Ingrosso, L., S. Marsigliante, V. Zonno, C. Storelli, and S. Vilella. An L-proline-dependent proton flux is located at the apical membrane level of the eel enterocytes. Am J Physiol Regulatory Integrative Comp Physiol 279: R1619-R1624, 2000.-This study has demonstrated the existence of an L-proline-dependent (Na independent) proton flux at the apical membrane level of the eel intestinal absorbing cells. Using isolated eel enterocytes and the pH-sensitive fluorescent dye

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... )-5-(and-6)-carboxyfluorescein, acetoxymethyl ester (BCECF), it was shown that a 20 mM concentration of the imino acid L-proline in the extracellular medium determined an intracellular acidification of ϳ0.28 pH units. However, neither sucrose nor other amino acids were able to significantly acidify the resting intracellular pH. A hyperbolic relationship between extracellular proline concentration and intracellular proton accumulation was observed. Using both isolated brush-border and basolateral membrane vesicles, it was demonstrated that this proline-proton cotransport mechanism was located at the apical membrane level only. In addition, the existence of a coupling mechanism between proline and proton fluxes was demonstrated by the observation that, in brush-border membrane vesicles, the presence of a pH gradient (pH in Ͼ pH out ) stimulated the uptake of L-proline.