Estrogen acutely stimulates nitric oxide synthase activity in fetal pulmonary artery endothelium

Estrogen (E) has nitric oxide (NO)-mediated effects in certain vascular beds, and fetal E levels rise acutely with parturition, suggesting that E may be involved in NO-mediated pulmonary vasodilation at birth. We tested the hypothesis that E acutely stimulates NO synthase (NOS) activity in ovine fetal pulmonary artery endothelial cells (PAEC) by measuring L-[³H]arginine conversion to L-[³H]citrulline in intact cells. NOS activity in the presence of 17β-estradiol (E₂β) rose in a dose-dependent manner, increasing 70- 100%, with a threshold concentration of 10^-10 M. This effect was detectable within 5 min of E₂β exposure, and the maximal response was comparable to that obtained with acetylcholine, which had a threshold concentration of 10^-8 M. Ca²⁺ removal completely inhibited E₂β- stimulated NOS activity, and activity with E₂β and the Ca²⁺ ionophore A- 23187 was not additive: In addition, the expression of the endothelial isoform of NOS (eNOS) was not altered, and the inducible and neuronal NOS isoforms were not detected by immunoblot analysis. These findings indicate that E₂β acutely stimulates eNOS by Ca²⁺ influx. Furthermore, E₂β- stimulated NOS activity was fully inhibited by the E receptor (ER) antagonists tamoxifen and ICI-182,780, and ER mRNA expression was evident in reverse transcription-polymerase chain reaction assays. Thus E acutely stimulates eNOS activity in fetal PAEC via the activation of endothelial ER and increases in intracellular Ca²⁺.