Ontogeny of Na +/ D-glucose cotransport in guinea-pig jejunal vesicles: only one system is involved at both 20°C and 35°C

Abstract

The kinetic parameters of Na +/ D-glucose cotransport were examined in fetal, newborn and adult guinea-pig jejunal brush-border membrane vesicles using a displacement curve and non-linear regression procedure. Our data indicated the presence of a single system with a K m of 0.34 ± 0.04 mM at both 20°C and 35°C. V max was increased by about 4-fold when the kinetic experiments were performed at 35°C. Since our results were not in agreement with the findings of Brot-Laroche et al. (J. Biol. Chem. (1986) 261, 6168–6176) which indicated the existence of a distinct D-glucose transport system in the adult guinea-pig jejunum at 35°C, we verified the influence of their experimental conditions on initial rate uptake measurements. In the presence of D-sorbitol instead of D-mannitol in the transport media, 70% inhibition of D-glucose uptake was observed, an effect which was attributable to contamination of sorbitol preparations by D-glucose. After removal of glucose contamination D-sorbitol did not significantly reduce the initial rate of D-glucose transport. These results led us to conclude the existence of a single D-glucose transport system in the guinea-pig small intestine and to stress the choice of experimental conditions as being crucial for an accurate estimation of kinetic parameters.