Altered calcium dynamics do not account for attenuation of endothelium-derived hyperpolarizing factor-mediated dilations in the female middle cerebral artery

Background and Purpose-The contribution of endothelium-derived hyperpolarizing factor (EDHF) to ATP-mediated dilations is significantly attenuated in the rat middle cerebral artery of intact and estrogen-treated ovariectomized (OVX) females compared with males and vehicle-treated OVX females. Since an increase in endothelial calcium appears to be a critical prerequisite in the EDHF response, we tested the hypothesis that endothelial cell intracellular calcium ([Ca2+](i)) fails to reach sufficient levels to elicit robust EDHF-mediated dilations in females and that this effect is mediated by estrogen. Methods-Vascular diameter and [Ca2+](i) were measured concomitantly in perfused middle cerebral artery segments with the use of videomicroscopy and fura 2 fluorescence, respectively. Results-In the presence of N-G-nitro-L-arginine methyl ester and indomethacin, the dilation to 10(-5) mol/L ATP was significantly reduced (P<0.05) in intact females (42+/-8%; n=6) and estrogen-treated OVX females (25+/-6%; n=9) compared with intact males (89+/-5%; n=6) and vehicle-treated OVX females (92+/-2%; n=7). Contrary to our initial hypothesis, endothelial cell [Ca2+](i) increased to comparable levels in intact females (461+/-116 nmol/L), estrogen-treated OVX females (417+/-50 nmol/L), intact males (421 77 nmol/L), and vehicle-treated OVX females (530+/-92 nmol/L). In response to luminal ATP (10(-5) mol/L), smooth muscle cell [Ca2+](i) decreased to a greater degree in males (37+/-4%; n=8) compared with females (21+/-5%; n=7) and in vehicle-treated OVX females (18+/-7%; n=7) compared with estrogen-treated OVX females (3+/-5%; n=9). Conclusions-Our data suggest that loss of a factor coupling EDHF to reduction of ionized smooth muscle cell [Ca2+](i) accounts for the attenuated EDHF-mediated dilations in the female middle cerebral artery.