Abstract
1. The uptake and release of [131I]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.
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Selected References
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