Changes in plasma thyroid hormone levels in pink salmon, Oncorhynchus gorbuscha, during their spawning migration in the Fraser River (Canada)

Plasma thyroid hormone levels were measured in pink salmon, Oncorhynchus gorbuscha , during their spawning migration in the Fraser River, British Columbia. The plasma levels of both l-thyroxine (T₄) and triiodo-l-thyronine (T₃) were significantly higher in males than in females. In both sexes the hormone levels were maintained, or increased somewhat, during the early stages of migration, but fell thereafter. In females the plasma T₄ and T₃ levels of salmon collected on the spawning grounds were at or below detectable levels of the assays. The changes in thyroid hormone levels are correlated with changes in plasma insulin, gonadotropin, gonadal steroid hormones, cortisol and vitellogenin levels measured in the same specimens.     

Migration tendency in juvenile steelhead trout, Salmo gairdneri Richardson, injected with thyroxine and thiourea

Juvenile steelhead trout, Salmo gairdneri injected with thyroxine (T₄) near the time of seaward migration exhibited a significantly lower migration tendency than untreated controls of fish injected with saline, thiourea, or a combination of T₄ and thiourea. Fish injected with thiourea alone or in combination with T₄ prior to the time of maximum migration tendency showed enhanced migration over untreated and saline injected controls and those injected with T₄ alone. Injections of T₄ or a combination of T₄ and thiourea elevated plasma T₄ and tri-iodothyronine (T₃) concentrations, while injection of thiourea alone depressed thyroid hormone levels relative to saline-injected controls. Plasma concentrations of T₃ and T₄ returned to control levels within 10 days in all groups. We suggest that thyroid hormones are antagonistic to mechanisms underlying seaward migration of steelhead trout and that these antagonistic effects must be overcome before migration tendency is expressed.     

Calcium metabolism and osmoregulation during sexual maturation of river running Atlantic salmon

As Atlantic salmon return from the ocean to undertake the anadromous spawning migration up the river of origin, profound changes in calcium metabolism and osmoregulation take place. Using tartrate resistant acid phosphatase as a marker, scale osteoclast activity was found to increase throughout sexual maturation and spawning migration. Thus, the participation of osteoclasts in the elevated scale resorption observed during this phase is established. As calcium was simultaneously accumulated in the female gonads, it is proposed that the scales are resorbed in order to provide calcium for the growing ovaries. Plasma oestradiol-17 β levels were elevated in females during sexual maturation, and had decreased at the time of spawning. Plasma testosterone levels were similar in males and females during the first part of the upriver migration, but had increased in males and decreased in females at spawning. In addition to the role of these sex steroids in the gonadal growth, their possible involvement in the increased scale resorption during this phase is discussed. Plasma growth hormone and thyroxine levels were elevated in both sexes at spawning, with the triiodothyronine/thyroxine (T₃/T₄) ratio declining sharply, indicating possible roles for these hormones in the maturational process. The relatively low gill Na⁺, K⁺-ATPase activity of salmon caught in the estuary implies that the fish had already adapted to a hypoosmotic environment. During the upriver migration, the gill Na⁺, K⁺-ATPase activity decreased further, indicating that the hypoosmoregulatory ability was suppressed further during sexual maturation and spawning migration.     

Seasonal changes of thyroid hormones in field-collected Atlantic cod in relation to condition indices, water temperature and photoperiod

Serum T₄ and T₃ in wild Atlantic cod Gadus morhua ranged from 1 to 12 ng ml⁻¹ and from 2 to 27 ng ml⁻¹ respectively over a 3-year period. In general, the concentrations increased from summer (T₃) or early autumn (T₄) to maxima in mid-winter and declined abruptly during spring. The T₄/T₃ monthly means were lowest in summer and highest in winter. The seasonal patterns of thyroid hormones were weakly correlated with changes in water temperature. However, both T₄ and T₃ co-varied simultaneously with photoperiod. In addition, T₃ was correlated with the hepatosomatic index and condition factor during summer and autumn. It is suggested that the seasonal changes in the release of T₄ from the thyroid were photoperioddriven, and that the course of T3 was regulated by the metabolic state of the fish during the somatic growth period.     

Effect of rearing density on thyroid and interrenal gland activity and plasma and hepatic metabolite levels in rainbow trout, Salmo gairdneri Richardson

There were significant inverse correlations between rearing density of rainbow trout, Salmo gairdneri Richardson, and final body weight, plasma L-thyroxine (T₄), trüodo-L-tryronine (T₃), cortisol and protein concentrations, plasma T₄/T₃ ratios and thyroid epithelial cell height. In addition, hepatosomatic indices and plasma free fatty acid concentrations were higher in fish reared at low (134 g 1⁻¹) density compared with groups reared at medium (210g1⁻¹) or high density (299g 1⁻¹), and the post-feeding (3.5-4h) elevation in plasma glucose and triglyceride levels evident in trout maintained at low rearing density was not found in those fish reared at higher densities. There were no significant effects of rearing density on hematocrit, carcass composition, hepatic glycogen and lipid levels and interregnal nucleus size.     

Seasonal changes in thyroidal status in the plaice, Pleuronectes platessa L.

A chemically specific analysis for plasma thyroid hormones has been used to disclose a seasonal bimodality in the concentrations of thyroxine (T₄) and triiodothyronine (T₃) in the plaice; maxima in both hormone levels occur in winter and in summer. The ratio of T₄ concentrations to T₃ varies seasonally being at a minimum in summer. A highly significant correlation of T₄ plasma levels with the landings per unit fishing effort for the same area has been observed.     

Dose-related effects of 17βoestradiol (E2) on liver weight, plasma E2, protein, calcium and thyroid hormone levels, and measurement of the binding of thyroid hormones to vitellogenin in rainbow trout, Salmo gairdneri Richardson

Administration of 17β-oestradiol (E₂) to rainbow trout, in the form of hydrogenated coconut oil implants produced a stable, long-term elevation in plasma E₂ levels. The elevation was doserelated (over the range 1–10mg kg-¹ body weight) both 4 and 8 weeks after implantation. Dose-related increases were also observed with respect to liver weight-body weight ratios and plasma protein levels. Plasma T₃ and total calcium levels were depressed and elevated, respectively, by E₂ treatment but the responses were not linearly related to the dose of E₂ administered; there was no significant effect of E₂ on plasma T₄ levels.
E₂ induced a shift in the binding of T₃ to plasma proteins, with T₃ binding to smaller molecular weight proteins; neither T₄ nor T₃ bound to vitellogenin which was present at high levels in the plasma of E₂-treated fish.     

Studies on the relationship between osmotic or ionic regulation and thyroid gland activity in two salmonid fishes, Salmo gairdneri Richardson and Oncorhynchus kisutch Walbaum

In freshwater-acclimated rainbow trout a single intraperitoneal injection of ovine TSH significantly elevated plasma thyroxine (T₄) levels within 1 h after the injection. In seawater adapted trout the increase in T₄ after TSH-treatment was not evident until 6 h after the injection. TSH caused a transient fall in plasma Na⁺ and Cl^- between 3 h and 9 h after the injection in seawater-adapted fish and plasma Na⁺ was lowered in freshwater-adapted trout 24 h after the injection. Although there were clear histological changes in the thyroid gland of freshwater-adapted trout after TSH-injection, no such changes were evident in seawater-adapted fish.
Plasma thyroid hormone levels and thyroid histology in freshwater-adapted rainbow trout and coho salmon transferred to sea water, and seawater-adapted trout transferred to fresh water showed no consistent changes compared with controls.
The data are interpreted to indicate that although ambient salinity may have indirect effects on thyroid activity there is no direct involvement in ionic or osmotic regulation in the two species.     

Early larval development and metamorphosis in the summer flounder: changes in per cent whole-body water content and effects of altered thyroid status

At the end of premetamorphosis, summer flounder Paralichthys dentatus larvae had 84·1% whole-body water content (WBW), which decreased to the lowest levels (8·5%) at the start of metamorphic climax (MC). During mid- and late MC, %WBW was slightly higher (82·1%) then returned to the lowest levels at the juvenile stage. In fish treated with thyroxine (T₄-Na salt, 100 ng ml⁻¹) beginning at premetamophosis, %WBW never differed from controls of the same age throughout metamorphosis, despite an earlier start of metamorphic climax and transitional settling behaviour. This suggests that thyroid hormones do not mediate the drop in %WBW which accompanies natural metamorphosis. Thiourea (TU, 30 μg ml⁻¹) treatment of fish over the same period induced a developmental stasis in early MC which was accompanied by initially higher %WBW than controls at 33 days post-hatch, followed by a progressive decrease to abnormally low %WBW by 42 and 45 days post-hatch. Since concurrent treatment with TU+T₄ rescued the fish from both the TU-induced developmental stasis and abnormally low %WBW, these findings suggest that thyroid hormones, or thyroid hormone-mediated developmental progression, are necessary for regulating %WBW.     

Changes in thyroid function and gonadotrophin activity during sexual development in ducks Anas sp. (Aves: Anatidae)

Age-related changes in plasma iodohormone concentrations in male and female Aylesbury and Khaki Campbell ducklings were determined between the second week after hatching and sexual maturity.
In both sexes and strains the concentration of total thyroxine (T₄) and the free thyroxine index (FT₄1) were low during the first 6–8 weeks after hatching. A marked increase in the T₄ and FT₄1 occurred when the ducklings were nine weeks old; T₄ progressively increased over the following four weeks and remained high thereafter. The T₄ concentration in both sexes was directly related to age.
Concentrations of total tri-iodothyronine (T₃) were more variable than T₄ but were highest before sexual maturation and inversely age-related. An age-related decline in free tri-iodothyronine index (FT₃1) was observed during the first nine weeks.
The possibility that these maturational changes in thyroid function may reflect changes in gonadal activity was assessed by measuring the concentration of luteinizing hormone (LH) in the same plasma samples. Although plasma LH increased with age, in all cases the increase occurred after changes in the thyroid hormones were observed. Moreover, the maturational pattern of LH was related to sex and strain whereas thyroid function was unaffected by sex or strain.
These results demonstrate reciprocal changes in plasma T₃ and T₄ levels in ducks during sexual development, although the stimuli responsible and physiological significance are uncertain.     

Seasonal and diurnal rhythms of thyroidal status in the rainbow trout, Salmo gairdneri Richardson

The concentrations of thyroxine (T₄) and triiodothyronine (T₃) in the blood plasma of rainbow trout ( Salmo gairdneri Richardson) at intervals throughout the year have been measured by a chemically specific gas-liquid chromatographic (GLC) method. Mean hormone levels showed a seasonal variation, maximal levels of both hormones occurring in winter and minimal concentrations in mid summer. An apparent secondary maximum in mean T₄ and T₃ concentrations was observed in spring. (T₄)/(T₃) ratios have been found to be highest in winter and lowest in summer. A radioimmunoassay procedure, validated by GLC analyses, revealed the presence of a diurnal rhythm in serum T₄ levels of trout sampled in September and in April.     

Effects on temperature on spontaneous and thyroxine-stimulated locomotor activity of Atlantic cod

Spontaneous locomotor activity of cod Gadus morhua maintained at 6° C tripled from February to May. In contrast, locomotor activity of cod held at 2° C was significantly lower than at 6° C (between 25 and 65% lower) and the seasonal increase was smaller. Plasma levels of both thyroxine (T₄) and triiodothyronine (T₃) did not differ between 2 and 6° C. T₄ injection increased locomotor activity by 10% for both temperature regimes. These data indicate that low water temperature reduces locomotor activity associated with migration in cod and that thyroid hormones are not involved in this decrease. This study provides a possible mechanism through which cold waters may affects migration and distribution of cod via its Effects on locomotor activity and swimming speed.     

Relationship between eyed-egg percentage and levels of cortisol and thyroid hormone in masu salmon Oncorhynchus masou

A possible cause of the low eyed-egg percentage that afflicts pond-reared masu salmon Oncorhynchus masou at a fish hatchery (Kumaishi Fish Hatchery, Hokkaido, Japan) was investigated. Serum cortisol levels of masu salmon during the spawning period were compared between individuals from Kumaishi and those from another station (Mori). Cortisol, thyroxine (T₄) and triiodothyronine (T₃) hormones were also measured in eyed eggs and their levels were compared to the eyed-egg percentage. Serum cortisol levels were significantly higher at Kumaishi than at Mori in May and July of the breeding season. At the Kumaishi station, there was a linear positive relationship between serum cortisol and fertilized-egg cortisol levels of female masu salmon. As cortisol levels in both ovulating females and eyed eggs increased, the eyed-egg percentage decreased, indicating a highly significant negative relationship between cortisol levels and eyed-egg percentage. In contrast, as T₃ and T₄ levels in eyed eggs increased, the eyed-egg percentage increased. The strong positive correlation between high cortisol levels in serum and in eyed eggs at Kumaishi indicates that the frequently higher cortisol levels may have caused the lowering of the eyed-egg percentage.     

Characterization of the Thyroid Hormone Transport System of Cerebrocortical Rat Neurons in Primary Culture

Abstract: The uptake of 3',3,5-triiodo- l -thyronine (T₃) and l -thyroxine (T₄) by primary cultures derived from rat brain hemispheres was studied under initial velocity conditions, at 25°C. Uptake of both hormones was carrier mediated and obeyed simple Michaelis-Menten kinetics. The K _m of T₃ uptake was very similar to that of T₄, and did not vary significantly from day 1 to 4 in culture (310–400 n M ). The maximal velocity ( V _max) of T₃ uptake nearly doubled between day 1 and 4 of culture (41 ± 3 vs. 70 ± 5 pmol/min/mg of DNA, respectively). The V _max of T₄ uptake did not change (28 ± 8 and 31 ± 4 pmol/min/mg of DNA on days 1 and 4, respectively). The rank order of unlabeled thyroid hormone analogues to compete with labeled T₃ or T₄ uptakes were the same (T₃ > T₄ > 3',5',3-triiodo- l -thyronine > 3',3,5-triiodo- d -thyronine > triiodothyroacetic acid), indicating that the transport system is stereospecific. Unlabeled T₄ was a stronger competitor of labeled T₄ uptake than of labeled T₃ uptake, whereas unlabeled T₃ had the same potency for both processes. These results suggest that T₃ and T₄ are transported either by two distinct carriers or by the same carrier bearing separate binding sites for each hormone. They also indicate that the efficiency of T₃ uptake increases during neuronal maturation.     

Influences of photoperiod and alternate days of feeding on plasma growth hormone and thyroid hormone levels in juvenile rainbow trout

The diurnal patterns of plasma growth hormone (GH), thyroid hormone and cortisol concentrations in rainbow trout Oncorhynchus mykiss held under three photoperiod (L : D) regimes (6 : 18, 12 : 12 and 18 : 6), and fed either daily (DF) or on alternate days (ADF) with 2·0% body mass per day of a commercial trout diet were determined. The ADF groups had reduced total mass gain and specific growth rates compared with DF fish, but photoperiod had no affect on growth for either of the feeding regime groups. In the ADF groups, the mean 24 h plasma thyroxine (T₄) and triiodothyronine (T₃) concentrations were significantly lower, both on days of feeding and days of fasting, than in DF fish held under all three photoperiod regimes, but for GH, only the 18L : 6D DF group was higher than the comparable ADF groups. There were no significant differences in mean 24 h plasma cortisol concentrations of DF and ADF groups. Diurnal patterns of plasma GH, cortisol, T₄ and T₃ were found in DF fish held under all three photoperiod regimes. Increases in plasma cortisol changes were associated with the onset of the light phase; elevations in plasma GH and T₄ concentrations were more closely associated with clock time, regardless of photoperiod; increases in plasma T₃ concentrations were strongly associated with time of feeding. In ADF groups, these diurnal changes in plasma GH, T₄ and T₃ concentrations were suppressed for both the fed and fast days, and plasma cortisol concentrations were suppressed on the fasting day. The observations are discussed in terms of the proposed anabolic, catabolic and growth regulating roles of these hormones in different growth and metabolic modifying situations in teleosts.     

Distribution of immunoreactive thyroxine in the endostyle and thyroid tissue of the sea lamprey Petromyzon marinus L., 1758

Anti-thyroxine (T₄) immunostaining in the endostyle and thyroid tissue during metamorphosis of sea lamprey Petromyzon marinus suggested that T₄ synthesis occurs on the luminal surface of the apical membrane of selected cells of the endostyle, and that a 'classical' endocrine thyroid function was only evident in post-transformed animals.     

Cortisol and thyroid hormone responses to acid stress in the brown trout, Salmo trutta L.

Individual cannulated brown trout monitored during exposure to acidic water showed increased plasma cortisol after 3 h at pH 4.0 with low (0.05 mm) or high (2.8 mm) calcium (Ca) content, and after 2 days in acidic water with a high Ca content. Most fish did not survive for 2 days in acidic water with a low Ca content. Non-cannulated fish showed a similar increase in mean plasma cortisol after 2 days in high-Ca acidic water (pH 4.0–4.6), but not in acidic water of a low Ca content. After 7 days of exposure to acidic water, plasma cortisol appeared to recover when there was a high Ca content but increased 20-fold when Ca content was low. In cannulated fish severe acid stress resulted in a marked and rapid thyroid response. Plasma thyroxine (T₄) was elevated after 3 h exposure to acidic water of both low and high Ca content and remained elevated for 2 days of acid exposure with high Ca. In non-cannulated fish an increase in mean T₄ was apparent only after 7 days in low-Ca acidic water. Plasma triiodothyronine (T₃) levels were not significantly altered by any of the acid regimes. Plasma glucose of cannulated fish was elevated within 3 h of acid-exposure and remained elevated after 2 days in high-Ca acidic water.     

Evidence for Circadian Variations of Thyroid Hormone Concentrations and Type II 5'-Iodothyronine Deiodinase Activity in the Rat Central Nervous System

Abstract: The 24-h patterns of tissue thyroid hormone concentrations and type II 5'- and type III 5-iodothyronine deiodinase (5'D-II and 5D-III, respectively) activities were determined at 4-h intervals in different brain regions of male euthyroid rats entrained to a regular 12-h light/12-h dark cycle (lights on at 6:00 a.m.). Activity of 5'D-II, which catalyzes the intracellular conversion of thyroxine (T₄) to 3,3',5-triiodo- l -thyronine (T₃) in the CNS, and the tissue concentrations of both T₄ and T₃ exhibited significant daily variations in all brain regions examined. Periodic regression analysis revealed significant circadian rhythms with amplitudes ranging from 9 to 23% (for T₃) and from 15 to 40% (for T₄ and 5'D-II) of the daily mean value. 5'D-II activity showed a marked nocturnal increase (1.3–2.1-fold vs. daytime basal value), with a maximum at the end of the dark period and a minimum between noon and 4:00 p.m. 5D-III did not exhibit circadian patterns of variation in any of the brain tissues investigated. Our results disclose circadian rhythms of 5'D-II activity and thyroid hormone concentrations in discrete brain regions of rats entrained to a regular 12:12-h light-dark cycle and reveal that, in the rat CNS, T₃ biosynthesis is activated during the dark phase of the photoperiod. For all parameters under investigation, the patterns of variation observed were in part regionally specific, indicating that different regulatory mechanisms may be involved in generating the observed rhythms.     

Influence of plasma lipid changes in response to 17β-oestradiol stimulation on plasma growth hormone, somatostatin, and thyroid hormone levels in immature rainbow trout

Plasma total lipids were significantly higher in 17β-oestradiol(E₂)-treated immature rainbow trout Oncorhynchus mykiss at week 4 after implantation, due to increases in polar and neutral lipids. The lipid classes responding were phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine, sterols and sterol esters, in a proportion that approximately reflected the increase in plasma vitellogenin (VtG) levels as measured by a non-competitive enzyme-linked immunosorbent assay (ELISA). Plasma non-esterified fatty acids and triacylglycerol were not affected by E₂ treatment. Plasma growth hormone GH levels were increased, and plasma somatostatin-14 (SRIF) levels decreased in E₂-treated fish, responses which could be secondary to elevated plasma lipid (VtG) content, although a direct E₂ action on somatotroph function is possible. Plasma T₄ concentrations were not affected by E₂ treatment, but plasma T₃ concentrations were significantly lower than in controls 1 week after implantation when plasma E₂ concentrations were the highest; this is in support of the hypothesis that E₂ has a suppressive action on T₃ production.     

Plasma and ovarian thyroxine levels in relation to sexual maturation and gestation in female Sebastes inermis

Plasma T₄ (L-thyroxine) concentrations in the viviparous rockfish Sebastes inermis showed a peak (119·3±27·8 ng ml⁻¹) during the development of embryos. Ovarian T₄ concentrations increased during vitellogenesis and final maturation and decreased during embryogenesis.However, total T₄ content within the ovary increased continuously up to the larval stages. Plasma oestradiol-17β, (E₂) concentrations peaked at the final maturation stage and then recovered to the level seen during the non-reproductive periods. Plasma testosterone (T) concentrations showed a peak (5·33±1·08 ng ml⁻¹) at the embryo stage and high levels were maintained throughout the gestation period. Plasma T₄ concentrations during the embryo stage and ovarian T₄ content during vitellogenesis were much higher than the levels reported in oviparous fishes. These data suggest that in viviparous rockfish, T₄ may be required for sustaining gestation or embryo development within the maternal body. In addition, the high content of total T₄ in the ovary implies that there probably does exist a maternal-embryo relationship during gestation as well as during vitellogenesis.