H⁺‐coupled (Na⁺‐independent) proline transport in human intestinal (Caco‐2) epithelial cell monolayers

Abstract

Previously, absorption of l‐proline across the apical membrane of the intestinal enterocyte has been attributed to transport via the Na+‐dependent Imino system. However, net (absorptive) transport of proline across intact Caco‐2 cell monolayers was enhanced by acidification of the apical environment, under both Na+‐containing and Na+‐free conditions. This Na+‐independent pH‐dependent proline flux was associated with H+ flow across the apical membrane as determined by continuous measurement of intracellular pH. H+/proline symport was associated with an inward I sc in voltage‐clamped Caco‐2 epithelial layers demonstrating the electrogenic nature of this transport process. In conclusion Caco‐2 cells possess an apically‐localised, Na+‐independent, electrogenic H+/imino acid transporter which may play an important role in intestinal proline absorption.

Transport of Pharmacologically Active Proline Derivatives by the Human Proton-Coupled Amino Acid Transporter hPAT1

“…The uptake of L-[ 3 H]proline was strongly stimulated by an inwardly directed H ϩ gradient ( Fig. 1) as it has been reported by Thwaites et al (1993b) already 10 years ago. At an outside pH 6.0, the uptake rate was increased 6-to 7-fold compared with transport at an outside pH 7.5.…”

“…Conflicting results reported in the literature concern the driving force (Na ϩ or H ϩ gradients), the localization of the transporters and the contribution of diverse amino acid transporters such as systems A, B, IMINO, and others to the overall proline uptake (for review and discussion, see Chen et al, 2003). Even among studies using one particular model, the Caco-2 cell, the results are on first sight incompatible (Nicklin et al, 1992;Thwaites et al, 1993b;Berger et al, 2000;Chen et al, 2003). Now that PAT1 has been cloned and studied functionally, the remaining problems will soon be resolved.…”

Identification of a Disulfide Bridge Essential for Transport Function of the Human Proton-coupled Amino Acid Transporter hPAT1

“…Subsequently, mouse and human homologs were cloned from mouse intestine (3) and from Caco-2 cells (4), respectively. PAT1 is identical to the H ϩ /amino acid cotransporter that has been functionally described in Caco-2 cells (5). It is localized mainly to the apical membrane of intestine epithelial cells and is also found in lysosomes in brain neurons (4) facilitating the transport of amino acids from luminal protein digestion or lysosomal proteolysis, respectively.…”

Angiotensin‐converting enzyme (ACE) inhibitor transport in human intestinal epithelial (Caco‐2) cells

“…However, the effect of these substrates on the steady-state uptake of ['4C]-Gly-Sar showed a different pattern ( Figure 2b). The Caco-2 cell line expresses an apically-localized H+-coupled amino acid transporter (Thwaites et al, 1993c) that is involved in transepithelial proline transport. A possible role for this H+-coupled transporter in ACE inhibitor transport was investigated (Figure 4).…”

“…Flux measurements were performed in two ways. Firstly, bidirectional proline (0.2 JACi ml-1, 50 gIM) fluxes were performed as described previously (Thwaites et al, 1993c 36 SgM). After a 30 s incubation the apical solution was rapidly aspirated and the cell monolayer was washed by sequential transfer through 4 beakers containing 500 ml icecold Krebs (pH 7.4).…”