Hexose Transport and Its Regulation in Mammalian Cells

Abstract

The uptake of solutes across the cell-surface membrane can occur by either an active or passive transport mechanism. Active transport mechanisms are characterized by the uptake of solutes against their concentration gradient at the expense of metabolic energy. Active transport of hexoses occurs in two major tissues in mammals, kidney and intestine, and will not be dealt with in this review. Passive transport can be subdivided into the two categories of simple diffusion or facilitative diffusion. In both cases, the movement of solutes across the cell membrane is driven solely by the concentration gradient between the intracellular and extracellular environment without any metabolic energy being required. Net uptake ceases when the concentration of solutes between the inside and outside of the cell has reached equilibrium. Facilitative diffusion differs from simple diffusion in that the former process is mediated by membrane-bound proteins which exhibit a high degree of specificity and whose activity is competitively inhibited with appropriate analogues. The difference in transport rate between simple and facilitative diffusion is dramatically exemplified by the permeability of d-glucose across synthetic lipid bilayers with a permeability coefficient of 10−9–10−10 cm/sec (Lidgard and Jones, 1975; Jung, 1971a), whereas for the intact erythrocyte, the permeability coefficient is approximately 10−4 cm/sec (Jung, 1971b).