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

Abstract

When normal quiescent (G₀) cells are stimulated by mitogens to enter the cell cycle, the metabolic derepression which occurs^1–8 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 (pH_i). 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 pH_i (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 pH_i 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.