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. [ 125I]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 [ 125I]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 μl/g, respectively. Co-infusion of [ 125I]rTf with unlabeled rTf or with a high dose of OX26 monoclonal antibody to the rat transferrin receptor significantly reduced the [ 125I]rTf transport, and in the presence of 10% rat serum [ 125I]rTf transport was nearly entirely abolished. The transport of [ 125I]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, [ 125I]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 [ 125I]transferrin is greatly inhibited by a small contamination of the perfusate by rat serum, which contains high concentrations of competing transferrin.
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