TRANSPORT OF [I-125] TRANSFERRIN THROUGH THE RAT BLOOD-BRAIN-BARRIER

Transferrin receptors are present in the plasma membrane of brain endothelial cells but it is unclear if these receptors mediate transport of transferrin across the blood-brain barrier (BBB). In the present study, the transport of rat holo-transferrin (rTf) across the BBB in vivo was evaluated in ketamine anesthetized rats (250-300 g) both by in situ brain perfusion coupled with capillary depletion analysis and by thaw-mount autoradiography. [I-125]rTf was infused through the right internal carotid artery at a rate of 3.65 ml/min for 2.5-5 min. After a 5 min perfusion, the volume of distribution (VD) of [I-125]rTf in the brain homogenate, the postvascular supernatant, or the vascular pellet was 55.8 +/- 4.5, 43.5 +/- 4.8, and 8.7 +/- 2.3 mu l/g, respectively. Go-infusion of [I-125]rTf with unlabeled rTf or with a high dose of OX26 monoclonal antibody to the rat transferrin receptor significantly reduced the [I-125]rTf transport, and in the presence of 10% rat serum [I-125]rTf transport was nearly entirely abolished. The transport of [I-125]rTf across the BBB in vivo was demonstrated by thaw-mount autoradiography, which showed silver grains well within brain parenchyma following a 5 min internal carotid artery perfusion. These studies are consistent with the following conclusions: (a) in the absence of competing plasma transferrin, [I-125]holotransferrin is transported through the BBB at a rate comparable to the OX26 monoclonal antibody; and (b) the ability to detect measurable transport of perfused [I-125]transferrin is greatly inhibited by a small contamination of the perfusate by rat serum, which contains high concentrations of competing transferrin.