The uptake of thyroxine and tri-iodothyronine by perfused hearts.

1. An examination has been made of the uptake of radioactive thyroxine and tri-iodothyronine by the perfused hearts of the rat, rabbit, chicken and frog.2. In all species the uptake of both hormones reaches a steady state at which the uptake of tri-iodothyronine is 2-4 times that of thyroxine.3. The uptake of thyroid hormones by perfused hearts can be resolved into two distinct components-first, an initial transient process which takes up hormone and then releases it with a peak uptake at about 20 min from the beginning of perfusion and second, a basic, slow-uptake process which takes several hours to reach a steady state and which has a greater affinity for tri-iodothyronine. Analysis of the ;initial uptake-and-release effect' suggests that it is due to the presence of plasma thyroxine-binding protein in the interstitial fluid of the heart.4. These two uptake processes provide the basis for the biphasic release of thyroid hormones previously described for the rat heart.