Figure 1.

Dynamics of DNA synthesis in Swiss 3T3 cells restimulated with FGF-1 after removing FGF-1. (A) Schematic diagrams of the experiments using confluent Swiss 3T3 cells that were made quiescent by incubating for 48 h in a serum-free DMI supplemented with 10 μg/ ml insulin. Quiescent cells were stimulated for 3 h with FGF-1 (5 ng/ml) and heparin (10 μg/ml), culture medium was removed, and the monolayer was washed three times with DMEM containing heparin. The cells were incubated for either 0.5 or 1.5 h in DMI alone, and were restimulated with FGF-1 and heparin (transient stimulation). Three types of controls were also performed: (a) constant stimulation, FGF-1 was not removed; (b) short stimulation, cells were stimulated with FGF-1 for 3 h, washed with heparin, and incubated in DMI without FGF-1; and (c) delayed stimulation, FGF-1 was added to quiescent cells simultaneously with restimulation of the cells that were undergoing the transient stimulation. DNA synthesis was monitored using [³H]thymidine incorporation 13–21 h after restimulation. (B) Restimulation with FGF-1 and heparin after a 0.5-h interruption with a heparin wash. [³H]thymidine incorporation (± SD) as a function of time after restimulation with constant stimulation (▪), short stimulation (□), transient stimulation (♦), and delayed stimulation (⋄) of Swiss 3T3 cell populations. (C) Restimulation with FGF-1 and heparin after a 1.5-h interruption with a heparin wash. Description is identical to that described in B.