Biophysical and pharmacological characterization of hypotonically activated chloride currents in cortical astrocytes

Rat cortical astrocytes regulate their cell volume in response to hypotonic challenge. This regulation is believed to depend largely on the release of chloride or organic osmolytes through anion channels. Using whole-cell recordings, we identified weakly outwardly rectifying chloride currents that could be activated in response to hypotonic challenge. These currents exhibited the following permeability sequence upon replacement of chloride in the bathing solution with various anions: I->NO3->Cl->Gluc(-) greater than or equal toMeS(-)>Ise(-). Interestingly, extracellular I-, albeit showing the greatest permeability, blocked the currents with an IC50 of approximate to50 mM. Currents were almost completely inhibited by 123 V,M NPPB and partially inhibited by 200 V,M niflumic acid or 200 muM DIDS. Additionally, the total number of Cl- ions effluxed through the hypotonically activated channels was markedly similar to the total solute efflux during volume regulation. We therefore propose the hypotonically activated chloride channel as a major contributor to volume regulation of astrocytes. To examine potential candidate chloride channel genes expressed by astrocytes, we employed RTPCR to demonstrate the presence of transcripts for CIC-2, 3, 4, 5, and 7, as well as for VDAC and CFTR in cultured astrocytes. Moreover, we performed immunostaining with antibodies against each of these channels and showed the strongest expression of CIC-2 and CIC-3, strong expression of CIC-5 and VDAC, weak expression of CIC-7 and very weak expression of CIC-4 and CFTR. Intriguingly, although we found at least seven Cl- channel proteins from three different gene families in astrocytes, none appeared to be active in resting cells. (C) 2004 Wiley-Liss, Inc.