The 4-vessel Sampling Approach to Integrative Studies of Human Placental Physiology In Vivo.

The human placenta is highly inaccessible for research while still in utero. The current understanding of human placental physiology in vivo is therefore largely based on animal studies, despite the high diversity among species in placental anatomy, hemodynamics and duration of the pregnancy. The vast majority of human placenta studies are ex vivo perfusion studies or in vitro trophoblast studies. Although in vitro studies and animal models are essential, extrapolation of the results from such studies to the human placenta in vivo is uncertain. We aimed to study human placenta physiology in vivo at term, and present a detailed protocol of the method. Exploiting the intraabdominal access to the uterine vein just before the uterine incision during planned cesarean section, we collect blood samples from the incoming and outgoing vessels on the maternal and fetal sides of the placenta. When combining concentration measurements from blood samples with volume blood flow measurements, we are able to quantify placental and fetal uptake and release of any compound. Furthermore, placental tissue samples from the same mother-fetus pairs can provide measurements of transporter density and activity and other aspects of placental functions in vivo. Through this integrative use of the 4-vessel sampling method we are able to test some of the current concepts of placental nutrient transfer and metabolism in vivo, both in normal and pathological pregnancies. Furthermore, this method enables the identification of substances secreted by the placenta to the maternal circulation, which could be an important contribution to the search for biomarkers of placenta dysfunction.

In vivo uteroplacental release of placental growth factor and soluble Fms-like tyrosine kinase-1 in normal and preeclamptic pregnancies.

BACKGROUND: Preeclampsia is characterized by maternal endothelial dysfunction, which underlies a highly diverse clinical presentation. The pathophysiologic condition remains to be unraveled fully, but interplay between factors that are released from the placenta and maternal vascular vulnerability is likely. An imbalance in circulating angiogenic factors is a prominent feature of preeclampsia; placental growth factor and soluble Fms-like tyrosine kinase 1 have been implemented as biomarkers of placental function and preeclampsia. Their test accuracies are limited in a clinical setting, which urges better insight into their production and removal. Current data suggest that placental growth factor and soluble Fms-like tyrosine kinase 1 are released from the placenta. Both the circulating levels and the placental expression are altered in preeclamptic pregnancies. However, in vivo placental release has not been determined in human pregnancies. Moreover, there is evidence that extra-placental tissues might contribute to the circulating levels placental growth factor and soluble Fms-like tyrosine kinase 1 in normal and preeclamptic pregnancies. OBJECTIVES: We aimed to study the in vivo placental release of placental growth factor and soluble Fms-like tyrosine kinase 1 by determining the uteroplacental arteriovenous differences in human pregnancies. Further, we investigated whether this release was altered in early-onset preeclampsia compared with control subjects and whether there was a release of placental growth factor and soluble Fms-like tyrosine kinase 1 from maternal systemic endothelium. STUDY DESIGN: We conducted a case-control study at Oslo University Hospital and included 23 women with preeclampsia (diagnosis <34 weeks) and 20 control subjects. During cesarean delivery, we sampled blood from 3 vessels simultaneously (uterine vein, radial artery, and antecubital vein). We determined concentrations of placental growth factor and soluble Fms-like tyrosine kinase 1 and calculated the arteriovenous differences. A possible net placental and extra-placental release was evaluated with the use of a Wilcoxon signed rank test. Differences between groups were compared by a Mann-Whitney U-test. RESULTS: The median gestational age at delivery was 33.4 weeks (Q1, 28.3; Q3, 34.4 weeks) in the preeclamptic group and 39.3 weeks (Q1, 39.0; Q3, 39.6 weeks) in the control subjects. Women with preeclampsia had lower plasma concentrations of placental growth factor and higher concentrations of soluble Fms-like tyrosine kinase 1 compared with control subjects (P<.001). There were significant uteroplacental arteriovenous differences of soluble Fms-like tyrosine kinase 1 in preeclampsia (P<.001), but not in the control subjects. The uteroplacental arteriovenous differences of placental growth factor were significant in both groups (P<.001). Despite lower concentrations of plasma placental growth factor in women with preeclampsia, the arteriovenous differences were not significantly different from normal pregnancies (P=.53), even when we corrected for placental weight (P=.79). We found no placental growth factor or soluble Fms-like tyrosine kinase 1 concentration differences between the radial artery and the antecubital vein. CONCLUSION: Our findings are consistent with a net release of soluble Fms-like tyrosine kinase 1 from the placenta in early-onset preeclampsia. This study demonstrated a placental release of placental growth factor to the maternal circulation but could not demonstrate that this release was impaired in the preeclamptic group. We could not find evidence of systemic endothelial release of placental growth factor and soluble Fms-like tyrosine kinase 1 by analyzing the arteriovenous differences in the forearm. This study contributes to the pathophysiologic understanding of preeclampsia by the use of the clinical setting to test current concepts in vivo and underscores that studies of in vivo degradation rates of placentally released compounds are needed.