Kinetic heterogeneity of Na-D-glucose cotransport in teleost gastrointestinal tract

Abstract

D-[3H]glucose transport properties of brush-border membrane vesicles (BBMV) of upper intestine and pyloric ceca of the Pacific copper rockfish (Sebastes caurinus) were characterized and compared. Vesicles from both organs exhibited Na-dependent, phloridzin-sensitive, carrier-mediated transport systems. Kinetic constants for D-[3H]glucose influx across vesicle membranes were as follows: upper intestine, apparent affinity of glucose (Kt) = 0.14 +/- 0.02 mM, maximal glucose influx (JM) = 1,649 +/- 57 pmol.mg protein-1.10 s-1; pyloric ceca, Kt = 0.58 +/- 0.12 mM, JM = 2,439 +/- 178 pmol.mg protein-1.10 s-1. A hyperbolic relationship, following Michaelis-Menten kinetics, occurred between D-glucose influx and external Na concentration for pyloric ceca, while a sigmoidal function, following Hill cooperativity kinetics (n = 1.71 +/- 0.31), was disclosed between the variables for the intestine. External phloridzin, D-glucose, methyl alpha-D-glucopyranoside, and D-galactose were the most potent inhibitors of D-[3H]glucose influx in each organ. Other compounds were generally more inhibitory in vesicles from the pyloric cecum than those of the intestine except for D-mannose which was considerably more potent in the intestine. Results suggest that there may be proximal-to-distal hexose- and Na-binding gradients in the teleost gut for optimizing sugar absorption during passage of food through the gastrointestinal tract.