The distribution and metabolism of thyroxine and 3:5:3'-triiodothyronine in the rabbit

Estimation of thyroxine distribution in man

A group of 13 normal subjects were evaluated for their extrathyroidal thyroxine distribution. The method employed the measurement of the acute plasma disappearance of a thyroxine-(131)I tracer and its concomitant uptake into the liver and forearm. The analysis of these parameters allowed the theoretical construction of a four compartmental mathematical model system comprised of the plasma, extracellular fluid, hepatic, and extrahepatic thyroxine pools. The results of this analysis revealed that the exchange of thyroxine from the plasma into the hepatic and extrahepatic cellular fluid spaces appeared, in general, to be rapid, while the uptake into the extrahepatic tissues was relatively slow. The calculated distribution of thyroxine at equilibrium was estimated to be 14% in liver, 34% in extrahepatic tissues, and 26% each in the plasma and extracellular fluid pools in this group of normal subjects.

An examination of the extent of reversibility of thyroxine binding within the thyroxine distribution space in the rabbit

1. New Zealand white rabbits were injected intraperitoneally with [(125)]L-thyroxine. Experiments were performed after 24 hr had elapsed.2. Thyroxine distribution space was calculated as 129 ml. kg(-1). Half-life of disappearance of thyroxine from the blood of the urethane anaesthetized rabbit was 653 min.3. After exchange of approximately 20 ml. kg(-1) whole blood with heparinized blood from a non-radioactive donor, equilibration of radioactivity in the recipient had produced a peak radioactivity by 2 hr.4. The thyroxine space in which complete exchange took place within 2 hr was 102.7 +/- 4.7 ml. kg(-1) in six rabbits. This is 79.6% of the thyroxine distribution space.5. These results are compared with observations in the rat.

The effect of amiodarone, a new anti-anginal drug, on cardiac muscle

1. Amiodarone (2-butyl, 3-(4-diethylaminoethoxy, 3,5-diiodo, benzoyl) benzofuran hydrochloride), an anti-anginal drug which causes coronary dilatation and depresses myocardial oxygen consumption, was found to protect anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation.2. A 5% (73.4 mM) solution of amiodarone had no local anaesthetic action on guinea-pig skin.3. Amiodarone, 20 mg/kg (29.4 mumol/kg) given daily for 6 weeks intraperitoneally, had no effect on the resting potential or action potential height, and only a small effect on the maximum rate of depolarization, of isolated rabbit atrial or ventricular muscle fibres as shown by intracellular recording. It caused a considerable prolongation of the action potential in both tissues.4. Simultaneous administration of thyroxine (5 mug; 6.26 nmol), given daily for 3 weeks intraperitoneally, prevented the prolongation by amiodarone of the duration of the action potential.5. Treatment of rabbits with 20 mg/kg of amiodarone daily intraperitoneally for 6 weeks had no effect on the weight of the thyroid gland, but was associated with a reduction in body growth rate.6. Treatment of rabbits with 10 mg/kg (60.3 mumol/kg) of potassium iodide (equal in its iodine content to that of 20 mg/kg of amiodarone), given daily for 6 weeks intraperitoneally, had no effect on body growth rate or the duration of cardiac action potentials.7. It was concluded that amiodarone had effects on cardiac action potentials similar to those which occur after thyroidectomy.

The distribution kinetics of triiodothyronine: studies of euthyroid subjects with decreased plasma thyroxine-binding globulin and patients with Graves' disease

The kinetics of distribution of 3,3',5-triiodo-L-thyronine (T(3)) have been studied employing both a single-injection and a continuous infusion of T(3-) (131)I. External monitoring of radioactivity in the liver during the infusion permitted estimation of the hepatic distribution volume (V(H)) and the one-way hepatic clearance (C(H)) of the hormone. Among 10 euthyroid control subjects, V(H) averaged 2.07 liters +/-0.50 (SD), and the mean value for C(H), 231 ml of plasma per min (+/-64). In three euthyroid men whose plasma showed decreased binding capacity by thyroxine-binding globulin (TBG) abnormally high V(H) and C(H) values were found, the increase in C(H) being proportional to the decrease in binding activity by plasma proteins. Among all 13 subjects, there was a high correlation (+ 0.86) between C(H) and the proportion of free hormone in plasma, measured in vitro. In four patients with hyperthyroid Graves' disease V(H) ranged from 3.2 to 4.2 liters and C(H) was elevated in every case, averaging 989 ml/min. The increase in C(H) in this group was out of proportion to the elevation of free hormone fraction in plasma. Seven patients who were either euthyroid or hypothyroid after treatment of Graves' disease showed a slight but significant increase in C(H) compared with the euthyroid controls without Graves' disease. The percentage of free hormone in the plasma of the treated group was normal or low and therefore could not explain the persistent elevation in unidirectional hepatic clearance observed. The rate of accumulation of labeled T(3) in the tissues of the thigh during the interval from 10 to 60 min of the sustaining infusion of tracer was slow compared to the rate of equilibration in the liver and did not differ significantly among the various groups studied. These latter findings suggest that in slowly equilibrating tissues such as the thigh the kinetics of T(3) distribution are relatively insensitive to alterations in hormone-binding activity by plasma proteins.

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.

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.

THE MEASUREMENT OF HUMAN THYROXINE-BINDING PROTEINS BY ELECTROPHORESIS ON CELLULOSE ACETATE MEMBRANES: STUDIES ON A SPURIOUS FOURTH THYROXINE-BINDING COMPONENT

Electrophoresis of human serum enriched with 131I-labeled L-thyroxine was performed on strips of cellulose acetate in a tris(hydroxymethyl)aminomethane –ethylenediaminetetraacetate – borate buffer, pH 9.0. High-sensitivity scanning of the strips revealed the presence of a poorly defined radioactive component between α2- and γ-globulins. This component was present in all sera in which the affinity of thyroxine-binding prealbumin (TBPA) was less than 30%, and was inversely related to TBPA at thyroxine levels of 0.02,1.0, and 5.0 μg/ml. The α2γ component, like the other three thyroxine-carrying proteins, was found to vary considerably with the volume of serum applied to the strip. With increasing sample volumes, the percentage of radioactivity in the α2γ component decreased but the absolute amount remained constant. Furthermore, the α2γ component contained increasing proportions of the total radioactivity as larger amounts of stable L-thyroxine were added to the serum. Soaking the strips in buffer containing 0.06% bovine albumin before the samples were applied virtually eliminated the α2γ component and substantially increased TBPA. It was concluded that the α2γ component was due to trailing of TBPA across albumin and thyroxine-binding globulin, and that all the electrophoretic data obtained with media where some radioactivity cathodal to thyroxine-binding globulin was present should be cautiously reappraised.

Lack of significant binding of L-triiodothyronine by thyroxine-binding globulin in vivo as demonstrated by acute disappearance of 131-I-labeled triiodothyronine

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