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To assess the pharmacological profile of a novel non-peptide endothelin antagonist (50–235) at endothelin receptors in human vascular smooth muscle preparations using radiolabelled binding techniques and in vitro pharmacological assays.The antagonist was investigated for its ability to inhibit specific [125l]-endothelin-1 binding to ETA and ETB receptors using cryostat sections of media of human coronary artery. Antagonism by 50–235 (1–30 μmol/l) of endothelin-1-induced vasoconstriction in isolated preparations of human coronary artery, saphenous vein and left internal mammary artery was also determined.In coronary artery 50–235 (10-11 to 10-4mol/l) inhibited specifically bound [125l]-endothelin-1 (0.1 nmol/l) in a biphasic manner. The ratio of ETA: ETB receptor was 79:21. Increasing concentrations of 50–235 produced progressive rightwards displacements of the endothelin-1 dose-response curve in each of the three types of blood vessel. The dose-response curves were parallel and no attenuation of the maximum endothelin-1 response was observed suggesting that 50–235 was antagonizing endothelin-1 vasoconstriction in a competitive manner. The pA2 values determined by analysis of the Schild regression lines were 6.05 in coronary artery, 6.12 in saphenous vein and 6.18 in left internal mammary artery, and the slopes were not significantly different from unity.The antagonist 50–235 exhibits nanomolar affinity for human ETA receptors and 500-fold selectivity for ETA compared with ETB receptors. Its novel non-peptide structure demonstrates that the carbon-nitrogen bond is not crucial for endothelin antagonist activity, and might provide important information for the development of therapeutic agents for conditions in which endothelins may be pathophysiologically relevant.