Thyroid hormone action: the mitochondrial pathway

Inotropic effect of triiodothyronine following myocardial ischemia and cardiopulmonary bypass: an experimental study in pigs

A significant reduction (p less than 0.0001) in plasma-free triiodothyronine (T3), which is known to have an inotropic effect, has been documented in patients undergoing open-heart procedures. To investigate the effect of this observation, 22 pigs underwent 2 hours (Group 1, r = 10) or 3 hours (Group 2, r = 12) of myocardial ischemia during cardiopulmonary bypass (CPB) at 26 degrees C; the myocardium was protected by cardioplegic solution and cold saline solution at 30-minute intervals. After the pig was rewarmed to 37 degrees C, CPB was discontinued, and measurements of hemodynamic function were made 10 and 70 minutes later. Half of the pigs (Subgroup B) received 6 micrograms of T3 intravenously immediately after removal of the aortic cross-clamp; the remainder (Subgroup A) received no T3. After 2 hours of ischemia, untreated pigs showed significantly reduced myocardial function 10 minutes after discontinuation of CPB. By 70 minutes after the end of CPB, 2 of 5 untreated pigs (Subgroup A) had died of low cardiac output, but all 5 treated pigs (Subgroup B) survived. After 3 hours of ischemia, both groups showed some reduced function at 10 minutes, though the reduction was more marked in untreated animals. By 70 minutes, 4 of 6 untreated pigs had died of myocardial failure and all treated pigs remained alive (p less than 0.03). Surviving pigs in both groups still demonstrated some reduced function compared with values obtained before CPB. When all pigs are considered together, overall survival of those that did not receive T3 was significantly less than those that did (p less than 0.006).(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone action: early calorigenic effect on dispersed rat liver cells in the absence of protein synthesis

Isolated dispersed rat liver cells were prepared by hypothyroid Sprague-Dawley rats. The cells were incubated under 95% O2/5% CO2 in Krebs-Ringer-bicarbonate buffer at pH 7.3-7.4 at 37 degrees C. The medium had been enriched with 2% bovine serum albumin (previously stripped of thyroid hormone) and 5-10 mM alanine as substrate. Two hour incubations were carried out with or without added triiodothyronine (T3) at 3 nM or 300-1,000 nM concentrations. Oxygen consumption determined at the end of the period of incubation with the Clark oxygen electrode showed stimulation above control values in the hormone treated flasks; parallel studies in which cycloheximide (100 microM) had been added to cells to block protein synthesis also showed enhanced oxygen consumption in response to T3. The results indicated a response to the hormone not dependent on new protein formation.

Thyroid hormone uptake into the cell and its subsequent localisation to the mitochondria

The nature of thyroid hormone uptake into the cell and the possible involvement of the serum carrier proteins and receptor-mediated endocytosis in this process are reviewed. The evidence that there is a specific thyroid hormone-binding receptor in the inner mitochondrial membrane and the relation of this to the adenine nucleotide translocator is discussed. Direct effects of thyroid hormone on mitochondrial function that might be mediated by such a receptor are also considered.

Evidence for ADP-ribosylation in the mechanism of rapid thyroid hormone control of mitochondria

Triiodothyronine in vitro at concentrations between 10(-13) and 10(-11) M very rapidly activates oxidative phosphorylation in hypothyroid rat liver mitochondria. Comparing the concentrations of hormone with estimates of the amounts of respiratory chain components present suggests that this activation may involve an amplification mechanism. Here we present evidence that while no changes in phosphorylation were detected following hormone administration, nicotinamide, an inhibitor of mono ADP-ribosylation reported to occur rapidly and reversibly in mitochondria, prevented activation by hormone. Moreover incubation with nicotinamide of euthyroid mitochondria and derived intact inner membrane vesicles revealed lowered ADP/O ratios under the same conditions as shown by hypothyroid preparations. While this lesion could be reversed simply by washing the intact mitochondria, the membrane vesicles required triiodothyronine addition.

Cellular mechanism of action of thyroid hormones

It has emerged in the last decade that the molecular mechanism of action of thyroid hormones resembles that of steroids; thyroid hormones indeed exert their effects mainly by directly regulating gene expression, on association with specific chromatin-bound receptors. Of the two thyroid hormones, thyroxine (T4) appears to be a sort of prohormone, whereas triiodothyronine (T3) seems to be the active form; in this respect, T4-deiodination, which occurs at the level of the target tissues, may be crucial in the local homeostasis of T3. Moreover, many cellular compartments, other than the nucleus, can bind thyroid hormone, and at least some of these further sites might play some role in modulating T3 supply to the nucleus. The binding of the T3-receptor complex to chromatin is likely to regulate the structural organization of specific genes and, in some instances, of the chromatin as a whole.

The influence of nanomolar calcium ions and physiological levels of thyroid hormone on oxidative phosphorylation in rat liver mitochondria. A possible signal amplification control mechanism

Using different conditions mitochondria from hypothyroid rats can show both unchanged ADP/O ratios and lowered ADP/O ratios without evidence of uncoupling when compared with euthyroid controls. Raising the free Ca2+ concentration to around 25 nM progressively lowered the ADP/O ratio in hypothyroid but not in euthyroid mitochondria. Ruthenium Red did not alter this behaviour and further increasing the Ca2+ concentration to levels below those which stimulate State 3 respiration had no additional effect. Measurements of the free Ca2+ concentration in the mitochondrial suspending medium using a Quin 2 fluorescence assay showed that the mitochondria did not buffer the free Ca2+ at these low concentrations. At 25 nM-free Ca2+, addition of 10-13) M-T3 to hypothyroid mitochondria produced an immediate and significant increase in the ADP/O ratio without altering the free Ca2+ concentration. The hormone effect was maximal by 10(-11) M. The concentration of ATP synthetase can be estimated to lie at about 10 nM in these experiments. Hence it appears possible that a substantial amplification of the hormone signal may have taken place. Comparison with binding studies suggests that T3 may have been maximally stimulating when somewhat less than half its receptor sites had been filled. The possible mechanisms by which this receptor mediated alteration of the ADP/O ratio might be achieved are discussed.

Hormonal effects on mitochondrial respiration: potential role of endogenous lipolytic activities

Hormonal effects on heart mitochondrial metabolism are investigated by comparing respiratory rates, Ca2+ uptake capacity, and lipolytic activities of mitochondria isolated from control rats to those of mitochondria isolated from thyroparathyroidectomized animals. Two biochemically and morphologically distinct populations of heart mitochondria are prepared--one derived from the region of the cell directly beneath the sarcolemma (subsarcolemmal mitochondria), the other originally between the myofibrils (interfibrillar mitochondria). Subsarcolemmal mitochondria isolated from normal rat cardiac tissue have both lower respiratory rates and Ca2+ uptake capacity than do interfibrillar mitochondria. However, when these mitochondrial populations are isolated from hearts from thyroparathyroidectomized rats, there is a selective increase in the maximal ability of the subsarcolemmal mitochondria to accumulate Ca2+, which is accompanied by a proportionate increase in their maximal respiratory rates. Neither Ca2+ uptake capacity nor respiratory rates are similarly increased in the interfibrillar mitochondria. Cytochrome contents and mitochondrial protein recoveries are not significantly changed in either of these mitochondrial preparations. The relationship between these selective increases in respiratory properties of the subsarcolemmal mitochondria to endogenous lipolytic activities is also investigated. It was previously demonstrated that, in the absence of Ca2+, both the rate and extent of formation of free fatty acids from endogenous phospholipids is greater in subsarcolemmal than interfibrillar mitochondria (J. W. Palmer et al. (1981) Arch. Biochem. Biophys. 211, 674-682). In this study it is shown that lipolysis is also more sustained in the subsarcolemmal mitochondria when Ca2+ is added. In the subsarcolemmal mitochondria isolated from thyroparathyroidectomized rats, however, the rates of release of stearic acid and oleic acid are reduced in both the presence and absence of Ca2+. In the presence of added Ca2+, the rate of release of arachidonic acid is also decreased compared to control subsarcolemmal mitochondria, suggesting that the expressed activity of Ca2+-activated phospholipase A2 is lower in those mitochondria isolated from the thyroparathyroidectomized animals, in which respiratory rates and Ca2+ uptake capacity are increased.

The rapid alteration by tri-iodo-L-thyronine in vivo of both the ADP/O ratio and the apparent H+/O ratio in hypothyroid-rat liver mitochondria

Mitochondria from the livers of thyroidectomized rats have a lowered ADP/O ratio, which can be restored to normal within 15 min after intravenous injection of a near-physiological dose of tri-iodothyronine. Thyroidectomy lowered the measured delta pH, which appears to be compensated by a rise (not statistically significant) in the membrane potential, so that the protonmotive force is unaltered. A simple simulation technique is described for use in estimating H+/O ratios by the oxygen-pulse technique, which circumvents the problem that this ratio can be seriously underestimated because of re-uptake of protons from the bulk phase by the mitochondria before their expulsion is complete. By this procedure the H+/O ratio of hypothyroid mitochondria is shown to be lowered by the same factor as the ADP/O ratio, and both these ratios are very rapidly restored in parallel by hormone administration. Although these findings could be consistent with a proposal that tri-iodothyronine rapidly modulates by some mechanism the efficiency of the respiratory-chain-linked proton pumps, the kinetic properties of the proton exchange suggest that the bulk-phase protons measured may not reflect faithfully those that drive the ATP synthetase.

Salinity adaptation in fish: interaction of thyroxine with fish gill mitochondria

When the freshwater fish Sarotherodon mossambicus is exposed to an ionoosmotic stress, extensive changes take place in the energetics of the gill mitochondria. These changes are reversed when thyroxine is administered to the fish prior to exposure to stress [K. Shivakumar and J. Jayaraman (1984) Arch. Biochem. Biophys. 233, 728]. The presence of a thyroxine binding component in the mitochondrial inner membrane, its characteristics, and its possible involvement in the salinity adaptation process are discussed.

Regulation of respiration and ATP synthesis in higher organisms: hypothesis

The present view on the regulation of respiration and ATP synthesis in higher organisms implies only Michaelis-Menten type kinetics and respiratory control as regulatory principles. Recent experimental observations, suggesting further regulatory mechanisms at respiratory chain complexes, are reviewed. A new hypothesis is presented implying regulation of respiration and ATP synthesis in higher organisms mainly via allosteric modification of respiratory chain complexes, in particular of cytochrome c oxidase. The allosteric effectors, e.g., metabolites, cofactors, ions, hormones, and the membrane potential are suggested to change the activity and the coupling degree of cytochrome c oxidase by binding to specific sites at nuclear coded subunits. Recent results on the structure and activity of cytochrome c oxidase, supporting the hypothesis, are reviewed.

Biochemical effects of mild iron deficiency and cold acclimatization on rat skeletal muscle

Most of the previous studies on the effects of iron deficiency on skeletal muscle respiratory capacity and work performance have been investigated in severe or moderate iron-deficiency anemia. We report here that even in mild iron deficiency where the hemoglobin concentration was 10 g/dl and the iron stores in livers and spleen were not completely depleted, a marked reduction in succinate dehydrogenase was observed in skeletal muscles but not in heart. Similarly, cytochrome oxidase activities were reduced. Although no significant change in glycerophosphate dehydrogenase was detected in the iron-deficient rats, exposure to cold in this group greatly reduced this enzyme activity. As cold acclimatization accelerates marrow erythropoiesis (20) which in turn, demands more iron, it seems that in the iron-insufficient state, this iron demand for marrow activity may persist at the expense of the tissue iron pool, resulting in a marked reduction in glycerophosphate dehydrogenase activities. Since succinate dehydrogenase plays a significant role in the impairment of mitochondrial function and early fatigue of iron-deficient muscle (11), the present study shows that even in mild iron deficiency, some loss of muscle functions could result as succinate dehydrogenase activities were greatly reduced.

The translation system of rat heart muscle mitochondria is stimulated following treatment with L-triiodothyronine

In vitro translation of mitochondrial translation products by mitochondria isolated from the myocardia of rats injected with L-triiodothyronine daily for 5 days was compared with those of euthyroid animals. The incorporation of 35S-methionine into proteins was greater by heart mitochondria isolated from T3-treated rats than by those isolated from euthyroid animals. This increase is due to a stimulation of mitochondrial protein synthesis rather than being the result of a reduction in the rate of protein degradation or an increase in the specific radioactivity of the amino acid pool. The results also established that the polypeptide profile of mitochondrial translation products is the same in heart mitochondria isolated from euthyroid and T3-treated animals. The relative increase in the rates of synthesis of mitochondrial translation products is non-uniform. These results suggest that the hormone acts by inducing a general but non-uniform increase in the activity of the mitochondrial transcription and/or translation system(s).

Effect of a failure of energy supply on adenine nucleotide breakdown in placentae and other fetal tissues from rat and guinea pig

The effects of ischaemia on adenylate energy charge of tissues from fetal rats and fetal guinea pigs were measured. Adult rat and guinea-pig tissues, as well as human placentae, were also studied. The largest differences observed were between the fetuses from different pregnant animals (P = 4.74 X 10(-15). Reductions in energy charge in placentae were slower than in other defined fetal tissues, especially brain. In the rat, an immature species at birth, greater 'stability' was observed in placentae of 14 days of gestation than near term at 20 days of gestation. As contrast, in the guinea pig, a mature species at birth, there was no difference in 'stability' in placenta or other fetal tissues between about 40 days of gestation and near term, about 60 days of gestation. In addition to these tissue and maturity effects in the fetus, it has been confirmed that fetal tissues are more 'resistant' than adult tissues to failures of energy supply. Concentrations of adenosine, uridine, guanosine and cytidine nucleotides in placenta show similar patterns in rats and guinea pigs. Fetal liver contains more uridine nucleotides and brain more cytidine nucleotides. It is suggested that the placenta retains an early fetal ability to maintain itself during ischaemia; this might be advantageous during parturition. Possible endocrine and other mechanisms 'damping' fetoplacental metabolism are linked with a discussion of the large maternal effect.

Heat exposure and hypothyroid conditions decrease hydrogen peroxide generation in liver mitochondria

Exposure of rats to heat (39 +/- 1 degree C) decreased H2O2 generation in mitochondria of the liver, but not of the kidney or the heart. The effect was obtained with three substrates, succinate, glycerol 1-phosphate and choline, with a decrease to 50% in the first 2-3 days of exposure, and a further decrease on longer exposure. The dehydrogenase activity with only glycerol 1-phosphate decreased, which is indicative of the hypothyroid condition, whereas choline dehydrogenase activity remained unchanged and that of succinate dehydrogenase decreased on long exposure. The serum concentration of thyroxine decreased in heat-exposed rats. Thyroxine treatment of rats increased H2O2 generation. Hypothyroid conditions obtained by treatment with propylthiouracil or thyroidectomy caused a decrease in H2O2 generation and changes in dehydrogenase activities similar to those with heat exposure. Treatment of heat-exposed or thyroidectomized rats with thyroxine stimulated H2O2 generation by a mechanism apparently involving fresh protein synthesis. The results indicate that H2O2 generation in mitochondria of heat-exposed animals is determined by thyroid status.

Oligomycin toxicity in intact rats

Intraperitoneal injection of oligomycin into the rat (0.5 mg per kg, corresponding to the LD33 dose) reduces the oxygen consumption by about 50%, whereas the arterial pO2 remains normal. The large extent of this decrease points to an involvement of liver and muscle tissue. Triiodothyronine pretreatment (3 doses of 0.075 mg/100 g body weight) is not able to prevent this effect. From the blood metabolites measured glucose, pyruvate and the parameters of lipid metabolism remain unchanged; only lactate is significantly increased, causing compensated metabolic acidosis. Heart rate, systolic blood pressure and electrocardiogram are essentially unchanged. Oliguria, reduced renal excretion of urea and increase of plasma urea also indicate a nephrotoxic action. The results are discussed in comparison with some effects of experimental uremia.

The influence of thyroid hormone on the degree of control of oxidative phosphorylation exerted by the adenine nucleotide translocator

Impaired phosphorylation efficiency in liver mitochondria from hypothyroid rats is paralleled by a defect in adenine nucleotide transport. Both of these lesions can be corrected within 15 min by a near-physiological dose of triiodo-L-thyronine. Measurement of the control strength of the translocator shows, however, that this step has a smaller share of the control for oxidative phosphorylation after thyroidectomy and that this is unaltered after 15 min by replacement therapy. Rapid control by triiodothyronine is thus exerted elsewhere than at this transfer and the effects of hormone on the translocator are likely to be indirect.

The efficiency of oxidative phosphorylation and the rapid control by thyroid hormone of nicotinamide nucleotide reduction and transhydrogenation in intact rat liver mitochondria

In confirmation of previous work enhancement of the fluorescence emission of reduced nicotinamide nucleotides in intact rat liver mitochondria was found to depend on incubation conditions. Under standard conditions the enhancement is constant at 4.8-fold in states 3 and 4 and is not altered by thyroidectomy of the animal 6 weeks prior to experiment. The ADP-induced (state 4----state 3----state 4) fluorescence changes are significantly different in intact mitochondria from normal and hypothyroid animals and reflect the decreased rate and efficiency of oxidative phosphorylation after thyroidectomy. Incubation of liver homogenates in vitro for 15 min with 1 microM triiodothyronine before isolating mitochondria significantly restores their ADP response towards normal. Direct addition of hormone to isolated mitochondria was ineffective. Enzymatic measurement of mitochondrial extracts shows that thyroidectomy leads to increases in the contents of NAD(H) by 22% and NADP(H) by 33%. With glutamate as substrate ADP-induced changes in the reduced/oxidized ratio of NAD+ are not significantly altered in hypothyroid preparations. By contrast the NADP+ ratio remains substantially more reduced in state 3 than it does in normal mitochondria. The hypothesis is advanced that the decreased efficiency of hypothyroid preparations in phosphorylating ADP may be the result of increased energy-linked transhydrogenase activity. This is needed to supply NADPH via the glutathione peroxidase for reducing endogenously formed peroxides. Direct reduction of mitochondrial glutathione with dithiothreitol had no substantial effect on ADP/O ratios or on ADP-induced redox cycles in either normal or thyroidectomised preparations. This decisively eliminates the possibility that lowered phosphorylation efficiency is the result of a leak of reducing equivalents via glutathione peroxidase.

Thyroid hormones and carbohydrate metabolism of brain in the teleost, Channa punctatus. I. Effect of T4 and thiourea on succinic dehydrogenase (SDH) activity and protein content

The brain of the fishes (Channa punctatus) subjected to cold acclimation (15 +/- 1 degree) in darkness exhibited low succinic dehydrogenase (SDH) activity after 3 and 7 days and high protein content after 7 days when compared with their warm acclimated (32 +/- 1 degree) counterparts. In warm acclimated fishes maintained in complete darkness, T4 (0.5 micrograms/g body weight/day) depressed the enzyme activity after 5 and 7 days of treatment and reduced the protein content after 3 days. But neither in cold acclimated fishes maintained in darkness for 3, 5, and 7 days nor in warm acclimated fishes maintained in 12 hr dark and 12 hr daylight for the same periods did T4 induce a significant change in the same biochemical parameters. It appears that T4 action in this fish is dependent on acclimation temperature and light:dark regimes. When warm acclimated control fishes maintained in complete darkness were compared with those maintained in 12 hr dark and 12 hr daylight, the enzyme activity was found to be higher and the protein content lower in the former than in the latter. The results suggest that natural photoperiod regulates the thyroid activity in vivo. In vitro studies revealed that the presence of T4 (3.12 microM) in the incubating medium stimulated the enzyme activity of brain homogenates possibly due to direct action on mitochondria. Immersion of fishes in thiourea solution (1 mg/ml) for 3, 5, and 7 days resulted in enhancement of enzyme activity after 7 days of treatment and of protein content after 5 days of treatment. Thyroid hormones in vivo appear to have an inhibitory effect on the carbohydrate metabolism of the nervous tissue.

Short-term control of mitochondrial adenine nucleotide translocator by thyroid hormone

An improved simple technique for measuring adenine nucleotide translocator activity at low medium substrate concentrations is described. Confirming previous reports, thyroidectomy was shown to lead to lowered translocator activity in rat liver mitochondria. The rapidly exchangeable portion of the matrix nucleotide also decreased in hypothyroid preparations even though the total nucleotides increased substantially. The apparent Km of translocator for ADP increased from 2.8 to 6.2 microM in hypothyroid preparations: Mg2+ ions raised this to about 20 microM. All of these changes in adenine nucleotide translocation were entirely reversed by 15 min after a single intravenous near-physiological dose of triiodothyronine.

Triiodothyronine depresses the NAD-linked glycerol-3-phosphate dehydrogenase activity of cultured neonatal rat heart cells

The activity of NAD-linked alpha-glycerol-3-phosphate dehydrogenase (NAD-G3PDH; EC 1.1.1.8) was depressed by 35% when the thyroid hormone 3,3',5-triiodo-L-thyronine (20 micrograms/liter) was added to the serum-free, hormonally supplemented medium of cultured neonatal rat heart cells. The degree of depression was greater (65%) when the medium contained normal serum levels of hydrocortisone and insulin. There is a dramatic inverse dose-response relationship between triiodothyronine levels and NAD-G3PDH activity. The classic elevation by thyroid hormones of the FAD-linked alpha-glycerol-3-phosphate dehydrogenase (FAD-G3PD; EC 1.1.99.5) was observed concurrently. The medium-glucose depletion rate in triiodothyronine-free cells was depressed 32% through 11 days-in-culture, indicating reduced glycolytic activity. The activities of nine other metabolically important enzymes which were measured during this study, including hexokinase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, phosphofructokinase, pyruvate kinase, malate dehydrogenase, NAD-isocitrate dehydrogenase, NADH cytochrome c reductase, and succinic cytochrome c reductase, did not respond to varying triiodothyronine concentrations.

Effect of thyroidectomy on the kinetics of ADP-ATP translocation in liver mitochondria

The role of adenine nucleotide translocase (AdNT) in the reduced oxidative metabolism of hypothyroidism has been examined. Both AdNT and respiratory activities in liver mitochondria of thyroidectomized rats were 30% below normal. Mitochondrial AdNT activities were determined by the back-exchange method of Pfaff and Klingenberg (Eur. J. Biochem. 6, 66, 1968). The Km and Vmax of the enzyme were temperature dependent. At physiological temperature, the Km and Vmax of the normal rat AdNT were 10 microM (for external ADP) and 4.73% s-1 (percentage efflux of the labeled adenine nucleotides), respectively. AdNT in hypothyroid rat liver mitochondria exhibited a 25-35% lower Vmax and 75% higher Km when assayed over the temperature range 0 to 37 degrees C. Dixonplot studies indicated that the AdNT in hypothyroidism was two- to threefold more sensitive to atractylate and palmitoyl-CoA inhibitions. In contrast the ADP-ATP translocase in hypothyroidism was more resistant than the control carrier to bongkrekate inhibition. The decrease in the transport of ADP, which is consistent with the decreased oxidative activity associated with hypothyroidism, apparently occurs secondary to changes in the lipid matrix of the inner mitochondrial membrane (F. L. Hoch (1977) Arch. Biochem. Biophys. 178, 535.).

Restraint-induced thermogenesis blunted by fasting in rats

Physical restraint was performed in fasted (F3, fasted for 3 days; F7, fasted for 7 days), and fed control (C) rats. In contrast to the C group, the F3 and F7 groups showed a low resting metabolism and a low calorigenic response to restraint. During restraint, plasma levels of norepinephrine (NE) and epinephrine (E) were markedly increased but were not significantly different between the C and F7 groups. Calorigenic responses to NE and E (6.0 and 12.0 nmol X kg-1 X min-1) were significantly less in the F3 and F7 groups than in the C group. Plasma concentrations of glucose were increased by the infusion of catecholamines in the C and F7 groups. The values were not different between the two groups. Plasma levels of free fatty acid in the F7 group were significantly higher than in the C group before and during catecholamine infusion. Plasma concentrations of thyroxine and triiodothyronine were significantly less in the F7 group than in the C group. The blunted restraint-induced thermogenesis in fasted rats may be mainly attributed to the decreased calorigenic response to catecholamines. The decreased calorigenic response may be due to suppressed utilization of energy substrates in calorigenic tissues produced by the lowered activity of thyroid hormone.

Age-dependence of the stimulatory effect of triiodothyronine on the DNA synthesis of rat hepatocytes

The effect of a single subcutaneous injection of triiodothyronine (T3) on DNA synthesis rate and liver growth was studied in young and old female Wistar rats. The hormonal treatment was effective in stimulating DNA synthesis, resulting in an increase of the specific liver volume as well as volume and numerical density of nuclei/100 g body wt. in young animals. In the old animals T3 injection induced a reduced and delayed synthesis of DNA, which did not affect the morphometric parameters of liver/100 g body wt.

Specific binding sites for the triiodothyronine in the plasma membrane of rat thymocytes. Correlation with biochemical responses

As a prerequisite to studies of whether the plasma membrane of the rat thymocyte contains specific, saturable binding sites for the thyroid hormone 3,5,3'-triiodothyronine (T(3)), a method was developed for the isolation of a plasma membrane fraction from these cells. As judged from both electron microscopic and marker enzyme studies, the fraction was composed principally of plasma membrane vesicles, was free of nuclear contaminants, and was only slightly contaminated with other subcellular components. At 37 degrees C and pH 7.4, binding of [(125)I]T(3) by the fresh membrane preparation was rapid, reaching a maximum at 5 min and then declining with time, so that by 60 min binding was virtually nil. Decreased binding with time was due to a loss of functional binding sites, but did not reflect desensitization, since the decrease in binding activity with time was independent of the presence or absence of T(3). Scatchard analysis of saturation studies revealed the presence of two binding sites, one with an apparent dissociation constant (K(d)) of 0.95 nM and a maximum capacity of 5.3 x 10(10) sites/100 mug protein, and the other with an apparent K(d) of 25 nM and a binding capacity of 1.4 x 10(12) sites/100 mug protein. Measurement of the ability of several thyronine analogues to inhibit the binding of [(125)I]T(3) revealed the following rank order of potency: l-T(3) > l-T(4) > d-T(3) = d-T(4) > l-3,5-T(2) > rT(3) > d,l-thyronine. Binding of T(3) was inhibited by the omission of calcium from the medium or by the addition of the beta adrenergic antagonist alprenolol. As judged from studies of the lower affinity binding site, these manipulations decreased the affinity, but not the number, of binding sites for T(3). The relative potencies of thyronine analogues to inhibit the binding of [(125)I]T(3) were generally parallel to their previously reported potencies in stimulating the uptake of the sugar analogue 2-deoxy-glucose (2-DG) in intact rat thymocytes in vitro. Further, the inhibition of T(3)-binding produced by l-alprenolol or by excluding calcium from the medium resembled the previously reported inhibition that these manipulations produce with respect to T(3)-induced enhancement of 2-DG uptake. These findings suggest that the binding sites for T(3) present in the plasma membrane of rat thymocytes act as functional receptors linked to the stimulation of 2-DG uptake that T(3) induces in these cells.

Cytochrome c reduction by triiodothyronine (T3)

Triiodothyronine (T3), the active thyroid hormone, reduced cytochrome c non-enzymatically. This reduction was partially inhibited by superoxide dismutase (SOD). Thyroxine (T4) was only minimally effective as a reductant in this system. The effect of T3 on mitochondrial oxidative phosphorylation may involve a direct redox mechanism.

Interactions of iron deficiency, anemia, and thyroid hormone levels in response of rats to cold exposure

We have earlier shown that iron-deficient rats have increased urinary norepinephrine (NE) excretion. They also have an exaggerated rise in urinary NE when placed in the cold, a stimulus known to cause increased NE excretion in normal rats. Nonetheless, they fall to maintain body temperature. We have now examined the thyroidal response to cold in iron-deficient rats. As others have shown, control rats had a rise in plasma levels of thyroxine (T4) and triiodothyronine (T3) soon after entering the cold environment (4 degree C); they also maintained a rectal temperature above 36 degree C. In the iron-deficient rats, basal levels of T3 and T4 were normal, but there was little or no increase after 6 hr in the cold, and, as before, body temperatures fell. Injections of T3, 10 microgram/kg, 15 min before cold exposure improved the ability of iron-deficient rats to maintain body temperature, but they still did not do as well as the controls. We conclude that the inability of iron-deficient rats to increase T3 levels after cold exposure is one factor in their poor resistance to cold. The defect could involve inability to augment thyroid secretion, imparied ability to convert T4 to T3 in peripheral tissues, or both. Preliminary data suggest that anemia is an important and perhaps critical factor in the cold sensitivity of iron deficiency. Transfusing iron-deficient rats from their usual hematocrit of 15-20 to one of 30 restores cold resistance to normal. Transfusion also allows a more normal thyroid response, with a rise in T3 and T4 levels, so thyroid hormones may be a factor in the improvement produced by transfusion.

Hypothalamic control of endocrine thermogenesis

Hypothalamic and ruminal cooling raised serum thyrotropin (TSH), adrenocorticotropin (ACTH), norepinephrine (NE), and glucose in conscious goats in 20 degree C ambient temperature. Cooling of the preoptic anterior hypothalamus (POAH) for 2 h initially evoked shivering and vasoconstriction, leading to 1.5 degree C rise in rectal temperature (Tr). Pituitary-thyroid activation by POAH cooling was shown by peak rises in TSH of 60% at 40 min, in triiodothyronine (T3) of 54% at 80 min, and in thyroxine (T4) of 40% at 140 min. At 60 min, ACTH and NE peaked at 57 and 65%, respectively. TSH, ACTH, and NE declined during the 2nd h of POAH cooling as Tr plateaued; when POAH cooling was stopped, these hormones fell below basal level as vasodilation and panting restored Tr to normal. In contrast to the core hyperthermia evoked by POAH cooling, ruminal intubation with O degree C water (1 liter/10 kg) led to general hypothermia, Tpoah and Tr falling 1.6 degree C at 40 min. Pituitary-thyroid responses were less but ACTH and NE more, compared with POAH cooling. TSH peaked at 37% at 20 min, T3 at 55% at 60 min, and T4 at 18% at 200 min. ACTH peaked at 250% at 30 min and NE at 120% at 20 min. Thermosensitive neurons in the POAH seem to mediate more sensitive and complete control over TSH than over ACTH, or NE release, whereas extrahypothalamic core thermosensitivity (e.g., brain stem, spinal cord, abdomen) may influence ACTH and NE more than TSH release.

Stereospecific determination of D-thyroxin and L-thyroxin in serum

A new method for separate determination of D-thyroxin and L-thyroxin in the serum was applied to study the response of serum levels of these isomers and of radioimmunologically determined total T3, total T4, free T4 and TSH upon administration of 2, 4 and 6 mg of a highly purified D-thyroxin preparation in six male patients with diffuse nontoxic goiter. D- and L-thyroxin are determined separately following extraction of the hormone from the serum and formation of diastereomeric dipeptides. The separation and final determination are accomplished by means of ion-pair chromatography on reversed-phase columns using an iodine-selective catalytic detector. A significant decrease in TSH takes place during the 3-days observation period. The values of D-T4, total T3 and free T4 are highest 4 hours after administration of the tablets and get to be to initial values after 24 hours. L-T4 shows no significant change. A direct suppressive effect of D-T4 on the pituitary gland may therefore be assumed as the cause of the suppression of TSH secretion.

In vitro binding of triiodothyronine to rat liver mitochondria

Saturable high affinity T3 binding sites were detected in a mitochondrial fraction enriched in internal membranes and partly solubilized by Triton X-100. Specific T3 binding to the solubilized sites, only detected at low T3 concentrations, was optimal at pH 8.0 and not dependent upon the presence of divalent cations or reducing agents; it was destroyed by heat and proteolytic enzymes. The solubilized T3 binding sites were distributed, after Sephadex G-200 gel filtration, between two peaks of similar affinity for T3 (Ka congruent to 5 x 10(10)l/mol) and similar binding characteristics. T3 was bound with a high stereospecificity, while some analogues of biological importance (L-T4; 3,5,3'-triiodothyroacetic acid; 3,3';-diiodo-L-thyronine) competed with L-T3 in the same range of low concentrations. This suggests that the high affinity mitochondrial T3 binding sites could be of biological relevance in the mitochondrial metabolism.

Deleterious influence of hypothyroidism on evolving myocardial infarction in conscious dogs

To study the influence of hypometabolism on evolving myocardial infarction in a model with intact autoregulation, we investigated 53 awake dogs after coronary artery occlusion. Severe hypothyroidism was induced by the intravenous administration of 131I. Animals were instrumented to obtain hemodynamic measurements, and regional myocardial blood flow was measured with radioactive microspheres. Infarct size was determined by the creatine kinase depletion method, and dysrhythmia analysis was performed from 24-h Holter monitor tapes in animals matched for infarct size. The microarchitecture of hypothyroid myocardium was determined by the electron microscope. Before coronary occlusion, mean systemic pressure in hypothyroid dogs was reduced by 14% and cardiac output reduced by 32%, with no change in left ventricular end-diastolic pressure, first derivative of left ventricular pressure rise, (dP/dt), or heart rate. After coronary occlusion, there was deterioration in hemodynamic measurements in both groups, with lower absolute levels of mean systemic blood pressure and cardiac output obtained in hypothyroid dogs. Hypothyroidism was detrimental to evolving infarction with a 36% increase in infarct size present in hypothyroid dogs (30 +/- 2%) compared to euthyroid controls (22 +/- 3%), P less than 0.05. Dysrhythmias were more severe in hypothyroid dogs. There were no changes in the relationship between regional myocardial blood flow and the extent of infarction after coronary occlusion. Abnormalities in microarchitecture were present in hypothyroid dog myocardium. Severe hypometabolism in this model was associated with alterations in hemodynamics, more severe dysrhythmias and changes in microarchitecture. The combined effect of these alterations resulted in an overall detrimental influence of hypothyroidism on evolving myocardial necrosis in this model.

Inhibition of cyclic 3',5' monophosphate synthesis in rat testis by L-triiodothyronine

Cytosolic adenylate cyclase activity from rat seminiferous tubules is inhibited by L-triiodothyronine (L-T3). In a typical dose-response curve, using Mn-ATP as substrate, no effect is observed at 10(-10) M L-T3; about 15 to 25% inhibition is found in the range between 10(-9) and 10(-6) M L-T3 and finally a sharp enzyme inhibition is evident at increasing hormone concentrations from 10(-6) to 10(-4) M. Incubation of decapsulated testes with L-T3 leads to a decrease of intracellular cyclic AMP levels. Dose-response relationships for such effect are similar to those found for adenylate cyclase activity. In this case a clear response is observed at 10(-8) M L-T3.

Rapid effect of triiodothyronine on the mitochondrial pathway in rat liver in vivo

Intravenous infections of minute doses of triiodothyronine were administered to thyroidectomized rats 30 minutes before they were killed. Hepatic mitochondria were isolated rapidly and formation of adenosine triphosphate and consumption of oxygen were assessed by a 2-minute incubation. Hormone injection enhanced formation of adenosine triphosphate 114 to 217 percent over control values, with a proportionate increase in consumption of oxygen. The ratio of phosphate to oxygen was about 2.0, signifying tightly coupled oxidative phosphorylation. Stimulation was not abolished by injection of cycloheximide, puromycin, actinomycin D, or chloramphenicol 1 hour before the rats were killed. This signifies direct mitochondrial stimulation by triiodothyronine in the absence of protein synthesis.

Differential effect of L-thyroxine on phospholipid biosynthesis in mitochondria and microsomal fraction

1. The action of L-thyroxine on the incorporation of radioactive choline or CDP-choline into phosphatidylcholine in vitro was explored in liver and brain microsomal fraction and mitochondria obtained from young adult rats. 2. In liver mitochondria isolated from animals treated with L-thyroxine (40 mg/kg body wt. during 6 days), the incorporation of both radioactive precursors into phosphatidylcholine was significantly decreased compared with normal controls, whereas in the total homogenate and in the microsomal fraction the incorporation was similar in the experimental and control groups. In subcellular fractions isolated from brain, the incorporation of precursors was similar in L-thyroxine-treated and normal animals. 3. Liver mitochondria isolated from normal animals incubated in vitro with CDP-choline, in the presence of different concentrations of L-thyroxine, showed also a marked decrease in the incorporation of label into phosphatidylcholine, whereas no significant changes were found in the total homogenate and in the microsomal fraction compared with control experiments. 4. The differential effect of L-thyroxine on the incorporation of radioactive precursors into phosphatidylcholine of isolated liver subcellular fractions gives further support to the hypothesis that liver mitochondria can independently synthesize part of their own phospholipids. 5. Possible mechanisms of the action of the hormone at the mitochondrial level are discussed.