Pdgf-BB Triggered Cytoplasmic Calcium Responses in Cells with Endogenous or Stably Transfected PDGF β-Receptors

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Abstract
Platelet-derived growth factor-BB (PDGF-BB) triggered signal transduction was investigated in human foreskin fibroblasts with endogenous PDGF β-receptors, and porcine aortic endothelial (PAE) cells with stably transfected PDGF β-receptors. Immunoprecipitation and immunoblotting showed that PDGF induced dose-dependent autophosphorylation of PDGF β-receptor, and that PLC-γ associates with autophosphorylated PDGF β-receptors and becomes phosphorylated. Activation of PLC-γ is known to induce fluctuations of the concentration of cytoplasmic calcium ([Ca2+]i). Microfluorometry and digital imaging were employed for measurements of the concentration of [Ca2+]i. In both cell types the growth factor induced four types of [Ca2+]i responses; no rise, a small and sluggish monophasic rise, a biphasic rise with an initial transient peak followed by a sustained elevation, and finally regular oscillations. The frequencies and amplitudes of the oscillatory responses were independent of agonist concentration after stimulation with PDGF-BB. Latency, the period from application of stimulus to the first [Ca2+]i peak, was reduced at higher concentrations of agonist. Also, the proportion of responding cells increased with higher concentrations of ligand. Oscillations of [Ca2+]i were elicited at submaximal concentrations of agonist. In PAE cells PDGF-BB triggered a single [Ca2+]i peak in absence of external Ca2+. Ligand-induced oscillations and sustained increases of [Ca2*]i were counteracted by the inorganic Ca2+ channel blocker Ce3 +. These results show that similar types of [Ca2+]i responses occur in different cell types independently of whether the PDGF β-receptors are expressed endogeneously or after transfection. Potentially, the different [Ca2+]i responses have distinct physiological consequences.