Heterogeneity of pig intestinald-glucose transport systems

Heterogeneity of intestinald-glucose transport is demonstrated using pig jejunal brush-border membrane vesicles in the presence of 100/0 (out/in) mM gradients each of NaCl, NaSCN, and KSCN. Two d-glucose transport systems are kinetically distinguished: high-affinity, low-capacity system 1, which is equivalent to the symporter SGLT1; and low-affinity, high-capacity system 2, which is not a member of the SGLT family but is a d-glucose and d-mannose transporter exhibiting no preference for Na over K . A nonsaturabled-glucose uptake component has also been detected; uptake of this component takes place at rates 10 times the rate of components characterizing the classical diffusion marker l-glucose. It is also shown that, in this kinetic work, 1) use of d-glucose-contaminatedd-sorbitol as an osmotic replacement cannot cause the spurious appearance of nonexistent transport systems and 2) a large range (≥50 mM) of substrate concentrations is required to correctly fit substrate saturation curves to distinguish between low-affinity transport systems and physical diffusion.