Follicular oocytes from Xenopus laevis display saturable and high affinity endothelin (ET)-1 binding sites. Competition binding experiments using unlabeled ET-1, ET-3, and sarafotoxin 6c indicated that these ET receptors belong to the ETA receptor subtype. However, the ETA receptor-selective antagonist BQ123 [cyclo(D-Trp,D-Asp,L-Pro,D-Val,L-Leu)] failed to inhibit 125I-ET-1 binding to these receptors, suggesting that these receptors belong to a novel subtype of ETA receptors (ETAX) distinct from the human ETA receptors. These endogenous receptors are present in the follicular layer, because defolliculated oocytes did not show any 125I-ET-1 binding. Addition of ET-1 to follicular oocytes led to Ca2+ mobilization, which was reversibly blocked by treatments that uncouple gap junctions, suggesting that these ETAX receptors mediate their response by transferring signals through gap junctions. On the other hand, the expressed human ETA receptor-mediated Ca2+ mobilization was not blocked by inhibitors of gap junctions. In agreement with the binding data, the endogenous ETAX receptor-mediated response was not inhibited by BQ123 even at 100 nM, whereas the expressed human ETA receptor-mediated response was inhibited by 50% at concentrations as low as 10 nM. This further confirms that the amphibian ETAX receptors are different from mammalian ETA receptors. Finally, ET-1 enhanced the rate of progesterone-induced maturation of follicular oocytes, implying the involvement of these endogenous ETAX receptors in an in vivo maturation process.