A Na+-dependent D-mannose transporter in the apical membrane of chicken small intestine epithelial cells

The presence of a Na+/D-mannose cotransporter in brush-border membrane vesi cles (BBMV) isolated from chicken small intestine was examined. In the pres ence of an electrochemical gradient for Na+, but not in its absence, D-mann ose was accumulated transiently by the BBMV. D-Mannose uptake into the BBMV was energized by both the membrane potential and the chemical gradient for Na+. The relationship between D-mannose transport and external D-mannose c oncentration was described by an equation that represented the superpositio n of a saturable component (Michaelis-Menten constant K-m 12.5 muM) and ano ther component unsaturatable up to 80 muM D-mannose. D-Mannose uptake was i nhibited by various substances in the following order of potency: D-mannose >>D-glucose>phlorizin>phloretin>D-fructose. For the uptake of a-methyl-gluc opyranoside the order was D-glucose=phlorizin>>phloretin=D-fructos=D-mannos e. The initial rate of D-mannose uptake increased as the extravesicular [Na +] increased, with a Hill coefficient of 1, suggesting that the Na+:D-manno se cotransport stoichiometry is 1:1. It is concluded that the intestinal ep ical membrane has a saturable, electrogenic and concentration- and Nac-depe ndent mannose transport mechanism that differs front the sodium-dependent g lucose transporter SGLT1.