Characterization of sodium-dependent glucose transport in sheep tracheal epithelium

The nonmetabolizable glucose analogue methyl(alpha-D-[U-14C]gluco)pyranoside ([14C]AMG) was used to study sodium-dependent glucose transport in two preparations: 1) discs punched from strips of sheep tracheal epithelium, and 2) freshly enzyme-isolated sheep tracheal epithelial cells. In discs, cellular accumulation of [14C]AMG was saturable and exhibited a Michaelis-Menten constant (Km) for AMG of 0.63 +/- 0.15 mM. Uptake was linear over 30 min and was inhibited maximally by 100 microM phlorizin [inhibition constant (Ki) approximately 20 nM], by replacement of external sodium with choline or by addition of 10 mM D-glucose (Ki = 0.19 +/- 0.02 mM). Accumulative uptake was activated, in a concentration-dependent manner, by external sodium [affinity constant (Ka) approximately 23 mM] with a Hill coefficient of greater than one but was abolished on depolarizing with high external potassium. In the presence of sodium, D-galactose and AMG both inhibited uptake of [14C]AMG, whereas L-glucose, D-fructose, and D-mannose were ineffective. In isolated cells, [14C]AMG accumulated only in the presence of external sodium and uptake was inhibited by the addition of D-glucose (Ki approximately 0.2 mM), D-galactose, and AMG but not by L-glucose or D-xylose. We conclude that sheep tracheal epithelium exhibits sodium-dependent glucose uptake with a very high affinity for phlorizin, which indicates the presence of a novel isoform of the transporter.