THE GLUCOSE-TRANSPORTER IN THE PLASMA-MEMBRANE OF THE OUTER SEGMENTS OF BOVINE RETINAL RODS

The facilitated diffusion glucose transporter in the plasma membrane o f intact outer segments isolated from bovine retinal rods (ROS) was ch aracterized by measurements of: (1) C-14-labeled 3-O-methylglucose flu xes; and (2) glucose-sensitive binding of H-3-labeled cytochalasin B t o ROS membranes. Inhibition of 3-O-methylglucose influx- into ROS, inh ibition of 3-O-methylglucose efflux from ROS and glucose-sensitive bin ding of cytochalasin B to ROS showed very similar cytochalasin B inhib ition/dissociation constants of 0.9 mu M, 1.3 mu M and 1.3 mu M, respe ctively. D-glucose inhibited both C-14-labeled 3-O-methylglucose trans port and cytochalasin B binding. The above results suggest that D-gluc ose-sensitive cytochalasin B binding reflects specific binding to the ROS glucose transporter and the density of glucose transporter in the ROS plasma membrane was determined to be 800 mu m(-2), comparable to r elatively abundant ROS plasma membrane proteins such as the cGMP-gated channel and the Na-Ca+K exchanger. Displacement of H-3-labeled cytoch alasin B by non-transportable hexoses was used to localize the hexose transporters to the ROS plasma membrane and to examine a simple single -site, alternating conformation model for hexose transport. A comparis on between the Glut1 glucose transporters of bovine ROS, bovine erythr ocytes and human erythrocytes suggests that kinetic and pharmacologica l characteristics of glucose transporters cannot be predicted in a sim ple manner from gene type and species.