[Characteristics of the transport of structural analogs of thyroid hormone through hemato-encephalic barrier].

The kinetics of competitive inhibition of 125I-T3 transport across the blood-brain barrier by two groups of thyroid hormone analogs (THA) was studied using the carotid injection technique. We confirm previous data that the transport of 125I-T3 was saturable; the Km of T3 transport was 1.2 microM. Our results also indicate that the mechanism of cross inhibition of T3 transport by THA with alanine side chain is competitive inhibition. The replacement of alanine side chain by an acetic acid group greatly reduced the affinity of the transport system. Consequently, the development of THA with selective tissue effects must take into account differential penetration rates of THA in the brain, when correlating biological effects with nuclear binding.