Changes in cytosolic free Ca²⁺ concentration are observed during the cell cycle and the extent of Ca²⁺ entry is related to cell proliferation (Golovina et al. 2001; Bodding, 2001). We have compared the intracellular Ca²⁺ stores contents and the amplitudes of capacitative Ca²⁺ entry (CCE) and non-capacitative Ca²⁺ entry (NCCE) in Hep G2 and HEK 293 cells in control cells, under conditions known to block cell-cycle progression, and after recovery.

Cells were kept in serum-free medium for 72 h and recovery measured 1, 4, 8 and 24 h after readmission of serum, or in complete culture medium for the same time period. Thapsigargin (1-3 µM) was used to estimate the intracellular Ca²⁺ store contents in Ca²⁺-free medium, and CCE amplitude was measured after the addition of 2 mM Ca²⁺.

In Hep G2 cells, a 72 h incubation in the absence of serum led to a 40 % decrease in CCE from 332 ± 23 nM (mean ± S.E.M., n = 6) to 206 ± 10 nM (n = 6). A clear recovery was observed after 1 h (224 ± 11 nM, n = 6) and 4 h (284 ± 11 nM, n = 7) readmission of 10 % FCS. The thapsigargin-sensitive Ca²⁺ store contents were not modified under the same experimental conditions. In HEK 293 cells, a 72 h incubation in the absence of serum led to a 70 ± 2 % (n = 14) decrease in CCE. CCE amplitudes were reduced only by 60 ± 6, 32 ± 4 and 28 ± 6 % after 1, 4 and 8 h readmission of 10 % FCS and a full recovery was observed after 24 h. The thapsigargin-sensitive Ca²⁺ store contents showed a similar pattern in these cells. In HEK 293 cells, a 72 h incubation in the absence of serum led to a 48 ± 2 % (n = 14) decrease in the thapsigargin-sensitive Ca²⁺ store contents. These contents were reduced only by 37 ± 2 % (n = 12), 28 ± 2 % (n = 12) and 17 ± 3 % (n = 8) after 1, 4 and 8 h readmission of 10 % FCS and a full recovery was observed after 24 h. No change in the amplitude of arachidonic acid-induced non-capacitative Ca²⁺ entry (NCCE; Mignen & Shuttleworth, 2000) was observed in HEK 293 cells in the same conditions. We propose that the amplitude of CCE is tightly related to cell-cycle progression and that the level of expression of plasma membrane Ca²⁺ channels might be under the control of growth and transforming factors in Hep G2 and HEK 293 cells.

This work was supported by grants from the Association pour la Recherche contre le Cancer (grants 5621 and 4615) and Comité de l’Essonne de la Ligue contre le Cancer (to T.C.) and National Institutes of Health grant GM40457 (to T.J.S.).