Background Because of the marked difference in the incidence and severity of cardiovascular diseases between men and premenopausal women, several groups have studied the effect of sex steroids, particularly estrogen, on vascular endothelial prostacyclin (PGI2) release.No previous studies have addressed the effect of estrogen on endocardial endothelial cells (EECs), which are involved in the modulation of the myocardium and potentially in downstream pulmonary and systemic vascular tone. Furthermore, all previous studies of estrogen effects on cultured endothelial cell function have used cells grown under standard static cell culture conditions, thereby ignoring the contribution of flow, the ubiquitous environmental endothelial stimulus.Methods and Results The effect of 17 beta -estradiol pretreatment (100 ng/mL, 72 hours) on cultured sheep EEC PGI2release in response to multiple physiologically relevant stimuli was studied. EECs were grown in six-well plates (static conditions) or on microcarrier beads and perfused at a constant flow with normoxic (Po2=150 mm Hg, Pco2=35 mm Hg) or hypoxic (Po2=35 mm Hg, Pco2=35 mm Hg) Krebs solution. The stable metabolite of PGI2, 6-keto-PGF1alpha, was determined in samples from both static and perfusion experiments by direct radioimmunoassay. 17 beta -Estradiol pretreatment did not alter basal or stimulated (arachidonic acid, 1 micromole/L, 10 micromole/L; A23187, 10 micromole/L; and bradykinin, 1 micromole/L) PGI2release in static conditions. Untreated and acutely treated (100 ng/mL added to perfusate) EECs responded to flow with a time-dependent increase in PGI2release that plateaued between 60 and 100 minutes. In contrast, 17 beta -estradiol-pretreated, perfused EECs did not increase PGI2release over time. During perfusion, acute hypoxia increased PGI2release: 140+-65 (normoxia) to 296+-113 pg (hypoxia) 6-keto-PGF1alpha/mg per minute. 17 beta -Estradiol inhibited hypoxia-induced PGI2release: 296+-113 pg (untreated EECs, hypoxia) versus 159+-60 pg (17 beta -estradiol pretreated, hypoxia) 6-keto-PGF1alpha/mg per minute.Conclusions This study demonstrates for the first time an inhibitory effect of 17 beta -estradiol on flow- and acute hypoxia-induced increase in PGI2release from perfused EECs in the absence of any effect on pharmacologically stimulated PGI2release from static cultures. These effects of 17 beta -estradiol may explain in part the well-recognized gender and estrogen effects in cardiovascular diseases and highlight the importance of flow in studies of endothelial cell function. (Circulation. 1994;90:2519-2524.)