Contribution of Na+-K+-Cl−cotransporter to high-[K+]o- induced swelling and EAA release in astrocytes

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Abstract
We hypothesized that high extracellular K+ concentration ([K+]o)-mediated stimulation of Na+-K+-Cl− cotransporter isoform 1 (NKCC1) may result in a net gain of K+ and Cl−and thus lead to high-[K+]o-induced swelling and glutamate release. In the current study, relative cell volume changes were determined in astrocytes. Under 75 mM [K+]o, astrocytes swelled by 20.2 ± 4.9%. This high-[K+]o-mediated swelling was abolished by the NKCC1 inhibitor bumetanide (10 μM, 1.0 ± 3.1%; P < 0.05). Intracellular36Cl− accumulation was increased from a control value of 0.39 ± 0.06 to 0.68 ± 0.05 μmol/mg protein in response to 75 mM [K+]o. This increase was significantly reduced by bumetanide ( P < 0.05). Basal intracellular Na+ concentration ([Na+]i) was reduced from 19.1 ± 0.8 to 16.8 ± 1.9 mM by bumetanide ( P < 0.05). [Na+]i decreased to 8.4 ± 1.0 mM under 75 mM [K+]o and was further reduced to 5.2 ± 1.7 mM by bumetanide. In addition, the recovery rate of [Na+]i on return to 5.8 mM [K+]o was decreased by 40% in the presence of bumetanide ( P < 0.05). Bumetanide inhibited high-[K+]o-induced 14C-labeledd-aspartate release by ∼50% ( P < 0.05). These results suggest that NKCC1 contributes to high-[K+]o-induced astrocyte swelling and glutamate release.