Vascular activities of endothelin‐1 and some alanyl substituted analogues in resistance beds of the rat

1 The effects of endothelin‐1 and of three analogues in which alanyl residues had been substituted in place of cysteinyl residues were studied in the rat, isolated, Krebs‐Henseleit‐perfused mesenteric bed and the in situ, blood‐perfused, mesenteric and hindquarters circulations of the rat. The effects on the vascular responses to these peptides of removing the endothelium by detergent perfusion or of cyclo‐oxygenase inhibition by indomethacin were also determined. 2 In all three preparations, endothelin‐1 was a potent vasoconstrictor (ED50 values ranged from 40 to 400 pmol) although it was rather less potent in the hindquarters than in the mesentery. Also, the maximum response was very much smaller in the isolated mesentery (24.7 ± 2.1 mmHg) than in the in situ mesentery (81.8 ± 2.6 mmHg) or hindquarters (107 ± 10 mmHg). 3 Removal of the endothelium by perfusion with detergent significantly enhanced the potency of endothelin as a vasoconstrictor in the in situ messentery, but reduced the maximum response obtained, whereas removal of the endothelium in vitro significantly increased the maximum response without changing the ED50. The presence or absence of indomethacin had no significant effects in the blood‐perfused hindquarters preparation or the isolated mesentery but, after administration of 5 mg kg−1 indomethacin to the in situ mesenteric preparation, the maximal response to endothelin‐1 was enhanced. 4 When the preparations were preconstricted with α1‐adrenoceptor agonists, endothelin‐1 had modest vasodilator effects. These vasodilator effects were abolished when the endothelium was destroyed by detergent perfusion. 5 Both [Ala3,11]endothelin‐1 and [Ala1,15]endothelin‐1 were also vasoconstrictor agents in the mesenteric preparations but they were less potent than endothelin‐1 itself; [Ala3,11]endothelin‐l was intermediate in potency between endothelin‐1 and [Ala1,15]endothelin‐1. In the in situ preparation these analogues gave similar maximal responses to the parent peptide but, in vitro, they gave maximal responses that were much greater than that of endothelin‐1 and which were similar to those found with all 3 peptides in the in situ mesentery. Destruction of the endothelium in vitro had no effect on the responses to these 2 analogues and the log dose‐response curve for [Ala1,15]endothelin‐l in the isolated mesentery was biphasic. 6 A third analogue possessing no disulphide bridges ([Ala1,3,11,15]endothelin‐l) was a partial agonist in the in situ preparations but had no vasoconstrictor effect in the in vitro mesentery. It had no vasorelaxant effect in the hindquarters preparation but it enhanced the responses to endothelin‐1 when the 2 peptides were administered together in all 3 preparations. 7 It is concluded that it is not essential for the endothelin family of peptides to possess 2 disulphide bridges for them to be vasoconstrictor agents. However, only endothelin‐1, of the 4 peptides studied, showed either endothelium‐dependent vasorelaxant activity or modulation by the endothelium of its pressor effects. This, together with some differences in the vasoconstrictor log dose‐response curves to the peptides and the results of co‐administration of [Ala1,31115]endothelin‐1 and endothelin‐1, suggests that there may be more than one receptor type mediating vascular responses to the peptides studied.