Absence of albumin receptor on brain capillaries in vivo or in vitro

Hormones and drugs are known to be available for transport into brain and liver in vivo from the circulating albumin-bound pool. An albumin receptor-mediated mechanism is one possible way in which the transport of ligands from the circulating albumin-bound pool into the tissue may be catalyzed. The albumin receptor model was tested for brain in the present studies using both 125I-albumin (labeled by lactoperoxidase) and [3H]albumin (labeled by reductive methylation). The interaction of the labeled albumin with brain capillaries was assessed in vivo with the carotid injection technique in rats and in vitro with isolated bovine brain capillaries. Artifactually high nonspecific binding in both the in vivo and in vitro assays was observed with 125I-albumin. Conversely, the transit time of [3H]albumin through the brain capillaries in vivo was no greater than the transit time of [14C]sucrose. The binding of [3H]albumin to isolated microvessels in vitro was low, less than [3H]inulin and was nonsaturable. In conclusion these studies do not support the albumin receptor model for the transport of albumin-bound ligands into tissues such as brain.