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In order to investigate the involvement of endothelium-derived vasoactive substances in deoxycorticosterone acetate (DOCA)-salt hypertension, the responses to noradrenaline, acetylcholine, sodium nitroprusside and papaverine were studied in the absence and presence of indomethacin. Noradrenaline was equally effective in evoking a constrictor response of aorta, with or without endothelium, isolated from DOCA-salt hypertensive rats, while in controls, noradrenaline induced higher submaximal responses in rubbed than in unrubbed preparations. A decreased response to acetylcholine, an endothelium-dependent vasodilator, was observed in aorta with endothelium which had been precontracted with noradrenaline isolated from hypertensive rats. The relaxant response was lost after removal of the endothelium in both control and DOCA-salt hypertensive groups. The response to sodium nitroprusside, an endothelium-independent agent, in aorta isolated from hypertensive rats as well as the response to papaverine, an agent partially dependent on the endothelium, was not altered. Indomethacin treatment altered the response to noradrenaline only in unrubbed aorta of hypertensive rats. In these preparations, a biphasic response to noradrenaline was observed. At lower concentrations noradrenaline induced the characteristic constrictor response, while at higher concentrations a relaxant response was obtained that was abolished by methylene blue, a guanylate cyclase inhibitor. This could indicate that noradrenaline induced the release of endothelium-derived relaxing factor (EDRF) in aorta of hypertensive rats. Furthermore, indomethacin treatment restored the decreased response to acetylcholine in aorta isolated from DOCA-salt hypertensive rats. It is suggested that DOCA-salt hypertension causes an impaired response to acetylcholine and an increased response to noradrenaline in aorta because of an alteration of some endothelial cell function which is not related to a diminished release and/or production of EDRF, but to a simultaneous release of an endothelium-derived contracting factor (EDCF) which is sensitive to indomethacin blockade.