Whole-cell chloride currents in rat astrocytes accompany changes in cell morphology

Astrocytes can change shape dramatically in response to increased physiological and pathological demands, yet the functional consequences of morphological change are unknown. We report the expression of Cl- currents after manipulations that alter astrocyte morphology. Whole-cell Cl- currents were elicited after (1) rounding up cells by brief exposure to trypsin; (2) converting cells from a flat polygonal to a process-bearing (stellate) morphology by exposure to serum-free Ringer's solution; and (3) swelling cells by exposure to hypo-osmotic solution. Zero-current potentials approximated the Nernst for Cl-, and rectification usually followed that predicted by the constant-field equation. We observed heterogeneity in the activation and inactivation kinetics, as well as in the relative degree of outward versus inward rectification. Cl- conductances were inhibited by 4, 4-diisothiocyanostilbene-2,2'-disulfonic acid (200 microM) and by Zn2+ (1 mM). Whole-cell Cl- currents were not expressed in cells without structural change. We investigated whether changes in cytoskeletal actin accompanying changes in astrocytic morphology play a role in the induction of shape-dependent Cl- currents. Cytochalasins, which disrupt actin polymers by enhancing actin-ATP hydrolysis, elicited whole-cell Cl- conductances in flat, polygonal astrocytes. In stellate cells, elevated intracellular Ca2+ (2 microM), which can depolymerize actin, enhanced Cl- currents, and high intracellular ATP (5 mM), required for repolymerization, reduced Cl- currents. Modulation of Cl- current by Ca2+ and ATP was blocked by concurrent whole-cell dialysis with phalloidin and DNase, respectively. Phalloidin stabilizes actin polymers and DNase inhibits actin polymerization. Dialysis with phalloidin also prevented hypo-osmotically activated Cl- currents. These results demonstrate how the expression of astrocyte Cl- currents can be dependent on cell morphology, the structure of actin, Ca2+ homeostasis, and metabolism.

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