Heterogeneity of pig intestinal D-glucose transport systems

Heterogeneity of intestinal D-glucose transport is demonstrated using pig j ejunal 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 ar e kinetically distinguished: high-affinity, low-capacity system 2,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 nonsaturable D-glu cose uptake component has also been detected; uptake of this component take s place at rates 10 times the rate of components characterizing the classic al diffusion marker L-glucose. It is also shown that, in this kinetic work, 1) use of D-glucose-contaminated D-sorbitol as an osmotic replacement cann ot cause the spurious appearance of nonexistent transport systems and 2) a large range (greater than or equal to 50 mm of substrate concentrations is required to correctly fit substrate saturation curves to distinguish betwee n low-affinity transport systems and physical diffusion.