Effect of cAMP on growth factor–induced DNA synthesis in human erythroid progenitors. (A) 8-Br-cAMP (cAMP, 1 mmol/L) in the extracellular medium caused a significant inhibition (**P < .01 by paired t-test) of Epo (0.5 U/mL)-induced thymidine incorporation while SCF (50 ng/mL)-dependent incorporation remained unaffected. Values represent means ± SEM of 6 to 15 separate cultures from 2 to 5 different batches of cells. (B) Effect of 8-Br-cAMP on the thrombin-induced inhibition of Epo and SCF-dependent DNA synthesis. cAMP alone left basal DNA synthesis (dotted line) unchanged (leftmost column). Note that cAMP antagonized the effect of thrombin in the presence of SCF but not in the presence of Epo. Columns show means ± SEM from 6 to 9 separate cultures from 2 to 3 different batches of cells (**P < .001; #P < .05 by t-test).