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 Isc 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.