Ovine placental vascular response to the local application of prostacyclin

Rankin et al. have shown that prostacyclin does not dilate the ovine placenta, yet Clark et al. have shown that retrograde infusion of prostacyclin dilates the uterine vasculature. To determine the effects of prostacyclin on the sheep placenta, the two groups have collaborated. In five chronically catheterized near-term sheep, blood flows were measured by the radioactive microsphere technique. Control blood flows were measured and prostacyclin infused into the retrograde uterine artery catheter at 10 micrograms/min. The blood flows were again measured, and the area served by the catheter was then delineated by the injection of a green dye at the time of maternal euthanasia. The placenta and cotyledons were separated and the kidneys were obtained for assay. The systemic effects of prostacyclin were minimal and have been observed previously. The resistance of the myometrium fell from 566 +/- 99.9 to 322 +/- 63.9 mm Hg X ml-1 X min X gm (p less than 0.002). The placental tissue showed no change in resistance. When the tissues were combined, we observed a decrease in resistance from 0.38 +/- 0.18 to 0.32 +/- 0.14 mm Hg X ml-1 X min (p less than 0.004), thereby confirming the observation that there was indeed vasodilation downstream. However, the vasodilation was not in the placenta but in the myometrium, and we reconfirm our previous conclusion that prostacyclin does not dilate the sheep placenta.