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Corticosterone decreases the activity of rat glutamate transporter type 3 expressed in Xenopus oocytes
Authors:Maehwa Kang  Junghee Ryu  Jin-Hee Kim  Hyoseok Na  Zhiyi Zuo  Sang-Hwan Do
Institution:1. Department of Orthopaedic Surgery, Juntendo University Shizuoka Hospital, 1129 Nagaoka, Izunokuni,Shizuoka 410-2295, Japan;2. Department of Surgery, Juntendo University Shizuoka Hospital, 1129 Nagaoka, Izunokuni, Shizuoka 410-2295, Japan;3. Department of Orthopaedic Surgery, Juntendo University School of Medicine, 2-1-1 Hongo, Bunkyo, Tokyo 113-8421, Japan
Abstract:Glucocorticoids can increase the extracellular concentrations of glutamate, the major excitatory neurotransmitter. We investigated the effects of corticosterone on the activity of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1; also called excitatory amino acid transporter type 3 EAAT3]), and the roles of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) in regulating these effects. Rat EAAC1 was expressed in Xenopus oocytes by injecting mRNA. l-Glutamate (30 μM)-induced membrane currents were measured using the two-electrode voltage clamp technique. Exposure of these oocytes to corticosterone (0.01–1 μM) for 72 h decreased EAAC1 activity in a dose-dependent fashion, and this inhibition was incubation time-dependent. Corticosterone (0.01 μM for 72 h) significantly decreased the Vmax, but not the Km, of EAAC1 for glutamate. Furthermore, pretreatment of oocytes with staurosporine, a PKC inhibitor, significantly decreased EAAC1 activity (1.00 ± 0.06 to 0.70 ± 0.05 μC; P < 0.05). However, no statistical differences were observed between oocytes treated with staurosporine, corticosterone, or corticosterone plus staurosporine. Similar patterns of responses were achieved by chelerythrine or calphostin C, other PKC inhibitors. Phorbol-12-myristate-13-acetate (PMA), a PKC activator, inhibited corticosterone-induced reduction in EAAC1 activity. Pretreating oocytes with wortmannin or LY294002, PI3K inhibitors, also significantly reduced EAAC1 activity, but no difference was observed between oocytes treated with wortmannin, corticosterone, or wortmannin plus corticosterone. The above results suggest that corticosterone exposure reduces EAAC1 activity and this effect is PKC- and PI3K-dependent.
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