Expression of endothelin-1 (ET-1) as well as ETA and ETB receptor mRNA in rat placentas (P), uterine placental beds (UPB), and the uterine free wall (UFW). Comparative mRNA expression was determined by real-time quantitative reverse transcription polymerase chain reaction in tissues from rats treated with NG-nitro-L-arginine-methyl ester (L-NAME; 2.5 mg/kg/h, gestational days 14-21, n = 6) or with saline vehicle (n = 6). Results are expressed as a percentage of the internal control gene, cytoplasmic β-actin, and each value represents the mean ± SEM. *P < .05 versus vehicle-treated control, by ANOVA.