Slow fractional removal of nonextractable iodine from rat tissue after injection of labeled L-thyroxine and 3,5,3'-triiodo-L-thyronine. A possible clue to the mechanism of initiation and persistence of hormonal action.

Previous studies have shown that a small but significant proportion of radioiodine from labeled L-thyroxine (T(4)) and 3,5,3'-triiodo-L-thyronine (T(3)) is incorporated into plasma and tissue proteins and is not, therefore, extractable with ethanol or other organic solvents. Other studies have shown that the complex consists, at least in part, of the iodothyronine in apparent covalent linkage with protein. In the present series of experiments the disappearance rate of nonextractable iodine (NEI) was determined in plasma, liver, and kidney after the injection of rats with a single dose of T(4) and T(3) labeled with radioiodine in the phenolic ring. The t(1/2) of NEI decay was substantially longer than the t(1/2) of the initial metabolic removal of T(4) (16 hr) and T(3) (4-6 hr). Thus, between days 3 and 11 the average t(1/2) of plasma NEI derived from T(4) was 2.2 days, from T(3), 1.9 days; kidney NEI from T(4), 7.4 days, from T(3), 6.1 days; hepatic NEI from T(4), 4.3 days, from T(3), 5.2 days. The slow disappearance of liver NEI was of special interest in connection with an analysis of previously published data by Tata and associates dealing with the sequential tissue effects after the injection of a single dose of T(3) into thyroidectomized rats. The t(1/2) of decay of the various biological effects measured, primarily in the liver, appeared similar to each other, averaging between 4 and 6 days. These findings are compatible with the existence of a single long-lived intermediate governing the tissue expression of thyroid hormone. The t(1/2) of hepatic NEI in similarly prepared animals (thyroidectomized and injected with 25 mug of T(3)) was found to be 4.5 days. The coincidence in the slow fractional disappearance rates of hepatic NEI and the dissipation of hormonal tissue effects raises the distinct possibility that T(3) interacts with specific cellular receptor sites to form covalent complexes which are slowly removed and serve both to initiate and to perpetuate hormonal action. A mathematical analysis of hormonal reaction mechanisms, based on the assumption of a linearly responsive system, a t(1/2) of T(3) of 4 hr, and a t(1/2) of 4.5 days for the postulated long-lived "messenger" suggests that maximal expression of hormonal activity cannot be attained before 20 hr after the injection of a hormone pulse. This value is broadly consonant with the observed data accumulated by Tata and associates. The existence of a long-lived messenger, possibly a species of NEI, would therefore explain not only the slow dissipation of hormonal effects but also the well-recognized "lag-time" in the expression of hormonal action.