ET(A) and ET(B) endothelin receptors in human myometrium characterized by the subtype selective ligands BQ123, BQ3020, FR139317 and PD151242

ETA selective (BQ123, FR139317, PD151242) and ET(B) selective (βQ3020) ligands were used to define the binding characteristics and contractile function of endothelin receptor subtypes in human myometrium. In saturation binding assays with 10 μm-thick tissue sections [¹²⁵I]endothelin-1 (ET-1) bound with a single affinity to receptors in the myometrium (K(d), 1.19 ± 0.17 nM) and adjacent endometrium (K(d) 1.39 ± 0.51 nM). Competition binding assays in myometrium revealed a heterogeneous population of receptors with BQ123 (K(d) ET(A), 1.43 ± 0.33 nM; K(d) ET(B), 39.91 ± 9.06 μM), FR139317 (K(d) ET(A) 2.54 ± 0.87 nM; K(d) ET(B) 89.79 ± 24.34 μM) and BQ3020 (K(d) ET(A), 4.57 ± 0.58 μM; K(d) ET(B), 90.07 ± 19.53 nM). The presence of these receptors in myometrium was confirmed by saturation assays with the new ET(A) selective ligand [¹²⁵I]PD151242 (K(d), 0.93 ± 0.08 nM; B(max) 138.7 ± 1.0 fmol/mg protein) and the ET(B) selective [¹²⁵I]BQ3020 (K(d), 0.62 ± 0.07; B(max) 44.5 ± 1.1 fmol/mg protein). Reverse-transcriptase PCR assays detected mRNA encoding both receptor subtypes in myometrium. Autoradiography with radiolabelled PD151242 and BQ3020 demonstrated that ET(A) receptors were the predominant subtype in the myometrium and identified a population of ET(B) receptors in the endometrium. In tissue bath experiments, an ET-1-induced increase in contractility of myometrial strips was antagonized by 10 μM FR139317 but not by BQ123 at the same concentration. The ET(B) agonist BQ3020, which is a potent agonist in animal tissue, did not increase contractility when tested at concentrations up to 2 μM. The results of this study suggest that ET(A) endothelin receptors and a smaller population of ET(B) receptors are found in human myometrium. ET(A), but not ET(B), receptors mediate myometrial contractility. The function of ET(B) receptors in human uterus remains to be elucidated.