Refeeding increases hepatic insulin-like growth factor-I (IGF-I) gene expression and plasma IGF-I concentration in fasted chicks

1. We examined the influence of refeeding after 2 d of fasting on plasma insulin-like growth factor-I (IGF-I) concentration and hepatic IGF-I gene expression in chickens at 6 weeks of age. 2. Hepatic IGF-I mRNA was measured by ribonuclease protection assay and plasma IGF-I concentration was determined by radioimmunoassay. 3. Plasma IGF-I concentration decreased following fasting, increased to the level of fed controls after 2 h of refeeding but then fell back to the level of fasted chickens after 6 h of refeeding. 4. Fasting reduced hepatic IGF-I mRNA concentrations to less than half of those in the fed controls. Refeeding increased IGF-I mRNA sharply at 2 h after refeeding, but by 6 h after refeeding they had taller back again to levels significantly lower than at 2 h. 5. A significant correlation between plasma IGF-I concentration and hepatic IGF-I gene expression was found, suggesting that when chicks are refed after 2 d of fasting, the short-term increase in plasma IGF-I concentration may be partly regulated by the alteration in hepatic IGF-I mRNA.     

Insulin administration suppresses an increase in insulin-like growth factor binding protein-2 gene expression stimulated by fasting in the chicken

1. We examined the changes in plasma IGF-I concentration and tissue IGFBP-2 gene expression of young fasted chickens refed a commercial diet or administered bovine insulin intravenously. 2. Plasma IGF-I concentration was decreased by fasting for 2 d. Although plasma IGF-I concentration was increased by refeeding, it didn't recover to the level of chickens fed a commercial diet ad libitum. 3. Insulin administration lowered plasma IGF-I concentration compared to other groups. 4. Hepatic IGFBP-2 mRNA was increased by fasting for 2 d and decreased by refeeding for 6 h. Insulin administration also decreased hepatic IGFBP-2 gene expression stimulated by fasting to the level of refed chickens. 5. IGFBP-2 mRNA in the gizzard was increased by fasting for 2 d and tended to decrease after refeeding for 6 h. Insulin administration decreased gizzard IGFBP-2 gene expression to less than that in refed chickens. 6. There was no between-treatment difference in IGFBP-2 mRNA in the brain and kidney. 7. These results suggest that the changes in IGFBP-2 gene expression in the liver and gizzard by fasting and refeeding might be partly regulated by the alteration in plasma insulin concentration.     

Influence of nutritional deprivation on insulin-like growth factor I, somatotropin, and metabolic hormones in swine

Although growth hormone (GH) is a primary stimulus for the synthesis of insulin-like growth factor I (IGF-I), other factors such as nutritional status, insulin, and thyroid hormones are important modulators of circulating IGF-I levels. Thus, the effects of feed restriction and subsequent refeeding on plasma levels of IGF-I, GH, insulin, and thyroid hormones were studied in swine. Despite an elevation in plasma GH levels after 48 h of feed restriction, circulating IGF-I levels were decreased by 53% (P less than .05). Plasma triiodothyronine (T3) and insulin were lower (P less than .05) within 24 h after the feed restriction began, whereas thyroxine (T4) did not decrease until 48 h after removal of feed. Blood glucose levels remained unchanged throughout the experiment. Refeeding after the 48-h fast was associated with a decline (P less than .05) in circulating GH levels within 2 h, concomitant with an elevation (P less than .05) in plasma insulin and T3. Refeeding fasted pigs was associated with an increase (P less than .05) in plasma IGF-I; however, levels still had not returned to prefast concentrations within 24 h after refeeding. These data indicate that the GH-IGF-I axis becomes uncoupled during nutritional restriction in swine and that inadequate nutrient supply may limit the expression of the anabolic effects of GH.     

Nutrient supply enhances both IGF-I and MSTN mRNA levels in chicken skeletal muscle

Nutrient supply may control muscle growth directly and indirectly through its influence on regulatory factors. The present study focuses on its effects on muscle insulin-like growth factors (IGF-I and -II) and myostatin (MSTN). Their mRNA levels were quantified by real time RT-PCR in pectoralis major (PM) and sartorius (SART) muscles from broiler chickens submitted to different feeding regimens (fed or fasted for 48 h) between hatch and 2 days of age and at 4 weeks of age. In the PM of 4 weeks old broilers, mRNA levels were also evaluated after a 16 h-fast and a refeeding period (refed 24 or 48 h after a 48 h-fast). In the PM muscle, both IGF-I and MSTN mRNA levels increased between 0 and 2 days of age in the fed group, while they remained low in the unfed one. A comparable trend was observed in the SART, but with lesser amplitude. In both muscles of 4 weeks old chickens, a 48 h-fast induced a significant reduction in MSTN mRNA levels (20% of fed state). In the PM, this effect required more than 16 h of fasting to occur and was fully reversed by only 24h of refeeding. IGF-I mRNA levels also varied with nutritional state. They decreased significantly with fasting in the SART muscle. By contrast, IGF-II mRNA levels did not vary significantly. Our data shows for the first time that two major paracrine regulators of muscle growth, IGF-I and MSTN, are sensitive to nutrient supply in hatching chicks, and also that fasting reduced IGF-I and MSTN mRNA levels in muscles of older chickens.     

Effects of fasting and refeeding on plasma concentrations of leptin, ghrelin, insulin, growth hormone and metabolites in swine

The effects of nutritional status, such as fasting and refeeding, on leptin and ghrelin secretion in swine were examined. The swine (n = 4) were fasted for 54 h and plasma hormone levels were measured before, during and after fasting. Plasma leptin and insulin concentrations began to decrease 12 and 6 h into the fasting period, respectively (P < 0.05), and maintained a low level for the remaining period of fasting. Plasma leptin and insulin returned to the pre‐fasting value 6 and 12 h after refeeding, respectively. Plasma ghrelin concentrations showed a nocturnal periodicity during the fasting period; it increased nocturnally at 36 and 42 h into the fasting period (P < 0.05). Plasma growth hormone levels did not show any remarkable changes during the fasting. Plasma glucose levels showed a modest fall during fasting and significantly decreased (P < 0.05) at 24 h into the fasting period, returning to pre‐fasting levels after refeeding. Plasma nonesterified fatty acid levels increased (P < 0.05) at 12 h into the fasting period and returned to the pre‐fasting level 6 h after refeeding. These results indicate that plasma leptin, insulin and ghrelin play an important role in maintaining energy homeostasis in swine. The plasma ghrelin did not continuously increase, but showed nocturnal periodicity during fasting. This may suggest that ghrelin is also involved in physiological processes other than energy homeostasis.     

Effect of short-term fasting on plasma concentrations of leptin and other hormones and metabolites in dairy cattle

We determined the effects of short-term fasting and refeeding on temporal changes in plasma concentrations of leptin, insulin, insulin-like growth factor- 1 (IGF-1), growth hormone (GH), glucose, and nonesterified fatty acids (NEFA), in early lactating cows, non-lactating pregnant cows, and postpubertal heifers. In experiment 1, Holstein cows in early lactation were either fed ad libitum (Control, n=5) or feed deprived for 48 h (Fasted, n=6). Plasma leptin, insulin, and glucose concentrations rapidly declined (P<0.05) within 6h, and IGF-1 by 12h, but all these variables sharply returned to control levels (P>0.10) within 2h of refeeding. Plasma NEFA and GH concentrations were elevated (P<0.05) by 4 and 36 h of fasting and returned to control levels (P>0.10) by 8 and 24h after refeeding, respectively. In experiment 2, four ruminally cannulated pregnant non-lactating Holstein cows were used in a cross-over design and were fasted for 48 h (Fasted) or fasted with partial evacuation of rumen contents (Fasted-Evac). The plasma variables measured did not differ (P>0.10) between Fasted and Fasted-Evac cows. Plasma leptin, insulin, and IGF-1 concentrations were reduced by 10, 6, and 24h of fasting, respectively, in Fasted-Evac cows; and these variables were reduced by 24h in Fasted cows (P<0.05). Plasma glucose levels were reduced (P<0.05) by 48 h of fasting in both groups of fasted animals. Plasma NEFA and GH levels were increased (P<0.05) by 12 and 48 h of fasting, respectively. In experiment 3, postpubertal Holstein heifers were either fed ad libitum (Control, n=4) or feed deprived for 72 h (Fasted, n=5). Concentrations of leptin, insulin, IGF-1, and glucose in plasma were reduced (P<0.05) by 24, 10, 24, and 48 h of fasting, respectively. Plasma NEFA concentrations increased (P<0.05) by 4h, of fasting while GH levels were not significantly (P>0.10) affected by fasting. Collectively, our data provide evidence that plasma leptin concentrations are reduced with short-term fasting and rebound on refeeding in dairy cattle with the response dependent on the physiological state of the animals. Compared to the rapid induction of hypoleptinemia with fasting of early lactation cows, the fasting-induced hypoleptinemia was delayed in non-lactating cows and postpubertal heifers.     

Effect of exogenous corticosterone in genetically fat and lean chickens

1. The effects of daily injections of corticosterone (1 or 5 mg/bird) on growth, fat deposition, liver lipid and plasma concentrations of uric acid, glucose, insulin and growth hormone were studied using genetically selected lines of fat (FL) and lean (LL) chickens. 2. Both doses of corticosterone depressed body weight gain and increased the liver lipid and the abdominal fat to the same extent in both lines. 3. In both lines, corticosterone caused a dose-dependent increase in the plasma concentrations of uric acid, glucose and insulin in the fasted and refed states. 4. In untreated birds, plasma concentrations of growth hormone (GH) were slightly higher in FL than in LL chickens and slightly decreased during refeeding. The response was not modified by injection of 1 mg corticosterone. Injections of 5 mg decreased plasma GH in both lines in the fasting state and in LL chickens during refeeding. In contrast, the same dose increased GH in FL chickens during refeeding. This contradiction remains unexplained. 5. The results suggest that corticosterone sensitivity is not involved in difference of fattening between FL and LL chickens.     

Increased responsiveness to dietary lysine deficiency of pectoralis major muscle protein turnover in broilers selected on breast development

It has been previously established that growth and carcass qualities of chicks are modified by genotype and dietary amino acid supply. In this study, we evaluated the effects of lysine deficiency and genetic selection on muscle protein metabolism. Chicks originating from an experimental line selected for breast development (QL) and its control line (CL) were provided ad libitum access to isoenergetic diets containing 20% crude protein but differing in their lysine content (0.75 or 1.01%). Protein fractional synthesis rates (FSR) were measured in vivo in the pectoralis major and sartorius muscles of 3-wk-old chickens (flooding dose of [3H]phenylalanine). Fractional breakdown rates (FBR) were estimated as the difference between synthesis and deposition. Lysine deficiency reduced (P < 0.001) growth performance and muscle weights and increased (P < 0.05) muscle FSR, capacity for protein synthesis (muscle RNA:Protein, Cs) and FBR. Although QL birds grew faster and had heavier pectoralis major muscles than CL birds (P < 0.05), there was no line difference in sartorius weight (P = 0.15). No difference between the lines was observed in sartorius protein metabolism (P > 0.14). In the pectoralis major muscle, chicks of both lines receiving an adequate lysine intake also exhibited similar protein turnover rates. However, in chicks fed the lysine-deficient (0.75% lysine) diet, FSR and Cs were higher in QL than in CL chicks (P < 0.05), and FBR tended (P = 0.07) to be higher in QL chicks. This increased protein turnover in the QL birds on the lysine-deficient diet suggests that the responsiveness of muscle protein metabolism to amino acid supply is modified by genetic selection for breast development.     

Localization of insulin-like growth factor I (IGF-I) in the chicken liver after fasting and refeeding: demonstration by using antigen retrieval immunohistochemistry

Immunohistochemical localization of insulin-like growth factor-I (IGF-I) was investigated in the liver of fasted and refed chickens by using an antigen retrieval method. The present study is the first one showing the localization of IGF-I in the chicken liver. Immunoreactivity for IGF-I was detected on the paraffin sections of livers from the fed and refed chickens after the treatment with the antigen retrieval agent. A moderate number of cells showing IGF-I immunoreactivity were scattered in the parenchyma of the liver from fed chickens. These cells were relatively large and polygonal in shape and seemed to be hepatocytes. Reaction products were observed as a granular structure in the cytoplasm of IGF-I-immunoreactive hepatocytes. The number of immunoreactive hepatocytes was increased in the liver from refed chickens compared with fed chickens. Diffuse reaction products as well as granular ones were observed throughout the cytoplasm of IGF-I-immunoreactive hepatocytes of livers from refed chickens. There are, however, no regular patterns of the distribution of immunoreactive hepatocytes in the parenchyma of both fed and refed chickens. In the liver of the fasted chickens, clear immunoreactivity for the peptide was not observed. These data show that IGF-I is located in the chicken hepatocytes and influenced by the nutriture.     

Effects of food intake and food withholding on plasma ghrelin concentrations in healthy dogs

OBJECTIVE: To investigate the physiologic endocrine effects of food intake and food withholding via measurement of the circulating concentrations of acylated ghrelin, growth hormone (GH), insulin-like growth factor-I (IGF-I), glucose, and insulin when food was administered at the usual time, after 1 day's withholding, after 3 days' withholding and after refeeding the next day in healthy Beagles. ANIMALS: 9 healthy Beagles. PROCEDURES: Blood samples were collected from 8:30 AM to 5 PM from Beagles when food was administered as usual at 10 AM, after 1 day's withholding, after 3 days' withholding, and after refeeding at 10 AM the next day. RESULTS: Overall mean plasma ghrelin concentrations were significantly lower when food was administered than after food withholding. Overall mean plasma GH and IGF-I concentrations did not differ significantly among the 4 periods. Circulating overall mean glucose and insulin concentrations were significantly higher after refeeding, compared with the 3 other periods. CONCLUSIONS AND CLINICAL RELEVANCE: In dogs, food withholding and food intake were associated with higher and lower circulating ghrelin concentrations, respectively, suggesting that, in dogs, ghrelin participates in the control of feeding behavior and energy homeostasis. Changes in plasma ghrelin concentrations were not associated with similar changes in plasma GH concentrations, whereas insulin and glucose concentrations appeared to change reciprocally with the ghrelin concentrations.     

Serum hormone and metabolite concentrations in fasted young bulls and steers.

The effect of dietary energy restriction on serum insulin, insulin-like growth factor I (IGF-I), growth hormone, (GH), cortisol, plasma urea nitrogen (PUN) and nonesterified fatty acid (NEFA) concentrations was examined. Angus bulls and steers (10 mo) were allotted to two groups of 12 animals and assigned a treatment order. In a switchback design, animals in order 1 were fed a high grain diet, then fasted, while order 2 animals were fasted, and then fed. Animals were allowed 60 hr to acclimate between treatments. Serum and plasma were obtained at 20 min intervals and 60 min, respectively, for 6 hr after feeding and for the last 6 hr of a 30 hr fast. Serum was assayed for insulin, IGF-I, GH, and cortisol (total and free). Plasma was assayed for PUN and NEFA. Mean insulin (ng/ml) differed between fed (.95 +/- .08) and fasted (.26 +/- .08) animals (P less than 01). Both mean total and free cortisol (ng/ml) were lower in fed (11.48 +/- .99) (1.06 +/- .12) than in fasted (17.10 +/- .93) (1.62 +/- .12) animals, respectively (P less than .01). Animals in order 1 differed in mean IGF-I (ng/ml) between fed (199.0 +/- 8.0) and fasted (116.5 +/- 7.2) treatments (P less than .01). Mean IGF-I for animals in order 2 was 146.7 +/- 7.2 in fed and 213.9 +/- 7.2 in fasted animals (P less than .01). Mean GH did not differ between treatments. Mean PUN and NEFA were higher in fasted than in fed animals (P less than .01). Except for % free cortisol (P less than .05), the hormones did not differ between bulls and steers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Insulin-glucose interactions characterised in newly hatched broiler chicks

1. A study was conducted to identify the specificity of insulin-glucose interactions in newly hatched broiler chicks. 2. Plasma insulin concentrations in fed chicks at one day post-hatch were lower than those at later ages and tended to increase up to d 7, while the concentrations from 1 to 7 d of age were lower than those in chickens of 10 to 28 d of age. 3. Plasma glucose concentrations were lowered for 60 min by injection of insulin at 10 and 40 microg/kg body weight (BW) in both 1- and 21-d-old chicks, showing that the hypoglycaemic effect of exogenous insulin is of larger magnitude but shorter duration in 1-d-old chicks. 4. The decrease in plasma glucose concentration at 60 min after insulin injection (10 and 40 microg/kg BW) was larger in 1- to 7-d-old chicks than in 14- to 21-d-old chickens. 5. These results indicate that newly hatched broiler chicks are under the control of specific insulin-glucose interactions characterised by low plasma insulin concentrations with high sensitivity to insulin.     

The effect of increasing photoperiod and food restriction in sexed, broiler-type birds. II. Plasma thyroxine, triiodothyronine, insulin-like growth factor-I and insulin.

1. Sexed broiler-type chicks were raised either under a continuous (CON) 23 h light (L) and 1 h dark (D) schedule or an increasing photoperiod (INC). From 5 to 11 d of age birds were fed either: ad libitum (AL), energy intake (kJ ME) restricted to 9.414 x gBW0.67 (R1) or energy intake (kJ ME) restricted to 6.276 x gBW0.67 (R2). 2. Blood samples were taken at 4, 7, 11, 14 d of age and weekly thereafter to 49 d of age. Plasma thyroxine (T4), triiodothyronine (T3), insulin-like growth factor-I (IGF-I) and insulin were determined. 3. CON birds had elevated plasma T3 concentrations to 21 d of age, and greater plasma T4 concentrations at 11 and 21 d of age concurrent with greater food intake. Elevated plasma T3 concentrations in INC birds at 28 d of age coincided with lower plasma IGF-I concentrations at a time when growth and food intake were greater than CON birds. 4. Food restriction elevated plasma insulin and T4 but depressed plasma T3 and IGF-I. Plasma T3 was greater for food-restricted birds at 21 d of age, but subsequently, was generally lower than ad libitum-fed birds which may account for a lack of complete 'catch-up' in growth. 5. Plasma T3 was higher in females at 11 d of age when growth was equivalent for both sexes. From 28 to 42 d, when sex differences in growth became most apparent, plasma T3 was greater in males.     

Effect of nutrient intake on the development of the somatotropic axis and its responsiveness to GH in Holstein bull calves

We investigated the effect of increasing nutrient intake on the responsiveness of the GH/IGF-I system in calves fed a high-protein milk replacer. Fifty-four Holstein bull calves were fed one of three levels (low, medium, and high; n = 18 per treatment) of a 30% crude protein, 20% fat milk replacer to achieve target rates of gain of 0.50, 0.95, or 1.40 kg/d, respectively, for low, medium, and high. Six calves per treatment were slaughtered at approximately 65, 85, and 105 kg BW. Additionally, six calves were slaughtered at 1 d of age to provide baseline data. Plasma aliquots from blood samples collected weekly were analyzed for IGF-I, insulin, glucose, NEFA, and plasma urea nitrogen (PUN). Plasma IGF-I and insulin, measured weekly, increased (P < 0.001) with greater nutrient intake from wk 2 of life to slaughter. Plasma glucose and NEFA also increased (P < 0.05) with nutrient intake. In addition, each calf underwent a GH challenge beginning 4 d before the scheduled slaughter. Plasma from blood collected before the first GH injection and 14 and 24 h after the third injection was analyzed for IGF-I and PUN. Response to challenge, calculated as the absolute difference between the prechallenge and 14-h postchallenge plasma IGF-I concentrations, was significant in calves on all three treatments. Plasma urea nitrogen was not different among treatments as measured weekly but decreased (P < 0.001) following GH challenge in all calves. Results of ribonuclease protection assays showed increased expression of hepatic mRNA for GH receptor 1A and IGF-I with increased intake. The amounts of GH receptor and IGF-I mRNA in muscle and adipose, however, were not affected by intake. In summary, plasma IGF-I was elevated in calves with increased nutrient intake, and the elevations in plasma IGF-I following short-term administration of GH were significant in all calves by 65 kg BW. Data demonstrate that in well-managed milk-fed calves the somatotropic (GH/IGF-I) axis is functionally coordinated and sensitive to nutrient intake and GH.     

Effects of thiamine and clenbuterol on body composition, plasma metabolites and hepatic oxygen consumption in broiler chicks.

1. We examined the effects of thiamine-hydrochloride (10 mg/kg body weight) and a beta-agonist, clenbuterol (50 microg/kg body weight), on plasma metabolites and hepatic oxygen consumption in female broiler chicks. 2. Clenbuterol, thiamine or both, dissolved in saline, were injected into thigh muscle on 2, 4 and 6 d of age. At 7 d of age blood samples in each treatment group were obtained and breast muscle and liver were weighed; liver slices were used for measurement of oxygen consumption. 3. Body weight gain was reduced by clenbuterol. Thiamine increased breast muscle weight as a proportion of body weight regardless of clenbuterol dose. Clenbuterol increased relative liver weight markedly, especially when chicks received thiamine also. 4. Clenbuterol increased plasma free fatty acid concentration in chicks treated with thiamine. Thiamine decreased plasma triglyceride regardless of clenbuterol dose. Plasma glucose concentration was decreased by both thiamine and clenbuterol. 5. The absolute rate of oxygen consumption in liver slices was greater in the thiamine-treated chicks; clenbuterol did not affect hepatic oxygen consumption. 6. These findings suggest that thiamine-induced energy expenditure results not only from thermogenesis in the liver, but also from increasing energy utilisation for muscle hypertrophy and this vitamin supplementation facilitates the lipolytic effects of the beta-agonist.     

Hematological and Blood Biochemical Effects of Fasting and Subsequent Oral Administration of Endotoxin in Prepubertal Gilts

The main objective of the present study was to investigate the effects of short-term fasting in gilts on endocrinological and blood biochemical parameters and, further, the effects of subsequent oral endotoxin (ET) administration. Group 1 was fasted for 30 h and then received feed with ET added. Group 2 was fasted for 30 h but received standard feed at refeeding. In group 3, gilts were fed every 6 h for 30 h. The major effects of fasting were: gradually increased concentration of plasma prostaglandin F_2α metabolite, serum total bilirubin, serum free fatty acids, and decreased serum glucose. The values were normalized within 1–4 h of refeeding. Twelve hours after refeeding, the ET-refed gilts showed higher levels of serum total bile acids and polymorphonuclear leukocytes than those in group 2. It is possible that the observed changes during fasting reflect either an increased intestinal uptake of naturally present endotoxin or a reduced endotoxin detoxifying capacity of the liver. The increased bile acid concentration and polymorphonuclear leukocyte count following refeeding with ET-feed may indicate that orally administered ET is to some extent absorbed from the gut.     

Metabolic responses to fasting and alloxan-induced diabetes mellitus in steers

To induce diabetes mellitus in 8 steers, they were fasted for 96 hours and given 110 mg of alloxan/kg of body weight (IV, in 1 dose) immediately before refeeding. Subsequently, 4 of the steers were treated with insulin (0.1 to 3 U/kg) to control hyperglycemia and 4 were not given insulin. Four control steers were fasted and refed. Fasting increased serum phosphorus, total protein, and bilirubin and decreased serum magnesium and potassium. Refeeding returned serum values of magnesium, potassium, total protein, and bilirubin toward base-line values, regardless of treatment group. However, serum phosphorus remained increased in steers with alloxan-induced diabetes and was not lowered by insulin injections. Sodium and chloride values were depressed in steers with alloxan-induced diabetes; these values remained significantly (P less than 0.05) lower than base-line values, even in steers given insulin. Fat infiltration was evident in the pancreas, liver, and to some extent, kidneys of steers with alloxan-induced diabetes, but was occasionally present in tissues of steers given insulin.     

Establishment of a time-resolved fluoroimmunoassay for measuring plasma insulin-like growth factor I (IGF-I) in fish: effect of fasting on plasma concentrations and tissue mRNA expression of IGF-I and growth hormone (GH) in channel catfish (Ictalurus punctatus)

A time-resolved fluoroimmunoassay (TR-FIA) was established and validated that allows for the determination of plasma concentrations of insulin-like growth factor I (IGF-I) in three domestically cultured fishes: channel catfish (Ictalurus punctatus), hybrid striped bass (Morone chrysopsxM. saxatilis), and rainbow trout (Oncorhynchus mykiss). Sensitivity of the assay was 0.20 ng/ml. Intra- and inter-assay coefficients of variation (CV) were <7 and <12%, respectively. Serial dilutions of plasma from each species were parallel to the standard curve. Recovery of IGF-I from spiked plasma samples was >90% for all three species of fishes. The IGF-I TR-FIA was biologically validated via its use to determine the effect of fasting on circulating IGF-I levels in channel catfish. Fasting-induced changes in plasma growth hormone (GH), hepatic IGF-I mRNA expression, and pituitary GH mRNA expression were also determined. Fasted channel catfish lost 5.6 and 15.6% body mass after 2 and 4 weeks of fasting, respectively. Plasma IGF-I concentrations were depressed (P<0.05) relative to fed controls following 2 and 4 weeks of fasting. Plasma GH concentrations were not different (P>0.05) in fasted fish after 2 weeks, but significantly increased (P<0.05) by 4 weeks of fasting. Hepatic IGF-I mRNA expression after 2 and 4 weeks of fasting was reduced (P<0.05) relative to fed controls. Pituitary GH mRNA expression was similar (P>0.05) between 2-week-fasted catfish and fed controls, but was increased (P<0.05) in 4-week-fasted catfish. The IGF-I TR-FIA was sensitive, accurate, and precise for all three species of fishes, and provided a low-cost, and non-radioisotopic method for quantifying plasma IGF-I levels in fed and fasted channel catfish.     

Dietary L-carnitine increases plasma insulin-like growth factor-I concentration in chicks fed a diet with adequate dietary protein level

1. The effect of L-carnitine supplemented into experimental diets with varying dietary protein concentrations (50, 200 and 400 g/kg) on body weight gain and plasma insulin-like growth factor-I (IGF-I) concentration in chicks was examined. 2. Dietary L-carnitine supplementation provided 0, 200, 500 and 1000 mg/kg. Chicks were given the diet ad libitum for 10 d. 3. When L-carnitine was provided as 500 or 1000 mg/kg, body weight gain was significantly improved in birds receiving the 200 and 400 g protein/kg diets. 4. There was an interaction between dietary L-carnitine and protein content on plasma IGF-I concentration. L-carnitine supplementation had little influence on plasma IGF-I concentrations in birds receiving the low protein (50 g/kg) diet. When dietary L-carnitine concentrations were increased from 0 to 1000 mg/kg in the adequate protein (200 g/kg) diet, plasma IGF-I concentrations were also increased. However, when dietary L-carnitine content was more than 500 mg/kg in the 400 g/kg protein group, plasma IGF-I concentration decreased with increasing dietary L-carnitine content. 5. Body weight change correlated significantly with the alteration in plasma IGF-I concentrations in chicks given diets with adequate dietary protein. 6. In conclusion, the improvement in body weight gain caused by dietary L-carnitine supplementation was achieved when chicks were given their dietary protein requirement, which may be partially explained by an increase in plasma IGF-I concentration.