Abstract
The presence of a Na +/ d-mannose cotransport activity in brush-border membrane vesicles (BBMV), isolated from either rat small intestine or rat kidney cortex, is examined. In the presence of an electrochemical Na + gradient, but not in its absence, d-mannose was transiently accumulated by the BBMV. d-Mannose uptake into the BBMV was energized by both the electrical membrane potential and the Na + chemical gradient. d-Mannose transport vs. external d-mannose concentration can be described by an equation that represents a superposition of a saturable component and another component that cannot be saturated up to 50 μM d-mannose. d-Mannose uptake was inhibited by d-mannose≫ d-glucose>phlorizin, whereas for α-methyl glucopyranoside the order was d-glucose=phlorizin≫ d-mannose. The initial rate of d-mannose uptake increased as the extravesicular Na + concentration increased, with a Hill coefficient of 1, suggesting that the Na +: d-mannose cotransport stoichiometry is 1:1. It is concluded that both rat intestinal and renal apical membrane have a concentrative, saturable, electrogenic and Na +-dependent d-mannose transport mechanism, which is different from SGLT1.
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