Effect of ADP on Epo- and SCF-induced DNA synthesis in progenitor cells. ADP (20 μmol/L) significantly (**P < .001, paired t-test) enhanced SCF-stimulated thymidine incorporation while no significant effect on Epo-stimulated activity was observed. In the absence of growth factors, ADP had a significant (P < .01) stimulating effect on the basal rate of DNA synthesis (dotted line) that was completely blocked by the PKC inhibitor Gö 6976 (##P < .001). Columns show means ± SEM from five to eight independent experiments.