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.