The uptake and release of thyroxine and triiodothyronine by the perfused rat heart.

1. The uptake and release of [(131)I]L-thyroxine and L-3,5,3'-tri-iodothyronine have been studied in the rat heart perfused by Krebs-Ringer bicarbonate solution.2. After 30 min perfusion thyroxine equilibrates to a ;space' of 15 ml./g of heart (i.e. each gram of tissue accumulates thyroxine equivalent to 15 ml. of perfusion fluid). Tri-iodothyronine takes about 120 min to equilibrate to a ;space' of about 75 ml./g of heart. These results are independent of the concentration of the hormone between 1 x 10(-5) and 1.0 mug/ml.3. The uptake of thyroxine at 17 degrees C is almost identical with that at 37 degrees C.4. Radioactive thyroxine and tri-iodothyronine are both released from the heart by two distinct exponential processes, viz. a rapid process with a half-time of 14 min and a slow process with a half-time of 60 min. About 30% of the total thyroxine and about 83% of the total tri-iodothyronine are present in the slow-release compartment.