The acute effects of alteration in the dietary concentrations of carbohydrate, protein, and lipid on plasma T4, T3, and glucose levels in rainbow trout, Oncorhynchus mykiss

Effect of dietary carbohydrate on non-specific immune response, hepatic antioxidative abilities and disease resistance of juvenile golden pompano (Trachinotus ovatus)

The present study was conducted to investigate the effects of dietary carbohydrate (CHO) levels on non-specific immune responses, hepatic antioxidative status and disease resistance of juvenile golden pompano. Fish were fed six isonitrogenous and isoenergetic diets containing various CHO levels for 8 weeks. After the feeding trial, fish were challenged by Vibrio harveyi and survival rate was recorded for the next 12 days. Plasma total protein and albumin content, respiratory burst activity, alkaline phosphatase, slightly increased with dietary starch level from 0% to 16.8%, but significantly decreased at dietary starch levels of 16.8%-28%. Plasma lysozyme, complement 3 and complement 4 levels increased with increasing dietary carbohydrate up to 11.2% and then declined (P < 0.05). Contrary to glutamic-oxalacetic transaminase and triiodothyronine, plasma cortisol content increased with increasing dietary carbohydrate up to 22.4%, and then levelled off. The hepatic total antioxidative capacity, reduced glutathione and catalase levels reached the peak at the fish fed diet with 16.8% carbohydrate (P < 0.05). This also held true for hepatic superoxide dismutase activities, whereas the hepatic malondialdehyde content of fish fed dietary starch level of 16.8% was significantly lower than that of fish fed no CHO diet, but showed little difference (P > 0.05) with those of the other treatments. After challenge, fish fed 11.2% and 16.8% dietary CHO showed higher survival rate than that of fish in 0% CHO group (P < 0.05). However, survival rate showed little difference among 0%, 5.6%, 22.4% and 28% CHO groups (P > 0.05). The results of this study suggest that ingestion of 11.2-16.8% dietary CHO can enhance the non-specific immune responses, increase the hepatic antioxidant abilities, and improve resistance to V. harveyi infection of juvenile golden pompano.

Influence of dietary carbohydrate level on endocrine status and hepatic carbohydrate metabolism in the marine fish Sparus sarba

Silver sea bream, Sparus sarba, were fed two diets of different carbohydrate levels (2 and 20% dextrin) for 4 weeks, and the effects on organ indices, liver composition, serum metabolite and hormone levels and gene expression profile of key enzymes of carbohydrate metabolism in the liver were investigated. By using real-time PCR, mRNA expression levels of carbohydrate metabolic enzymes including glucokinase (GK, glycolysis), glucose-6-phosphatase (G6Pase, gluconeogenesis), glycogen synthase (GS, glycogenesis), glycogen phosphorylase (GP, glycogenolysis) and glucose-6-phosphate dehydrogenase (G6PDH, pentose phosphate pathway) in liver of sea bream have been examined, and it was found that high dietary carbohydrate level increased mRNA level of GK but decreased mRNA levels of G6Pase and GP. However, mRNA levels of GS and G6PDH were not significantly influenced by dietary carbohydrate. Silver sea bream fed high dietary carbohydrate had higher hepatosomatic index (HSI), liver glycogen and protein, but there were no significant changes in gonadosomatic index (GSI), serum glucose and protein level, as well as liver lipid and moisture level. Pituitary growth hormone (GH) and hepatic insulin-like growth factor I (IGF-I) transcript abundance were assayed by real-time PCR, and it was found that both parameters remained unchanged in fish fed different dietary carbohydrate levels. Serum triiodothyronine (T(3)) and thyroxine (T(4)) were not significantly affected by dietary carbohydrate levels, but lower serum cortisol level was found in fish fed high dietary carbohydrate level. These results suggest that silver sea bream is able to adapt to a diet with high carbohydrate content (up to 20% dextrin), the consumption of which would lead to fundamental re-organization of carbohydrate metabolism resulting in hepatic glycogen deposition.

Thyroid hormones regulate lipid metabolism in a teleost Anabas testudineus (Bloch)

We compared the long-term action of 3,5,3'-triiodo-L-thyronine (T3) and 3,5-diiodo-L-thyronine (T2) on lipid metabolism in a teleost Anabas testudineus. Among the six groups of animals used in this experiment, except for the control group, all received 6-propylthiouracil (6-PTU) to create a hypothyroid state in order to analyse the action of iodothyronines on lipid metabolism. Injections of 6-PTU reduced T3 concentration in the circulation by 79.6% and injections of iodothyronines enhanced the level of T3 in the plasma, and a maximum increase was observed in T3 (500 ng)-treated specimens. Analysis of lipogenic enzymes in liver and heart showed that a tissue-specific variation exists in the action of thyroid hormones and, in many cases, activity is higher in T2-treated groups. Analysis of various lipid classes showed that long-term administration of T2 is also effective in producing a comparable effect with that of T3 on lipid metabolism.

Influence of high-carbohydrate enriched diets on plasma insulin levels and insulin and IGF-I receptors in trout

Insulin and insulin growth factor-I (IGF-I) binding to skeletal muscle semipurified receptors were assessed in rainbow trout (Oncorhynchus mykiss) fed with different enriched carbohydrate diets. The animals were fed for 2 months, either in spring, summer or autumn with a control diet (C, commercial diet containing 21% raw carbohydrates) or with two diets supplied with highly digestible carbohydrates (E1, 22% expanded wheat; and E2, 37% expanded wheat). Insulin and IGF-I receptors were semipurified by affinity chromatography (WGA-agarose). Fish fed with a carbohydrate enriched diet did not show lower growth rates than those fed with the control diet. Independently of the season, rainbow trout fed E1 and E2 presented higher insulin and glucose plasma levels as well as higher tissue glycogen reserves than fish fed C. An increase in the number of insulin receptors during the diet adaptation was observed especially in fish fed with E2. No differences in the affinity of receptors were observed. IGF-I specific binding in skeletal muscle was higher than that of insulin in all groups and in all seasons. Furthermore, IGF-I receptors showed the same tendency as insulin receptors, with increases in their number in experimentally fed fish, especially those fed with E2. Insulin and IGF-I receptors TKA increased only slightly, as a consequence of E1 and E2 diet adaptation. In conclusion, rainbow trout can be fed high-carbohydrate levels and show good rates of growth. This adaptation determines increases in circulating glucose and insulin, and muscle insulin receptors, which indicate an adaptation of the fish to higher levels of glucose supply. The response of IGF-I receptors also suggests a possible role in the regulation of metabolism.

CRF effect on thyroid function is not mediated by feeding behavior in goldfish

In the present study we examined the effects of acute corticotropin-releasing factor (CRF) administration and refeeding treatment on glucose levels and thyroid hormones (plasma levels and thyroid contents) in 48-h food-deprived goldfish. Central CRF administration (2 micrograms) decreased food intake and the thyroid T3 free fraction, without significantly modifying either thyroid hormones bound fractions (T3 and T4) or plasma glucose levels. Subsequently, we tested whether CRF affects thyroid activity by itself, or whether this effect is mediated by CRF-induced feeding reduction. CRF treatment in fasted fish reduced thyroid-free T3 and increased thyroid-free T4, which could be mediated by a decreased intrathyroidal 5'-monodeiodinase activity. These data suggest a CRF effect upon thyroid activity independent of feeding reduction. On the other hand, refeeding after 48-h fasting caused a significant increase in thyroid free T4 content and plasma thyroid hormone levels. Thus, a relationship between nutritional status and thyroid function, which could overlap with CRF effects, cannot be discarded. Plasma glucose levels were only significantly modified by refeeding, which seems to be the signal triggering the increase in glucose titers. Our results support the existence of both CRF-thyroid activity and nutritional status-thyroid function interactions in goldfish.

Dietary effects on thyroid hormones in the red drum, Sciaenops ocellatus

Juvenile red drum (Sciaenops ocellatus) were cultured at 25°C on a variety of diets and blood sampled over eight weeks to examine the relationship between growth and plasma thyroid hormone levels. Maximum growth rates were achieved on formulated experimental diets and a simulated natural shrimp diet. Associated with these maximal rates was a significant increase in triidothyronine (T3), but no consistent change in thyroxine (T4). Reduced rations of diets resulted in low growth rates associated with significantly lowered levels of T3 but not T4. To determine whether weight gain could be increased by application of exogeneous hormone, diets were supplemented with T3 or T4 at 2, 10, and 50 mg hormone/kg diet. Significantly elevated T3 was induced by supplementation with 10 and 50 mg T3/kg diet, although there were no indications of an anabolic effect of T3 incorporation, and 50 mg T3/kg diet was in fact associated with decreased weight gain. Incorporation of T4 into diets had no effect on growth or T3, and had effects on T4 which were small and inconsistent, indicating that T4 may not be effectively absorbed from the gut. No difference was found in response to hormone feeding between low (6 ppt) or high (35 ppt) water salinity. T3 levels thus appear to closely parallel growth in fish on unsupplemented diets, whereas T4 which were small and manipulation. Supplementation with T3 is not an effective means of stimulating growth in red drum fed optimum diets. Whereas thyroid hormones may function to regulate intermediary metabolism in red drum, elevated endogenous thyroid hormone levels appear adequate to supply tissue needs during juvenile growth in culture.