H+-COUPLED (NA+-INDEPENDENT) PROLINE TRANSPORT IN HUMAN INTESTINAL (CACO-2) EPITHELIAL-CELL MONOLAYERS

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.