Figure 1: Effect of serum and second messengers on the density of DHP receptors in normal human fibroblasts. (a) On day 2 after the plating of fibroblasts at high (bar 1) or low density (bars 2–6), the growth medium was supplemented with 1 mM 8Br-cAMP (bar 3), 1 μM Ca2+ ionophore A23187 (bar 4), 1 μM D-cis-diltiazem (bar 5), or growth medium was replaced by serum-free Eagle’s medium containing 0.1% BSA (bar 6). After 4 days, the density of DHP receptors was measured at 2 nM [3H]-PMD as pmol [3H]PMD specifically bound per 106 cells of confluent (bar 1, >2 × 104 cells/cm2) or nonconfluent monolayers (bars 2–6, 3–5 × 103 cells/cm2). Mean of 5-6 measurements ± SEM. . (b) Effect of serum deprivation on the density of DHP receptors in fibroblasts. On day 2 after plating, sparse cultures of fibroblasts were incubated for up to 4 days in serum-free Eagle’s medium supplemented with 0.1% BSA. The horizontal bars indicate when serum-free medium was replaced by standard growth medium containing 10% serum. The time course of changes in the density of DHP binding sites was measured as in (a) at different time points (not shown for simplicity) and compared with that in cells not subjected to serum-deprivation (blue line).