Swelling of astrocytes causes membrane potential depolarization

Rat brain astrocytes growing in primary monolayer cultures were swollen by exposing them to media of decreasing osmolality caused by removal of NaCl, and the effects of this treatment on their membrane potentials were measured by intracellular recording. Depolarizations were seen that were proportional to the degree of swelling, reaching a maximum of around 60 mV when 80-100 mM NaCl was removed from the reaction media, which had an original total osmolality of 290 mosmolal. These effects were completely reversible, since restoring the cells to iso-osmotic medium after a 2-min exposure caused an immediate repolarization back to the original membrane potentials, and depolarizations were not seen when isotonicity was maintained by replacing NaCl with sucrose. Partial repolarization was seen during an extended period (30 min) of exposure to hypo-osmotic medium, mirroring a regulatory volume decrease we have previously described in these cells under identical conditions. In ion-replacement studies depolarizations were seen when the solution was made hypo-osmotic with the large cation N-methyl-D-glucamine totally replacing Na+ in the medium. Removal of Cl- from the medium also had no effect on the initial swelling-induced depolarization. These results show that even moderate swelling of astrocytes in primary culture results in marked depolarization of their membrane potentials; possible mechanisms for this effect and the potentially profound implications for the swelling of astrocytes seen in situ under pathological conditions are discussed.