Figure 2. Ionic dependence of spermine uptake in Drosophila S2 cells.

(A) Na⁺, K⁺, Cl⁻ and Ca²⁺ were iso-osmotically replaced using either ionic or non-ionic osmolytes. Cells were incubated with 500 nM [¹⁴C]spermine for 5 min, and the values obtained for each replacement experiment were normalized to uptake under control conditions using standard MDM (broken line). Transport was not significantly changed by replacement by Na⁺, K⁺, Li⁺, sucrose, NMDG, NO₃⁻, gluconate or Mg²⁺, but was inhibited by choline (*, P<0.01). (B) Concentration-dependent inhibition of spermine uptake by choline. Increasing concentrations of choline, Na⁺, Li⁺ or NMDG were added to cells incubated in standard MDM with 500 nM [¹⁴C]spermine. A four-parameter logistic sigmoid curve was fitted to the choline data (R²=0.7879) to yield an IC₅₀ of 66.4±39.8 mM (mean±S.E.M.). Results are means±range for two independent experiments performed in triplicate.