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 vasopresin, 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 F_2α 47%. The dihydropyridine Ca²⁺-channel antagonist nifedipine at 10 μM 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 μM 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⁺.