Depletion and refilling of intracellular calcium pools in bovine chromaffin cells.

To investigate Ca2+ uptake by Ca(2+)-depleted bovine chromaffin cells, we depleted these cells of Ca2+ by incubating them in Ca(2+)-free buffer, then measured changes in cytoplasmic Ca2+ concentration ([Ca2+]i), 45Ca2+ uptake, and Mn2+ uptake in response to added Ca2+ or MN2+. In depleted cells, the increase in [Ca2+]i after Ca2+ addition, and the Mn2+ and 45Ca2+ uptakes were higher than in control cells, and were inhibited by verapamil. The size of the intracellular Ca2+ pools in depleted cells increased after Ca2+ addition. The times for [Ca2+]i rise and Mn2+ entry to reach plateau levels were much shorter than the time for refilling of intracellular Ca2+ stores. In Ca(2+)-depleted cells and cells which had been loaded with BAPTA, 45Ca2+ uptake was much higher than in control cells. These results suggest that extracellular Ca2+ enters the cytoplasm first before refilling the intracellular stores. The rate of Mn2+ influx depended on the level of filling of the Ca2+ stores, suggesting that some signalling takes place between the intracellular stores and Ca2+ entry pathways through the plasma membrane.