A MINIMAL ROLE FOR ENDOTHELIAL NITRIC OXIDE IN VASOCONSTRICTOR AND VASODILATOR RESPONSES OF GUINEA-PIG AORTA

Abstract

Objective: Acetylcholine (Ach) dilates rat isolated aorta by releasing nitric oxide (NO) from the vascular endothelium. The vasoconstrictors, U46619 (thromboxane mimetic), phenylephrine (α-adrenoceptor agonist) and β-phenylethylamine (PEA) (trace amine associated receptor (TAAR) agonist), also release NO causing opposing vasodilatation. The roles of NO and endothelium in these vasoconstrictor responses were examined in guinea-pig aorta, along with the status of the endothelium. Design and method: Isometric contractions of guinea-pig aortic rings immersed in Kreb's solution gassed with 5% CO2 in O2 at 37 ± 0.5oC were recorded. Aortae were either intact or denuded of endothelium by rubbing. Cumulative concentration-response curves (CRC) for acetylcholine chloride, β-phenylethylamine hydrochloride (PEA) or (-)-phenylephrine HCl were obtained. Contractions were expressed as a percentage of the contraction to KCl (60 mM). Responses were compared by Student's paired t-test, P less than 0.05 indicating significance. Results: Contractions to phenylephrine and PEA were not significantly different in endothelium intact and denuded aortae. The maxima for phenylephrine were 80.4 ± 3.0 (n = 11) and 77.1 ± 8.1% (n = 6), respectively and the maxima for PEA were 71.7 ± 3.6 (n = 22) and 60.2 ± 3.4% (n = 17), respectively. Contractions to U46619 (1 μM) were also not significantly different in intact (1.32 ± 0.14 g n = 4) and denuded tissues (1.24 ± 0.38 (n = 12). We tested whether endothelium was present in intact tissues by adding Ach to U46619-pre-contracted aortae (1 or 0.5 μM). Of 16 intact tissues, only 6 responded with small falls in tension (0.07 ± 0.02 g), representing only 7.11 ± 3.04% of the contraction to U46619 (1.33 ± 0.27 g). Sodium nitroprusside, however, relaxed U46619-precontracted (1 μM) aorta reaching a maximum of 97.3 ± 5.1% of the U46619 contraction (1.58 ± 0.21 g, n = 4). No denuded preparations responded to Ach. Ach in non-contracted intact aortae produced small dose-related contractions. After washout (x2), tissues were raised from the bath and the endothelium removed before returning to the bath for a second CRC, which was potentiated, the maximum increasing from 25.1 ± 10.5% to 48.9 ± 21.5% (n = 4). Phenylephrine contractions also increased after rubbing to remove endothelium. Conclusions: Intact guinea-pig aortae displayed poor relaxations to Ach suggesting a minimal role of endothelial muscarinic responses. In uncontracted aortae, Ach caused constriction which was potentiated when repeated after endothelium removal, indicating a minor opposing endothelium-dependent vasodilatation.