Dose-response curves of the uterine and placental vascular beds to prostaglandin I2

Abstract

Local infusion of prostaglandin I2 (PGI2) has been reported to dilate the uteroplacental vasculature in a dose-dependent manner. In this experiment we attempted to distinguish the placental and nonplacental (uterine) components of this response over four concentrations of PGI2. Eleven near-term sheep were chronically instrumented for determination of regional blood flows by the use of radioactive microspheres. PGI2 was administered in a retrograde manner via a branch of the middle uterine artery at 1, 3, 10, and 20 μg/min. Flows were measured before (control) and after 5-minute infusions at each of the four concentrations (test). The uterine vasculature vasodilated in response to local PGI2 infusion. The 10 μg/min dose, for example, produced a mean (±SEM) flow of 0.70 ± 0.07 ml/min/gm; the control value was 0.41 ± 0.03 ml/min/gm (p < 0.001). At 20 μg/min the test and control flows were 0.75 ± 0.16 and 0.36 ± 0.06 ml/min/gm (p < 0.05), respectively. Uterine vascular resistance fell in a dose-dependent manner as well. There was no evidence of placental vasodilation at any of the doses tested. Renal vasodilation and decreased systemic arterial pressure at higher PGI2 doses suggest a recirculation effect. We conclude that PGI2 does not dilate the placental vasculature over the dose range of 1 to 20 μg/min and that the reported vasodilation of the uteroplacental vasculature is a result of decreased resistance in the uterine vasculature alone.