Triiodothyronine binding in rat anterior pituitary, posterior pituitary, median eminence and brain.

In vivo studies of the exchange of tracer [125-I]-L-triiodothyronine (T3) between plasma (P), and the anterior pituitary (AP), posterior pituitary (PP), median eminence (ME) and the frontal lobes of the brain (B), in the rat show that from 2.5 h onwards the concentration of new [125I]T3 in AP, PP and B were parellel to that of the plasma, with a t1/2 of 7.4 h; the t1/2 for ME was 10.3 h. The extrapolation of these curves to zero time was assumed to indicate the relative concentration of T3 per unit weight in terms of total body T3. T3 content of these tissues was determined by radioimmunoassay. The values obtained validated the steady state parameters derived from the radio-isotopic measurements. As an indicator of the concentration gradient between tissue and plasma the organ/plasma (O/P) ratio was calculated; these data indicate that under steady state conditions, the order of T3 concentration is AP greater than PP greater than ME greater than B. Binding studies have shown that AP and PP contain "specific," saturable binders while ME and B do not. Evaluation of the binding parameters of the high affinity binders in both AP and PP gave similar association constants. These associations constants, when corrected for the binding strength of T3 to plasma proteins, resulted in values similar to those of neuclear T3 binders.