The relationship between transplacental O2 diffusion and placental expression of PlGF, VEGF and their receptors in a placental insufficiency model of fetal growth restriction

Abstract

Placental growth factor (PlGF) and vascular endothelial growth factor (VEGF) are involved in placental angiogenesis through interactions with the VEGFR-1 and VEGFR-2 receptors. The placenta of pregnancies whose outcome is fetal growth restriction (FGR) are characterized by abnormal angiogenic development, classically associated with hypoxia. The present study evaluated the near-term expression of this growth factor family in an ovine model of placental insufficiency-FGR, in relationship to uteroplacental oxygenation. Compared to controls, FGR pregnancies demonstrated a 37% increase in uterine blood flow (FGR vs. control, 610.86+/-48.48 vs. 443.17+/-37.39 ml min(-1) (kg fetus)(-1); P<0.04), which was associated with an increased maternal uterine venous PO2 (58.13+/-1.00 vs. 52.89+/-1.26 mmHg; P<0.02), increased umbilical artery systolic/diastolic ratio (3.90+/-0.33 vs. 2.12+/-0.26, P<0.05), and fetal hypoxia (arterial PO2; 12.79+/-0.97 vs. 18.65+/-1.6 mmHg, P<0.005). Maternal caruncle PlGF mRNA was increased in FGR (P<0.02), while fetal cotyledon VEGF mRNA was reduced (P<0.02). VEGFR-1 mRNA was also reduced in FGR fetal cotyledon (P<0.001) but was not altered in caruncle tissue. Immunoblot analysis of PlGF and VEGF demonstrated single bands at 19,000 and 18,600 Mr, respectively. Caruncle PlGF concentration was increased (P<0.04), while cotyledon VEGF was decreased (P<0.05) in FGR placentae. The data establish that uterine blood flow is not reduced in relationship to metabolic demands in this FGR model and that the transplacental PO2 gradient is increased, maintaining umbilical oxygen uptake per unit of tissue. Furthermore, these data suggest that an increased transplacental gradient of oxygen generates changes in angiogenic growth factors, which may underline the pathophysiology of the post-placental hypoxic FGR.

Figure 1

Umbilical arterial systolic/diastolic (Umb S/D), umbilical placental resistance (Umb PI) and umbilical resistance index (Umb RI) in TN (□) and HT (▪) pregnancies, determined following HT treatment. ** P < 0.01, *** P < 0.005, between-treatment comparisons.

Figure 2

Comparison of uterine venous and umbilical venous P_O2 and P_CO2 differences in TN (□) and HT (▪) pregnancies. ** P < 0.01, between-treatment comparisons.

Figure 3

Concentration of ovine PlGF mRNA (A) and VEGF mRNA (B) (GF/actin mRNA densitometry units) in TN (□) and HT (▪) maternal caruncle and fetal cotyledon tissues at 135 dGA.

Figure 4

Concentration of ovine VEGFR-1 mRNA (A) and VEGFR-2 mRNA (B) (VEGFR/actin mRNA densitometry units) in TN (□) and HT (▪) maternal caruncle and fetal cotyledon tissues at 135 dGA.

Figure 5

RT-PCR amplification demonstrates PlGF (A) and VEGF (B) isoform expression in pooled 135 dGA TN and HT caruncle and cotyledon samples.

Figure 6

Representative immunoblots of PlGF standards, PlGF₁₄₉ and PlGF₁₇₀ (A), and VEGF standards, VEGF₁₂₁, VEGF₁₆₅ and VEGF₁₈₉ (B), and representative immunoblots of PlGF and VEGF in pooled placental (caruncle and cotyledon) samples.

Figure 7

Ovine PlGF (A) and VEGF (B) protein measured by Western immunoblot (GF/Actin) in TN (□) and HT (▪) maternal caruncle and fetal cotyledon tissues at 135 dGA.

Figure 8

Schematic representation of fetal and uteroplacental oxygenation and blood flows in TN (Control) and HT (FGR) pregnancies at 135 dGA, together with the observed changes in PlGF, VEGF and VEGFR-1 mRNA concentrations. The P_O2 values (mmHg) for maternal artery (M_A), uterine vein (Ut_V), fetal pedal artery (F_a) and umbilical vein (Umb_v) are presented, as are uterine and umbilical blood flows per 100 g of placenta. P determined by unpaired Student's t test, * P < 0.01 and *** P < 0.005, between-treatment comparisons.