Autacoids affecting vascular tone in the human fetal extracorporeal circulation

Abstract

1. The human fetal extracorporeal circulation is normally a vasodilated, low pressure system. 2. As this vasculature lacks innervation, autacoids have been postulated as being of great importance in controlling its tone. 3. This has now been confirmed by pharmacological in vitro techniques, particularly utilizing perfusion of the isolated umbilical cord and placental lobule. 4. The fetal umbilical-placental vessels are sensitive to a wide range of vasoconstrictor autacoids, some of which can cause intense vasospasm. 5. Thromboxane A2 receptors have been identified in both umbilical vein and placental villous vessels. 6. Prostacyclin and endothelial cell-derived relaxing factor (or nitric oxide) may be largely responsible for the low vascular resistance normally found in the fetal extracorporeal circulation. 7. Immediately after birth it is likely that stimuli such as cooling, stretching and handling of the umbilical cord cause release of eicosanoids and other autacoids from the vessels, leading to their complete closure.