The transport protein responsible for the absorption of di/tripeptides across the apical membrane of the human intestinal epithelium (the di/tripeptide transporter hPepT1) has been cloned and when expressed in HeLa cells functions in a Na⁺-independent, H⁺-coupled fashion (Liang et al. 1995). In contrast, dipeptide transport across the apical membrane of an intestinal epithelium (Caco-2) is indirectly dependent on extracellular Na⁺, Na⁺ being required to mediate action of the apical Na⁺-H⁺ exchanger NHE3, which maintains the driving force (the H⁺-electrochemical gradient) for absorption (Kennedy & Thwaites, 2001). Although immunocytochemical studies demonstrate that hPepT1 is absent from the basolateral membrane of the intestinal epithelium (Walker et al. 1998) a dipeptide uptake mechanism has been identified functionally (Thwaites et al. 1993). The purpose of this investigation was to determine whether this basolateral dipeptide uptake mechanism is dependent on extracellular Na⁺ either directly or indirectly (due to dependence on the basolateral Na⁺-H⁺ exchanger NHE1).

Caco-2 cell monolayers (passage number 101-126) were cultured on polycarbonate permeable supports and used 14-17 days post-seeding. [¹⁴C] Gly-Sar (10-100 µM, 0.5 µCi ml^-1, 30 min) and ²²Na⁺ (137 nM, 1 µCi ml^-1, 5 min) uptake (37 °C) across the basolateral membrane were measured in Na⁺-containing (137 mM NaCl) or Na⁺-free (137 mM choline chloride) Krebs Ringer solution (pH 5.0-7.4) (results are means ± S.E.M. (n)). At basolateral pH 6, Gly-Sar uptake was significantly greater (P < 0.05, ANOVA) in Na⁺ compared with Na⁺-free conditions (9.9 ± 2.1 (4) vs. 4.4 ± 0.7 (4) pmol cm^-2). Na⁺-dependent Gly-Sar uptake was unaffected in the presence of the NHE1 inhibitor cariporide (HOE642; Scholz et al. 1995) at a concentration (10 µM) which, at basolateral pH 7.4, significantly reduces (P < 0.001, ANOVA) basolateral ²²Na⁺ uptake (from 145 ± 5 (6) to 53 ± 7 (6) fmol cm^-2 in the absence or presence of cariporide, respectively) to the level observed in the presence of 100 µM ethyl isopropyl amiloride (EIPA) (45 ± 1 (6) fmol cm^-2, P > 0.05 vs. cariporide).

In conclusion, dipeptide uptake at the basolateral membrane of Caco-2 cell monolayers is both pH and Na⁺ dependent. However, in contrast to the apical dipeptide transporter hPepT1, the inability of the NHE1 inhibitor to modulate basolateral dipeptide uptake suggests that the Na⁺ dependency is direct rather than indirect (through activity of the basolateral Na⁺-H⁺ exchanger NHE1).

F.D.H. is funded by a GlaxoSmithKline studentship. Cariporide (HOE642) was a kind gift from A. Weichert and H.J. Lang (Aventis Pharma Deutschland GmbH, Frankfurt, Germany).