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Barium triggers rapid endocytosis in calf adrenal chromaffin cells

Paolo G. P. Nucifora and Aaron P. Fox

1. Changes in cell capacitance were monitored in whole-cell patch-clamp recordings from calf adrenal chromaffin cells using a software-based phase-tracking technique. Rapid endocytosis and exocytosis were observed in extracellular solutions containing either Ca²⁺ or Ba²⁺.

2. There was no significant difference in the magnitude or the time course of rapid endocytosis of cells stimulated in Ca²⁺ as compared to Ba²⁺. When cells were pretreated with caffeine and thapsigargin in order to deplete intracellular Ca²⁺ stores, rapid endocytosis in Ba²⁺ was not affected. This indicates that Ba²⁺ itself is capable of supporting rapid endocytosis.

3. The application of the calmodulin inhibitor calmidazolium via the intracellular pipette solution did not inhibit rapid endocytosis. Although our findings are inconsistent with an immediate requirement for calmodulin in rapid endocytosis, they do not rule out an involvement on a longer time scale.

4. While rapid endocytosis was not affected by the substitution of Ca²⁺ with Ba²⁺, the maximum rate of exocytosis was higher in cells stimulated in Ca²⁺ than in Ba²⁺. Since Ba²⁺ currents were much larger than Ca²⁺ currents during depolarizations to +10 mV (the test potential used in these experiments), Ba²⁺ appears to be less efficient at promoting exocytosis than Ca²⁺.