The aim of the present study was to assess the role of superoxide anions in the relaxation induced by acetylcholine (ACh) in aortic segments from male rats, and to investigate if their production is altered by sex hormone deprivation. In segments precontracted with 10 nmol/l noradrenaline, ACh (0.1 nmol/–10 μ mol/l) induced concentration-dependent relaxation, which was greater in segments from castrated compared with control animals. ACh-induced relaxation was abolished in segments from control rats, and reduced in those from castrated rats, by 0.1 mmol/l N G -nitro- L -arginine methyl ester ( L -NAME; a nitric oxide synthase inhibitor). Indomethacin (1 μ mol/l; a cyclo-oxygenase blocker) decreased the ACh-induced relaxation in arteries from control males only. Incubation of segments with superoxide dismutase (SOD; 100 units/ml; a superoxide anion scavenger) enhanced and reduced relaxation in segments from control and castrated animals respectively. For arteries from castrated animals, the presence of SOD plus L -NAME abolished such responses. In these arteries, incubation with L -NAME abolished the relaxation caused by ACh when the segments were precontracted with 30 mmol/l KCl. In segments obtained from castrated rats and pretreated with L -NAME, 1 mmol/l tetraethylammonium or 0.4 μ mol/l charybdotoxin [blockers of Ca 2+ -sensitive and large-conductance Ca 2+ -sensitive (BK Ca ) K + channels respectively] abolished the relaxation induced by ACh. These results suggest that ACh generates endothelial NO and superoxide anions from the arterial wall in both control and castrated animals; these agents negatively modulate ACh-induced relaxation in control rats by destruction of NO, and positively modulate ACh-induced relaxation in castrated rats by activation of BK Ca channels.
The aim of this study was to determine the possible influence of sex hormones on the contractile responses induced by clonidine, an agonist of α 2 -adrenoceptors, as well as the endothelial modulation of these responses. For this purpose, thoracic aorta segments from male (control and castrated) and female (in oestrous phase and ovariectomized) rats were used. In intact segments from the four groups of rats, clonidine (0.01-10 µ mol/l) induced concentration-dependent contractions, which were increased by the nitric oxide synthase inhibitor N ω -nitro-⌊-arginine methyl ester (0.1 mmol/l) or by endothelium removal, but were reduced by 1 µ mol/l yohimbine (an α 2 -adrenoceptor antagonist) in all animals and by 1 µ mol/l indomethacin (a cyclo-oxygenase inhibitor) in control males only. The rank order of the magnitude of the maximal response was: oestrous females > ovariectomized females > control males > castrated males, whereas the sensitivity to clonidine (EC 50 value) was similar in all animals. In endothelium-denuded segments, the rank order was: oestrous females = control males > ovariectomized females = castrated males. These results suggest that: (1) the presence of oestrogen or androgen increases the contraction caused by α 2 -adrenoceptor activation with clonidine; (2) endothelium negatively modulates the response to this agonist in the four groups of rats, due to endothelial NO release (entirely in females and in part in males); (3) androgen also seems to modulate the response by stimulating the release of an endothelial contracting factor, probably a prostanoid; and (4) the endothelium of males has a greater capacity than that of comparable females for negative regulation of the tension generated by the underlying vascular smooth muscle.