Characterization of endothelin receptors in rat renal artery in vitro

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Abstract
1 The aim of this study was to investigate the function and characteristics of endothelin receptors in rat main branch renal artery in vitro. 2 Endothelin(ET)-1 (mean EC50 = 9.8 nm) was approximately 12 fold more potent than ET-3 (mean EC50 = 120 nm) as a contractile agonist and produced a greater maximum response. In contrast, neither of the ETB receptor-selective agonists, alanine1,13,11,15] ET-1 nor sarafotoxin S6c, (0.1 nm-1 μm), induced any contractile effect, or any relaxant effect in endothelium-intact preparations pre-contracted with the thromboxane A2 mimetic, U-46619. Sarafotoxin S6c (30 nm) also failed to induce any further contraction in tissues pre-contracted with an EC50 concentration of ET-1. 3 The ETA receptor-selective antagonist, BQ123, behaved as a weak and variable antagonist of the contractile effects of ET-1 (mean pA2 estimates in the range 5.8–6.3). In contrast, BQ123 antagonized ET-3 with a potency (mean pA2 = 7.6) consistent with its affinity for ETA receptors. Co-incubation of BQ123 (3 μm) with the putative ETB receptor-selective antagonist, IRL1038 (10 μm), produced no greater antagonism of ET-1 responses than was induced by BQ123 (3 μm) alone. 4 In conclusion, ETB receptors do not appear to be present in rat main branch renal artery. The contractile effects of ET-3 in this tissue seem to be mediated by ETA receptors. While ETA receptors partly mediate the contractile effects of ET-1, these data raise the possibility that a population of novel BQ123-insensitive endothelin receptors may also contribute to this response.

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