The substrate selectivities of the ‘imino’ transporters in rat or rabbit small intestine are distinct (Stevens & Wright, 1985; Munck et al. 1994). Little evidence exists for an ‘imino’ carrier in the human small intestine. cDNAs isolated from rat, mouse and human (Sagne et al. 2001; Boll et al. 2002; Chen et al. 2003) induce, following heterologous expression, H⁺-coupled amino (imino) acid transport with identical characteristics to system PAT in human intestinal Caco-2 cells (Thwaites et al. 1995). Experiments were designed to determine whether hPAT1 is the human homologue of either the rat imino acid or rabbit IMINO transporter.

Two sets of compounds (1, GABA, β-aminobutyric acid (β-ABA), and α-ABA; 2, isonipecotic, nipecotic and pipecolic acids) were chosen to discriminate between the rat and rabbit systems. Substrate (all 10 mM, pH 5.5, Na⁺-free buffers)-induced H⁺ flow, into Caco-2 cell monolayers (passage no. 101-114, 13-29 days post-seeding) (Thwaites et al. 1995) loaded with the pH-sensitive fluorescent marker BCECF (2â,7â-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein), and current flow into hPAT1-expressing Xenopus laevis oocytes (cRNA 50 ng oocyte^-1, 3-5 days post-injection) (Boll et al. 2002) were determined. The ability of each compound (all 10 mM) to inhibit ³H-labelled N-methylaminoisobutyric acid ([³H]MeAIB; 20 µM, 0.5-5 µCi ml^-1) uptake (pH 5.5, Na⁺-free buffers) via hPAT1 was determined in Caco-2 cells, hPAT1-expressing oocytes or HRPE (human retinal pigment epithelial) cells (Chen et al. 2003).

A close correlation (linear regression, r² = 0.95) was observed for each substrate to induce current flow and H⁺ flow via hPAT1 in either Caco-2 cells or oocytes. GABA (ΔpH_i min^-1 0.119 ± 0.010 (8), mean ± S.E.M. (n)) and β-ABA (0.110 ± 0.011 (8)) both significantly (P < 0.001, ANOVA, Bonferroni post test) changed pH_i whereas α-ABA was without effect (P > 0.05). Transported substrates inhibited [³H]MeAIB uptake in each cell-type, e.g. in Caco-2 cells, GABA and β-ABA reduced uptake to 8.2 ± 3.2 (13) and 22.6 ± 2.9 % (14) control (both P < 0.001 vs. control or α-ABA). The relative ability of substrates to undergo transport or inhibit [³H]MeAIB uptake by hPAT1 is consistent with the predicted pattern for the rat imino acid carrier but distinct from the rabbit IMINO system.

This work was supported by the MRC (G9801704) and BBSRC (13/D17277).