Abstract 291 Poster Session I, Saturday, 5/1 (poster 141)

Nitric oxide (NO) produced by endothelial nitric oxide synthase (eNOS) contributes to the maintenance of low vascular resistance in the fetal-placental circulation. Exposure of the ovine pregnancy to heat-stress (HS) early in gestation reduces fetal and placental growth, and decrease cotyledon eNOS protein content. We hypothesize that reduction of eNOS protein contributes to abnormal vascular tone accounting for abnormal umbilical velocimetry previously shown in this model. However, eNOS protein content in maternal and fetal vessels supplying the placenta in this model has yet to be studied. Our objective was to evaluate early gestation uterine and umbilical vessel eNOS protein expression and localization in a HS ovine model of placental insufficiency and intrauterine growth restriction (IUGR). At 40 days gestation (term=147 days), 5 ewes were exposed to HS in an environmental chamber and 5 control ewes were housed in ambient conditions. Sheep were euthanized after 3 weeks of HS exposure, and fetal and placentome weights were recorded. Western blot analysis were performed on umbilical artery and vein, and uterine artery homogenates using a monoclonal antibody against eNOS. Immunohistochemistry (IHC) was used to localize eNOS on fixed sections. Fetal weights were similar between groups. However, HS placentomes weighed less than control placentomes (117.8±16.3 vs 177.8±17.3; p=0.036). There were no differences between groups for eNOS protein content in uterine or umbilical vessels. However, there was a trend for higher eNOS protein content in the umbilical artery of the HS group (p=0.08). IHC localized eNOS to the endothelium of the vessels. We conclude that 3 weeks of HS exposure early in the ovine pregnancy does not alter eNOS protein content in the uterine or umbilical vessels. The trend for an increase in eNOS protein content in the HS group is of interest given that these animals are hypoxic in later gestation and hypoxia has been shown in other models to upregulate eNOS. Other aspects of the NO-cGMP cascade, such as reduced substrate, altered O2 levels, smooth muscle cell responsiveness, or changes in other vasoactive mediators, may account for the abnormal velocimetry and blood flow in this model. We speculate that the trend for increased eNOS in the HS animals may represent an early effort by the fetus to compensate for the uteroplacental insufficiency previously shown in this model of IUGR.

(Supported in part by AAOGF)