BARIUM TRIGGERS RAPID ENDOCYTOSIS IN CALF ADRENAL CHROMAFFIN CELLS

1. Changes in cell capacitance were monitored in whole-cell patch-clam p recordings from calf adrenal chromaffin cells using a software-based phase-tracking technique. Rapid endocytosis and exocytosis were obser ved in extracellular solutions containing either Ca2+ or Ba2+. 2. Ther e was no significant difference in the magnitude or the time course of rapid endocytosis of cells stimulated in Ca2+ as compared to Ba2+. Wh en cells were pretreated with caffeine and thapsigargin in order to de plete intracellular Ca2+ stores, rapid endocytosis in Ba2+ was not aff ected. This indicates that Ba2+ itself is capable of supporting rapid endocytosis. 3. The application of the calmodulin inhibitor calmidazol ium via the intracellular pipette solution did not inhibit rapid endoc ytosis. Although our findings are inconsistent with an immediate requi rement for calmodulin in rapid endocytosis, they do not rule out an in volvement on a longer time scale. 4. While rapid endocytosis was not a ffected by the substitution of Ca2+ with Ba2+, the maximum rate of exo cytosis 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+.