alpha.-amino N to urea N (functional hepatic N clearance). The functional hepatic N clearance was 22.4 1/h in control subjects, and 13.7 1/h (P < 0.025) in the patients with cirrhosis. It was correlated with quantitative measures of the liver function (the galactose elimination capacity, r = 0.84 and the clearance of antipyrine, r = 0.80). The abundant capacity of the urea synthesis system was confirmed. A given urea synthesis rate apparently requires a higher .alpha.-amino level in patients with reduced liver function.