Interactions among calcium compartments in C6 rat glioma cells: involvement of potassium channels.
1. Variations in intracellular free Ca2+ concentration ([Ca2+]i) induced by alteration of the extracellular concentrations of Ca2+ ([Ca2+]o) and K+ ([K+]o) were imaged in single fluo-3-loaded C6 glioma cells. In addition, the effect of membrane potential on [Ca2+]i was investigated in fura-2-loaded, voltage-clamped cells. 2. Step alterations of [Ca2+]o from 0 to 10 nM were followed by proportional variations in [Ca2+]i, with a maximum 7-fold increase and an apparent half-maximum at [Ca2+]o of 1.5 mM. 3. The time to half-maximum change (t1/2) of [Ca2+]o-associated [Ca2+]i variations ranged between 10 and 50 s, and was inversely related to the amplitude of [Ca2+]o steps. 4. Transient, serotonin-induced [Ca2+]i elevations, used as a measure of Ca2+ availability in inositol 1,4,5-trisphosphate-sensitive stores, were diminished within 10 min in 0 mM [Ca2+]o, but were unaffected by [Ca2+]o changes in the 1-5 mM range. 5. Restoration of normal [Ca2+]i following its elevation by serotonin was delayed by removal of external Na+ or Cl- and was enhanced by warming the medium to 37 degrees C. These conditions did not affect [Ca2+]o-associated [Ca2+]i variations. 6. [Ca2+]o-associated [Ca2+]i variations were depressed by La3+ and Ba2+, while blockers of voltage-activated Ca2+ channels were ineffective. 7. Elevated [K+]o depressed the basal level of [Ca2+]i, and in high concentrations (70-140 mM) also diminished the response to serotonin. 8. Depolarizing the membrane potential of voltage-clamped cells reversibly reduced [Ca2+]i. These membrane-potential associated [Ca2+]i variations were blocked by La3+, Ba2+ and TEA, all of which also depolarized membrane resting potential. 9. Apamin (at 1-10 microM), a blocker of [Ca2+]i-activated K+ channel, totally and reversibly prevented [Ca2+]o-associated [Ca2+]i variations. 10. These studies indicate that C6 cells are responsive to variations in [Ca2+]o, and that a K+ channel is a possible path through which Ca2+ penetrates into the cell.
Department of Neurobiology, Weizmann Institute, Rehovot, Israel.
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D000818:Animals; D001932:Brain Neoplasms / Q000378:metabolism*; D002118:Calcium / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D016257:Fura-2; D005910:Glioma / Q000378:metabolism*; D007091:Image Processing, Computer-Assisted; D007811:Lanthanum / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D012701:Serotonin / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D002121:Calcium Channel Blockers; D015221:Potassium Channels; D012701:Serotonin; D007811:Lanthanum; D002118:Calcium; D016257:Fura-2