Endothelium‐dependent vascular activities of endothelin‐like peptides in the isolated superior mesenteric arterial bed of the rat

1 The vasoconstrictor activities of endothelin‐2, endothelin‐3, sarafotoxin S6b, human proendothelin 1–38 and mouse vasoactive intestinal contractor (VIC) were studied in the isolated Krebs‐Henseleit perfused mesenteric arterial bed of the rat in the presence and absence of the endothelium. The vasoconstrictor properties of endothelin‐1 were studied in control preparations and in preparations treated with methylene blue or N ω ‐nitro‐ l ‐arginine methyl ester (NAME). Finally, the direct vasodilator properties of endothelin‐2, endothelin‐3 and sarafotoxin S6b were studied in preparations preconstricted with methoxamine. 2 In the presence of an intact endothelium, all of the peptides caused dose‐dependent increases in perfusion pressure and sarafotoxin S6b was a full agonist relative to the other peptides studied (maximum increase in perfusion pressure, R max = 106 ± 11mmHg). Endothelin‐1, endothelin‐2 and VIC were more potent vasoconstrictors (ED 50 93.0 ± 40.0, 90.8 ± 20.5 and 106 ± 63 pmol, respectively) than endothelin‐3 and sarafotoxin S6b, which were found to be equipotent (ED 50 values 411 ± 195 and 345 ± 86 pmol, respectively). A full dose‐response relationship could not be constructed for proendothelin, but the highest dose used (4 nmol) increased the perfusion pressure by 15.4 ± 1.6mmHg. 3 Destruction of the endothelium with the zwitterionic detergent 3‐[(3‐cholamidopropyl)‐dimethylammonio]‐l‐propanesulphonate (CHAPS) significantly enhanced the pressor activity of all 5 peptides. The R max for sarafotoxin S6b was not significantly altered by removal of the endothelium but its potency was significantly increased (ED 50 = 115 ± 15 pmol). Although their R max values were significantly increased, endothelin‐2 and VIC were still partial agonists relative to sarafotoxin S6b in CHAPS‐pretreated preparations; their potencies were unchanged (ED 50 values 118 ± 53 and 416 ± 196 pmol, respectively). Removal of the endothelium significantly reduced the potency of endothelin‐3 (ED 50 , 6.3 ± 2.2 nmol) but this peptide then exhibited full agonist activity (R max = 106 ± 14 mmHg). After endothelial cell destruction, the pressor responses to proendothelin were increased; 4 nmol gave a response of 38.8 ± 5.5 mmHg. 4 Exposure of preparations to either 100 μ m NAME (R max = 42.6 ± 2.4 mmHg and ED 50 = 57.5 ± 13.7 pmol) or 10 μ m methylene blue (R max = 36.0 ± 5.1 mmHg and ED 50 = 81.5 ± 26.1 pmol) significantly enhanced the maximum pressor responses to endothelin‐1 (control: R max = 22.5 ± 2.6mmHg; ED 50 = 93.0 ± 40.0 pmol). The values in the presence of NAME or methylene blue were not significantly different from those found previously for endothelin‐1 after removal of the endothelium with CHAPS. 5 Endothelin‐2, endothelin‐3 and sarafotoxin S6b all caused vasorelaxation in preparations which had been precontracted with 100 μ m methoxamine. This action was endothelium‐dependent as it was abolished by perfusing the mesentery with CHAPS. Endothelin‐3 and sarafotoxin S6b caused relaxation at much lower doses than were needed with endothelin‐1 and endothelin‐2. 6 The endothelium significantly modulates the vasoconstrictor activity of all the endothelin‐like peptides studied, including the precursor peptide proendothelin (which was the least potent of the peptides). This modulation is likely to be due to the release of endothelium‐derived relaxing factor, since similar results to destruction of the endothelium were obtained when endothelin‐1 was investigated in the presence of either methylene blue or NAME (an inhibitor of nitric oxide formation) in the perfusion fluid. The vasodilator effects of the peptides were also endothelium‐dependent. There was a different order of potency for vasoconstriction and vasodilatation supporting the suggestion that there are sub‐types of receptor for the endothelin‐like peptides in the vasculature; one type on the vascular smooth muscle and a second type on the endothelium.