Estradiol alters nitric oxide production in the mouse aorta through the alpha-, but not beta-, estrogen receptor

Although estradiol (E(2)) has been recognized to exert several vasculoprotective effects in several species, its effects in mouse vasomotion are unknown, and consequently, so is the estrogen receptor subtype mediating these effects. We investigated the effect of E(2) (80 microg/kg/day for 15 days) on NO production in the thoracic aorta of ovariectomized C57Bl/6 mice compared with those given placebo. E(2) increased basal NO production. In contrast, the relaxation in response to ATP, to the calcium ionophore A23187, and to sodium nitroprusside was unaltered by E(2), whereas acetylcholine-elicited relaxation was decreased. The abundance of NO synthase I, II, and III immunoreactive proteins (using Western blot) in thoracic aorta homogenates was unchanged by E(2). To determine the estrogen receptor (ER) subtype involved in these effects, transgenic mice in which either the ERalpha or ERbeta has been disrupted were ovariectomized and treated, or not, with E(2). Basal NO production was increased and the sensitivity to acetylcholine decreased in ERbeta knockout mice in response to E(2), whereas this effect was abolished in ERalpha knockout mice. Finally, these effects of E(2) on vasomotion required long-term and/or in vivo exposure, as short-term incubation of aortic rings with 10 nmol/L E(2) in the isolated organ chamber did not elicit any vasoactive effects. In conclusion, this study demonstrates that ERalpha, but not ERbeta, mediates the beneficial effect of E(2) on basal NO production.

Failure of L-nitroarginine to inhibit the activity of aortic inducible nitric oxide synthase

Nitric oxide (NO) is produced by a family of three isoenzymes: the endothelial, inducible and neuronal NO synthases. L-Nitroarginine methyl ester (L-NAME) is the most commonly used inhibitor of NO synthase activity. The goal of the present study was to evaluate to what extent L-nitroarginine (L-NA), the in vivo circulating metabolite of L-NAME, blocks NO production in the rat aorta depending on the NO synthase isoform expressed (and evidenced by Western blotting) and on the presence or absence of the extracellular NO synthase substrate L-arginine (100 microM, i.e. the plasma concentration). Intact [endothelium present (E+)] control aortic rings express mainly endothelial NO synthase. L-NA (30--100 microM) induced a dose-dependent contraction (due to blockade of the relaxant properties of NO) irrespective of the presence or absence of L-arginine. In deendothelialized (E-) control aortic rings, the three isoforms of NO synthase are virtually absent (as demonstrated by Western blotting) and L-NA does not elicit any contractile effect. E- aortic rings from lipopolysaccharide (LPS)-treated rats express mainly inducible NO synthase. In these rings, L-NA induced a dose-dependent (0--100 microM) contraction in the absence of extracellular L-arginine, whereas L-arginine (100 microM) completely abrogated the contractile effect of the NO synthase inhibitor. Chronic L-NAME administration (50 mg/kg/day for 4 weeks) elicited the aortic expression of inducible NO synthase, but to a lesser extent (about 5-fold) than in LPS-treated rat aorta. The average plasma concentration of L-NA was 50 +/- 10 microM in these rats. In E- rings from these L-NAME-treated rats, L-NA induced a similar contractile response (but smaller in magnitude) to that observed in LPS-treated rat aorta. Altogether, these data suggest that (1) in the presence of a physiological concentration of extracellular L-arginine, L-NA fails to inhibit inducible NO synthase, and (2) chronic L-NAME administration, at a dose commonly given to block NO production in vivo, leaves the activity of inducible NO synthase unaffected.