Endothelium-independent contractile and relaxant responses to histamine in the rabbit aorta and common carotid, mesenteric, renal, and femoral arteries☆

Abstract 1. 1. The role of the vascular endothelium in the relaxant and contractile responses to histamine of the isolated rabbit aorta; common carotid, mesenteric, renal, and femoral arteries; as well as receptor types mediating these responses were analyzed. 2. 2. Histamine (10 −8 to 10 −4 mol/l) contracted resting rings and caused a further concentration. dependent contraction of rings of the arteries precontracted by phenylephrine. 3. 3. Pyrilamine abolished the contractile response to histamine in resting rings of the arteries, whereas it reversed that response into a concentration-dependent relaxant response in precontracted rings of the arteries. The relaxant effect of histamine was abolished by metiamide, but it was not affected by sotalol and atropine. Moreover, in control experiments, the phenylephrine-induced contractions and acetylcho-line-induced relaxations were not changed by pyrilamine and metiamide, respectively. 4. 4. Endothelial removal did not influence the contractile and relaxant responses of the arteries to histamine. 5. 5. These findings indicate that, in the isolated rabbit aorta and common carotid, mesenteric, renal, and femoral arteries, the contractile effect of histamine resulting from the activation of H 1 receptors overcomes its relaxant effect resulting from the activation of H 2 receptors. The effects of histamine are neither mediated nor modulated by the endothelial cells.