AMMONIA PRODUCTION BY THE PREGNANT SHEEP UTERUS

Abstract

It is generally assumed that, during gestation, nitrogen metabolism of the uterus and conceptus is directed exclusively toward the synthesis of new tissues. The finding of large quantities of urea excreted by the fetal lamb prompted us to question that assumption and to examine the role of ammonia as an excretory product of the pregnant uterus in sheep with chronically implanted catheters. Seventeen pregnant ewes (GA 47-155 days) were studied with uterine vein and femoral artery catheters; 7 fetuses (114-144 days) had umbilical vein and pedal artery catheters. Two nonpregnant sheep were also studied. After postoperative recovery, simultaneous fetal and maternal blood samples were obtained for NH3 and O2 content. Between-group differences in mean arterial [NH3] (fetus 49.27 ± 2.35 μM; pregnant ewe 36.70 ± 0.93 μM; nonpregnant ewe 23.04 ± 1.01 μM) were statistically significant. In all cases, NH3 was excreted from the gravid uterus into both the uterine and umbilical circulations by tissues other than the fetus. Thus, the placenta is the probable site of NH3 formation. Quantitatively, at 70 days of gestation, the ratio of nitrogen excreted by the uterus as NH3 to the combined total nitrogen requirements for both growth and NH3 excretion is ~ 44%. Toward the end of gestation, uterine NH3 production represents ~ 12% of fetal nitrogen requirements. Ammonia is an important metabolic end product of the ovine gravid uterus, especially in early pregnancy.