8-pCPT-modified cAMP analogs inhibit [3H]thymidine uptake. PC12 cells were preincubated with the indicated concentration (A) or 10 μm 8-pCPT-cAMP, 8-pCPT-ado, Sp-8-pCPT-cAMPS, 8-pCPT-2′-O-methyl-cAMP, or 8-pCPT-2′-O-methyl-ado (B and C) for 30 min. Then, [3H]thymidine uptake into the cells was determined as described under “Materials and Methods.” Values are means ± S.D. (error bars) of three independent determinations. Asterisks indicate a significant difference between cAMP/ado analog-treated and vehicle-treated cells; n.s., not significant as compared with the control.