Transport of thyroid and steroid hormones through the blood-brain barrier of the newborn rabbit: primary role of protein-bound hormone.

Abstract

The transport of [125I]T3 [125I]T4, [3H]testosterone, and [3H]estradiol through the blood-brain barrier (BBB) of the newborn rabbit was studied with a tissue-sampling, single injection technique. The first pass extractions of T3 and T4 were 22 ± 2% and 14 ± 1%, respectively, after carotid injection of Ringer's solution (0.025% albumin). Thyronine transport was saturated and cross-competitive, but was not inhibited by a large excess of leucine, a neutral amino acid. The extraction of T3 was reduced to 7 ± 1% after injection of hormone mixed in a 10% T3- specific rabbit antiserum, and this value approximated the extraction of an extracellular space marker, such as sucrose (6 ± 1%). Therefore, antibody-bound T3 was not available for transport in vivo. Conversely, T3 bound to the plasma proteins in newborn rabbit antiserum, e.g. albumin, was transported into the brain. The concentration of plasma protein required to inhibit T3 transport 50% in vivo was 28-fold greater than the serum concentration that resulte...