Fig 1. Characterization of [³H]dAMP uptake into Caco-2 cells.

(A) Time course of [³H]dAMP uptake by Caco-2 cells. The uptake of [³H]dAMP (2 nM) was estimated at 37°C and pH 7.4. (B) pH- and Na⁺-dependent uptake of [³H]dAMP. The uptake of [³H]dAMP (2 nM) was estimated at each of the indicated conditions for 3 min. The pH of the transport buffer was varied by appropriately adjusting the concentrations of MES, HEPES, and Tris. When required, Na⁺ was isosmotically substituted by N-methyl-d(−)-glucamine. *, **Significantly different from uptake at pH 7.4 in the absence of Na⁺ at p < 0.05, p < 0.01, respectively. †Significantly different from uptake at pH 7.4 in the presence of Na⁺ at p < 0.05. (C) The inhibitory effects of various compounds on the uptake of [³H]dAMP by Caco-2 cells. Caco-2 cells were incubated with [³H]dAMP (2 nM) at 37°C and pH 7.4 for 3 min in the absence or presence of each of the indicated compounds (100 μM). **Significantly different from the control at p < 0.01. All the data are presented as the mean ± S.E. of at least 3 independent experiments performed in triplicate.