Maternal metabolizable protein restriction during late gestation leads to greater gravid uterine uptake of total serum nitrites in sheep

Abstract Text: We have previously reported that maternal metabolizable protein restriction during late gestation increases uterine blood flow per fetal weight.  To examine the effects of metabolizable protein (MP) restriction during late gestation on total serum nitrite concentration in the uterine and umbilical vessels and transplacental exchange of total serum nitrites, 11 pregnant ewes were assigned to one of three isocaloric diets that were formulated to provide 60% of MP (MP60), 80% of MP (MP80), or 100% of MP (MP100) requirements from day 100 to 130 of gestation.  On day 130 of gestation, uterine and umbilical blood flows were measured by Doppler ultrasonography.  Serum samples from the maternal saphenous artery, gravid uterine vein, umbilical artery, and umbilical vein were obtained and analyzed for total nitrite concentration.  Total serum nitrites in the maternal artery and gravid uterine vein were less (P ≤ 0.05) in MP60 ewes compared with MP100 ewes (maternal artery: 5.65 vs. 14.5 ± 2.33 µM; uterine vein: 4.65 vs. 16.1 ± 2.28 µM), with MP 80 ewes being intermediate (maternal artery: 10.7 ± 2.33 µM; uterine vein: 10.3 ± 2.28 µM).  Total uterine flux of serum nitrites was similar in the MP60 and MP80 ewes (863 and 204 ± 348 µM/ml/min) and indicated there was uterine uptake of total serum nitrites, but this differed (P = 0.03) from the MP100 ewes, where flux calculations indicated uterine release of total serum nitrites (-704 ± 348 µM/ml/min).  Total serum nitrite concentration in both the umbilical artery and umbilical vein were similar (P ≥ 0.08) among treatments (umbilical artery: 44.7, 47.9, and 90.9 ± 14.3 µM for MP60, MP80, and MP100 ewes, respectively; umbilical vein: 49.1, 54.2, and 97.7 ± 15.9 µM for MP60, MP80, and MP100 ewes, respectively). Fetal flux of total serum nitrites was similar (P = 0.35) among treatments (1619, 2960, and 3409 ± 1078 µM/ml/min for MP60, MP80, and MP100 ewes, respectively).  Similarly, uteroplacental flux of total serum nitrites was similar (P= 0.09) among treatments (-756, -2757, and -3740 ± 1033 µM/ml/min for MP60, MP80, and MP100 ewes, respectively).  In conclusion, the observed increase in uterine blood flow per fetal weight in the MP60 ewes may be due to increases in nitric oxide uptake.  

Keywords: ewes, metabolizable protein, nitrites