<i>In vivo</i> physiological ramifications of placental <i>SLC2A3-</i> RNA interference throughout gestation in sheep

RNA interference (RNAi) of the placental glucose transporter SLC2A3 resulted in smaller hypoglycemic fetuses with reduced umbilical artery insulin and glucagon concentrations at mid-gestation (75 days of gestation [dGA]) in sheep. Our current objective was to determine the ramifications of SLC2A3-RNAi throughout gestation, and to assess fetal insulin secretion in response to glucose and arginine challenges. We successfully generated SLC2A3-RNAi (n = 6) and NTS-RNAi (non-targeting sequence) control sheep pregnancies (n = 6). Near-term they underwent surgical catheterization followed by in vivo metabolic studies at 133 ± 2 dGA. A baseline metabolic study, which included assessment of uterine and umbilical blood flow rates, was followed by a square-wave hyperglycemic clamp of the fetus (GSIS), followed by infusion of an arginine bolus to assess maximal fetal insulin secretion. The baseline metabolic study determined that uterine glucose uptake (mmol/min) was reduced 29% (P = 0.03), as was placental glucose utilization by 40% (P = 0.05), but umbilical glucose uptake was not impacted in SLC2A3-RNAi pregnancies. By contrast, amino acid carbon uptake/kg of uterus increased 76%, as did placental utilization of amino acid carbon (P = 0.09), indicating placental compensation supported by increased mRNA concentrations of NOS3, IGF2, IGF1R and IGF2R (P ≤ 0.05 to ≤ 0.10). Unlike the findings at 75 dGA, near-term fetal body and pancreas weights were no longer significantly (P ≥ 0.10) impacted by SLC2A3-RNAi. While baseline umbilical artery concentrations of glucose and insulin were not different (P ≥ 0.10), two-way ANOVA revealed a significant (P ≤ 0.01) SLC2A3-RNAi treatment effect during GSIS and arginine stimulated insulin secretion (ASIS), demonstrating significant enhancement of fetal insulin secretion capacity. In summary, placental compensatory mechanisms appeared to rescue fetal growth and umbilical glucose concentrations between mid-gestation and near-term. Decreasing placental glucose utilization while increasing amino acid utilization may be a mechanism aiding recovery of glucose transfer to the fetus. While microvillous glucose uptake to the placenta appears to be rate-limiting to fetal growth and development early in gestation, placental glucose transfer and fetal growth are rescued later in gestation. However, increased nutrient stimulated fetal insulin secretion persists near-term, as a possible sequela of the earlier impacts.