Effect of excess iodide on thyroid function of rainbow trout,Salmo gairdneri

The acute and chronic effects of excess iodide (KI or NaI) were studied on thyroid function of rainbow trout at 11±1°C. No Wolff-Chaikoff effect, characteristic of mammals, was observed and instead plasma L-thyroxine (T₄) levels increased 6 hr after a single iodide injection. Plasma 3,5,3′-triiodo-L-thyronine (T₃) did not change and by 24 hr plasma T₄ returned to normal. This iodide-induced elevation in plasma T₄ was probably not due to toxic effects demonstrated at higher NaI or KI doses. A single iodide injection also decreased the plasma iodide distribution space, decreased the fractional rate of plasma iodide loss and completely blocked thyroidal uptake of radioiodide. Injections of iodide over a 22-day period elevated plasma iodide 200X with no mortality and no influence on plasma T₄ or T₃. It is concluded that: (i) apart from the transient 6h increase in plasma T₄, trout thyroid function, as judged by plasma hormone levels, is insensitive to considerable iodide excess, (ii) non-invasive iodide suppression of thyroidal radioiodide recycling may be useful in kinetic studies of¹²⁵I-labeled thyroid hormones, and (iii) fundamental differences in intrathyroidal iodine metabolism appear to exist between mammals and fish.     

Effects of thyroid hormone on gonadotropin-induced steroid production in medaka,Oryzias latipes,ovarian follicles

Blood and ovarian samples were collected at intervals of 4h prior to spawning time from medaka (Oryzias latipes) that were maturationally synchronized with artificial photoperiod (14h light: 10h dark). Plasma estradiol-17β (E₂) levels increased rapidly from 16h before spawning and peaked at 8h before spawning. Follicle-enclosed oocytes (ovarian follicles) at different stages of development were isolated from the ovaries and used to study the in vitro effects of thyroid hormone (triiodothyronine; T₃) on pregnant mare serum gonadotropin (GTH)-induced E₂ production. GTH at a concentration of 100 IU/ml stimulated E₂ production by ovarian follicles collected between 32 and 16h before spawning. At 32h before spawning, T₃ (5 ng/ml) administered along with GTH (100 IU/ml) resulted in a 3.5 fold increase in E₂ production, compared with GTH administered alone. These results suggest that T₃ can act on ovarian follicles directly to modulate GTH-stimulated E₂ production in the medaka.     

Molecular cloning of the cDNA encoding the β subunit of thyrotropin and regulation of its gene expression by thyroid hormones in the goldfish, Carassius auratus

A cDNA encoding the subunit of thyrotropin (TSH) was isolated from a goldfish (Carassius auratus) pituitary gland cDNA library. By comparing the sequence with other teleost TSHs, a signal peptide of 19 amino acids and a mature hormone of 131 amino acids were predicted for goldfish TSH subunits. The resulting putative mature hormone of 131 amino acids had well-conserved cysteine positions and a putative N-linked glycosylation site; homology was 51–67% with TSHs from other teleosts, 38–43% with tetrapod TSHs, but only 27 and 29% with goldfish GTH-I and -II, respectively. We also examined the effects of thyroid hormones (TH) and thiourea (TU, an inhibitor of TH production) treatments on TSH and GTH subunit gene expressions in the goldfish pituitary gland. After thyroxine (T₄) treatment, circulating T₄ concentration increased and TSH mRNA level decreased. Supressing the amount of circulating T₄ and triiodothyronine (T₃) by TU treatment increased the TSH mRNA level. Moreover, T₄ replacement therapy (simultaneous treatment of both TU and T₄) caused a high level of circulating T₄ and a low level of circulating T₃, and a decrease in the TSH mRNA level. Thus, changing levels of circulating TH exert a negative feedback on the level of TSH subunit mRNA in goldfish in vivo. On the other hand, GTH subunit mRNA levels were not affected by changes in the levels of circulating TH.     

Changes in plasma T3 during fasting/refeeding in tilapia (Oreochromis niloticus) are mainly regulated through changes in hepatic type II iodothyronine deiodinase

Fasting and refeeding have considerable effects on thyroid hormone metabolism. In tilapia (Oreochromis niloticus), fasting results in lower plasma T₃ and T₄ concentrations when compared to the ad libitum fed animals. This is accompanied by a decrease in hepatic type II (D2) and in brain and gill type III (D3) activity. No changes in kidney type I (D1) activity are observed. Refeeding results in a rapid restoration of plasma T₄ values but not of plasma T₃. Plasma T₃ remains low for two days of refeeding before increasing to normal levels. Liver D2 and gill D3 also do not increase until two days after refeeding. Brain D3, on the other hand, rises immediately upon refeeding. These results suggest that the change in hepatic D2 activity is one of the main factors responsible for the changes in plasma T₃ observed during starvation and refeeding in tilapia. This finding supports the hypothesis that, in contrast to mammals and birds, liver D2 is the primary source of plasma T₃ in fish. Although the deiodinases important for the gross regulation of plasma T₃ during fasting/refeeding differ (mammals: D1 and D3, birds: D3, fish: D2), they all occur in the liver, suggesting that the organ itself may play a crucial role. In addition, the changes in brain and gill D3 suggest that these enzymes constitute a fine tuning mechanism for regulation of T₃ availability at the cellular or plasma levels, respectively.     

The relationship between T3 production and energy balance in salmonids and other teleosts

Extrathyroidal T₄ 5′-monodeiodination, demonstrated in several teleost species, generates T₃ which binds more effectively than T₄ to putative nuclear receptors and is probably the active thyroid hormone. T₄ to T₃ conversion is sensitive to the physiological state and provides a pivotal regulatory link between the environment and thyroid hormone action. T₃ generation is enhanced in anabolic states (positive energy balance or conditions favoring somatic growth; food intake or treatment with androgens or growth hormone) and is suppressed in catabolic states (negative energy balance or conditions not favoring somatic growth; starvation, stress, or high estradiol levels associated with vitellogenesis). In fish, as in mammals, thyroidal status may be finely tuned to energy balance and through T₃ production regulate energy-demanding processes, which in fish include somatic growth, development and early gonadal maturation.     

Determination of 3,5,3′-triiodo-L-thyronine (T3) levels in tissues of rainbow trout (Salmo gairdneri) and the effect of low ambient pH and aluminum

Tissue T₃ (3,5,3′-triiodo-L-thyronine) concentrations were measured in rainbow trout, Salmo gairdneri, after digestion by Pronase or collagenase and extraction with ethanolic ammonia (99:1, v/v) followed by 2N NH₄OH and chloroform. Recoveries of [¹²⁵I]T₃ administered in vivo or in vitro were high and consistent and there was close parallelism between sample dilutions and the radioimmunoassay curve, but recoveries of unlabeled T₃ administered in vitro were low and variable. Alternatively, trout were brought to isotopic equilibrium by [¹²⁵I]T₃ infusion for 96 h, the extracted [¹²⁵I]T₃ determined by gel filtration and the tissue T₃ content calculated from the specific activity of plasma [¹²⁵I]T₃. By the latter method, tissue T₃ concentrations were: intestine (4.2 ng/g), kidney (2.5), liver (2.8), stomach (1.5), heart (1.0), muscle (0.7), gill (0.6) and skin (0.3). Muscle (67% of body weight) comprised the largest tissue T₃ pool (82% of all tissues examined). Seven days exposure of trout to water acidified with H₂SO₄ (pH 4.8) or acidified water containing aluminum (21.6 mM), decreased tissue T₃ content generally and particularly in muscle (14% of controls). In conclusion, skeletal muscle is the largest T₃ tissue pool and seems highly responsive to altered physiologic state.     

Does the time of feeding affect the diurnal rhythms of plasma hormone and glucose concentration and hepatic glycogen content of rainbow trout?

The diurnal rhythms of plasma glucose, cortisol, growth hormone (GH) and thyroid hormone (T₄, T₃) concentrations and hepatic glycogen content were measured in rainbow trout that had been entrained to a specific time of daily feeding (post-dawn, midday, pre-dusk); the purpose of the study was to investigate the significance of feeding time on hormones and metabolite patterns. Plasma GH, cortisol and T₄ concentrations all showed evidence of a diurnal rhythm in some treatment groups. There was a significant interaction between the time of feeding and plasma GH and cortisol concentration rhythms; for GH, this appeared to be related to the phase-shifting of the post-prandial increases in plasma GH concentrations, and for cortisol, the rhythms were only evident in fish fed in the post-dawn period [diurnal rhythms were not evident in treatment groups fed in at midday or pre-dusk]. Peak plasma T₄ concentrations were evident during the photophase in all three treatment groups; however, the time of feeding had a negligible effect on the timing of those peaks. There were no apparent diurnal rhythms of plasma T₃ and glucose concentrations, hepatic glycogen content or hepatosomatic index in any of the three treatment groups. To whom correspondence should be addressed     

In vivo calcitropic effect of some vitamin D compounds in the marine Antarctic teleost,Pagothenia bernacchii

The Antarctic notothenioid, Pagothenia bernacchii, were found to have plasma total and free calcium levels, plasma inorganic phosphate and whole body calcium efflux rates which were similar to those seen in other teleosts. But total bone calcium was lower than reported for other teleosts. A single injection of vitamin D₃ (5 ng g^–1 fish) increased plasma total and plasma free calcium and these increases were associated with an increase in whole body calcium efflux and bone calcification. Conversely, the same treatment with 1,25-(OH)₂-D₃ reduced plasma free calcium. This seco-steroid also increased the specific activity of ⁴⁵Ca in bone at 40h post-injection but did not significantly effect total bone calcium, plasma total calcium or whole body calcium efflux. 25-OH-D₃ at the same dose had no effect on any of the parameters tested and none of the seco-steroids tested had any effect on plasma total inorganic phosphate. These data show that both D₃ and 1,25-(OH)₃-D₃ can have calcitropic effects in this marine teleost and that these two forms of vitamin D can exert different effects within the same species.     

Circadian pattern of hepatosomatic index,liver glycogen and lipid content,plasma non-esterified fatty acid,glucose, T3, T4, growth hormone and cortisol concentrations in Oncorhynchus mykiss held under different photoperiod regimes and fed using demand-feeders

The circadian patterns of several tissue and plasma metabolites, and several plasma hormone concentrations are described in rainbow trout (Oncorhynchus mykiss) that were held in groups under three different photoperiod regimes, and given free access to a demand-feeder. Regardless of photoperiod regime, all the measured parameters showed significant diel rhythms that appeared to be synchronized by dawn; dawn was represented by the concomitant onset of both light and feeding. The diel increases in hepatic glycogen content, and plasma T₄ and cortisol concentrations were in phase with the main period of feeding activity, whereas the peaks in plasma T₃ and glucose concentrations that may also be triggered by feeding activity, were delayed by several hours. The peaks in hepatosomatic index, plasma non-esterified fatty acids and plasma growth hormone concentrations were 180° out of phase with the main period of feeding activity, and associated with periods of hypophagia and low activity.     

Seasonal changes in channel catfish thyroid hormones reflect increased magnitude of daily thyroid hormone cycles

Channel catfish (Ictalurus punctatus) in pond culture, sampled once per day, have been reported to exhibit significant seasonal cycles in the thyroid hormones thyroxine (T₄) and 3,5,3′-triiodothyronine (T₃), rising from levels generally below 2 ng/ml in January to above 8 ng/ml in July. To determine if daily thyroid hormone cycles underlie these seasonal changes, we blood sampled groups of 20 catfish (10 males and 10 females) in the morning (approx. 1 h after sunrise), midday, and evening (approx. 1.5 h before sunset) on January 9, April 4, and July 29. From January to July, pond temperatures rose from 7 ° to 32 °, associated with significant (p < 0.05) increases in mean fish weight (from 477 to 1052 g) and in monthly mean food consumption (from 34 to 474.7 g/kg fish). On all three dates, significantly (p < 0.05) greater levels of both hormones (except T₃ in April) were found in midday and evening compared to morning samples. In January, the daily change was small (from morning to midday, mean T₃ rose from 2.2 to 3.6 ng/ml and mean T₄ from 2.3 to 4.8 ng/ml), whereas in July it was considerably greater (from morning to evening, mean T₃ rose from 7.2 to 17.8 ng/ml, and T₄ from 9.0 to 22.4 ng/ml). No significant differences were found between midday and evening levels, or between males and females. Additionally, no seasonal phase-shifting of cycles was apparent. A subset of animals was examined to evaluate the potential contribution of peripheral mechanisms in generating these seasonal and daily cycles. Whereas we observed only minor changes in thyroid hormone binding to plasma proteins during any single day, a significant seasonal increase in the ratio of free T₄:free T₃ indices (from a mean of 1.3–1.5 in January to 2.0–2.1 in July) indicated enhanced T₃ binding by plasma proteins in July. Furthermore, in vitro hepatic T₄ and T₃ deiodination activities showed across dates no significant change in T₄ outer-ring deiodination to produce T₃ (ranging from a mean of 53.1 to 70.1 pmol T₄ deiodinated/h/mg microsomal protein), but a significant (p < 0.05) decrease in T₄ inner-ring deiodination to degrade T₄ to 3,3′5′-triiodothyronine (from a mean in January of 2.4 to 0.65 pmol T₄ deiodinated /h/mg protein in April) and a significant (p < 0.05) decrease in T₃ inner-ring deiodination to degrade T₃ to 3,3′-diiodothyronine (from a mean in January of 115.5 to 3.1 pmol T₄ deiodinated/h/mg protein in July). These results demonstrate that channel catfish under conditions of natural temperature and photoperiod exhibit robust daily cycles in total plasma T₄ and T₃ similar in magnitude to those reported for other fish species held under controlled laboratory conditions. These cycles maintain a similar phase throughout the year, indicating that apparent seasonal increases in thyroid hormones are not due to phase-shifting of daily cycles. However, seasonal studies sampling fish only in the morning would underestimate the magnitude of the annual changes in blood thyroid hormones. Thyroidal status, as judged from total plasma T₄ and T₃ levels in the afternoon, is greatest in July, coinciding with the postspawning peak in food consumption and growth. Enhanced T₃ plasma protein binding and a shift from predominantly hepatic inner-ring deiodination in winter to outer-ring deiodination in summer suggest that peripheral mechanisms contribute to the generation of these seasonal changes.     

Thyroid hormone deiodinase systems in salmonids,and their involvement in the regulation of thyroidal status

The trout thyroid secretes L-thyroxine (T₄) which undergoes enzymatic deiodination in liver and other tissues. Based on mammalian studies, T₄ outer-ring deiodination (ORD) or T₄ inner-ring deiodination (IRD) could generate respectively 3,5,3′-triiodo-L-thyronine (T₃) or 3,3′,5′-T₃(rT₃), while subsequent T₃ORD or T₃IRD could generate respectively 3,5-diiodo-L-thyronine (T₂) or 3,3′-T₂, and rT₃ORD or rT₃IRD could generate respectively 3,3′-T₂ or 3′,5′-T₂. In practice, T₄ in trout undergoes hepatic ORD to produce T₃ but negligible IRD to produce rT₃, and T₃ in turn undergoes negligible ORD but modest IRD to produce 3,3′-T₂. T₄ORD, which is particularly important in converting T₄ to the biologically more potent T₃, also occurs in gill, muscle and kidney. At least two isozymes are involved: i) a high-affinity, propylthiouracil (PTU)-sensitive T₄ORD which displays ping-pong kinetics, requires thiol as a cofactor, and is present in liver, gill and muscle, and ii) a low-affinity, PTU-insensitive T₄ORD with sequential kinetics with a thiol cofactor, and is present in liver and kidney. Receptor-bound T₃ is derived primarily from the plasma for kidney, mainly from intracellular sources for gill and about equally from both plasma and intracellular sources for liver. Thus, the high-affinity T₄ORD may produce T₃ for local intracellular use while the low-affinity 5′-monodeiodinase may produce T₃ for systemic use. T₄ORD activity responds to nutritional factors and the physiologic state of the fish. Furthermore, T₃ administered orally for either 6 weeks or 24h reduces the functional level (V_max) of hepatic T₄ORD, and T₃ added to isolated hepatocytes also reduces activity, indicating direct T₃ autoregulation of T₄ORD to maintain hepatocyte T₃ homeostasis. However, T₃ administration also induces T₄IRD to produce biologically inactive rT₃ and induces T₃IRD to produce 3,3′-T₂. Thus, the trout liver has several iodothyronine deiodinase systems which in a coordinated manner regulate tissue T₃ homeostasis in the face of a T₃ challenge. It does this by decreasing formation of T₃ itself, by diverting T₄ substrate to biologically inactive rT₃ and by increasing the degradation of T₃. These deiodinases differ in many respects from any mammalian counterparts.     

Osmoregulation in the mudskipper,Boleophthalmus boddaerti I. Responses of branchial cation activated and anion stimulated adenosine triphosphatases to changes in salinity

The mudskipperB. boddaerti, was able to survive in waters of intermediate salinities (4–27). Fish submerged in dechlorinated tap water suffered 60% mortality by the fifth day while 60% of those in 100% sea-water (sw) died after the third day of exposure. After being submerged in 50% or 80% sw for 7 days, the plasma osmolality, plasma Na⁺ and Cl^– concentrations and the branchial Na⁺ and K⁺ activated adenosine triphosphatase (Na⁺,K⁺-ATPase) activity were significantly higher than those of fish submerged in 10% sw for the same period. However, the activities of the branchial HCO₃ ^– and Cl^– stimulated adenosine triphosphatase (HCO₃ ^–,Cl^–-ATPase) and carbonic anhydrase of the latter fish were significantly greater than those of the former. Such correlation suggests that Na⁺,K⁺-ATPase is important for hyperosmotic adaptation in this fish while HCO₃ ^–-Cl^–-ATPase and carbonic anhydrase may be involved in hypoosmotic survival.     

Influence of 11-ketotestosterone, 17β-estradiol, and 3,5,3′-triiodo-L-thyronine on distribution and metabolism of carotenoids in Arctic charr, Salvelinus alpinus L.*

This investigation examines the influence of implants containing 11-ketotestosterone (11KT), 17-estradiol (E₂), and 3,5,3-triiodo-l-thyronine (T₃) on astaxanthin metabolism in sexually immature individually tagged Arctic charr. The fish (initial average weight 427 g) were maintained in freshwater for 40 days, and weekly implanted intraperitoneally with oil-based injections containing either 11 KT, E₂ or T₃ at levels of 0.1, 1.0 and 0.1 mg (100 g body weight (BW))^–1, respectively. The control fish were given the oil medium alone (0.2 ml 100 g BW^–1). The diet contained ca. 50 mg astaxanthin kg^–1. Carotenoid composition was monitored in plasma, fillet, liver and skin, and 11 KT, E₂ and testosterone (T) levels in plasma. All hormone treatments reduced plasma T compared to the control. E₂-treated fish had a higher (p<0.05) hepatosomatic index (HSI) than the other treatments. Hormone treatment did not influence gonadosomatic index (GSI). T₃ administration induced a silvery skin appearance. The fillet and plasma carotenoid content decreased during the experiment. 11 KT implantation reduced astaxanthin and idoxanthin concentrations of plasma and fillets, and increased the amount in liver and skin, compared to the other treatments. The relative proportion of astaxanthin to idoxanthin was higher in the control fish and T₃ implanted fish, than in fish implanted with 11 KT or E₂ (p<0.05). Fish treated with E₂ had the highest skin carotenoid concentration. Male fish had significantly higher carotenoid content in plasma, fillet and skin than female fish. This study reveals that sex hormones affect carotenoid metabolism and partitioning among body compartments of Arctic charr, effects differently displayed by the sexes.     

Hormonal changes accompanying sexual maturation in captive milkfish (Chanos chanos Forsskal)

Steroid hormone profiles accompanying sexual maturation in captive milkfish are described. There were no significant differences in levels of serum estradiol 17- (E₂) and testosterone (T) between immature male and female fish. Mean E₂ levels rose from 0.54±0.11 ng/ml in immature females (Stage 1) to 4.53±1.16 ng/ml in vitellogenic females (Stage 5), while T levels increased from 2.06±0.28 ng/ml to 38.4±9.26 ng/ml. E₂ and T levels were positively correlated to GSI and oocyte diameter. In males, serum T levels increased from 2.5±0.40 ng/ml in immature males to 27.73±5.02 ng/ml in spermiating males. A significantly higher T level was found in males with thick and scantly milt (spermiation index, SPI, 2) compared to males with scanty milt (SPI, 1) or males with copious, fluid milt (SPI, 3).Serum levels of E₂ and T, and the GSI in females rose significantly during the breeding season (April–June 1983). The levels of both steroids dropped below 1 ng/ml in spent females sampled in succeeding months. In immature males, T levels ranged from 1.11 ng/ml to 2.78 ng/ml and rose significantly to 21.52±8.38 ng/ml during the breeding season when GSI peaked. Serum T levels dropped to around 10 ng/ml in the succeeding months when only spent or regressed males were sampled.     

Influence of cortisol, growth hormone, insulin-like growth factor I and 3,3′,5-triiodo-l-thyronine on hypoosmoregulatory ability in the euryhaline teleost Fundulus heteroclitus

The capacity of cortisol, ovine growth hormone (oGH), recombinant bovine insulin-like growth factor I (rbIGF-I) and 3,3,5-triiodo-l-thyronine (T₃) to increase hypoosmoregulatory capacity in the euryhaline teleost Fundulus heteroclitus was examined. Fish acclimated to brackish water (BW, 10 ppt salinity) were injected with a single dose of hormone suspended in oil and transferred to seawater (SW, 35 ppt salinity) 10 days post-injection. Fish were sampled 24 h after transfer and plasma osmolality and gill Na⁺, K⁺-ATPase activity were examined. Transfer from BW to SW induced significantly increased plasma osmolality but not gill Na⁺, K⁺-ATPase activity. Cortisol (50 g g^–1 body weight) improved the ability to maintain plasma osmolality and to increase gill Na⁺, K⁺-ATPase activity. oGH (5 g g^–1 body weight) also increased hypoosmoregulatory ability and gill Na⁺, K⁺-ATPase activity. A cooperation between oGH and cortisol was observed in increasing hypoosmoregulatory ability but not in increasing gill Na⁺, K⁺-ATPase activity. rbIGF-I (0.5 g g^–1 body weight) alone was without effect in increasing salinity tolerance or gill Na⁺, K⁺-ATPase activity. rbIGF-I and oGH showed a positive interaction in increasing salinity tolerance, but not gill Na⁺, K⁺-ATPase activity. Treatment with T₃ (5 g g^–1 body weight) alone did not increase salinity tolerance or gill Na⁺, K⁺-ATPase activity, and there was no consistent significant interaction between cortisol and T₃ or between GH and T₃. The results confirm the classical role of cortisol as a seawater-adapting hormone and indicate an interaction between cortisol and the GH/IGF-I axis during seawater acclimation of Fundulus heteroclitus.     

T3 enhancement ofin vitro estradiol-17β secretion by oocytes of rainbow trout,Salmo gairdneri,is dependent on cAMP

The cellular mechanism of action of 3,5,3-triiodo-L-thyronine (T₃) in enhancing SG-G100 gonadotropin-induced ovarian secretion of 17-estradiol (E₂) was studiedin vitro using oocyte follicular preparations of rainbow trout. The dependence of the T₃ stimulatory action on the level of intracellular 3,5-cyclic adenosine monophosphate (cAMP) was shown in experiments in which forskolin or dibutyryl cAMP enhanced E₂ secretion. In the presence of partially purified salmon gonadotropin (SG-G100), T₃ stimulation of E₂ secretion was prevented by theophylline, suggesting that T₃ may exert part of its stimulatory action by inhibiting phosphodiesterase.     

Response of rainbow trout gill (Na++K+)-ATPase and chloride cells to T3 and NaCl administration

With the aim of comparing the effects of oral T₃ and NaCl administration on trout hypoosmoregulatory mechanisms, three groups of rainbow trout (Oncorhynchus mykiss Walbaum) held in freshwater (FW) were fed a basal diet (C), the same diet containing 8.83 ppm of 3,5,3-triiodo-L-thyronine (T₃) (T) or 10% (w/w) NaCl (N) respectively for 30 d. They were then transferred to brackish water (BW) for 22 d and fed on diet C. Gill (Na⁺+K⁺)-ATPase activity and its dependence on ATP, Na⁺ and pH, number of gill chloride cells (CC), serum T₃ level as well as fish growth, condition factor (K) and mortality were evaluated. During the FW phase, as compared to C trout, T trout showed a two fold higher serum T₃ level, had unchanged gill (Na⁺+K⁺)-ATPase activity and increased CC number, whereas N trout showed higher gill (Na⁺+K⁺)-ATPase activity and CC number. At the end of the experiment the enzyme activity was in the order T>N>C groups and all groups showed similar CC number. Both treatments changed the enzyme activation kinetics by ATP and Na⁺. A transient increase in K value occurred in N group during the period of salt administration. In BW, T and N groups had higher and lower survival than C group respectively. Other parameters were unaffected by the treatments. This trial suggests that T₃ administration promotes the development of hypoosmoregulatory mechanisms of trout but it leaves the (Na⁺+K⁺)-ATPase activity unaltered till the transfer to a hyperosmotic environment.     

Effect of triiodothyronine on the growth and survival of larval striped bass (Morone saxatilis)

This study was carried out to test the effect of triiodothyronine (T₃) on the growth and survival of larval striped bass (Morone saxatilis). Growth and survival of striped bass held in 5 ppt seawater and treated with various doses of T₃ were measured beginning at 5 and 16 days after hatching. Body content of T₃ was measured by radioimmunoassay. T₃ dissolved in the 5 ppt seawater was taken up by larval striped bass in a dose-dependent manner, and affected the growth and survival of the fish. At 5 days after hatching, T₃ at 100 ng ml^–1 and 50 ng ml^–1 retarded the growth of larval striped bass and caused a lower survival rate than T₃ at 25 ng ml^–1 or the control treatment. At 16 days after hatching, T₃ at 100 ng ml^–1 retarded the growth of larval fish and caused a higher mortality. T₃ at 10 ng ml^–1 and 1 ng ml^–1 did not show any effect on either survival or growth. Body content of T₃ returns to control levels within days following end of treatment. The results indicate that exogenous T₃ can be detrimental to the growth and survival of larval striped bass.     

Histological changes in insulin-immunoreactive pancreatic β-cells,and suppression of insulin secretion and somatotrope activity in brook trout (Salvelinus fontinalis) maintained on reduced food intake or exposed to acidic environment

Immature brook trout (Salvelinus fontinalis) were randomly divided into a pH control, a pH and food control and an acid-stressed group. Fish in the first two groups were held at neutral pH and those in the last group were maintained at pH 4.2 for up to two months. The food supply to the pH and food control group was restricted to simulate the reduction in food intake demonstrated for acid-stressed trout. Plasma insulin levels were significantly decreased from 5–20 ng/ml to 1–2 ng/ml and plasma cortisol levels were significantly increased from 5–10 ng/ml to as high as 70 ng/ml in the acid-stressed brook trout. Concomitantly, a significant decrease of 21–39% in the proportion (volume density) of insulin immunoreactive -cells was observed within the principal pancreatic islets. Somatic growth was stunted and ultrastructural morphometry revealed the suppression of somatotrope secretory activity in the acid-stressed fish. Restriction of food supply induced a smaller but still significant decrease in circulating levels of insulin which was however not accompanied by a reduction in insulin immunoreactive -cells. The rise in plasma cortisol levels was not significant, and the plasma levels of glucose and protein were unaffected. Nevertheless, somatotrope secretory activity was suppressed and somatic growth was stunted. This study demonstrates for the first time the complexity of the endocrine response to acid stress and that some of the response to acid stress can be attributed to the lowering of food intake.     

Changes in thyroid hormone levels in eggs and larvae and in iodide uptake by eggs of coho and chinook salmon,Oncorhynchus kisutsch andO. tschawytscha

Developmental profiles of thyroxin (T₄), triiodothyronine (T₃) and radioactive iodide uptake were established for eggs and T₄ and T₃ profiles were established for larvae (whole-body, yolk-only and body-only) of coho and chinook salmon. T₄ and T₃ were consistently present in all samples. In eggs, hormone levels remained fairly constant in all cohorst for at least the first three weeks of incubation, but then fluctuated in both directions in some sample groups. Large increases in T₄ (from 9 ng/g to 245 ng/g) were seen in 1985 chinook eggs 28 days after fertilization. Radioactive iodide uptake (which was used as a possible indicator of thyroxinogenesis) increased at least 10-fold in both 1986 coho and chinook eggs from 23–30 days after fertilization. T₄ (62 ng/g) and T₃ (393 ng/g) were found in the bodies of 28-day-old 1986 chinook embryos. In whole larvae, hormone levels varied depending upon the cohort studied. In general, initial body-only concentrations of both T₄ and T₃ decreased as body weight increased, but before yolksac resorption was completed, both thyroid hormone content and concentration increased (except for chinook T₃). T₄ and T₃ content in larval yolk stayed constant as yolksac size decreased, resulting in increased thyroid hormone concentration in the yolksac. All of these data suggest that the initial source of thyroid hormones in coho and chinook salmon eggs is maternal, but that by approximately 3–4 weeks after fertilization, the developing embryos begin to produce their own thyroid hormones. After hatching, increases in tissue T₄ and T₃ concentration coupled with constant T₄ and T₃ content in diminishing yolksacs suggest that larvae also produce their own thyroid hormones; yolksac content then may reflect both the original maternal hormones and the larva-producted hormones.