Platelet-derived growth factor stimulates non-mitochondrial Ca²⁺ uptake and inhibits mitogen-induced Ca²⁺ signaling in Swiss 3T3 fibroblasts

Changes in intracellular free Ca²⁺ concentration ([Ca²⁺]_i) were used to study the interaction between mitogens in Swiss 3T3 fibroblasts. Platelet-derived growth factor (PDGF) produced an increase in [Ca²⁺]_i and markedly decreased the increases in [Ca²⁺]_i caused by subsequent addition of bradykinin and vasopressin. If the order of the additions was reversed the [Ca²⁺]_i response to PDGF was not inhibited by bradykinin or vasopressin. Inhibition of protein kinase C by staurosporine or chronic treatment of the cells with phorbol 12-myristate 13-acetate prevented the inhibitory effect of PDGF on the [Ca²⁺]_i response to vasopressin but not bradykinin. PDGF did not decrease the receptor binding of bradykinin and produced only a small decrease in the receptor binding of vasopressin. PDGF decreased the rise in [Ca²⁺]_i caused by the Ca²⁺ ionophores 4-bromo-A23187 and ionomycin and by a membrane perturbing ether lipid, 1-octadecyl-2-methyl-rac-glycero-3-phosphocholine, both in the presence and absence of external Ca²⁺. There was no change in cell ⁴⁵Ca²⁺ influx caused by PDGF, vasopressin, or bradykinin. ⁴⁵Ca²⁺ efflux from cells exposed to PDGF and vasopressin mirrored the changes in [Ca²⁺]_i caused by the agents, that is, PDGF added after vasopressin produced a further increase in ⁴⁵Ca²⁺ efflux but vasopressin did not increase ⁴⁵Ca²⁺ efflux after PDGF. PDGF but not vasopressin caused an increase in the uptake of ⁴⁵Ca²⁺ into an inositol 1,4,5-trisphosphate-insensitive non-mitochondrial store in permeabilized cells. The results suggest that the decreased [Ca²⁺]_i response to mitogens after PDGF represents an action of PDGF at a point beyond the release of intracellular Ca²⁺ and the influx of external Ca²⁺, caused by an increase in the rate of removal of cytoplasmic free Ca²⁺. This increased removal of cytoplasmic Ca²⁺ by PDGF is not due to the increased export of Ca²⁺ from the cell but results from increased Ca²⁺ uptake into non-mitochondrial stores.