ALBERT

Notes: This study was undertaken to determine acceptable dietary concentrations of high-fibre canola meal (CMHF) and low-fibre canola meal (CMLF) for juvenile shrimp, Penaeus vannamei. Four groups of 0.78 g shrimp held in running, 24.0–27.8°C sea water on a 12 h light: 12 h dark cycle were each fed one of seven isonitrogenous (340 g kg−1 protein) and isoenergetic (18.5 MJ of gross energy kg−1) diets to satiation four times daily for 56 days. Each of the test canola protein products comprised either 150, 300 or 450 g kg−1 of the protein in a basal (practical) diet by replacement of one-third, two-thirds or all of the menhaden meal protein.Shrimp that ingested diets in which CMHF and CMLF comprised 450 and 300 g kg−1 of the protein, respectively, exhibited significant reductions in growth and feed intake relative to those fed the basal diet. Feed and protein utilization were not significantly depressed unless menhaden meal in the basal diet was completely replaced by CMHF or CMLF. In general, percentage survival and final whole-body levels of protein, minerals, and thyroid hormones were not significantly affected by dietary treatment. Terminal whole-body levels of moisture were raised significantly in shrimp fed diets containing the highest levels of CMHF and CMLF. Potassium levels were significantly higher in shrimp fed the diet containing the lowest level of CMLF relative to those fed the basal diet and the diet with the highest level of CMLF. Water stability of the diet pellets was negatively correlated with their levels of CMHF and CMLF.It is concluded that commercial high-fibre canola meal can constitute 300 g kg−1 of the dietary protein of juvenile shrimp (Penaeus vannamei) without compromising growth, feed intake and feed and protein utilization. However, because of a trend towards reduced shrimp survival at this dietary concentration of canola meal, it is recommended that this protein source not exceed 150 g kg−1 of the protein in practical juvenile shrimp diets. Fibre-reduced canola meal did not have improved nutritive value for shrimp. However, we postulate that one or more fibre-reduced, and solvent-extracted canola protein products may be cost-effective substitutes for fish meal protein.

Notes: The effects of 17β-oestradiol (E2) on plasma kinetics of thyroid hormones (T4, l-thyroxine; T3, 3,5,3′-triiodo-l-thyronine) were studied in immature rainbow trout. E2-3-benzoate (0.5 mg/100 g) was injected intraperitoneally on days 0 and 3, and on the morning of day 4 each trout received an intracardiac injection of either [125I]T4 and Na 131I or [I25I]T3. Groups of trout were bled and killed from 5 min to 4 days post-injection of tracer. E2 did not alter the plasma T4 level but depressed the T4 plasma clearance rate, plasma-to-total tissue flux of T4 and thyroidal T4 secretion rate. Monodeiodination of T4 to T3 was also depressed, as judged from plasma [I25I]T3 and I25I − levels in [125I]T4-injected trout. E2 had no major effect on T3 plasma clearance rate but depressed the plasma T3 level, plasma-to-total tissue flux of T3 and the T3 plasma appearance rate. E2 had no influence on biliary transport of [I25I]T4 or [125I]T3. The above results suggest that E2, at the dose range employed, depresses extrathyroidal T4 to T3 conversion, which may in turn decrease plasma T4 clearance and thyroidal T4 secretion.

Notes: Abstract 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.

Notes: Summary We have reviewed the stages in teleost thyroid function and its regulation, from the initial biosynthesis of the TH to their eventual interaction with putative receptors. TH biosynthesis depends on an adequate plasma iodide level, determined partly by dietary iodide and partly by active branchial iodide uptake from the water, Pulse-injected radioiodide can be used to evaluate thyroidal iodide uptake, aspects of TH biosynthesis and TH thyroidal secretion. However, owing to variable plasma iodide levels, care is required in interpretating these parameters. TH biosynthesis, thyroglobulin properties and intrathyroidal secretion mechanisms have received limited recent attention. Histological indices of thyroid tissue changes, while useful in many situations, do not always correlate with more direct estimates of thyroidal secretion and can be misleading. Thyroid function is regulated by the hypothalamo-pituitary-thyroid axis, but neither the identities of the hypothalamic factors nor a reliable immunoassay for TSH have been established. Currently, activity of the hypothalamic-pituitary axis is usually determined by pituitary thyrotrope histological appearance or bioassay of pituitary TSH. Plasma free T4 feeds back at both the pituitary and hypothalamic levels and inhibits TSH release. Thyroidal T4 secretory activity is presumably adjusted to maintain a constant plasma T4level according to physiologic state. Plasma T4 is probably the most commonly used index of thyroidal status. However, (1) T4 is probably not the active form of TH, (2) the T4 plasma level may be influenced by the binding properties of plasma proteins, and (3) the T4 concentration alone makes no provision for the rate of T4 turnover in plasma. The most practical way to measure thyroidal T4SR is to determine plasma T4DR, and assuming steady-state conditions, equate it to T4SR. The T4DR is determined from kinetic studies employing*T4, which also enable estimates of sizes of vascular and extravascular T4 pools and their rates of exchange. Excretion of T4 or its derivatives in urine or bile can be determined also. A high proportion of T4 is enzymatically monodeiodinated in liver and other tissues, generating T3 for local (intracellular) and vascular systemic compartments. Bothin vivo andin vitro methods have been used to quantify T4 deiodinase activity, which is highly responsive to physiologic state and environmental variables. T3 production is inhibited by a moderate T3 excess indicating an autoregulatory system, whereby tissue T3 levels are maintained at a set-point appropriate for a particular physiologic state. The rate of T3 production provides an informative measure of thyroidal status in a given tissue. However, other pathways also contribute to the maintenance of T3 homeostasis at a particular set-point. These include the rate of T3 degradation to 3,3′-T2, the rate of T4 substrate diversion to rT3 (an inactive isomer) and by the excretion of parent compounds or conjugates in bile and urine. Potential losses across branchial or integumentary surfaces have yet to be evaluated. The most fundamental measure of thyroidal status is represented by the amount of T3 saturably bound to receptors/nucleus for the cell type of interest. This is estimated most accurately in double isotope studies in which T3 contributions from both vascular and intracellular compartments are evaluated. Less satisfactory but meaningful indices of T3 availability to receptor sites may be obtained from the plasma T3 (or free T3) level and from the tissue T3 level. The former is appropriate if the cell type in question obtains its T3 primarily from plasma; the latter should be measured if the cell type derives its T3 mainly through intracellular deiodinase activity. If the proportion of vascular T3/intracellular T3 bound to receptors is known, it may indicate the degree of receptor activation. However, even cytosolic T3 levels may not vary in proportion to nuclear T3 levels. Differences in thyroidal function between teleosts and homeotherms can be attributed to distinctive strategies in iodide economy and to fundamental differences in control of thyroidal status. Owing to more certain iodide availability (branchial iodide pump and plasma iodide-binding proteins), teleosts are probably more liberal in their iodide use and have less efficient mechanisms for recovery and retention of hormonal iodide than homeotherms. Also, primary control of teleost thyroidal function appears peripheral. It is the finely regulated conversion of T4 to T3 in tissues which may largely determine the T4 secretion rate. Thus, T4, as a prohormone, may be produced more to satisfy the substrate needs for T4 conversion rather than to drive T3 production. Because TH are mainly implicated in tissue- or cell-specific processes involved in development, growth and reproduction in teleosts, it may be advantageous for their thyroidal status to be determined locally through T4-to-T3 deiodination. In homeotherms, primary control is mainly central through the hypothalamic-pituitary axis, which regulates thyroidal secretion of T4 and significant amounts of T3. The level of T4 (free T4) is believed to drive the production of T3 in most peripheral tissues. Because TH are extensively involved in the systemically integrated adjustment of basal metabolic rate in homeotherms, it may have been advantageous to evolve a system leaning towards central control by the hypothalamus, the brain centre associated with thermoregulation.

Notes: Abstract 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.

Notes: Abstract 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.

Notes: Abstract The absorptions of 3,5,3′-triiodo-L-thyronine (T3) and L-thyroxine (T4) from the intestinal lumen of the rainbow trout were compared in vivo. Tracer doses of [125I]T4 (+T4) or [125I]T3 (*T3) were injected through an anal cannula into the duodenum of trout fasted for 3 days at 12°C, and radioactivity was measured in blood and tissues at 4–48 h. *T3 was removed more extensively than *T4 from the intestinal lumen and more radioactivity was absorbed into the blood and tissues of u+T3-injected trout than *T4-injected trout. HPLC analysis showed that a high proportion of the radioactivity in the plasma, liver, kidney and intestinal lumen of *T3-injected trout remained as the parent *T3. However, in *T4-injected trout most plasma radioactivity was in the form of 125I−, and by 24 h a high proportion of luminal radioactivity was 125I−. By 48 h, over 4% of the injected *T3 and 1% of the injected *T4 dose resided in the gall bladder, primarily as derivatives of *T3 or *T4. We conclude that T3 is absorbed more effectively than T4 from the intestinal lumen of fasted trout, indicating the potential for an enterohepatic T3 cycle.

Notes: Résumé La thyroîde de truite sécrète la L-thyroxine (T4) qui subit des désiodations enzymatiques dans le foie et d'autres tissus. D'après les études réalisées chez les mammifères, la désiodation du cycle externe de la T4 (DCE) ou cell du cycle interne (DCI) pourrait conduire, respectivement, à la 3,5,3′-triiodo-L-thyronine (T3) ou à la 3,3′,5′-T3 (rT3). Ensuite, la T3DCE ou la T3DCI pourrait générer, respectivement, la 3,5-diiodo-L-thyronine (T2) ou la 3,3′-T2, tandis que la rT3DCE ou la rT3DCI pourrait générer la 3,3′-T2 ou la 3′,5′-T2. Pratiquement chez la truite, la T4 est soumise à une DCE hépatique pour donner la T3, mais à une DCI (donnant de la rT3) négligeable. La T3, elle, subit une DCE négligeable mais une DCI modeste pour donner de la 3,3′-T2. La T-4DCE, particulièrement importante dans la conversion de T4 en T3 biologiquement plus active, existe aussi dans la branchie, le muscle et le rein. Deux isoenzymes, au moins, sont impliquées: i) une T4DCE, de haute affinité et sensible au propyle-thio-uracile (PTU), présentant une cinétique de type pingpong et nécessitant un thiol comme cofacteur; elle est trouvée dans le foie, la branchie et le muscle, ii) une T4-DCE de faible affinité et insensible au PTU, présentant une cinétique de type séquentielle et nécessitant également un thiol comme cofacteur, elle est trouvée dans le foie et le rein. La T3 susceptible de se lier à son récepteur provient essentiellement du plasma sanguin pour le rein, principalement de sources intracellulaires pour la branchie, et, de manières à peu près équivalentes, du plasma et de sources intracellulaires pour le foie. Ainsi, la T4DCE de forte affinité peut produire de la T3 pour une utilisation locale intracellulaire, tandis que la T4DCE de faible affinité peut produire de la T3 pour une utilisation locale intracellulaire, tandis que influencée par les facteurs nutritionnels et l'état physiologique du poisson. Par ailleurs, la T3 administrée par voie orale, pendant 6 semaines ou 24h, réduit le niveau fonctionnel (Vmax) de la T4-DCE hépatique, et la T3, ajoutée à des hépatocytes isolés, réduit également cette activité, indiquant l'existence d'une autorégulation directe de la T3 sur la T4-DCE pour maintenir l'homéostasie en T3 des hépatocytes. Toutefois, l'administration de T3 induit aussi une activité T4-DCI pour produire de rT3), biologiquement inactive, et une activité T3-DCI pour produire la 3,3′-T2. Ainsi, le foie de truite possède plusieurs systèmes de désiodases de l'iodothyronine, qui, de manière coordonnée, régulent l'homéostasie de la T3 face à une stimulation par la T3. Cela se fait par une réduction de la synthèse de T3, elle même, via une conversion de la T4 (substrat) en en rT3 biologiquement inactive, et par une augmentation de la dégradation de la T3. Ces désiodases diffèrent, par plusieurs aspects, de leurs homologues mammaliennes.

Notes: Abstract Changes in tissue and plasma isoenzymes of alkaline phosphatase (ALP) were qualitatively and quantitatively determined for male and female Atlantic salmon parr, silvery parr, smolt, immature grilse, prespawning grilse and postspawning grilse using cellulose acetate electrophoresis, densitometry and spectrophotometry. Tissue ALP isoenzymes were isolated from intestine, kidney, bone, liver, and gonad and compared to plasma isoenzymes. Parr plasma displayed three isoenzymes from bone and liver (slow and fast). During smoltification, ALP activity increased in tissue extracts from liver, gonad, and kidney of males and females. Total plasma ALP activity also increased and was due to slow and fast liver isoenzymes. During ovarian development, total ALP plasma activity increased in females and was due mostly to liver isoenzymes and an incompletely identified isoenzyme or isoenzyme mixture (band 2). However, in males total ALP plasma activity did not increase during maturation and no band 2 was evident. In male and female maturing adult grilse, bone ALP activity declined and the isoenzyme band evident in parr plasma could not be detected. ALP activity declined in the plasma of postspawning males and females. In females this was due partly to the total clearance of band 2 from the plasma, together with lowered levels of liver isoenzymes. Treatment of postspawned grilse in February and March with triiodothyronine and thyroxine elevated plasma thyroid hormone levels and increased plasma ALP levels. In conclusion, plasma ALP isoenzyme activities change with physiological state, and knowledge of the conditions governing these changes is important when using these enzymes as a diagnostic tool.