Early PDGF-induced alterations in cytosolic free calcium are required for mitogenesis

Early alterations in cytosolic free Ca²⁺ concentration (Ca²⁺(i)) (occurring within seconds to minutes) following platelet-derived growth factor (PDGF) stimulation were demonstrated to be required, in both BALB/c3T3 fibroblasts and vascular smooth muscle cells, for subsequent DNA synthesis by introduction of Ca²⁺-antagonists at different times in relation to growth factor stimulation. Blockade of PDGF-stimulated increases in Ca²⁺(i) correlated with inhibition of PDGF-stimulated DNA synthesis in both systems, although the mechanism of increased Ca²⁺(i) is different in the two cell types. In vascular smooth muscle cells, voltage-sensitive Ca²⁺-channel antagonists, TPA, and pertussis toxin inhibited both PDGF-induced increases in Ca²⁺(i) and DNA synthesis when added immediately before PDGF, but did not do so when added for the same time period 4 hr after PDGF. Similarly, pretreatment of fibroblasts with TMB-8 inhibited PDGF-induced alterations in Ca²⁺(i) and DNA synthesis, but had no effect on DNA synthesis when added after PDGF exposure. These findings demonstrate for the first time that early increases in Ca²⁺(i) stimulated by PDGF play a critical role in PDGF-stimulated mitogenesis.