Figure 1. Drosophila S2 cells show specific spermine and spermidine uptake.

(A) Time course of spermine and spermidine uptake in Drosophila S2 cells. At zero time, 500 nM [¹⁴C]spermine (●, ○) or [¹⁴C]spermidine (■, □) was added to Drosophila S2 cells as described in the Materials and methods section. A robust time-dependent increase in the amount of label associated with S2 cells was observed at 22 °C, but not at 0 °C, indicating that these measurements reflect temperature-dependent transport rather than non-specific binding. Results are means±S.E.M. for three independent experiments performed in triplicate. (B) Inhibition of [¹⁴C]spermine uptake by different polyamines. Varying concentrations of unlabelled spermine (●), spermidine (■), putrescine (△) and cadaverine (◇) were added to Drosophila S2 cells incubated with 500 nM [¹⁴C]spermine. Results are means±range for two independent experiments performed in triplicate. See the text for IC₅₀ values for spermine and spermidine.