ESTROGEN REGULATES MYOGENIC TONE IN PRESSURIZED CEREBRAL-ARTERIES BY ENHANCED BASAL RELEASE OF NITRIC-OXIDE

Second-order middle cerebral arteries (135.0 +/- 4.6 mu m ID) from mal e, female, ovariectomized female (no endogenous estrogen), and estroge n-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-o mega-nitro-L-arginine methyl ester (L-NAME; 2 x 10(-4) M), an inhibito r of nitric oxide (NO) synthase, and after endothelium removal, to exa mine the contribution of NO to net myogenic tone. With intact endothel ium, there were no differences in myogenic tone between the groups, bu t in the presence of L-NAME and after endothelium removal, estrogen-ex posed vessels developed significantly greater tone at high transmural pressure. There were no differences in sensitivity to sodium nitroprus side, an NO donor, or A-23187, a calcium ionophore. These results sugg est an increase in basal release of NO in cerebral arteries exposed to estrogen, without change in NO sensitivity or maximally stimulated NO release.