Nitric-Oxide-Dependent Pial Arteriolar Dilation in the Female Rat: Effects of Chronic Estrogen Depletion and Repletion

Abstract

In this study, we compared endothelial nitric oxide synthase (eNOS)-mediated cerebral vasodilating responses in intact female rats, chronically ovariectomized (OVX) rats, and OVX rats treated for 2 weeks with 17β-estradiol (E2). Under anesthesia, using intravital microscopy and a closed cranial window system, pial arteriolar diameter changes were monitored during sequential cortical suffusions of an eNOS-dependent dilator [acetylcholine (ACh)] and a direct NO donor [S-nitrosoacetylpenicillamine (SNAP)]. In separate rats from the same groups, we compared eNOS and caveolin-1 (CAV-1) protein abundance in pial arterioles (via immunofluorescence analyses). In untreated and low-dose E2-treated (1.0 μg · kg−1 · day−1) OVX rats, ACh-induced vasodilations were virtually absent. High-dose E2 treatment (100 μg · kg−1 · day−1) restored ACh-induced pial arteriolar dilations to levels seen in intact females. The vasodilations elicited by SNAP and ADO were unaffected by chronic estrogen changes, indicating no direct estrogen influence on vascular smooth muscle (VSM) reactivity. Pial arteriolar eNOS protein abundance was diminished by ovariectomy and restored by high-dose E2 treatment. Pial arteriolar CAV-1 expression was higher in OVX versus intact and E2-treated OVX females. These results suggest that long-term changes in estrogen directly influence brain eNOS functional activity. The estrogen-related changes in eNOS-dependent vasodilating function appear to be related, in part, to a capacity for E2 to increase eNOS protein expression and, in part, to an E2-associated diminution in endothelial CAV-1 expression.