We have used aequorin as an indicator for the intracellular free calcium ion concentration ([Ca++]i) of Swiss 3T3 fibroblasts. Estimated [Ca++]i of serum-deprived, subconfluent fibroblasts was 89 (±20) nM, almost twofold higher than that of subconfluent cells growing in serum, whose [Ca++]i was 50 (±19) nM. Serum, partially purified platelet-derived growth factor (PDGF), and fibroblast growth factor (FGF) stimulated DNA synthesis by the serum-deprived cells, whereas epidermal growth factor (EGF) did not. Serum immediately and transiently elevated the [Ca++]i of serum-deprived cells, which reached a maximal value of 5.3 µM at 18 s poststimulation but returned to near prestimulatory levels within 3 min. Moreover, no further changes in [Ca++]i were observed during 12 subsequent h of continuous recording. PDGF produced a peak rise in [Ca++]i to ∼1.4 pM at 115 s after stimulation, and FGF to ∼1.2 µM at 135 s after stimulation. EGF caused no change in [Ca++]i. The primary source of calcium for these transients was intracellular, since the magnitude of the seruminduced rise in [Ca++]i was reduced by only 30% in the absence of exogenous calcium. Phorbol 12-myristate 13-acetate (PMA) had no effect on resting [Ca++]i. When, however, quiescent cells were treated for 30 min with 100 nM PMA, serum-induced rises in [Ca++]i were reduced by sevenfold. PMA did not inhibit growth factor-induced DNA synthesis and was by itself partially mitogenic. We suggest that if calcium is involved as a cytoplasmic signal for mitogenic activation of quiescent fibroblasts, its action is early, transient, and can be partially substituted for by PMA. Activated protein kinase C may regulate growth factor-induced increases in [Ca++]i.
ASJC Scopus subject areas
- Cell Biology