AJP - Endocrinology and Metabolism, Vol 251, Issue 2 204-E208, Copyright © 1986 by American Physiological Society
CBG does not restrict blood-brain barrier corticosterone transport in rabbits
W. M. Pardridge, J. Eisenberg, G. Fierer and R. W. Kuhn
The metabolic clearance rate of corticosterone in rabbits is unrelated to
the physiological concentration of corticosteroid binding globulin (CBG) in
rabbit plasma. This suggests that corticosterone is available for transport
into peripheral tissues in rabbits from the circulating CBG-bound pool,
similar to what is known to occur in rat liver. This hypothesis was tested
in the present studies, which investigate the transport of corticosterone
into rabbit brain from the circulating rabbit or human CBG-bound pool.
Corticosterone was readily exchangeable in brain capillaries in vivo from
the circulating albumin-bound and rabbit or human CBG-bound pools. The
involvement of specific CBG receptors on brain capillary endothelia in this
process was investigated with [3H]-labeled human CBG prepared by reductive
methylation. The transport of [3H]CBG across rabbit brain capillaries in
vivo was immeasurably low, and no specific binding of this radiolabeled
plasma protein to isolated brain capillaries in vitro was observed at 37
degrees C during incubations up to 120 min. These studies indicate that the
rabbit is a novel system for assessing the role of CBG in delivering
corticosterone to peripheral tissues in vivo and that specific endothelial
CBG receptors may not participate in the transport process.
