Allosteric inhibition of endothelin ETA receptors by 3,5-dibromosalicylic acid

Derivatives of salicylic acid (SA) and benzoic acid prevent endothelin-1 (E T-1) binding to ETA receptors. This study analyzed actions of 30 derivative s of benzoic acid and salicylic acid on I-125-ET-1 binding to recombinant r at ETA receptors. The most active compounds were 3,5-dibromosalicylic acid (Br2SA, K-i = 0.5 mM) and 3,5-diiodosalicylic acid (K-i = 0.3 mM). They wer e about 50 times more potent than SA and aspirin. Br2SA inhibited equilibri um I-125-ET-1 binding in an apparently competitive manner. It accelerated 8 -fold the dissociation of I-125-ET-1 receptor complexes and did not modify the second order rate constant of association of I-125-ET-1 to its receptor s. Br2SA also decreased the affinity of ETA receptors for receptor antagoni sts BQ-123 and bosentan. Br2SA accelerated dissociation of I-125-ET-1-solub ilized ETA receptor complexes and decreased the apparent molecular size of solubilized receptors. Br2SA and 3,5-diiodosalicylic acid inhibited two cel lular actions of ET-1: the mobilization of intracellular Ca2+ stores in iso lated cells and contractions of rat aortic rings. They accelerated the rela xing action of BQ-123 and bosentan in ET-1-treated aortic rings. The result s suggest the existence of an allosteric modifier site on ETA receptors tha t recognizes selected derivatives of SA. SA derivatives might be of therape utic interest to relieve tight ET-1 binding and to favor actions of recepto r antagonists.