CONVERSION OF BIG ENDOTHELIN-1 IN RAT UTERUS CAUSES CONTRACTION MEDIATED BY ETA RECEPTORS

Like endothelin-1 (ET-1), its immediate human precursor big ET-1 (1-10 0 nM) increased the rate of spontaneous phasic contractions and caused graded tonic contractions of isolated rat uterus strips. The tonic co ntraction to big ET-1 (10 nM) was markedly blocked by phosphoramidon ( 100 muM), which did not modify the response to an equipotent concentra tion of ET-1 (3 nM). Responses to big-ET-1 (30 nM) were abolished in c alcium-free medium, but those to ET-1 (10 nM) were only reduced by thi s condition. The EC50 of big ET-1 for inducing tonic contraction was o nly sevenfold greater than that of ET-1, and both peptides produced a maximal response similar to that evoked by KCl 80 mM. ET-3 was much le ss potent. The selective ET(A) receptor antagonist BQ-123 (40-600 nM) caused graded rightward shifts of the ET-1 curve without affecting the maximal response, yielding a Schild plot with a slope not different f rom unity and a pA2 Value of 7.76. BQ-123 (100 nM) did not affect cont ractions induced by oxytocin (5 nM), acetylcholine (3 muM), or bradyki nin (0.3 nM), but inhibited responses to both big ET-1 and ET-1. There fore, the rat uterus contains a phosphoramidon-sensitive, calcium-depe ndent endothelin-converting enzyme that readily converts big ET-1 into ET-1, which then contracts the myometrium via activation of ET(A) rec eptors.