Maternal prostacyclin, thromboxane, and placental blood flow

The vasoactive prostanoids--prostacyclin (PGI2) and thromboxane A2 (TxA2)--and their metabolites--6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TxB2), respectively--have been implicated as regulators of uteroplacental blood flow in animals. To study their roles in placental blood flow in human beings, plasma or serum samples were collected for the measurement of 6-keto-PGF1 alpha and TxB2 from 42 women during late pregnancy on the same occasion, when placental intervillous blood flow (IVBF) was determined with 133Xe isotope method. The concentrations of 6-keto-PGF1 alpha in plasma or those of TxB2 in plasma or serum were not related to the IVBF. The intravenous infusion of ritodrine for 1 hour up to the dose of 200 micrograms/min increased (p less than 0.05) the plasma 6-keto-PGF1 alpha concentrations, but caused no changes in the TxB2 levels or IVBF in seven women with premature uterine contractions. We conclude that if PGI2 and TxA2 participate in the control of placental blood flow, their changes are located in the fetoplacental unit and, thus, are not reflected by the levels of their metabolites in maternal circulation, and ritodrine may stimulate PGI2 synthesis in human beings.