UPTAKE OF CA2+ AND REFILLING OF INTRACELLULAR CA2+ STORES IN EHRLICH-ASCITES-TUMOR CELLS AND IN RAT THYMOCYTES

We have studied the uptake of Ca2+ and its redistribution between the cytoplasm and the intracellular stores in Ehrlich-ascites-tumour cells and rat thymocytes previously depleted of Ca2+ by incubation in Ca2+-free medium. Measurements included changes of the cytoplasmic Ca2+ concentration ([Ca2+](i)), uptake of 45Ca2+ and uptake of Mn2+, a Ca2+ surrogate for Ca2+ channels. Refilling of the Ca2+ stores in thymocytes was very fast (half-filling time: 4 s at 37°C) and very sensitive to temperature (10 times slower at 20°C). It was always preceded by an increase of [Ca2+](i). In the Ehrlich cell, both refilling and increase of [Ca2+](i) were about one order of magnitude slower. The increase of [Ca2+](i) and the refilling of the intracellular stores were both almost completely blocked by Ni2+ in thymocytes, but only partially in the Ehrlich cell. The rates of 45Ca2+ and Mn2+ uptake varied consistently with temperature and the kind of cell. These results suggest that the intracellular stores are refilled by Ca2+ taken up from the cytoplasm. We also find that filling of the Ca2+ stores decreases by about 90% the rate of Mn2+ uptake in thymocytes. This is direct evidence of modulation of the plasma-membrane Ca2+ entry by the degree of filling of the intracellular stores. This modulation occurs in the absence of agonists, suggesting some kind of signalling between the intracellular stores and the Ca2+ entry pathways of the plasma membrane.