A NOVEL CA2+ ENTRY MECHANISM IS TURNED ON DURING GROWTH ARREST INDUCED BY CA2+ POOL DEPLETION

Ca2+ pool depletion with Ca2+ pump blockers induces growth arrest of r apidly dividing DDT(1)MF-2 smooth muscle cells and causes cells to ent er a stable, quiescent G(o)-like growth state (Short, A. D., Bian, J., Ghosh, T. K., Waldron, R. T., Rybak, S. L., and Gill, D. L. (1993) Pr oc. Natl. Acad. Sci. U.S.A. 90, 4986-4990). Here we reveal that induct ion of this quiescent growth state with the Ca2+ pump blocker, thapsig argin, is correlated with the appearance of a novel caffeine-activated Ca2+ influx mechanism. Ca2+ influx through this mechanism is clearly distinct from and additive with Ca2+ entry through store-operated chan nels (SOCs). Whereas SOC-mediated entry is activated seconds after Ca2 + pool release, caffeine-sensitive influx requires at least 30 min of pool emptying, Although activated in the 1-10 mM caffeine range, this mechanism has clearly distinct methylxanthine specificity from ryanodi ne receptors and is not modified by ryanodine. It is also unaffected b y the Ca2+ channel blockers SKF96365 or verapamil and is independent o f modifiers of cyclic nucleotide levels. Growth arrest by thapsigargin -induced Ca2+ pool depletion can be reversed by treatment with 20% ser um (Waldron, R. T., Short, A. D., Meadows, J. J., Ghosh, T. K., and Gi ll, D. L. (1994) J. Biol. Chem. 269, 11927-11933). The serum-induced r eturn of functional Ca2+ pools and reentry of cells into the cell cycl e correlates exactly with the disappearance of the caffeine-sensitive Ca2+ influx mechanism. Therefore, appearance and function of this nove l Ca2+ entry mechanism are closely tied to Ca2+ pool function and cell , growth state and may provide an important means for modifying exit f rom or entry into the cell cycle.