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

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Abstract
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− ≥MeS−>Ise−. Interestingly, extracellular I−, albeit showing the greatest permeability, blocked the currents with an IC50 of ≈50 mM. Currents were almost completely inhibited by 123 μM NPPB and partially inhibited by 200 μM niflumic acid or 200 μM 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 RT‐PCR to demonstrate the presence of transcripts for ClC‐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 ClC‐2 and ClC‐3, strong expression of ClC‐5 and VDAC, weak expression of ClC‐7 and very weak expression of ClC‐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.