| Popis: |
Urea is synthesized only in the liver, at a rate dependent on nitrogen intake. Several investigators have reported impaired urea synthesis in cirrhotics during nitrogen loading, but heretofore only indirect methods have been used to measure this process. We have now investigated urea synthesis in 5 normals and 8 cirrhotics with a kinetic tracer method. Urea synthesis was stimulated by infusion for 12 hr of a 5% solution of crystalline amino acids at 115 mg amino acid nitrogen/kg ideal body wt3/4/hr. Simultaneously, urea kinetics were studied with a constant infusion of [14C]urea. Plasma urea and urea specific activity were monitored hourly and plasma alphaamino nitrogen was measured every 3 hr. Because specific activity stabilized 6-8 hr after the infusion began, rate of urea synthesis during hours 8-12 could be calculated as [14C]urea infusion rate divided by •urea specific activity. In normals, alpha-amino nitrogen reached a plateau by 6 hr at about 12 mg/dl and average urea synthesis rate during hours 8-12 was 95 mg urea nitrogen/kg ideal body wt3/4/hr, range 81-107. In 6 cirrhotics, average alpha-amino nitrogen level increased to 20 mg/dl by the 12th hr, and average urea synthesis rate during hours 8-12 was 68 mg urea nitrogen/kg ideal body wt3/4/hr, range 60-82. The present study thus confirmed that the urea synthesis response to a load of amino acids is impaired in cirrhotic patients. In normals, synthesis rate of urea nitrogen approached infusion rate of amino acid nitrogen; hence, their maximal rate is probably greater than 107 mg urea nitrogen/kg ideal body wt3/4/hr. Whether the rates of 60-80 in cirrhotics represent maxima for these patients can be determined only by additional experiments at a higher infusion rate. |
