Effects of estradiol on ischemic factor-induced astrocyte swelling and AQP4 protein abundance

In the early hours of ischemic stroke, cerebral edema forms as Na, Cl, and water are secreted across the blood-brain barrier (BBB) and astrocytes swell. We have shown previously that ischemic factors, including hypoxia, aglycemia, and arginine vasopressin (AVP), stimulate BBB Na-K-Cl cotransporter (NKCC) and Na/H exchanger (NHE) activities and that inhibiting NKCC and/or NHE by intravenous bumetanide and/or HOE-642 reduces edema and infarct in a rat model of ischemic stroke. Estradiol also reduces edema and infarct in this model and abolishes ischemic factor stimulation of BBB NKCC and NHE. There is evidence that NKCC and NHE also participate in ischemia-induced swelling of astrocytes. However, little is known about estradiol effects on astrocyte cell volume. In this study, we evaluated the effects of AVP (100 nM), hypoxia (7.5% O(2)), aglycemia, hypoxia (2%)/aglycemia [oxygen glucose deprivation (OGD)], and estradiol (1-100 nM) on astrocyte cell volume using 3-O-methyl-d-[(3)H]glucose equilibration methods. We found that AVP, hypoxia, aglycemia, and OGD (30 min to 5 h) each significantly increased astrocyte cell volume, and that estradiol (30-180 min) abolished swelling induced by AVP or hypoxia, but not by aglycemia or OGD. Bumetanide and/or HOE-642 also abolished swelling induced by AVP but not aglycemia. Abundance of aquaporin-4, known to participate in ischemia-induced astrocyte swelling, was significantly reduced following 7-day but not 2- or 3-h estradiol exposures. Our findings suggest that hypoxia, aglycemia, and AVP each contribute to ischemia-induced astrocyte swelling, and that the edema-attenuating effects of estradiol include reduction of hypoxia- and AVP-induced astrocyte swelling and also reduction of aquaporin-4 abundance.