Intracellular Ca²⁺ pool content is linked to control of cell growth

A close correlation was observed between intracellular Ca²⁺ pool depletion and refilling and the onset of DNA synthesis and proliferation of DDT₁MF-2 smooth muscle cells. The intracellular Ca²⁺ pump inhibitors 2,5- di-tert-butyl-hydroquinone (DBHQ) and thapsigargin (TG) specifically emptied identical inositol 1,4,5-trisphosphate (InsP₃)-sensitive Ca²⁺ pools and both arrested cell growth at concentrations corresponding to Ca²⁺ pump blockade. However, an important distinction was observed between the two inhibitors with respect to their reversibility of action. Upon removal of DBHQ from DBHQ-arrested cells, Ca²⁺ pools immediately refilled, and 14 hr later cells entered S phase followed by normal cell proliferation; the time for entry into S phase was identical to that for cells released from confluence arrest. Although TG irreversibly blocked Ca²⁺ pumping and emptied Ca²⁺ pools, high serum treatment of TG-arrested cells induced recovery of functional Ca²⁺ pools in 6 hr (via probable synthesis of new pump); thereafter cells proceeded to S phase and normal cell proliferation within the same time period (14 hr) as that following release of DBHQ- arrested cells. The precise relationship between Ca²⁺ pump blockade and growth arrest indicates that Ca²⁺ pool emptying maintains cells in a G₀- like quiescent state; upon refilling of pools, normal progression into the cell cycle is resumed. It is possible that a specific cell cycle event necessary for G₀ to G₁ transition depends upon signals generated from the InsP₃-sensitive Ca²⁺ pool.