NOVEL ENDOTHELIN RECEPTORS IN THE FOLLICULAR MEMBRANES OF XENOPUS-LAEVIS OOCYTES MEDIATE CALCIUM RESPONSES BY SIGNAL-TRANSDUCTION THROUGH GAP-JUNCTIONS

Follicular oocytes from Xenopus laevis display saturable and high affi nity endothelin (ET)-l 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 ET(A) rece ptor-selective antagonist BQ123 [cyclo(D-Trp,D-Asp,L-Pro,D-Val,L-Leu)] failed to inhibit I-125-ET-1 binding to these receptors, suggesting t hat these receptors belong to a novel subtype of ET(A) receptors (ET(A X)) distinct from the human ET(A) receptors. These endogenous receptor s are present in the follicular layer, because defolliculated oocytes did not show any I-125-ET-1 binding. Addition of ET-1 to follicular oo cytes led to Ca2+ mobilization, which was reversibly blocked by treatm ents that uncouple gap junctions, suggesting that these ET(AX) recepto rs mediate their response by transferring signals through gap junction s, On the other hand, the expressed human ET(A) receptor-mediated Ca2 mobilization was not blocked by inhibitors of gap junctions. In agree ment with the binding data, the endogenous ET(AX) receptor-mediated re sponse was not inhibited by BQ123 even at 100 nm, whereas the expresse d human ET(A) receptor-mediated response was inhibited by 50% at conce ntrations as low as 10 nM. This further confirms that the amphibian ET (AX) receptors are different from mammalian ET(A) receptors. Finally, ET-1 enhanced the rate of progesterone-induced maturation of follicula r oocytes, implying the involvement of these endogenous ET(AX) recepto rs in an in vivo maturation process.