The high K_m glucose transporter of islets of langerhans is functionally similar to the low affinity transporter of liver and has an identical primary sequence

The liver has been shown to contain a facilitated diffusion glucose transporter with high K_m for glucose that is structurally distinct from the low K_m glucose transporters found in most other tissues. We find that 3-O-methyl glucose is greater than 90% equilibrated across dispersed islet cells within 60 s, consistent with a facilitated diffusion transport mechanism. L-Glucose uptake was minimal throughout the time course, indicating stereospecificity. Measurement of glucose transport over a range of 3-O-methyl glucose concentrations from 0.05 to 60 mM revealed the presence of a component of glucose transport with an apparent K_m of 17 mM, a value essentially identical to that previously reported for liver. Interestingly, a second component of glucose transport was also observed with an apparent K_m of 1.4 mM, as has been reported for other tissues such as erthyrocytes that are known to contain the "HepG2" or "erythroid/brain" type glucose transporter. Further evidence for the existence of two transport components is provided by the observation that a low concentration of cytochalasin B (0.4 μM) completely inhibits the low Km transport activity but has no effect on the high K_m transporter. The kinetic similarity of high K_m glucose transport in liver and islets is readily understood in light of our structural analysis. Sequence analysis of cDNA clones indicates that the liver and islet glucose transporters have identical sequences and, thus, are the products of the same gene.