1 Introduction + [1–4] l l [3–5] + + [6,7] + + + [8–10] + + [11] + + + + + [11,12] + + + i + + + + + [7,10,13] [14] i [15] i 2 Materials and methods 2.1 Materials 14 − 1 3 − 1 2.2 Cell culture [16] 2.3 Measurement of dipeptide uptake 14 − 1 3 − 1 [16] 4 2 4 2 4 2 + 2 4 2.4 i i [10,17] + + i + i i i i + [18] 2.5 Measurement of intracellular cAMP levels i 2.6 Statistical analysis t t p 3 Results 3.1 Inhibition of dipeptide uptake by caffeine is via inhibition of NHE3 − 2 − 1 n p Fig. 1 + [12] Fig. 1 + − 2 − 1 n p + + [10,12,18] + Fig. 1 + p [19] + [12] Xenopus laevis [7] p + [12] + [7,12] Fig. 1 p 3.2 The inhibition of hPepT1 by caffeine and related compounds is consistent with inhibition of phosphodiesterase activity Fig. 2 Fig. 2 Fig. 2 Fig. 2 + p + p [18] 1 i Fig. 2 [20–22] i [12] i i − 2 n − 2 n i − 2 n + Fig. 3 50 [20,21,23–25] + 3.3 Pentoxifylline decreases the maximal capacity for dipeptide uptake Fig. 4 V max − 2 − 1 p n K m p + V max [12] 3.4 i + [10,12] + i i + [10,12,18,26] i Fig. 5 + + + + i i Fig. 5 + i Fig. 5 + − 1 n p Xenopus laevis [7] 3.5 + + + [27] [28] + + [26,29,30] + + [26] + + + [26,29,30] [16] Fig. 6 p + p + 4 Discussion [3,4] [14] [31] [32] + Figs. 1 and 2 + i + [18] i [33] i 50 Fig. 3 50 50 [20–22,34–36] 2 [37] 2 i [22,37] [38] [39] [40] [24,41] [24,33] [42] [25,43,44] [45] [46,47] + + [8–10] + Fig. 6 +