Direct assessment of dipeptide/H+ symport in intact human intestinal (Caco-2) epithelium: a novel method utilising continuous intracellular pH measurement.

Direct demonstration of intact peptide transport by the intestinal H+/dipeptide carrier is limited both by luminal/cytosolic hydrolysis and the availability of suitable radiolabelled substrates. Perfusion of Val-Val (20mM) at the apical surface of human intestinal epithelial (Caco-2) cell monolayers resulted in a marked intracellular acidification, due to dipeptide-induced H(+)-flow across the apical membrane. Val-Val (20mM) also inhibited both the pH-dependent apical-to-basal transport and intracellular accumulation of [14C]Gly-Sar. Valine (20mM) had no effect on [14C]Gly-Sar transport (and intracellular accumulation) or pHi. We conclude that this novel method for studying H(+)-coupled transport clearly differentiates between the mechanisms responsible for absorption of an intact substrate and products of hydrolysis.