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.