Jacques Leloup

Fish growth hormone enhances peripheral conversion of thyroxine to triiodothyronine in the eel (Anguilla anguilla L.).

In the eel, a low dose of tilapia growth hormone (tGH) (45 ng/g body wt), like ovine GH (oGH), induces a decrease in plasma thyroxine and a concomitant increase in plasma triiodothyronine, which result from a stimulation of peripheral conversion of thyroxine to triiodothyronine. Salmon prolactin (sPrl), unlike ovine Prl (oPrl), has no such action. Recognition of this specific action of growth hormone (GH) on production of active thyroid hormone (T3) opens up a new approach to the problem of the action of both hormones (GH, T3) in growth and in seawater adaptation of fish.

The endocrine control of reproduction and molt in male and female emperor (Aptenodytes forsteri) and adelie (Pygoscelis adeliae) penguins: II. Annual changes in plasma levels of thyroxine and triiodothyronine

Changes in plasma thyroxine (T4) and triiodothyronine (T3) levels were studied during a breeding season and in more detail during the postbreeding molt in male and female emperor (Aptenodytes forsteri) and adelie (Pygoscelis adeliae) penguins under natural conditions in the Antarctic. During the 4-month natural fast that accompanies courtship and incubation in male emperors, plasma T4 and T3 levels were maintained around 11 and 0.6 ng/ml, respectively. In courting, fasting female emperors plasma T4 levels were maintained around 10 ng/ml for more than 1 month; plasma T3 levels were around 0.8 ng/ml but were markedly depressed (0.1 ng/ml) at the time of copulation although they increased again (2.2 ng/ml) at oviposition. During the 5-month period of chick rearing, plasma T3 (males and females) and T4 (females) were maintained at the same levels as during courtship and incubation, but plasma T4 levels in male emperors were slightly lower (7 ng/ml). Similar plasma T4 and T3 levels were observed in breeding adelie penguins. These results do not provide any convincing evidence for thyroid-gonadal interrelations in breeding penguins, but demonstrate their capacity to maintain plasma thyroid hormone levels during very prolonged natural fasts. During the heavy postnuptial molt when the birds were fasting, in both species and sexes, marked but separate peaks in plasma T4 and T3 levels occurred concurrently with the initial growth of the new feathers, and with the subsequent shedding of the old plumage, respectively. Peak plasma T4 levels were observed at the time of the emergence of the new feathers out of the skin, and peak plasma T3 levels were roughly concurrent with the maximum daily body weight loss. This is the first strong evidence that increases in plasma T4 and T3 levels are correlated with different stages of molt in a wild seabird. Increased plasma T4 but not T3 levels at the time of feather papilla eruption suggest that T4 is concerned with feather growth, but is not exclusive of a role of T3. Increased plasma T3 but not T4 levels during the reduction in thermal insulation in molting penguins suggest that this hormone rather than T4 might be active in energy metabolism in penguins.

Effet de l'oestradiol sur l'axe hypophyso-thyroïdien de l'anguille

Abstract The effect of estradiol (E2) treatment on the hypophyso-thyroidal axis has been investigated in male and female eels in fresh water. The animals were injected with 150 μg/100 g body wt daily or every other day for 5 to 78 days; another group was treated for 40 days and sacrificed 40 days after the last injection. In the pituitary, the thyrotrophs (TSH cells) are activated and degranulated in long-term-treated eels. The histological structure of the thyroid follicles suggests a hypoactivity as shown by a reduced epithelial cell height in male and female eels injected with E2. Using radioimmunoassay of thyroxine (T 4 ) and triiodothyronine (T 3 ) in the plasma, a sharp decrease of both hormones is demonstrated which is highly significant. The ratio T 3 T 4 increases gradually. In animals sacrificed 40 days after the last injection, the thyroid function is still depressed although less than in eels sacrificed at the end of the treatment. These data suggest that thyroid hormone release is depressed by E2 although TSH release seems to be increased. The very important hypercalcemia induced by E2 could explain this paradoxical finding. The increased T 3 T 4 ratio could result from stimulation of prolactin releaseby E2.

Effects of vertebrate prolactins and growth hormones on thyroxine 5′-monodeiodination in the eel (Anguilla anguilla): A potential bioassay for growth hormone

Growth hormones (GHs) and prolactins (Prls) purified from representatives of each vertebrate class from bony fish onwards were tested for their ability to stimulate in vivo peripheral deiodination of labeled thyroxine (T4*) into triiodo-L-thyronine (T3*) in the eel. Plasma T3*/T4* ratio was used as parameter. All GHs significantly increased T3*/T4*, the magnitude of the response being unrelated to the phylogenic position of species. No significant stimulation was shown with the various Prl, with the exception of ovine Prl, suggesting a heterosomatotropic effect of this preparation in the eel. Furthermore, both tilapia and ovine GH produced a dose-related effect on plasma T3*, T4*, and T3*/T4*. The stimulation of the peripheral deiodination of T4* into T3* estimated in vivo in the eel could become a specific, sensitive, and rapid fish bioassay for GH.

The effect of severe starvation and captivity stress on plasma thyroxine and triiodothyronine concentrations in an antarctic bird (emperor penguin).

The effect of confinement and severe starvation on the plasma thyroxine (T4) and triiodothyronine (T3) concentrations was determined in emperor penguins (Aptenodytes forsteri). During their annual cycle, emperor penguins fast freely for periods of up to 4 months and may thus represent a unique subject to study endocrine adaptations to fasting. Plasma T4 concentrations progressively decreased following capture and confinement of naturally fasting penguins, and within 15-20 days stabilized at levels three times lower than in free-living penguins. A transient fourfold increase in plasma T3 concentration developed within the day following confinement in parallel with a rise in daily body mass loss. Both plasma T3 concentration and mass loss subsided to normal levels within 15 days. The decrease in plasma T4 concentration is in accordance with the well-known inhibitory effect of stress on thyroid function in birds and mammals, whereas the transient increase in plasma T3 concentration seems related to enhancement of energy expenditure as a consequence of restlessness. Starvation severe enough to exhaust fat stores and to activate protein catabolism induced a 6- and 5 to 10-fold fall in plasma T4 and T3, respectively. This is in marked contrast with maintenance of plasma thyroid levels during long-term natural fasting associated with protein sparing (R. Groscolas and J. Leloup (1986) Gen. Comp. Endocrinol. 63, 264-274). Surprisingly, there was a final reincrease in plasma T4 concentration in very lean penguins. These results suggest that the effect of starvation on plasma thyroid hormones seems to depend on how much protein catabolism is activated and demonstrate the acute sensitivity of thyroid hormone balance to stress in penguins.

Effects of ovine prolactin on iodide metabolism and thyroid activity in the eel

SummaryIn the eel, ovine prolactin (oPrl) treatment (0.018 IU/day·g body weight), for 8 to 13 days modifies neither iodide absorption from the water nor excretion, extrathyroidal metabolism and plasma level of iodide.Thyroid activity, evaluated by epithelial cell height, radioiodine uptake and absolute iodide uptake is approximately twice that of controls. However, the amounts of total iodine, thyroxine (T4) and triiodothyronine (T3) in thyroid are unaltered by oPrl. Therefore, the decrease of plasma T4 and the increase of plasma T3, previously observed in oPrl-treated eels, do not result from a preferential thyroidal secretion of T3, but only from a stimulation of peripheral conversion of T4 to T3. Furthermore, the increased thyroid activity probably originates from a decreased feedback inhibition following the fall of circulating T4 induced by oPrl.

Thyroid function of Gerbillidae (Meriones (M) libycus, M. shawi shawi and Psammomys (P) obesus) : field studies in Southern Tunisia

Abstract 1. Thyroid function was studied in three desert rodents P. obesus, M. shawi and M. libycus. Of the three, Psammomys had a diet richer in water, salt and iodine than the others. 2. Despite the difference in iodine supply the thyroid weights, iodine and T4 concentrations and the plasma T4 levels were not significantly different for the three species. 3. This was because the major part of food iodine in Psammomys was non-assimilable and, was thus excreted in the faeces. 4. Only thyroid and plasma T3 concentrations and plasma T3/T4, ratios were lower in Psammomys than in Meriones as a result of a slight overload of iodine. 5. T4 production rates were not different between Psammomys and M. shawi and were analogous to those found in rodents of the temperate zone. 6. Thus, these species do not present hypothyroid characteristics, a hypothesis proposed to explain the low metabolic rate of desert rodents.