ABSTRACT
Placental development requires adequate and organized interaction of vascular growth factors and their receptors, including vascular endothelial growth factor (VEGF) and placental growth factor (PlGF). Both VEGF and PlGF, acting through the tyrosine kinase receptors VEGFR-1 and VEGFR-2, have been implicated in playing a role in ovine placental vascular development. The present studies describe the placental expression of components of the VEGF family at two maturational time points (55 and 90 days post coitus, dpc) in a hyperthermic-induced ovine model of placental insufficiency-intrauterine growth restriction (PI-IUGR). Both caruncular and cotyledonary VEGF and PlGF mRNA concentration increased with gestational age (P< 0.05), whereas only cotyledonary VEGF and PlGF protein concentration increased over gestation (P< 0.002). At 55 dpc, VEGF mRNA concentration was elevated in hyperthermic (HT) ewes, compared to control thermoneutral (TN) animals (TN; 0.52+/-0.08 vs HT; 1.27+/-0.17 VEGF/GAPDH, P< 0.001). At 90 dpc, expression of PlGF and VEGF mRNA was not altered by the HT treatment. Both TN cotyledonary VEGFR-1 and VEGFR-2 mRNA expression levels rose significantly over the period studied (P< 0.05 and P< 0.01 respectively). Receptor mRNA concentration in HT cotyledonary tissue was significantly reduced at 90 dpc (VEGFR-1; TN 0.21+/-0.02 vs HT 0.11+/-0.01 VEGFR-1/actin, P< 0.05, VEGFR-2; TN 0.18+/-0.05 vs HT 0.07+/-0.01 VEGFR-2/actin, P< 0.01). Soluble VEGFR-1 (sVEGFR-1) mRNA was not detected in these tissues. These alterations in growth factor and growth factor receptor mRNA expression, as a result of environmental heat stress early in placental development, could impair normal placental vascular development. Furthermore, alterations in VEGF, VEGFR-1 and VEGFR-2 mRNA expression, during the period of maximal placental growth, may contribute to the development of placental insufficiency, and ultimately intrauterine growth restriction.
Subject(s)
Endothelial Growth Factors/metabolism , Fetal Growth Retardation/veterinary , Lymphokines/metabolism , Placenta/metabolism , Placental Insufficiency/veterinary , Proteins/metabolism , Receptors, Growth Factor/metabolism , Adult , Animals , Disease Models, Animal , Endothelial Growth Factors/genetics , Female , Fetal Growth Retardation/metabolism , Gestational Age , Humans , Lymphokines/genetics , Membrane Proteins , Pregnancy , Proteins/genetics , RNA, Messenger/metabolism , Receptors, Growth Factor/genetics , Reverse Transcriptase Polymerase Chain Reaction , Sheep/physiology , Species Specificity , Vascular Endothelial Growth Factor A , Vascular Endothelial Growth FactorsABSTRACT
Pregnant ewes were exposed chronically to thermoneutral (TN; 20+/-2 degrees C, 30% relative humidity; n=8) or hyperthermic (HT; 40+/-2 degrees C 12 h/day, 35+/-2 degrees C 12 h/day, 30% relative humidity, n=6) environments between days 37 and 93 of pregnancy. Ewes were killed following 56 days of exposure to either environment (days in treatment (dit)), corresponding to 93+/-1 day post coitus (dpc). Maternal core body temperatures (CBT) in HT ewes were significantly elevated above the TN ewes (HT; 39.86+/-0.1 degrees C vs TN; 39.20+/-0.1 degrees C; P<0.001). Both groups of animals displayed circadian CBT, though HT ewes had elevated amplitudes (HT; 0.181+/-0.002 degrees C vs TN; 0.091+/-0.002 degrees C; P<0.001) and increased phase shift constants (HT; 2100 h vs TN; 1800 h; P<0.001). Ewes exposed to chronic heat stress had significantly reduced progesterone and ovine placental lactogen (oPL) concentrations from 72 and 62 dpc respectively (P<0.05), corresponding to approximately 30 dit. However, when compared with the TN ewes, HT cotyledonary tissue oPL mRNA and protein concentrations were not significantly different (P>0.1). Prolactin concentrations rose immediately upon entry into the HT environment, reaching concentrations approximately four times that of TN ewes, a level maintained throughout the study (HT; 216.31+/-32.82 vs TN; 54. 40+/-10.0; P<0.0001). Despite similar feed intakes and euglycemia in both groups of ewes, HT fetal body weights were significantly reduced when compared with TN fetuses (HT; 514.6+/-48.7 vs TN; 703. 4+/-44.8; P<0.05), while placental weights (HT; 363.6+/-63.3 vs TN; 571.2+/-95.9) were not significantly affected by 56 days of heat exposure. Furthermore, the relationship between body weight and fetal length, the ponderal index, was significantly reduced in HT fetuses (HT; 3.01+/-0.13 vs TN; 3.57+/-0.18; P<0.05). HT fetal liver weights were also significantly reduced (HT; 27.31+/-4.73 vs TN; 45.16+/-6.16; P<0.05) and as a result, the brain/liver weight ratio was increased. This study demonstrates that chronic heat exposure lowers circulating placental hormone concentrations. The observation that PL mRNA and protein contents are similar across the two treatments, suggests that reduced hormone concentrations are the result of impaired trophoblast cell development, specifically trophoblast migration. Furthermore, the impact of heat exposure during maximal placental growth is great enough to restrict early fetal development, even before the fetal maximal growth phase (100 dpc-term). These data highlight that intrauterine growth retardation (IUGR) may result primarily from placental trophoblast cell dysfunction, and secondarily from later reduced placental size.