Endothelin (ET) binding sites and phosphoinositide hydrolysis induced by ET peptides were studied in the nonpregnant human myometrium. Saturation binding experiments revealed that the proportion of [125I]ET-1 binding sites was 4-fold higher than that of [125I]ET-3 binding sites, whereas Kd values were not significantly different. In competition binding studies, unlabeled peptides displaced [125I]ET-1 binding with the following order of affinity ET-1 > sarafotoxin 6b > ET-3 > sarafotoxin 6c, whereas very similar Ki values were obtained with the four peptides for the displacement of [125I]ET-3 binding. Approximately 75% of [125I]ET-1 binding sites exhibited high affinity to BQ 123 ([cyclo(D-Trp,D-Asp,L-Pro,D-Val,L-Leu)]), an ETA selective antagonist. ET-1 elicited a time-dependent accumulation of [3H]inositol phosphates in myometrial explants prelabeled with myo-[3H]inositol. All the peptides examined, ET-1, ET-3, sarafotoxin 6b and sarafotoxin 6c were able to induce phosphoinositide hydrolysis in a dose-dependent manner, ET-1 being more potent than ET-3 with corresponding EC50 values of 32 +/- 12 and 441 +/- 37 nM, respectively. Sarafotoxin 6c induced a moderate stimulation of inositol phosphates accumulation. ET-1- and ET-3-induced accumulation of [3H]inositol phosphates was (40-45%) inhibited in part by 100 microM BQ 123, whereas sarafotoxin 6c response was not affected by the ETA-antagonist. All these results indicate the presence of ETA and ETB receptors coupled to phospholipase C in human myometrium. Although ET-1 and ET-3 bind to both subtypes, sarafotoxin 6c only interacts with the ETB subtype.