Homologous competitive binding of endothelin-1 (ET-1) and ET-3 by ETA and ETB receptors in rat placenta (P), uterine placental bed (UPB), and uterine free wall (UFW). Placental membranes were prepared from vehicle-treated (n = 6) and L-NAME-treated (2.5 mg/kg/h, gestational days 14-21, n = 6) pregnant rats. (A, C, E) Representative binding curves for P, UPB, and UFW membrane preparations, respectively, from vehicle-treated rats. (B, D, F) Representative binding curves for P, UPB, and UFW membrane preparations, respectively, from L-NAME-treated rats. Each experiment was performed in triplicate. (G) The maximum binding coefficient, Bmax, for both receptors is expressed as fmoles endothelin/μg protein, and each value represents the mean ± SEM. ET in captions and axis labels refers to both ET-1 and ET-3. **P < .01 versus vehicle-treated control by ANOVA.