ROLE of POSITION-9 and POSITION-10 in the ENDOTHELIN MOLECULE for BIOLOGICAL-ACTIVITY and DISCRIMINATION of RECEPTOR SUBTYPES

Abstract

The importance of residues 9 and 10 in endothelin-1 was assessed by studying the responses of the guinea-pig ileum to [Ala(9)]endothelin-1 and [Ala(10)]endothelin-1. Both analogues induced relaxation followed by contraction. [Ala(9)]Endothelin-1 showed similar ED(50) values and maximum response to those of endothelin-1, whereas [Ala(10)]endothelin-1 showed a larger ED(50) value and was a partial agonist. Endothelin-1 and [Ala(10)]endothelin-1 induced similar degrees of tachyphylaxis, whereas [Ala(9)]endothelin-1 induced very little tachyphylaxis, indicating that Lys(9) is important for inducing tachyphylaxis. Apamin inhibited the relaxation induced by endothelin-1 and [Ala(9)]endothelin-1 but not that induced by [Ala(10)]endothelin-1. BQ-123 (cyclo[D-Trp-D-Asp-Pro-D-Val-Leu), a specific endothelin ET(A) receptor antagonist, inhibited [Ala(9)]endothelin-1-, but not [Ala(10)]endothelin-1-induced contraction. Cross-tachyphylaxis and additivity studies indicated that [Ala(9)]endothelin-1, like endothelin-1, acts at the endothelin ET(A) receptor, whereas [Ala(10)]endothelin-1 behaved as an endothelin ET(B) receptor agonist, like sarafotoxin S6c. Thus, the residue at position 10 plays a significant role in receptor activation and is a candidate for further exploration of receptor antagonism.