Contribution of external and internal Ca2+ to changes in intracellular free Ca2+ produced by mitogens in Swiss 3T3 fibroblasts: the role of dihydropyridine sensitive Ca2+ channels

Changes in intracellular free Ca2+ concentration ( Ca2+]i) produced by growth factors and mitogens have been studied using aequorin-loaded Swiss 3T3 cells. Decreasing free Ca2+ in the external medium by using EGTA had no significant effect on the increase in Ca2+]i produced by vasopressin, bradykinin, bombesin or prostaglandin E2, but reduced the increase in Ca2+]i produced by platelet derived growth factor (PDGF) by 58%, by prostaglandin E1 44% and by prostaglandin F2 alpha 47%. The dihydropyridine Ca2+-channel antagonist nifedipine at 10 microM inhibited the Ca2+]i response to PDGF by 41% in both the presence of and in the absence of external Ca2+. Methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl) pyridine-5-carboxylate (BAY K8644), a Ca2+-channel agonist, at 10 microM produced an increase in Ca2+]i and decreased the Ca2+]i response to PDGF by 39%. Nifedipine did not block 45Ca2+ uptake or release by inositol 1,4,5-trisphosphate in saponin-permeabilized Swiss 3T3 fibroblasts but BAY K8644 inhibited 45Ca2+ release by inositol 1,4,5-trisphosphate. The results suggest that the increase in Ca2+]i caused by PDGF in Swiss 3T3 fibroblasts is due to the influx of external Ca2+ through dihydropyridine sensitive Ca2+ channels, as well as release of internal Ca2+.