The uptake and release of thyroxine and tri‐iodothyronine 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 × 10 −5 and 1·0 μg/ml. 3. The uptake of thyroxine at 17° C is almost identical with that at 37° 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.