A common sequence of calcium and pH signals in the mitogenic stimulation of eukaryotic cells

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Abstract
When normal quiescent (G0) cells are stimulated by mitogens to enter the cell cycle, the metabolic derepression which occurs1–8 is similar in a variety of cells. The mechanisms initiating these responses and their relationship to subsequent progression through G1 to DNA synthesis in S phase, however, are generally undefined. The clearest evidence has been obtained in sea urchin eggs9, where fertilization by sperm causes a rapid, transient increase in the concentration of free cytoplasmic Ca2+ ([Ca]i), followed by a sustained increase in cytoplasmic pH (pHi). It has been demonstrated clearly that these ionic responses are obligatory for progression to DNA synthesis by the normal pathway after fertilization, although the Ca2+ signal can be bypassed by parthenogenetic agents which elevate directly pHi (for example, NH+4 ions)9. These observations raise the questions of whether other eukaryotic cells show the same sequence of ionic responses when stimulated by mitogens and whether such signals are an obligatory component of their mitogenic pathways. We show here that a common sequence of [Ca]i and pHi responses occurs in both quiescent mouse thymocytes and Swiss 3T3 fibroblasts stimulated by appropriate mitogens. Furthermore, ‘opportunistic’ mitogens (those that do not act on the cells in vivo, such as concanavalin A (Con A)10, the Ca2+ ionophore A2318711 and 12-o-tetradecanoyl phorbol 13-acetate CTPA)12,13) that are mitogenic for both mouse thymocytes and 3T3 fibroblasts14, each produce characteristic ionic responses that are the same in both types of cell.