Rat skeletal muscle, liver and brain have different fetal and adult forms of the glucose transporter.

Rabbit antibodies made against the human erythrocyte glucose transporter were used to determine whether or not embryonic glucose transporters of rat skeletal muscle, liver and brain are identical to the transporters of adult animals. The results indicate that in both skeletal muscle and liver, the transporter switches from a highly antibody-reactive embryonic form to a low antibody-reactive adult form within 2 days of birth. This suggests that there are two different forms of glucose transporter in embryonic and adult skeletal muscle and liver. In contrast, these antibodies have equal reactivity toward the glucose transporters of embryonic and adult brain. In embryonic brain, two forms of the transporter coexist, with different molecular weights (Mr = 45,000 and 40,000), while in the adult brain the Mr = 40,000 form is predominant. The dissociation constant for glucose for the embryonic liver transporter was measured by displacement of bound cytochalasin B. The results indicate that the embryonic liver transporter has a low affinity for glucose and for cytochalasin B, similar to the adult liver transporter, even though the antibody reactivity toward these two transporters is different.