H2O2, signal, substrate, mutagen and chemorepellent from physiology to biochemistry and disease

The history of the study by our group of the generation, the role and the effects of H2O2 in the thyroid, is summarized. The relations with thyroid diseases are discussed: myxedematous cretinism, thyroiditis, thyroid cancer, congenital hypothyroiddism, are discussed. A new role of H2O2 in the chemorepulsion of bacteria is proposed.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Thiocyanate induces cell necrosis and fibrosis in selenium- and iodine-deficient rat thyroids: a potential experimental model for myxedematous endemic cretinism in central Africa

Thyroid destruction leading to endemic myxoedematous cretinism is highly prevalent in central Africa, where iodine (I) and selenium (SE) deficiencies as well as thiocyanate (SCN) overload are combined. All three factors have been studied experimentally in the etiology of the disease, but they have never been studied in combination. In a model using rats, we have previously shown that combining I and SE deficiencies increases the sensitivity of the thyroid to necrosis after iodide overload, an event unlikely to occur in the African situation. To develop a model that would more closely fit with the epidemiological findings, we have determined whether an SCN overload would also result in thyroid necrosis as does the I overload. The combination of the three factors increased by 3.5 times the amount of necrotic cells, from 5.5 +/- 0.3% in the I-SE+ thyroids to 18.9 +/- 1.6% in the I-SE-SCN-overloaded ones. Methimazole administration prevented the SCN-induced necrosis. SE- thyroids evolved to fibrosis, whereas SE+ thyroids did not. TGFbeta was prominent in macrophages present in SE- glands. Thyroid destruction in central Africa might therefore originate from the interaction of three factors: I and SE deficiencies by increasing H(2)O(2) accumulation, SE deficiency by decreasing cell defense and promoting fibrosis, and SCN overload by triggering follicular cell necrosis.

Fetal tissues are exposed to biologically relevant free thyroxine concentrations during early phases of development

Maternal hypothyroxinemia in early pregnancy is often associated with irreversible effects on neuropsychomotor development. To evaluate fetal tissue exposure to maternal thyroid hormones up to midgestation, we measured total T(4) and free T(4) (FT(4)), T(3), rT(3), TSH, and possible binding proteins in first trimester coelomic and amniotic fluids and in amniotic fluid and fetal serum up to 17 wk. Samples were obtained before interruption of maternal-fetal connections. The concentrations in fetal compartments of T(4) and T(3) are more than 100-fold lower than those in maternal serum, and their biological relevance for fetal development might be questioned. We found, however, that in all fetal fluids the concentrations of T(4) available to developing tissues, namely FT(4), reach values that are at least one third of those biologically active in their euthyroid mothers. FT(4) levels in fetal fluids are determined by both their T(4)-binding protein composition and the T(4) or FT(4) in maternal serum. The binding capacity is determined ontogenically, is independent of maternal thyroid status, and is far in excess of the T(4) in fetal fluids. Thus, the availability of FT(4) for embryonic and fetal tissues would decrease in hypothyroxinemic women, even if they were euthyroid. A decrease in the availability of FT(4), a major precursor of intracellular nuclear receptor-bound T(3), may result in adverse effects on the timely sequence of developmental events in the human fetus. These findings ought to influence our present approach to maternal hypothyroxinemia in early pregnancy regardless of whether TSH is increased or whether overt or subclinical hypothyroidism is detected.

Selenium status in pregnant women of a rural population (Zaire) in relationship to iodine deficiency

Endemic myxoedematous cretinism has been associated with combined selenium and iodine deficiency in several areas of Zaire. To determine selenium and iodine status across the country, serum selenium and thyroid function parameters including urinary iodide were determined at prenatal clinics in 30 health centres of rural villages distributed over the whole country. Only in Bas-Zaire was the mean serum selenium level similar to that in non-deficient areas (80-120 ng/ml); in the regions of Bandunda and Kasai levels were marginally decreased (55-80 ng/ml), and in Kivu, Haut-Zaire, Equateur and Shaba they were marginally or moderately decreased (< 55 ng/ml). The frequency of abnormally low urinary iodide (< 5 micrograms/dl) varied from 20% in the region of Bas-Zaire to 50% in Kasai (P < 0.001), and to still higher percentages in the 5 other regions of Zaire (Bandundu, 57%; Kivu, 63%; Equateur, 72%; Shaba, 76%; Haut-Zaire, 84%). With the exception of Bas-Zaire, biochemical maternal hypothyroidism (serum TSH > 5mU/l) was present in every region, with a frequency ranging from 3% in Kivu to 12% in Equateur. Iodine deficiency affects most of the Zairean population and requires public health measures on a larger scale than previously estimated. Combined iodine and selenium deficiency affects Equateur, Haut-Zaire and Kivu, where endemic myxoedematous cretinism occurs, but also Shaba, where it was not previously described. Besides combined iodine and selenium deficiency which is permissive, another factor (thiocyanate?) must be taken into account to explain the peculiarly elevated prevalence of endemic myxoedematous cretinism in Central Africa.

Detection of thyroid hormones in human embryonic cavities during the first trimester of pregnancy

Transfer of maternal thyroxine (T4) to the human fetus near term has recently been demonstrated. We investigated whether maternal thyroid hormone is available to the conceptus during the first trimester of pregnancy as well. Transvaginal ultrasound-guided puncture of the embryonic cavities was performed during the first trimester of pregnancy to obtain coelomic fluid between 6 and 11 weeks, and amniotic fluid between 8 and 11 weeks of pregnancy. T4 was found in coelomic fluid with mean values (+/- SEM) being 961 +/- 193 pmol T4/L (747 +/- 150 pg/mL). Concentrations increased both with gestational age and with rising maternal serum T4. Concentrations of 3,5,3'-triiodothyronine (T3) were at least 30 times lower, and those of 3,3',5'-triiodothyronine (rT3) four times higher, than coelomic fluid T4. Thyroxine and rT3 in amniotic fluid (8-11 weeks) were markedly lower than in the coelomic fluid, and T3 was undetectable. These results show that maternal thyroxine can cross the placental barrier as early as the second month of pregnancy. T4 from the coelomic fluid may reach the embryo via the yolk sac. This finding raises the possibility that the increase in maternal T4 occurring during the first trimester may be functionally important for the developing embryo, when its thyroid is not yet functioning.

Selenium deficiency mitigates hypothyroxinemia in iodine-deficient subjects

Studies were performed to assess the role of combined selenium and iodine deficiency in the etiology of endemic myxedematous cretinism in a population in Zaire. One effect of selenium deficiency may be to lower glutathione peroxidase activity in the thyroid gland, thus allowing hydrogen peroxide produced during thyroid hormone synthesis to be cytotoxic. In selenium-and-iodine-deficient humans, selenium supplementation may aggravate hypothyroidism by stimulating thyroxin metabolism by the selenoenzyme type I iodothyronine 5'-deiodinase. Selenium supplementation is thus not indicated without iodine or thyroid hormone supplementation in cases of combined selenium and iodine deficiencies.

Selenium and the thyroid: how the relationship was established

Several hypotheses concerning consequences of selenium deficiency on iodine metabolism can be proposed on the basis of experimental studies in rats and from epidemiological and experimental studies in humans. By decreasing intracellular GSH peroxidase activity, selenium deficiency may increase hydrogen peroxide (H2O2) supply and lead over several weeks to the thyroid atrophy observed in myxoedematous cretins. By improving thyroid hormone synthesis and by decreasing peripheral thyroxin (T4) deiodination, selenium deficiency could protect fetal brain T4 supply and thus prevent neurologic cretinism. Selenium deficiency may protect against iodine deficiency by decreasing T4 metabolism--and thus iodide leakage and--perhaps also by increasing H2O2 supply and thyroid hormone synthesis and thus thyroid efficiency.

Effect of selenium supplementation on thyroid hormone metabolism in an iodine and selenium deficient population

OBJECTIVE

Severe selenium deficiency has been documented in northern Zaïre, already known as one of the most iodine deficient regions in the world and characterized by a predominance of the myxoedematous form of cretinism. This has been attributed to the double deficiency of essential trace elements. A short selenium supplementation programme was conducted in this area to evaluate the effects of a selenium supplementation on thyroid diseases.

DESIGN

Placebo or selenium 50 micrograms as selenomethionine was administered once daily for 2 months. Blood and urine samples were collected before and after supplementation.

PATIENTS

Fifty-two healthy schoolchildren from northern Zaire.

MEASUREMENT

Selenium status, thyroid function and urinary iodide were determined.

RESULTS

After 2 months of selenium supplementation, mean +/- SD serum T4 decreased from 73.1 +/- 45.4 to 48.3 +/- 23.7 nmol/l (P less than 0.001), serum FT4 from 11.8 +/- 6.7 to 8.4 +/- 4.1 pmol/l (P less than 0.01), and serum rT3 from 124 +/- 115 to 90 +/- 72 pmol/l (P less than 0.05), without significant change in serum T3 and serum TSH.

CONCLUSION

Deiodinase type I which has been shown to be a seleno-enzyme could account for the changes in thyroid hormones in our subjects. Our data show that selenium plays a definite role in thyroid hormone metabolism in humans. Selenium could be an important cofactor in the clinical picture of iodine deficiency in Central Africa and could be involved in the aetiology of both forms of cretinism.

Iodine and selenium deficiency associated with cretinism in northern Zaire

Selenium status was determined in an endemic-goiter area and in a control area of Zaire. Compared with the reference values of a noniodine-deficient area, serum selenium in subjects living in the core of the northern Zaire endemic-goiter belt (Karawa villages) was seven times lower in 52 school-children and similarly low in 23 cretins; erythrocyte glutathione peroxidase (RBC-GPX) was five times lower in schoolchildren and still two times lower in cretins (P = 0.004). In a less severely iodine-deficient city of the same endemia (Businga), selenium status was moderately altered. RBC-GPX activity was linearly associated with serum selenium concentration up to a value of 1140 nmol/L and leveled off at approximately 15 U/g Hb at greater selenium concentration. At Karawa villages, selenium supplementation normalized both the serum selenium and the RBC-GPX. This combined iodine and selenium deficiency could be associated with the elevated frequency of endemic myxedematous cretinism in Central Africa.

Bretylium tosylate versus lidocaine in experimental cardiac arrest

Bretylium tosylate has been shown effective in the treatment of ventricular fibrillation and in the prevention of its recurrence. However, lidocaine is generally preferred because bretylium could have adverse hemodynamic effects related to its antiadrenergic action. To explore further the differences between these two antiarrhythmic agents, the authors compared the effects of bretylium, lidocaine, and saline on a standardized dog model of ventricular fibrillation followed by electromechanical dissociation (EMD). The protocol included three successive episodes of cardiac arrest in each animal. Three minutes before each episode of ventricular fibrillation, 5 mg/kg of bretylium tosylate (n = 11), 1 mg/kg of lidocaine (n = 9) or saline (n = 12) were administered blindly. There was no difference in the duration of cardiac arrest (bretylium, 8 min 18 sec; lidocaine, 7 min 54 sec; saline, 8 min 20 sec) or the total doses of epinephrine required to resuscitate the animals. Both bretylium and lidocaine appeared to preserve cardiac function 5 minutes after recovery, as stroke volume increased from 17.8 +/- 6.7 to 18.7 +/- 6.7 mL (NS) after bretylium and from 17.7 +/- 7.7 to 19.0 +/- 7.0 mL (NS) after lidocaine, but decreased from 19.0 +/- 5.3 to 14.6 +/- 6.0 mL (P less than .05) after saline. During the first 10 minutes of EMD, ventricular fibrillation or ventricular tachycardia recurred in 4 dogs treated with lidocaine, 3 dogs treated with saline, but no dog treated with bretylium (P less than .05 between bretylium and saline).(ABSTRACT TRUNCATED AT 250 WORDS)

Transthoracic electrical bioimpedance versus thermodilution technique for cardiac output measurement during mechanical ventilation

To study the possible influence of mechanical ventilation on the accuracy of thoracic electrical bioimpedance (TEI) in the measurement of cardiac output, we determined cardiac output concurrently by TEI using Kubicek's equation and by thermodilution in 8 acutely ill patients who were mechanically ventilated (assist/control mode) but who had no underlying respiratory failure. Cardiac outputs were lower with TEI than with thermodilution (3.97 +/- 0.80 vs 4.83 +/- 1.16 l/min p = 0.004) and there was poor correlation between the values (r = 0.41). Although there is a need to develop non-invasive techniques to measure cardiac output, the present study indicates that TEI is not reliable in mechanically ventilated patients.

Right ventricular dysfunction in septic shock: assessment by measurements of right ventricular ejection fraction using the thermodilution technique

Right ventricular ejection fraction (RVEF) was measured by the thermodilution technique in a series of 127 consecutive critically ill patients monitored with a modified pulmonary artery (PA) catheter equipped with a fast response thermistor. Thermodilution RVEF was significantly lower in septic shock (23.8 +/- 8.2%, 93 measurements) than in sepsis without shock (30.3 +/- 10.1%, 118 measurements) or in the absence of sepsis or cardiopulmonary impairment (32.5 +/- 7.1%, 62 measurements). Both myocardial depression and pulmonary hypertension could account for this impairment of RV function. RVEF decreased from 35.1 +/- 9.8 to 24.2 +/- 10.4% (P less than 0.01) during development of septic shock and increased from 25.0 +/- 7.6 to 29.8 +/- 8.5% (P less than 0.05) during recovery (14 patients). Initial RVEF in septic shock was 27.8 +/- 8.6% in 11 patients who survived but only 20.9 +/- 6.7% (P less than 0.02) in the 23 patients who eventually died. Thus, RV dysfunction is common during septic shock, is directly related to its severity, and can easily be recognized in patients monitored with a PA catheter.

Amrinone administration in endotoxin shock

This study explored the hemodynamic effects of amrinone, a phosphodiesterase inhibitor, in association with intravenous fluids, in the treatment of endotoxin shock. Mongrel dogs were anesthetized with pentobarbital and mechanically ventilated with room air. Treatment was started 30 min after slow intravenous administration of 3 mg/kg of E. coli endotoxin. In the first part of the study, ten dogs were resuscitated for 30 min with intravenous saline alone (10 ml/kg) and for the next 3 h by saline (10 ml/kg/h) and amrinone 40 micrograms/kg/min. During this latter period, arterial pressure remained stable while cardiac output significantly increased from 3.1 +/- 0.5 to 5.2 +/- 0.7 l/min (P less than 0.01), and oxygen delivery increased from 616 +/- 92 to 983 +/- 156 ml/min (P less than 0.01). Comparison with control animals revealed that amrinone infusion prevented the decrease in left ventricular stroke work and markedly increased oxygen delivery. In the second part of the study, 18 dogs were treated by saline infusion titrated to maintain pulmonary artery balloon-occluded pressure at baseline level. In ten dogs, amrinone was added 60 min after endotoxin administration at a dose of 40 micrograms/kg/min. Total amount of fluids infused averaged 87 +/- 14 ml in the amrinone-treated dogs and 64 +/- 15 ml in the control dogs (differences nonsignificant). Oxygen delivery and oxygen consumption increased significantly in the amrinone-treated dogs (from 541 +/- 36 to 1063 +/- 176 ml/min, P less than 0.01, and from 145 +/- 23 to 202 +/- 38 ml/min, P less than 0.01, respectively) but not in the control dogs. The amrinone-treated dogs had lower PaO2 and higher venous admixture than the control dogs.(ABSTRACT TRUNCATED AT 250 WORDS)