Figure 4.

Inhibition of cAMP synthesis by added and released nucleotides. After loading with [³H]adenine, PC12 cells were incubated in RO 20-1724 (100 μm) for 105 min. During the last 15 min of this incubation period, 1 μm CGS 21680 was present either alone or together with the indicated concentrations of additional agents. The amount of radioactivity retrieved within the fraction of cAMP was calculated as percentage of the total radioactivity extracted from the cell cultures, and values obtained in the presence of CGS 21680 were normalized to the data obtained in its absence (normalized to basal). A, Cells were incubated in the presence (open bar) or absence (filled bar) of 1 μm CGS 21680; n = 41. B, Cells were incubated in the presence of the indicated concentrations (micromolar) of CGS 21680 or 2MeSAMP, or both; n = 6-9; ^**p < 0.01 versus the value obtained in the presence of CGS 21680 only. C, Cells were incubated in the presence of the indicated concentrations (micromolar) of CGS 21680 and ADP or 2MeSAMP, or both; n=6-9; ^***p < 0.001 versus the value obtained in the presence of CGS 21680 only; n.s. indicates no significant difference. D, Cells were incubated in the presence of the indicated concentrations of CGS 21680, apyrase, and KCl (NaCl was reduced accordingly) or CdCl₂, or both; n = 6-9; ^**p < 0.01 and ^***p < 0.001, respectively, versus the value obtained in the presence of CGS 21680 only; n.s. indicates no significant difference.