Biochemical and ontogenic characterization of thyroxine 5'-monodeiodinase in brown adipose tissue from fetal and newborn lambs

Thyroid gland in vitro activity and reactivity to TRH in newborn lambs

The aim of the study was to determine the thyroid gland tissue concentration and activity on T4 and T3 in vitro release to the medium without (control) or with TRH (10 μg/ml) for 90 min of incubation in lambs just after birth and after the first 6h of postnatal life. The results obtained show that T4 concentration in the blood plasma in newborn lambs as well after 6h of life is higher than in thyroid gland tissue. In the case of T3 the concentration in blood plasma of lambs just after birth is lower than after 6h of postnatal life. Inversely, the higher level of that hormone concentration in blood plasma just after birth is higher than in the thyroid gland tissue. The results clearly indicate the inhibitory TRH effect on in vitro T4 especially, and in lesser degree T3 release from the thyroid gland tissue of lambs just after birth. Stimulation of the TRH thyroid gland tissue from lambs after 6h of life caused an increase in T4 release, especially after 60 min of the experiment. Similarly, a significant increase of T3 release was found only after 60 min of incubation. The transient promoting TRH effect on the amount of iodothyronine release from the thyroid gland in older lambs is probably connected with this gland's function not occurring before the designated time of postnatal adaptation. The described in vitro experiment visibly shows that changing thyroid gland activity in newborn lambs is not only conditioned by the entire HPT activity, but by the immanent thyroid gland attributes as well.

Short communication: quantitative comparison of iodothyronine deiodinase I and II mRNA expression in ovine tissues

Iodothyronine deiodinases I and II (DIO1 and DIO2) remove iodine from T4 to convert it to a more biologically active T3. The relative contribution of different tissue deiodinases to the establishment of a euthyroid state in sheep is not known. The objective of this study was to quantitate the amounts of transcription of DIO1 and DIO2 deiodinases in different ovine tissues. Using RT-qPCR, we found that DIO1 deiodinase is transcribed in skeletal muscle, kidney, and heart, more than thyroid, in diaphragm in quantities very similar to thyroid, and in liver, spleen, lung, and mammary gland lower than thyroid. We also found that the level of DIO2 transcription in all other tissues was lower than that in thyroid. In clinical settings, measurement of DIO1 and DIO2 expression in a given tissue may provide important clues on the intensity of selenium deficiency and its effects on the metabolism of thyroid hormones.

Postnatal changes in lambs of two pathways for thyroxine 5'-monodeiodination in brown adipose tissue

We have recently shown that ovine fetal brown adipose tissue (BAT) contains two distinct iodothyronine 5'-monodeiodinase (5'MDI) activities, one with a high Km (type I) and another with a low Km (type II). Both activities increased to maximum levels near term (150 days gestation). BAT plays a major role in neonatal temperature regulation in lambs, and available evidence suggests that BAT 5'MDI activity is closely linked to thermogenic capacity. To better characterize the changes in 5'MDI after birth, we studied both type I and type II 5'MDI in lamb BAT from the time of birth to 30 days of postnatal age. Type I 5'MDI activity [pmol 3,5,3'-triiodothyronine (T3).mg protein-1.h-1] showed no significant changes during the first 11 days after birth [newborn (NB), 95 +/- 16; 1 day, 83 +/- 20; 3-4 days, 80 +/- 11; 10-11 days, 92 +/- 28]. Activity decreased significantly at 30 days (24 +/- 8.9, P less than 0.05). On the other hand, the type II 5'MDI activity (fmol I- released.mg protein-1.h-1) increased significantly (P less than 0.01) during the first 4 days, (NB, 348 +/- 23; 1 day, 679 +/- 37; 3-4 days, 785 +/- 199), decreased toward NB values (401 +/- 87) at 10-11 days of age, and fell to 66 +/- 31 at 30 days (P less than 0.05 vs. NB). Kinetic analysis of BAT type II thyroxine 5'MDI revealed a rise in maximum velocity from NB to 1 and 3-4 days of age without a change in the enzymatic activity Km.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid system ontogeny in the sheep: a model for precocial mammalian species

There is now extensive data in two animal models to characterize the events of thyroid system maturation in mammalian species. Data from the rat have contributed importantly to our understanding of thyroid development in the altricial species while studies in the sheep have characterized thyroid ontogenesis in precocial mammals. More limited data are available from human studies and this information, with the animal data, indicate that the major features of thyroid system development are qualitatively similar in all three species. The combined data has provided an important overview of mammalian thyroid ontogenesis. However the picture remains unfinished. We have limited insights into the maturation of the molecular events of hypothalamic pituitary control, the maturation of TSH and thyroid nuclear receptors and postreceptor events, and the mechanisms of the thyroid effects on brain maturation. We can look forward to continuing contributions in these areas.

Induction of type II iodothyronine 5'-deiodinase and mitochondrial uncoupling protein in brown adipocytes differentiated in cell culture

Brown adipocytes differentiated in primary cell culture were found to contain a type II iodothyronine 5'-deiodinase (5'D). Incubation of confluent cells with norepinephrine or dibutyryl-cAMP caused up to 17-fold increase in 5'D activity with a maximum after 8 h. Activation of 5'D required mRNA and protein synthesis and was accompanied by parallel, up to 5.8-fold increase in the amount of mitochondrial uncoupling protein with a maximum after 24 h. Analysis of adrenergic stimulation of 5'D suggested predominant involvement of the beta-receptors and increased intracellular cAMP levels, while the contribution of alpha 1-receptors was small.

Induction of type II T4-5'-monodeiodinase activity in brown adipose tissue in fasted mice

In order to study the effect of starvation on brown adipose tissue (BAT) type II 5'-monodeiodinating activity (5'MDI), type II 5'MDI was measured in vitro in the presence of 20 mM dithiothreitol, 1 mM propylthiouracil, 2 nM thyroxine (T4) and appropriate amounts of 600 X g infranatant of BAT from fed control or 3 day fasted mice, with or without daily T4 replacement (1.2 micrograms/100 g bw) during starvation. I- released from 125I-T4 was measured by ion-exchange column chromatography. Activity of BAT 5'MDI was markedly elevated in the 3 day fasted group (133 +/- 28 fmol I-/h per mg protein vs. 26 +/- 6.4; p less than 0.05). Kinetic studies using BAT infranatant suggested that fasting-induced activity is associated with a similar change in the Vmax, but no demonstrable change in apparent Km of T4 monodeiodination. T4 replacement during fasting, which normalized both serum T4 and T3 in fed and 3 day fasted groups, did not stop the increase of BAT 5'MDI in the fasted group (p less than 0.01). The data suggest that: (1) the fasting-induced increase in BAT 5'MDI is due mainly to the changes in capacity rather than the affinity of the enzyme, and (2) the fasting-induced increase in BAT 5'MDI is not mediated entirely through changes in serum thyroid hormone concentration.

The endocrine control of the onset of thermogenesis at birth

The experimental studies in the fetal sheep demonstrate that the central hypothalamic mechanisms for responding to a cold stress have differentiated well before birth. There are several major determinants of the initiation of maximal thermogenesis at birth. These are cutaneous cooling, oxygenation and separation from the placenta. Firstly the stimulation of cutaneous cold receptors regulates sympathetic nervous system activity, primarily local noradrenaline release from sympathetic nerve termini to the brown adipocyte adrenoreceptors. Circulating catecholamines, the euthyroid state and other hormones also play a role. Secondly an increase in oxygen delivery to brown adipose tissue through increased oxygen content and increased blood flow is required. Finally, our observations suggest that separation from the placenta is necessary for maximal nonshivering thermogenesis. The effects on thermogenesis of interrupting and reestablishing placental flow are rapid and reversible. Umbilical cord occlusion is the signal for a rapid increase in thermogenesis, while the release of cord occlusion is followed by an equally rapid fall in thermogenesis. This strongly suggests the presence of a placental inhibitor of brown adipose tissue thermogenesis. The problem of the regulation of growth and recruitment of brown adipose tissue in the fetus despite the lack of thermal stress in utero may be resolved by this evidence for an inhibitory factor of thermogenesis produced by the placenta. The withdrawal of the inhibitor at birth by separation from the placenta will allow the rapid initiation of thermogenesis in response to sympathetic nervous system stimulation of the brown adipose tissue.

Iodothyronine 5'-deiodinase activity as an early event of prenatal brown-fat differentiation in bovine development

Iodothyronine 5'-deiodinase activity appears to be a type I enzyme in bovine brown adipose tissue, on the basis of its high Km for 3,3',5'-tri-iodothyronine ('reverse T3') (in the micromolar range) and sensitivity to propylthiouracil inhibition. This enzyme activity is already detectable in perirenal adipose tissue of bovine fetuses in the second month of gestation, reaches peak values around the seventh month of fetal life, declines before birth, becomes lower after parturition and finally undetectable in the adult cow. Iodothyronine 5'-deiodinase activity is present in the pericardic, peritoneal and intermuscular adipose depots of the neonatal calf, but it is always undetectable in the subcutaneous adipose tissue. It is concluded that iodothyronine 5'-deiodinase is a specific feature of brown fat in the bovine species that is not shared by white adipose tissue. white adipose tissue. Peak values of 5'-deiodinating activity appear as an early event in the prenatal differentiation programme of bovine brown-fat cells as they occur when uncoupling-protein-gene expression first starts.

Sequential changes in the expression of mitochondrial protein mRNA during the development of brown adipose tissue in bovine and ovine species. Sudden occurrence of uncoupling protein mRNA during embryogenesis and its disappearance after birth

Samples of adipose tissue were obtained from different sites in bovine and ovine foetuses and newborns. RNA was isolated and analysed using bovine cDNA and ovine genomic probe for uncoupling protein (UCP), cDNA for subunits III and IV of cytochrome c oxidase and cDNA for ADP/ATP carrier. UCP mRNA was characterized for the first time in foetal bovine and ovine adipose tissue. It appeared later than mRNA of cytochrome c oxidase subunit III, and increased dramatically at birth (10-fold). ADP/ATP carrier mRNA was expressed at a lower level but also increased 10-fold at birth. It was demonstrated that UCP mRNA reached its highest level at birth in all bovine adipose tissues studied, except subcutaneous tissue. It disappeared quickly afterwards, being no longer detectable two days after birth. Similar variations were observed in newborn lambs. ADP/ATP carrier mRNA showed the same pattern of expression as UCP mRNA; although it was still lightly expressed two days after birth, it disappeared soon afterwards. Only mRNAs for cytochrome c oxidase subunits III and IV remained at the same level during the first postnatal week. On the basis of these data and of observations reported in the literature a sequence of events for the development of brown adipose cells in vivo is proposed. Soon after birth the perirenal adipose tissue of ruminants, which still contains mitochondria of typical brown adipose tissue morphology and high levels of cytochrome c oxidase mRNA, lacks UCP mRNA. Can it still be considered as brown fat? Ruminant species appear to be attractive models to study both the differentiation of brown adipose tissue and its possible conversion to white fat in large animals.

Serum thyroid hormones and tissue 5'-monodeiodinase activity in acutely thyroidectomized newborn lambs

After either total thyroidectomy or sham operation in full-term fetal sheep, fetuses were delivered and serial blood samples were obtained for measurements of thyroxine (T4), triiodothyronine (T3), and catecholamines. Despite comparable serum T4 values (T4 means +/- SE, sham 7.1 +/- 0.6 micrograms/dl, thyroidectomized 6.8 +/- 0.7 micrograms/dl at 60 min after birth), serum T3 values were lower in the thyroidectomized animals (T3 means +/- SE, thyroidectomized 39 +/- 4.8 ng/dl, sham 153 +/- 20.1 ng/dl at 60 min after birth). Four hours after birth, the animals were killed with an intravenous overdose of barbiturate. Brain, thyroid, liver, kidney, and brown adipose tissues were dissected and analyzed for thyroxine 5'-monodeiodinase (5'-MDI) activity in vitro. 5'-MDI activity was comparable in all tissues from sham-operated and thyroidectomized lambs. Plasma epinephrine and norepinephrine concentrations, mean arterial pressure, mean pulse, rectal temperature, and arterial blood gas values were similar in the two groups of animals. These data support the hypothesis that the thyroid gland is the major source of T3 for the T3 surge in the immediate newborn period. They also indicate that the neonatal T3 surge has limited immediate metabolic significance in euthyroid newborns.