Estrogen regulates myogenic tone in pressurized cerebral arteries by enhanced basal release of nitric oxide.

Second-order middle cerebral arteries (135.0 +/- 4.6 microns ID) from male, female, ovariectomized female (no endogenous estrogen), and estrogen-treated ovariectomized female Sprague-Dawley rats were harvested and mounted in a pressure myograph. Myogenic response was recorded over a pressure range of 10-100 mmHg and was repeated in the presence of N omega-nitro-L-arginine methyl ester (L-NAME; 2 x 10(-4) M, an inhibitor of nitric oxide (NO) synthase, and after endothelium removal, to examine the contribution of NO to net myogenic tone. With intact endothelium, there were no differences in myogenic tone between the groups, but in the presence of L-NAME and after endothelium removal, estrogen-exposed vessels developed significantly greater tone at-high transmural pressure. There were no differences in sensitivity to sodium nitroprusside, an NO donor, or A-23187, a calcium ionophore. These results suggest an increase in basal release of NO in cerebral arteries exposed to estrogen, without change in NO sensitivity or maximally stimulated NO release.