Figure 3. Effect of model cations, polyamines and polyamine analogs on MPP⁺ transport.

Uptake of 0.1 μM [³H]MPP⁺ in oocytes expressing OCT1 (A), OCT2 (B) or OCT3 (C) was measured in the absence or in the presence of TMA, TEA, natural polyamines (spermidine, a7; and spermine, a10), and selected polyamine analogs (b–e), as detailed in Figure 1. a7 and a10 were used at 1 and 10 mM, and the rest were tested at 1 mM. For TMA, TEA, b10, c10-a, and d10, data are means ± S.E. of 3 experiments with different donor frogs, and were normalized to the MPP⁺ uptake in the absence of external inhibitors (control), 180 ± 15 (OCT1), 231 ± 15 (OCT2) and 274 ± 20 (OCT3) fmol/oocyte/30 min. For a7, a10, b7, c6, c10-b, d6, e6 and e10, data are from individual experiments, where each compound was assayed in parallel in OCT1, OCT2 and OCT3-expressing oocytes from the same frog, and normalized to their own controls; each data set was confirmed in at least one additional trial with oocytes from a different preparation. Uptake in non-injected oocytes was fmol/oocyte/30 min (mean ± S.E. of 10 experiments), and was not affected by any of the test compounds. *p < 0.05 as compared to uptake in absence of external inhibitors (paired Student’s t test).