title: A novel barium-sensitive calcium influx into rat astrocytes at low external potassium.
abstract: Cultured rat cerebellar astrocytes, loaded with the Ca2+-sensitive fluorescent dyes Fura-2 or Fluo-3, responded with cytoplasmic Ca2+ transients, when the external K+ concentration was reduced from 5 mM to below 1 mM. Ca2+ transients were generated after changing to a saline containing 0.2 mM K+ in 82% of the cells (n =303) with a delay of up to 4 min. Cultured rat cortical neurones, which responded in high-K+ saline (50 mM) with Ca2+ transients, showed no Ca2+ responses in low K+ (n =22). In acute rat hippocampal brain slices, presumed glial cells responded with Ca2+ transients in low K+ similar to astrocytes in culture (88%, n =17). The Ca2+ transients were observed both in somatic and dendritic regions of cultured astrocytes, as examined with confocal laser scanning microscopy. Patch-clamped astrocytes hyperpolarized in 0.2 mM K+ from an average resting potential of -65 +/- 4 mV to -98 +/- 20 mV (n =15). The Ca2+ transients in low K+ were suppressed in Ca2+-free saline, buffered with 0.5 mM EGTA, but not after depletion of intracellular Ca2+ stores by thapsigargin, cyclopiazonic acid or by Ruthenium Red, indicating that they were due to Ca2+ influx into the cells, and not caused by intracellular Ca2+ release. The addition of different divalent cations revealed that Ba2+, but not Ni2+, Cd2+, Sr2+ or Mg2+, reversibly blocked the Ca2+ transients in low K+. There was a significant reduction of the Ca2+ responses at micromolar Ba2+ concentrations (Ki = 3.8 microM). The application of different K+ channel blockers, tetraethylammonium, dequalinium, tolbutamide, clotrimazole, or quinidine had no effect on the Ca2+ responses. Removal of external Na+, or intracellular acidification by the addition of 40 mM propionate to the saline, had also no influence on the generation of the Ca2+ transients. The results suggest that reducing the external K+ concentration elicits a Ca2+ influx into rat astrocytes which is highly sensitive to Ba2+. It is discussed that this Ca2+ influx might occur through K+ inward rectifier channels, which become Ca2+-permeable when the extracellular K+ concentration decreases to 1 mM or below.
authors.affiliation: Abteilung für Allgemeine Zoologie, FB Biologie, Universität Kaiserslautern, Kaiserslautern, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Dallwig
authors.forename: H
authors.initials: H
authors.lastname: Vitten
authors.forename: J W
authors.initials: JW
authors.lastname: Deitmer
mesh_terms: D000814:Aniline Compounds; D000818:Animals; D001253:Astrocytes / Q000378:metabolism*; D001464:Barium / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020013:Calcium Signaling; D048429:Cell Size; D002478:Cells, Cultured; D002586:Cesium / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism; D005456:Fluorescent Dyes; D006624:Hippocampus; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D018613:Microscopy, Confocal; D009474:Neurons / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism*; D026902:Potassium Channel Blockers; D024661:Potassium Channels, Inwardly Rectifying; D051381:Rats; D012413:Rubidium / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D014966:Xanthenes
chemical_list: D000814:Aniline Compounds; D002121:Calcium Channel Blockers; D005456:Fluorescent Dyes; D026902:Potassium Channel Blockers; D024661:Potassium Channels, Inwardly Rectifying; D014966:Xanthenes; D002586:Cesium; C059715:Fluo-3; D001464:Barium; D012964:Sodium; D012413:Rubidium; D011188:Potassium; D002118:Calcium