Timing and duration of maternal nutrient restriction during mid to late gestation influence fetal glucose and amino acid flux in sheep

Previous research has demonstrated that maternal nutrient restriction during mid to late gestation can influence fetal flux of glucose and amino acids in sheep. However, what is unclear is how the timing and duration of nutrient restriction during midgestation influence net uterine, uteroplacental and fetal flux of glucose and amino acids. On day 50 of gestation, 41 ewes carrying singletons (mean initial body weight [BW] = 106 ± 1.3 pounds) were assigned to these dietary treatments: 100% of nutrient requirements (control; CON; n = 20) or 60% of nutrient requirements (restricted; RES; n = 21) from day 50 to day 90 of gestation (midgestation). At day 90, 14 ewes were euthanized (CON, n = 7; RES, n = 7) and the remaining ewes were subjected to treatments of nutrient restriction or remained on a control diet from day 90 until day 130 of gestation (late gestation; CON-CON, n = 6; CON-RES, n = 7; RES-CON, n = 7; and RES-RES, n = 7) and euthanized for sample collection. We found that maternal nutrient restriction during midgestation increases fetal glucose flux, but maternal nutrient restriction during late gestation decreases fetal glucose flux. Fetal amino acid flux was decreased with nutrient restriction during midgestation; however, this was not apparent when fetal amino acid flux was measured on day 130. These data demonstrate that fetal glucose and amino acid flux are influenced by the timing and duration of nutrient restriction during mid to late gestation, which might have implications for offspring growth and efficiency.

Several animal models of fetal and placental growth restriction have been developed to better unravel the relationship among uteroplacental blood flow, placental vascularity and nutrient delivery to the fetus. In sheep, increasing uterine blood flow during the last half of gestation is vital for maintaining continual delivery of sufficient oxygen and nutrients to the exponentially growing fetus (Ford, 1995).

Compromised pregnancies show a decrease in umbilical cord blood flow and a decrease in fetal plasma total α-amino acid concentrations (Kwon et al., 2004). Low birth weight offspring from compromised pregnancies have increased incidences of adult onset diseases, poor growth rates and lower daily rates of gross energy accretion.

Limited information is available on uteroplacental nutrient delivery, uptake by the fetus and maternal nutrient supply from compromised pregnancies. Previous research has demonstrated that maternal nutrient restriction can influence net uteroplacental flux of glucose and amino acids in gestating ewes (Lemley et al., 2013). However, what is unclear is how the timing and duration of nutrient restriction during mid to late gestation influences fetal nutrient delivery. The objective of this experiment was to evaluate how nutrient restriction during mid to late gestation influences net uterine, uteroplacental and fetal flux of glucose and amino acids.

Forty-one ewe lambs carrying singletons were mated by natural service, pregnancy was confirmed, and they were housed at the North Dakota State University Animal Nutrition and Physiology Center. On day 50 of gestation, 41 ewes carrying singletons (mean initial BW = 106 ± 1.3 pounds) were assigned to these dietary treatments: 100% of nutrient requirements (control; CON; n = 20) or 60% of nutrient requirements (restricted; RES; n = 21) from day 50 to day 90 of gestation (midgestation). At day 90, 14 ewes (CON, n = 7; RES, n = 7) were euthanized and the remaining ewes were subjected to treatments of nutrient restriction or remained on a control diet from day 90 until day 130 of gestation (late gestation; CON-CON, n = 6; CON-RES, n = 7; RES-CON, n = 7; and RES-RES, n = 7) and euthanized for sample collection.

On day 90 (n = 14) and day 130 (n = 27), anesthesia was induced with 3 milligrams per kilogram (mg/kg) of BW sodium pentobarbital. A jugular catheter was inserted to maintain anesthesia through intermittent infusion of sodium pentobarbital. The uterus was exposed with a midventral laparotomy for measurements of uteroplacental blood flow, blood collection and fetal extraction as described by Lemley et al. (2012).

Blood serum was analyzed for glucose and amino acids. Net uterine, fetal and uteroplacental flux were calculated as the arteriovenous concentration difference multiplied by blood flow. Positive flux represents tissue uptake whereas a negative flux represents tissue release.

Fetal glucose uptake tended to increase (P = 0.08) with nutrient restriction during midgestation (Table 1). Nutrient restriction during midgestation decreased (P ≤ 0.05) uterine and uteroplacental release of total AA and tended to decrease (P = 0.07) total AA uptake by the fetus.

1 - Treatments: CON = control, 100% of National Research Council (NRC) requirements; RES = restricted, 60% of NRC requirements.
2 - CON, n = 7; RES, n = 7.

Uteroplacental release and fetal uptake of essential AA were decreased (P = 0.03) with RES by 53.4% and 45%, respectively. Uterine and uteroplacental release of nonessential AA were decreased (P = 0.03) with RES but fetal uptake was not affected (P = 0.14).

Nutrient restriction during midgestation increased (P = 0.04) fetal glucose flux measured on day 130 (Table 2). This indicates that increased fetal glucose flux resulting from midgestational nutrient restriction is a persistent effect that alters fetal metabolism throughout late gestation, which might suggest that offspring growth and metabolism could be altered.

1 - Treatments: CON = control, 100% of NRC requirements; RES = restricted, 60% of NRC requirements. The second row is midgestational treatments (MG Trt) and the third row is late gestational treatments (LG Trt).
2 - CON-CON, n = 6; CON-RES, n = 7; RES-CON, n = 7; RES-RES, n = 7.

Nutrient restriction during late gestation decreased (P = 0.02) fetal glucose uptake and increased (P = 0.02) uteroplacental glucose uptake. Bidirectional changes in fetal glucose flux resulting from maternal nutrient restriction during either midgestation (increase) or late gestation (decrease) suggests that programming outcomes for offspring is dependent on the timing and duration of maternal nutrient restriction.

Uterine, uteroplacental and fetal fluxes of total, essential or nonessential amino acids were not influenced (P > 0.08) by maternal nutrient restriction during mid to late gestation (Table 2). These data indicate that the decreased fetal essential amino acid flux that occurs with nutrient restriction during midgestation does not persist during late gestation. This might suggest that uterine and umbilical blood flows, placental amino acid metabolism and/or fetal metabolism adapt to meet amino acid requirements of the placenta and the fetus to support proper growth and development.

We thank the staff and graduate students at NDSU who helped with sample collections and analysis. This work was partially supported by the U.S. Department of Agriculture - Agriculture and Food Research Initiative [2016-67016-24884] and the North Dakota State Board of Agricultural Research and Education.

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