PRODUCTION AND EXCRETION OF UREA BY THE FETAL LAMB

Abstract

The urea production rate of the fetal lamb has been measured in 10 pregnant ewes studied who were free of operative stress. The measurement of the fetal urea production rate was made by utilizing an independent measurement of placental urea clearance by the infusion of urea tagged with radioactive carbon (14C) into the fetal circulation and the endogenous concentration difference for urea between umbilical arterial blood and maternal arterial blood. It was shown that the concentration difference of urea between umbilical arterial and maternal arterial blood is significantly elevated during the first four days of recovery from surgery, with a mean value of 0.06 mg/ml plasma water on the first day. By the fifth postoperative day this concentration difference has decreased to a mean value of 0.034, which does not change from the fifth to the twelfth postoperative days. This study demonstrates that urea is excreted across the placenta at a rate of approximately 0.54 mg/min/kg fetal weight. By comparison with the urea excretion rate of adult sheep, this is a high rate of fetal urea excretion. It was demonstrated that the urea excretion rate across the placenta could be increased threefold by an infusion of ammonium lactate into the fetus. Thus, the endogenous urea excretion rate of 0.54 mg/min/kg fetal weight does not represent the maximum rate of urea production for the fetus. The finding of a high urea production rate in fetal life is new and contrary to the common belief that nitrogen catabolism plays a minor role in fetal life. These studies demonstrate that as much as 25% of the fetal oxygen consumption could be accounted for by the catabolism of amino acids.