Optical imaging reveals cation--Cl(-) cotransporter-mediated transient rapid decrease in intracellular Cl(-) concentration induced by oxygen--glucose deprivation in rat neocortical slices.

In brain slices from young (postnatal day (P) 10--15) rat somatosensory cortex, real-time neuronal intracellular Cl(-) concentration (Cl(-)) recordings were made by an optical technique measuring 6-methoxy-N-ethlquinolinium iodide (MEQ) fluorescence. Oxygen--glucose deprivation (in vitro model of ischemia) induced a long-lasting Cl(-) increase preceded by a rapid, transient Cl(-) decrease that could not be inhibited by blockers of Cl(-) pumps, Cl(-) channels, or Cl(-) antiporters, but was sensitive to cation-Cl(-) cotransporter inhibitors (bumetanide and furosemide). Use of low external Na(+) or high external K(+) revealed that the Na(+),K(+)-2Cl(-) cotransporter was inhibited by bumetanide and furosemide, whereas the K(+)-Cl(-) cotransporter was preferentially inhibited by furosemide under our experimental conditions. With a reduced inward driving force for Na(+) (reducing Na(+),K(+)-2Cl(-) cotransport), the transient Cl(-) decrease was only rarely induced by oxygen-glucose deprivation. In contrast, with a reduced outward driving force for K(+) (reducing K(+)-Cl(-) cotransport), the transient Cl(-) decrease still occurred. These results suggest that the transient Cl(-) decrease was primarily mediated by a rapid inhibition of the inwardly directed Na(+),K(+)-2Cl(-) cotransporter. Reverse transcriptase-polymerase chain reaction (RT-PCR) experiments suggested that the isoform involved is NKCC1. We hypothesize that the initial rapid Cl(-) efflux might effectively delay the irreversible Cl(-) influx that mediates neuronal injury.