Influence of thyroid status on water metabolism and survival of normal and dehydrated desert rodents Meriones libycus.

The relationship between thyroid status and resistance to water deprivation in a desert rodent, Meriones libycus, has been studied in normal, radiothyroidectomized (Tx), and thyroidectomized T4-supplemented (1.5 microg T4/day) (Tx + T4) animals. In animals given free access to water, 1 month after thyroidectomy water influx and efflux decreased 3-fold. This decrease was partially corrected after 5 days of T4 administration. Thyroidectomy did not modify urinary osmolality nor affect survival. In dehydrated animals, the body weight decreased (about 15%) over 2 weeks in all groups and then stabilized. Water flux decreased sharply in normal or Tx + T4 animals during the 1st week and then stabilized. A further decrease of water flux occurred in hypothyroid animals, which continued over 4 weeks, when fluxes were half those of normal or Tx + T4 animals. The urinary osmolality increased equally sharply in the three groups, at least during the first 5 days of dehydration when sampling was possible. Whereas dehydrated normal and Tx + T4 animals survived at least 7 weeks, 70% of Tx animals had died after 4 weeks and none survived more than 7 weeks. The daily metabolic energy intake was estimated from water flux and metabolic water of the dietary barley. After 4 weeks, when water influx represented only metabolic water from food, metabolic energy intake decreased 2.5-fold in hypothyroid compared with normal or Tx + T4 animals. This low metabolic energy intake led to a trend of body dehydration, hypothermia, and death. Thus, although an effect of thyroidectomy on survival of hydrated animals beyond 4 weeks cannot be excluded, we infer that thyroid hormones play a significant role in the survival of desert rodents under conditions of hydric stress.

Excess iodide and thyroid function in the developing chick embryo.

The effects of adding excess iodide (I-) to egg yolks were studied on the quantity of iodide transferred to the chick embryo and on thyroid function from Days 8 to 18 of incubation. Following injection of either 0.5 or 1 mg of iodide into the yolk on the second day of incubation, embryonic plasma iodine levels increased significantly over levels of controls at the same age. These increases were proportional to the amount of excess iodide in the yolk. Moreover, a constant ratio between the iodine levels of the allantoic fluid and the plasma was maintained for any given day of incubation. Up to Day 10, the iodine content of the embryonic thyroid was closely correlated with the increased plasma iodine concentrations. After Day 10, concomitant with the establishment of the hypothalamo-adenohypophyseal-thyroid axis, the increase in thyroid iodine no longer paralleled that of plasma iodine, but was only twice that of controls. As thyroid weight doubled in these treated embryos, thyroid iodine concentrations of control and treated embryos were similar. Iodide excess did not block thyroid iodine organification, nor was there any modification of iodine distribution among the different iodo-amino acids. Moreover, despite the doubling of thyroid hormone content in the goiters of treated embryos, circulating levels of thyroid hormones in control and treated embryos were not different.

[Effect of prolonged water deprivation on weight gain and water metabolism in a gerbillid rodent (Meriones shawi shawi)]. / Effet d'une carence hydrique prolongée sur la croissance pondérale et sur le métabolisme hydrique chez un rongeur Gerbillidé (Meriones shawi shawi).

The effect of prolonged water deprivation, applied progressively since weaning, was studied on body weight in growing sub-desert rodents (Meriones shawi). Then, water metabolism, food consumption and kidney weight were measured in these adults animals. During partial water deprivation the weight curve was similar to that of controls, whereas with total water deprivation three different stages were noticed: a decrease, a stabilization and an increase in body weight as a function of age. At the end of the experiment, the body water volume was similar in dehydrated and in control animals. However, in water deprived animals, there was a lower rate of water influx and efflux compared to controls. Their food consumption was two-fold that of controls. Dehydrated meriones showed a significant increase in kidney weight as compared to controls.

Effect of hypophysectomy by partial decapitation and treatment with thiourea on iodine metabolism in the developing chick embryo.

The hypothesis of an action of the pituitary gland of the developing chick embryo in the transfer of iodide from the yolk of the egg to the circulation of the embryo, through the yolk sac, was tested. Plasma iodide levels and thyroidal iodine contents were determined in hypophysectomized (by partial decapitation), thiourea-injected and control embryos. From day 11 of incubation these parameters were always lower in the 'hypophysectomized' embryos than in controls, and plasma iodide levels of the thiourea-treated embryos were higher than those of controls. These results indicate a reduced iodide transfer from the yolk to the 'hypophysectomized' embryo, and an increased iodide transfer to the thiourea-treated embryo. This occurred in spite of a reduced thyroid hormonal secretion in both series. The pituitary gland could therefore have a direct action (not through the thyroid gland) at the yolk sac level, to augment the transfer of iodide from the yolk in intact embryos from day 11 to the end of incubation. Thyroid-stimulating hormone (TSH) could be the pituitary hormone acting at the yolk sac level, the increased iodide transfer observed in the thiourea-injected embryos being due to a raised TSH secretion responding to the decreased plasma thyroxine levels.

Thyroidal iodine and thyroglobulin in hibernating and active Richardson's ground squirrels Spermophilus richardsoni.

Thyroidal iodine, protein and thyroglobulin (TG) were investigated in a hibernator, Spermophilus richardsoni, sampled in the field and from laboratory-held winter colonies. In field animals, thyroidal iodine and protein contents were similar at onset of hibernation and at terminal arousal. Iodine content was increased in laboratory-held animals, a function of alimentary supply, but no differences were observed between hibernators and non-hibernators. Density sucrose gradients showed that approx. 20% of the TG was present as the 12S precursor sub-unit. No variations in the iodine content, nor the iodoamino acid composition of the TG occurred as a function of hibernation.

Iodine supply to the thyroid gland in the developing chick embryo.

During embryonic development of the chick it was found that 81% of the iodine deposited in the egg yolk, principally in the form of iodine, I-, was transferred to the embryo. Changes in plasma iodide levels, [I-], showed that transport is low between days 8 and 9 of incubation ([I-] = 2 micrograms/dl) and increased from day 10 ([I-] = 3 micrograms/dl) to day 20 ([I-] = 77 micrograms/dl). This increase of [I-], coupled with an increase in total blood flow through the thyroid gland, results in an increasing supply of iodide to the thyroid throughout the incubation period. Our results and those of other workers suggest the existence of bidirectional exchanges of iodide between the embryo and the yolk which may be under pituitary control.

[Thyroid function and iodine metabolism in the chicken embryo]. / Fonction thyroïdienne et métabolisme de l'iode chez l'embryon de Poulet.

Iodine is accumulated in the egg yolk during oogenesis. It is transferred during incubation through the walls of the yolk sac (extraembryonic endomesoderm surrounding the yolk) to the extraembryonic, and thence to the embryonic circulation. Iodine exchanges would probably occur between both compartments, the egg yolk and the embryonic blood in which the iodine level increases from day 10 to 20 of incubation. The increase of thyroid gland iodine content parallels that of the plasma iodine level. Our results suggested that the pituitary gland, which acts directly on the developing thyroid gland, might also influence the iodine exchanges across the yolk sac wall, thereby affecting the thyroid by increasing the iodine supply.

[Course of thyroxinemia during the development of the chick embryo. Influence of hypophysectomy]. / Evolution de la thyroxinémie au cours du développement de l'embryon de Poulet. Influence de l'hophysectomie

Plasma thyroxine (T4) level of intact and hypophysectomized Chick embryos has been determined for the incubation interval 10 to 18 days. T4, which is the main circulating thyroid hormone, exists as early as day 10. The plasma T4 level of controls increases as power function of time from day 10 to day 17. Hypophysectomy (on day 2) produces a decrease of the plasma T4 level, but only from day 11, which accounts for the beginning of a pituitary control of the thyroid secretion after day 10.

Thyroidal iodine metabolism during the development of the chick embryo.

Stable iodine was measured in the thyroid gland of the chick embryo from day 9 to day 20 of incubation in order to evaluate quantitatively the functional development of the gland. Total iodine content increased progressively during incubation. From day 9 to day 17 of incubation, this increase resulted from the increases of pellet-bound iodine and of soluble iodine. Afterwards, it essentially paralleled the increase of the soluble thyroglobulin-bound iodine which reflected the increase in both thyroglobulin content and the degree of iodination of the thyroglobulin. The total iodine, thyroglobulin-bound iodine and thyroglobulin (TG) content, increased as power functions of time during incubation, with critical times on days 11 and 15. Their concentrations also increased during the whole incubation period, while the iodide concentration remained roughly constant (25 ng/mg) from day 13 to day 19. Only one iodoprotein, 19.5 S TG, was found, and its heterogeneity of iodination was demonstrated during the whole period of incubation studied (from day 11 to day 20). The degree of dissociation with sodium dodecyl sulfate (SDS) of the TG into 12 S subunits decreased as the degree of iodination of the TG increased. Throughout embryonic development, iodine was bound more and more to TG molecules, which were resistant to dissociation with SDS. While the average iodine content of the TG increased, no appreciable changes were found in iodotyrosine and iodothyronine percentages of TG-bound iodine: monoiodotyrosine, 26%; diiodotyrosine, 43%; thyroxine 12%; 3,5,3'-triiodothyronine, 2.5%. As a consequence, a linear relationship existed for each iodoamino acid between the number of its residues per mole of TG and the iodine content of TG (127I atoms per mole)-- about 30 atoms of iodine was required to form 1 mole of T4. The low efficiency of the TG of the chick embryo as a thyroidal hormone-forming protein was compensated for by its high degree of iodination.