The aims of the present investigation were (a) to compare the lateral mobility of membrane receptors of human fibroblasts and polymorphonuclear leukocytes (PMNL) labelled with either platelet-derived growth factor (PDGF), or the lectin wheat germ agglutinin (WGA), and (b) to study effects of serum or PDGF on the mobility of these receptor molecules in human fibroblasts. Human foreskin fibroblasts (AG 1523) were grown on coverslips either under standard (10%) or under serum-free conditions yielding “normal” and “starved” cells, respectively. The receptor mobility was studied in response to exposure to PDGF, or serum, in short time or prolonged incubations. Human polymorphonuclear leukocytes (PMNL) were adhered to microscope slides by clotting drops of blood. They were stained with rhodaminated PDGF or fluoresceinated WGA. The diffusion of labelled receptors was assessed with fluorescence recovery after photobleaching (FRAP). It was found that (a) fibroblasts grown at normal serum concentration had a lower diffusion coefficient (D=3×10−10 cm2 s−1) for the PDGF-receptor and a slightly lower mobile fraction (R=60%) than starved cells (D=5×10−10 cm2s−1 and R=73%), (b) addition of serum to starved cells increased both D and R for the PDGF receptor to 12×10−10 cm2 s−1 and 96%, respectively, (c) a similar pattern was obtained for WGA-labelled glycoconjugates indicating general membrane effects of serum-induced cell stimulation, and (d) in PMNL the PDGF receptor displayed motility characteristics (D=3–4×10−10 cm2 s−1 and R=59%) similar to those in fibroblasts, possibly suggesting equivalent anchorage mechanisms in the membrane.

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