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

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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.