INTRACELLULAR CA2-GROWTH( POOL CONTENT IS LINKED TO CONTROL OF CELL)

A close correlation was observed between intracellular Ca2+ pool deple tion 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) specifical ly emptied identical inositol 1,4,5-trisphosphate (InsP3)-sensitive Ca 2+ 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 act ion. Upon removal of DBHQ from DBHQ-arrested cells, Ca2+ pools immedia tely refilled, and 14 hr later cells entered S phase followed by norma l cell proliferation; the time for entry into S phase was identical to that for cells released from confluence arrest. Although TG irreversi bly 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 h r) as that following release of DBHQ-arrested cells. The precise relat ionship between Ca2+ pump blockade and growth arrest indicates that Ca 2+ pool emptying maintains cells in a G0-like quiescent state; upon re filling of pools, normal progression into the cell cycle is resumed. I t is possible that a specific cell cycle event necessary for G0 to G1 transition depends upon signals generated from the InsP3-sensitive Ca2 + pool.