Endothelin derivatives showing potent effects in the guinea pig trachea

Endothelin (ET-1) was synthesized by solid-phase peptide synthesis. To prevent side reactions with tryptophan residue, a formyl group was used to protect the beta-indole moiety. The two disulfide bridges were formed selectively using S-methylbenzyl for the cysteine residues in positions 3 and 11, and S-acetamidomethyl for positions 1 and 15. Crude synthetic ET-1 was obtained after deprotection and cleavage from the resin by HF, followed respectively by potassium ferricyanide and iodine oxidations, in order to generate sequentially the monocyclic and bicyclic forms of the peptide. Finally, Trp(For) was deprotected by a treatment with piperidine. After each step, a sample of peptide was isolated and purified by RP-HPLC. The biological activity of these synthetic peptides was evaluated on guinea pig tracheal strips. The results suggest that oxidation of Met-7 and/or formylation of Trp-21 do not affect the biological activity of ET-1 in the guinea pig trachea. Moreover, although a decrease of potency and intrinsic activity was observed, the monocyclic analog containing Cys(Acm) residues at positions 1 and 15 exhibited substantial constrictor activity. This result, contrasting with that observed with several vascular preparations, suggests that the guinea pig trachea contains a distinct ET receptor population.