Effect of mitogens on the density of DHP receptors (open bars) and cell proliferation (dark bars) in normal human fibroblasts. On day 2 after cell plating, the growth medium was replaced for 4 days with serum-free Eagle’s medium supplemented with 0.1% BSA. Cells were incubated for 4 days in the presence of 10% serum (control), 0.1% BSA (serum-free), epidermal growth factor (EGF, 15 ng/mL), insulin (10 μg/mL), or basic fibroblast growth factor (bFGF, 2 ng/mL). The density of DHP receptors was measured at 2 nM [³H]-PMD as pmol [³H]-PMD specifically bound per 10⁶ cells of nonconfluent monolayers (3–5 × 10³ cells/cm²). Cell proliferation was estimated from the synthesis of DNA measured by [¹⁴C]thymidine incorporation.