The metabolic significance of exchangeable cellular thyroxine

Thyroxine uptake by perfused rat liver. No evidence for facilitation by five different thyroxine-binding proteins

Rates of hepatic uptake of thyroxine (T4) from dilute solutions of five different plasma T4-binding proteins were measured in the isolated perfused rat liver using an indicator dilution method. For each protein, this rate was compared with the rate of spontaneous dissociation of the T4-protein complex measured in vitro. Proteins studied were human T4-binding globulin (TBG), human T4-binding prealbumin (TBPA), human albumin, rat TBPA, and human albumin isolated from subjects with familial dysalbuminemic hyperthyroxinemia. For each of the five protein-hormone complexes studied, the rate of hepatic uptake of T4 (measured under conditions expected to result in dissociation-limited uptake) closely approximated the rate of spontaneous dissociation of the protein-hormone complex within the hepatic sinusoids. These findings indicate an absence of special cellular mechanisms that facilitate the hepatic uptake of T4 from its plasma binding proteins, and support the view that uptake occurs from the free T4 pool after spontaneous dissociation of T4 from its binding proteins.

Stereospecific transport of triiodothyronine from plasma to cytosol and from cytosol to nucleus in rat liver, kidney, brain, and heart

We have investigated the transport of L- and D-triiodothyronine (T3) from plasma to cellular cytoplasm and from cytoplasm to nucleus by estimating the concentration of free hormone in these compartments in rat liver, kidney, brain, and heart. We assessed the distribution of T3 in various tissues and its metabolism by standard isotopic techniques and measured plasma and cytosolic tissue T3 by radioimmunoassay. In addition, we determined the fraction of radiosensitive T3 associated with the cytosol in individual tissues and estimated the cytosolic volume per gram of tissue. Equilibrium dialysis allowed us to determine the binding power of cytosols and plasma, and in vitro saturation techniques provided values for the affinity (ka) for L- and D-T3 of isolated nuclei in aqueous solution at 37 degrees C. We calculated the free cytosolic hormone from the product of cytosolic T3 and the binding power of cytosol for T3, and the free intranuclear T3 from the ka and previously determined ratio of occupied-to-unoccupied binding sites under steady state conditions in euthyroid animals. Our results showed that the free cytosolic/free plasma concentrations for L-T3 and D-T3, respectively, were: liver 2.8, 21.6; kidney 1.17, 63.3; heart 1.31, 1.58; brain 0.86, 0.24. The free nuclear/free cytosolic ratios for L-T3 and D-T3, respectively, were: liver 58.2, 3.70; kidney 55.9, 1.54; heart 80.6, 24.9; and brain 251, 108.6. Our findings suggest that stereospecific transport occurs both from plasma to cytosol and from cytosol to nucleus. The high gradients from cytosol to nucleus imply that there is an energy-dependent process and appear to account for the differences in the nuclear association constant determined in vivo and in vitro.

Inhibition of hepatic binding of thyroxine by cholecystographic agents

Subsequent to studies indicating that cholecystographic agents and sulfobromophthalein (BSP) inhibit uptake of thyroxine (T(4)) by rat liver slices, the effect of such compounds on hepatic storage of T(4) in man has been examined. After intravenous administration of [(125)I]T(4) to five normal subjects, hepatic radioactivity, estimated by external gamma counting, rose to a peak in approximately 4 h and then declined in parallel with serum radioactivity. When a 6-g dose of the cholecystographic agent, tyropanoate (Bilopaque), was administered orally 3 d later, estimated hepatic extravascular radioactivity fell 50-60% within 4 h and then rose toward the pretyropanoate value. Concomitant with the fall in hepatic radioactivity, serum radioactivity rose 57-70%, as did stable T(4) levels in serum, suggesting that hormone discharged from the liver entered the serum. Both uptake of T(4) and discharge by tyropanoate were much less in two patients with liver disease. Ipodate (Oragrafin), 12 g orally in two subjects, caused much smaller changes in hepatic and serum radioactivity. However, ipodate also caused a doubling of the percent free T(4) in the serum as judged by equilibrium dialysis, and the ratio of hepatic radioactivity to free [(125)I]T(4) in serum declined markedly after ipodate, similar to the fall in hepatic:serum (125)I ratios after tyropanoate. BSP, 20 mg/kg i.v. in three subjects, caused a smaller change; the decline in hepatic T4 content averaged 19%. We conclude that these organic anions, tyropanoate, ipodate, and BSP, all can displace T(4) from the liver. The results imply a link between T(4) transport and organic anion transport, and indicate a mechanism for altering hepatic T(4) content in man that could be the site of physiologic regulation or of disease. A method is described whereby analysis of the change in hepatic and plasma radioactivity after tyropanoate could be employed to estimate hepatic T(4) content in diverse clinical circumstances.

The transcapillary exchange of thyroid hormones and thyroxine-binding proteins between blood and tissue fluids

A study has been made of the relative importance of potien bound and unbound hormone in the exchange of thyroid hormones between blood and interstitial fluid. 2. When [I] tyroxine (or thriiodothyronine) and [I] human serum albumin were injected simultaneously into the circulation of sheep with chronic lymphatic fistulae, the thyroid hormones were removed from the circulation and apperaed in all lymph samples at a greater fractional rate than human serum albumin. 3. The steady-state lymph/plasma concentrations ratios of the two specific thyroxine binding proteins were similar to each other and to those of albmin and total thyroxine. 4. Gel filtration studies indicated that the two specific thyroxine binding proteins, ovine serum albumin and human serum albumin, were all of similar molecular size. 5. Concentrations of unbound thyroxine in plasma and various samples of lymph from the one animal were similar. 6. Increasing the proportion of thyroid hormone that was unbound resulted in an increase rate of equilibration of labelled hormone between blood plasma and lymph. 7. Perfusion of the popliteal lymph node demonstrated that thyroid hormones were removed from lymph during its passage through the node. The amount removed was related to the proportion of hormone in the unbound state.

Relative rates of transcapillary movement of free thyroxine, protein-bound thyroxine, thyroxine-binding proteins, and albumin

The rate of appearance of labeled thyroxine (T4) and albumin in lymph from various areas after simultaneous i.v. injection of the labeled substances in conscious ambulatory sheep has been used to estimate the relative rates of transcapillary movement of stable T4 and albumin. Labeled T4 appeared in hepatic lymph at the same rate as albumin. In intestinal and leg lymph, labeled T4 appeared eight and four times as rapidly as albumin indicating that T4 crosses capillaries in these areas independently of and much more rapidly than albumin and other proteins having similar distribution kinetics. The lymph:plasma ratios for all the T4-binding proteins including albumin were very similar in any one area showing that the relative fractional rates of transcapillary movement of these proteins were very similar. Therefore in extrahepatic areas, transcapillary movement of T4 in the protein-bound form was quantitatively much less important than in the free form. The findings support earlier views, recently questioned, that free T4 is of considerable physiological significance.

Evidence for enhanced cellular uptake and binding of thyroxine in vivo during acute infection with Diplococcus pneumoniae

Previous work has demonstrated that acute pneumococcal infections in man and in the rhesus monkey are accompanied by accelerated metabolic disposal of L-thyroxine (T(4)). In order to study the influence of acute pneumococcal infection on the kinetics of hormone distribution, the early cellular uptake of T(4) (CT(4)), reflecting the net effect of plasma and cellular binding factors, was assessed in rhesus monkeys from the differences in instantaneous distribution volumes of T(4)-(131)I and albumin-(125)I during the first 60 min after their simultaneous injection. Hepatic and renal uptakes of (131)I were also determined. Plasma binding of T(4) was assessed by measuring the per cent of free T(4) (% FT(4)) in serum. Six monkeys were studied 12 hr (INF-12) and seven 24 hr (INF-24) after intravenous inoculation with Diplococcus pneumoniae; seven controls were inoculated with a heat-killed culture. CT(4) at 60 min as per cent administered dose was 31.5 +/-2.0 (mean +/-SE) in INF-12 and 33.0+/-0.8 in INF-24, values significantly greater than control (22.4+/-1.3). By contrast, mean% FT(4) was identical in control and INF-12 (0.028 +/-0.002 and 0.028 +/-0.001) and variably increased in INF-24 (0.034 +/-0.003). Thus, in the infected monkeys CT(4) and% FT(4) were not significantly correlated. The increased CT(4) in the infected monkeys could not be ascribed to an increase in vascular permeability and did not correlate with the magnitude of fever. Although the increased CT(4) could not be accounted for by increased hepatic or renal uptake of hormone, hepatic and renal T(4) spaces were increased, results consistent with increased binding by these tissues. Our data indicate that the cellular uptake of T(4) is increased early in acute pneumococcal infection and suggest that this results from a primary enhancement of cell-associated binding factors for T(4).

Active secretion of thyroxine into bile: the role of tissue thyroxine-binding sites

1. An investigation has been made into factors regulating the secretion of thyroid hormones into bile. The preparation employed was the isolated rat liver perfused with a modified Tyrode solution.2. Tracer amounts of radioactive thyroxine were injected into the Tyrode solution just before its entry into the liver. This allowed the thyroxine-binding sites in the tissue to capture most of the injected hormone. It was found that this tissue-bound hormone was then gradually secreted into the bile and that the process could be resolved into two components. There was a steady, continuous secretion of hormone, which maintained a fairly constant bile:liver ratio in thyroxine concentration of about 2.3. There was also, superimposed upon this, an extra transient secretion of thyroxine occurring just after the injection. This transient effect was abolished by injecting the hormone at 22 degrees C and it was probably due to the sudden flood of free thyroxine into the tissue during the 15 sec injection period.3. Very similar results were obtained with tri-iodothyronine except that the secretion process maintained a higher bile:liver ratio of about 3.7.4. The secretion mechanism was extremely sensitive to changes in temperature and it was able to generate very high concentrations of free thyroxine in the bile. It was concluded that the over-all process was probably an active one.5. The presence of 30% bovine serum in the perfusion fluid did not reduce the bile:liver thyroxine ratio even though it caused a massive fall in the amount of thyroxine in the tissue.6. These results show that the rate of thyroxine secretion into bile is determined by the total amount of thyroxine trapped in the tissue. It is not directly related to the concentration of free thyroxine in the perfusing medium.7. It is suggested that the secretion mechanism works from a small intracellular pool of free thyroxine which is in rapid exchange with the large pool of tissue-bound hormone, and that thyroxine exchange between this small intracellular pool and the free thyroxine pool in the plasma is relatively slow.

The mechanism of thyroxine transfer from plasma to tissue binding sites

1. An examination has been made of the uptake of thyroxine by rat livers perfused with very dilute solutions of thyroxine-binding globulin (TBG) and thyroxine-binding pre-albumin (TBPA).2. Under these circumstances the rate of transfer of thyroxine to the liver is very similar to the rate of thyroxine dissociation for both TBG and TBPA.3. It is concluded that thyroxine transfer from plasma to tissue-binding sites involves its initial release into the free state and not, as has been suggested, directed exchange.

Human thyroxine-binding globulin and thyroxine-binding pre-albumin: dissociation rates

1. A simple method is described for estimating the rate of thyroxine dissociation from binding sites in plasma.2. At tracer concentrations in human plasma thyroxine dissociates at two distinct exponential rates. These are due to the two thyroxine-binding proteins present in human plasma.3. The t((1/2)) for thyroxine dissociation from thyroxine-binding globulin is 8.1 +/- 0.7 (S.E.) min at room temperature and 38.6 +/- 2.1 sec at 37 degrees C.4. The t((1/2)) for thyroxine dissociation from thyroxine-binding pre-albumin is 53 +/- 5.0 sec at room temperature and 7.9 +/- 0.3 sec at 37 degrees C.5. Dissociation of thyroxine from both proteins is a single exponential process, suggesting that each protein has only one type of specific binding site.6. These results indicate that the minute pool of free thyroxine in plasma is turning over about 100 times each second, with about two thirds of this flux being due to release and binding of the hormone by pre-albumin.

Differences in primary cellular factors influencing the metabolism and distribution of 3,5,3'-L-triiodothyronine and L-thyroxine

Administration of phenobarbital, which acts exclusively on cellular sites, results in an augmentation of the liver/plasma concentration ratio of L-thyroxine (T4) in rats but no change in the liver/plasma concentration ratio of L-triiodothyronine (T3). Whereas phenobarbital stimulates the fecal clearance rate both of T3 and T4, it increases the deiodinative clearance rate of T4 only. These findings suggest basic differences in the cellular metabolism of T3 and T4. Further evidence pointing to cellular differences was obtained from a comparison of the distribution and metabolism of these hormones with appropriate corrections for the effect of differential plasma binding. The percentage of total exchangeable cellular T4 within the liver (28.5) is significantly greater than the corresponding percentage of exchangeable cellular T3 within this organ (12.3). Extrahepatic tissues bind T3 twice as firmly as T4. The cellular metabolic clearance rate (= free hormone clearance rate) of T3 exceeds that of T4 by a factor 1.8 in the rat. The corresponding ratio in man, 2.4, was determined by noncompartmental analysis of turnover studies in four individuals after the simultaneous injection of T4-(125)I and T3-(131)I. The greater cellular metabolic clearance rate of T3 both in rat and man may be related to the higher specific hormonal potency of this iodothyronine.