Effect of I3C in the presence or absence of putrescine on intracellular putrescine levels in SW480 cells following 24 hour treatment. Cells were cultured with DMSO alone (Control) or I3C (175 μM) in the presence of 0, 20 or 50 μM putrescine as indicated. Intracellular putrescine levels were determined by HPLC, as described in Materials and Methods. Results are presented as mean ± pooled SD (shown on control bar only, n = 4). nd = value not determined. Significant difference from the control (DMSO without putrescine), and from I3C without putrescine, is indicated by a and b respectively (P < 0.05).