Free and protein-bound insulin-like growth factor-I (IGF-I) and IGF-binding proteins in plasma of coho salmon, Oncorhynchus kisutch

Prolactin receptor and proliferating/apoptotic cells in esophagus of the Mozambique tilapia (Oreochromis mossambicus) in fresh water and in seawater

We have previously shown that esophageal epithelium of a euryhaline goby displays elevated cell proliferation in freshwater (FW) fish, but undergo apoptosis during seawater (SW) acclimation. Prolactin (PRL) injection into the goby induced the cell proliferation, whereas cortisol treatment stimulated the cell proliferation and apoptosis [Takahashi, H., Sakamoto, T., Narita, K., 2006a. Cell proliferation and apoptosis in the anterior intestine of an amphibious, euryhaline mudskipper (Periophthalmus modestus). J. Comp. Physiol. B 176, 463-468, 2006). In the euryhaline tilapia (Oreochromis mossambicus), the dynamics of changes in cortisol-glucocorticoid receptors (GR) during acclimation to different salinities also suggests a role for glucocorticoid signaling in the esophageal cell turnover, but the mode of PRL action remains largely unclear. In the present study, we report on effects in the tilapia esophagus that result from changes in environmental salinity. Specifically, we assessed: (1) mRNA expression of PRL receptor (PRLR) using quantitative real-time RT-PCR; (2) esophageal cell proliferation and apoptosis, using immunohistochemistry of proliferating cells nuclear antigen (PCNA) and in situ nick end-labeling of genomic DNA (TUNEL); (3) the possible localization of immunoreactive PRLR on proliferating/apoptotic cells. Plasma PRL increased after FW acclimation; PRLR mRNA levels in the esophagus of FW fish were significantly higher than those in SW-acclimated fish. Cell proliferation was induced randomly throughout the esophageal epithelium after FW acclimation, while cell division and increased apoptosis were concentrated at the tips of esophageal epithelial folds in SW-acclimated fish. Immunoreactive PRLR appeared to be localized at proliferating cells and at certain apoptotic cells, whereas immunoreactive GR was observed over the whole epithelium including the apoptotic/proliferating cells. Thus, PRL appears to affect cell turnover directly in the esophageal epithelium of the euryhaline tilapia.

Induction of three vitellogenins by 17beta-estradiol with concurrent inhibition of the growth hormone-insulin-like growth factor 1 axis in a euryhaline teleost, the tilapia (Oreochromis mossambicus)

The objective of the present study was to utilize the male Mozambique tilapia (Oreochromis mossambicus) as a model for examining the molecular mechanisms that mediate the physiological transition between somatic and gonadal growth in female teleost fish, and in vertebrates in general. Partial cDNAs that encode multiple forms of vitellogenin (Vtg), which is the major precursor of yolk proteins, were cloned from estrogen-treated males and utilized to develop real-time quantitative RT-PCR assays, which were supplemented by an assay for Vtg immunoreactivity in the plasma. Alignment analyses of the amino acid sequences deduced from the vtg cDNAs revealed three distinct tilapia Vtgs, which were categorized as Aa-, Ab-, and C-type Vtgs. A single injection of male tilapias with 17beta-estradiol (E(2)) at 5 microg/g body weight significantly increased the plasma E(2) and hepatic levels of all three vtg transcripts within 1 day. Plasma E(2) levels declined after 3 days, whereas the plasma Vtg immunoreactivity and hepatic levels of the three vtg transcripts continued to increase. Hepatic expression of the estrogen receptor (esr) 1 gene, but not the esr2 gene, also increased markedly 1 day after E(2) injection and remained elevated for 5 days. While plasma growth hormone (Gh) levels were unaffected, hepatic expression of transcripts that encoded the Gh receptor and insulin-like growth factor 1 (Igf1) was suppressed by E(2), as were the plasma Igf1 levels. These results clearly suggest a distinct negative interplay between the growth and reproductive axes at the molecular level of key hepatic regulatory pathways involved in the control of energy utilization by gonadal and somatic growth processes.

Insulin-like growth factor I (IGF-I) in a growth-enhanced transgenic (GH-overexpressing) bony fish, the tilapia (Oreochromis niloticus): indication for a higher impact of autocrine/paracrine than of endocrine IGF-I

Several lines of growth hormone (GH)-overexpressing fish have been produced and analysed for growth and fertility parameters. However, only few data are available on the growth-promoting hormone insulin-like growth factor I (IGF-I) that mediates most effects of GH, and these are contradictory. Using quantitative real-time RT-PCR, radioimmunoassay, in situ hybridization, immunohistochemistry, and radiochromatography we investigated IGF-I and IGF binding proteins (IGFBPs) in an adult (17 months old) transgenic (GH-overexpressing) tilapia (Oreochromis niloticus). The transgenics showed an around 1.5-fold increase in length and an approximately 2.3-fold higher weight than the non-transgenics. Using radioimmunoassay, the serum IGF-I levels were lower (6.22 +/- 0.75 ng/ml) in transgenic than in wild-type (15.01 +/- 1.49 ng/ml) individuals (P = 0.0012). Radioimmunoassayable IGF-I in transgenic liver was 4.2-times higher than in wild-type (16.0 +/- 2.21 vs. 3.83 +/- 0.71 ng/g, P = 0.0017). No hepatocytes in wild-type but numerous hepatocytes in transgenic liver contained IGF-I-immunoreactivity. RT-PCR revealed a 1.4-times higher IGF-I mRNA expression in the liver of the transgenics (10.51 +/- 0.82 vs. 7.3 +/- 0.49 pg/microg total RNA, P = 0.0032). In correspondence, in situ hybridization showed more IGF-I mRNA containing hepatocytes in the transgenics. A twofold elevated IGF-I mRNA expression was determined in the skeletal muscle of transgenics (0.33 +/- 0.02 vs. 0.16 +/- 0.01 pg/microg total RNA, P < 0.0001). Both liver and serum of transgenics showed increased IGF-I binding. The increased IGFBP content in the liver may lead to retention of IGF-I, and/or the release of IGF-I into the circulation may be slower resulting in accumulation of IGF-I in the hepatocytes. Our results indicate that the enhanced growth of the transgenics likely is due to enhanced autocrine/paracrine action of IGF-I in extrahepatic sites, as shown here for skeletal muscle.

Effects of fasting on growth hormone, growth hormone receptor, and insulin-like growth factor-I axis in seawater-acclimated tilapia, Oreochromis mossambicus

Effects of fasting on the growth hormone (GH)--growth hormone receptor (GHR)-insulin-like growth factor-I (IGF-I) axis were characterized in seawater-acclimated tilapia (Oreochromis mossambicus). Fasting for 4 weeks resulted in significant reductions in body weight and specific growth rate. Plasma GH and pituitary GH mRNA levels were significantly elevated in fasted fish, whereas significant reductions were observed in plasma IGF-I and hepatic IGF-I mRNA levels. There was a significant negative correlation between plasma levels of GH and IGF-I in the fasted fish. No effect of fasting was observed on hepatic GHR mRNA levels. Plasma glucose levels were reduced significantly in fasted fish. The fact that fasting elicited increases in GH and decreases in IGF-I production without affecting GHR expression indicates a possible development of GH resistance.

Effects of acute stress on osmoregulation, feed intake, IGF-1, and cortisol in yearling steelhead trout (Oncorhynchus mykiss) during seawater adaptation

Juvenile steelhead (Oncorhynchus mykiss) acclimated to freshwater (FW) were subjected for 3 h to confinement stress in FW, and subsequently saltwater (SW, 25 ppt) was introduced to all tanks. Fish were sampled immediately after the stress treatment, and 1, 7, and 14 days after introduction of SW. Electrolytes, cortisol, glucose, and lactate showed the typical stress response expected after stress treatment in FW. Fish regained osmotic balance within 24 h. Glucose concentrations were increasing throughout the experiment and lactate levels stayed elevated during the time spent in SW. Insulin-like growth factor-1 (IGF-1) did not show an immediate response to stress but after transfer to SW we detected significantly higher concentrations for control fish at days 1 and 14. The differences in IGF-1 levels between stressed and control fish are not reflected in SW adaptability but positive correlations between IGF-1 and electrolyte levels in control fish may indicate its role for osmoregulation. Confinement stress did not impair feed intake subsequently in SW, but our results suggest that feed intake was suppressed by the change of the media from FW to SW.

Effects of aquaculture related stressors and nutritional restriction on circulating growth factors (GH, IGF-I and IGF-II) in Atlantic salmon and rainbow trout

The effects of aquaculture related stressors on circulating levels of GH, IGF-I and for the first time, IGF-II in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) were investigated. Specifically, circulating growth factor levels were measured in four different experiments. Two 24 h confinement stressor procedures, (one with Atlantic salmon, the other with rainbow trout); following a hypo-osmotic stressor (freshwater bath) in salt water acclimated, adult, Atlantic salmon; and during a 22 day starvation and re-feeding protocol with juvenile Atlantic salmon. Handling and confinement resulted in significant decreases in circulating levels of all three growth factors in Atlantic salmon, and IGF-I and IGF-II (but not GH) in rainbow trout. A 2-3 h freshwater bath to remove gill parasites on a commercial Atlantic salmon aquaculture operation caused a significant decrease in circulating GH and IGF-I concentrations, but no significant change in IGF-II concentration, 2 days post bathing. Starvation for a period of 15 days in Atlantic salmon resulted in a significant increase in circulating GH levels and a significant decrease in circulating IGF-I and IGF-II. Re-feeding of starved fish for 7 days resulted in a significant decrease in GH to the concentration measured in continually fed fish, however re-feeding did not change plasma levels of IGF-I and IGF-II relative to continually starved fish. The results presented here confirm previously observed handling and confinement stressor induced effects on GH and IGF-I and, for the first time, on IGF-II in salmonids. Furthermore this study confirms the nutritional regulation of GH, IGF-I and IGF-II in juvenile Atlantic salmon.

Regulation of the somatotropic axis by dietary factors in rainbow trout (Oncorhynchus mykiss)

The activity of the somatotropic axis was analysed in juvenile rainbow trout (Oncorhynchus mykiss) fed either a fishmeal-based diet (FM) or graded levels of plant proteins to replace 50% (PP50 diet), 75% (PP75 diet) or 100% (PP100 diet) of the fishmeal protein. For this purpose, partial cloning and sequencing of the gene encoding rainbow trout growth hormone receptor (GHR) was first accomplished by RT-PCR, using degenerate primers based on the sequences of non-salmonid fish GHR. Growth rates and energy retention were lowered by the PP75 and PP100 diets and a concurrent and progressive increase in plasma levels of growth hormone (GH) was found. However, no changes in hepatic GH binding and total plasma insulin-like growth factor (IGF)-I levels were observed among the four experimental groups. This fact agrees with the lack of changes in hepatic measurements of GHR and IGF-I transcripts. No consistent changes in IGF transcripts were found in peri-visceral adipose tissue and skeletal muscle, but GHR mRNA was up-regulated in the peri-visceral adipose tissue of fish fed the PP75 and PP100 diets, which would favour the lipolytic action of GH. Two specific bands (47 and 33 kDa) of IGF-binding proteins were found in the plasma of all analysed fish, but the sum of the two integrated areas increased progressively with plant protein supply, which might reflect a reduced free IGF availability. Therefore, in our experimental model, the growth impairment could be due, at least in part, to a lowered availability of biologically active IGF (free IGF fraction) rather than to liver GH desensitization or defect in IGF synthesis and release at the systemic and/or paracrine-autocrine level.

Development of Non-Radioisotopic Immunoassay Systems for Measuring Flounder IGF-I

A time-resolved fluoroimmunoassay system (TR-FIA) for measuring flounder insulin-like growth factor-I (IGF-I) was developed using biotinylated flounder IGF-I, anti-fish IGF-I antiserum and europium-avidin conjugate. The detection limit per well was <5 pg/well corresponding to <0.5 ng/ml in a basic procedure for sample of 10 microl/well and to <0.08 ng/ml in a procedure modified for high volume samples (up to 70 microl/well). Specificity of the assay was validated using various IGF-Is and insulins. All IGFs except seabream IGF-I showed very low or no crossreactivity. Binding inhibition curves for flounder and seabream IGF-Is were completely identical to each other. Intra- and interassay variations ranged from coefficients of variations of 3.9% to 7.2%. Recovery tests using barfin flounder plasma varied from 82.7 to 101.6% in the added range from 20 to 160 ng/ml. This assay system was applied for measuring total plasma IGF-I in barfin flounder injected porcine growth hormone (GH). A group injected with GH at the dose of 0.05 IU/gBW showed a significant increase of total plasma IGF-I compared with those of albumin-injected (control) and initial groups. In addition, I was able to substitute time-resolved fluorometric detection in this assay system with enzymatic fluorometric detection (FIA). Binding inhibition curve for flounder IGF-I in this substituted assay system showed equal performance with that of the TR-FIA system. Correlation of IGF-I levels between TR-FIA and FIA was high (r(2)=0.957) in plasma samples from barfin flounders in various physiological conditions. Thus, the present study shows precision and efficiency of two non-radioisotopic immunoassay systems for measuring flounder IGF-I.

Long-term treatment of ghrelin stimulates feeding, fat deposition, and alters the GH/IGF-I axis in the tilapia, Oreochromis mossambicus

Recently, a new peptide, ghrelin, which specifically stimulates growth hormone (GH) release from the pituitary, was identified in the rat and human stomach. Ghrelin has been shown to stimulate GH release by acting through a growth hormone secretagogue receptor. We have identified two ghrelins (ghrelin-C8 and -C10) in the stomach of tilapia, a euryhaline fish. The current study was aimed at investigating the in vivo effect of the two tilapia ghrelins on feeding, fat deposition, and on the GH/IGF-I axis. Tilapia were implanted with micro-osmotic pumps containing either ghrelin-C8, ghrelin-C10 or saline (control). Ghrelin was delivered at a continuous rate of 10 ng/h for 21 days. Food consumption increased significantly in those animals that received ghrelin-C10 but not ghrelin-C8. Treatment with ghrelin-C10 increased body weight significantly without altering body length. Hence, the condition factor was significantly higher in the ghrelin-C10 group compared with the control. Liver weight and total fat content in the liver were also elevated significantly in the fish treated with ghrelin-C10. There was no effect of either ghrelin on plasma GH levels, whereas plasma IGF-I levels were reduced significantly in the ghrelin-C10 group. These findings demonstrate that ghrelin plays a role in feeding and fat metabolism in the tilapia, and suggest that the two forms of ghrelin may be acting through different receptors.

Growth hormone and insulin-like growth factors in fish: where we are and where to go

This communication summarizes viewpoints, discussion, perspectives, and questions, put forward at a workshop on "Growth hormone and insulin-like growth factors in fish" held on September 7th, 2004, at the 5th International Symposium on Fish Endocrinology in Castellon, Spain.

Photoperiod influences growth rate and plasma insulin-like growth factor-I levels in juvenile rainbow trout, Oncorhynchus mykiss

The effect of different photoperiod regimes and the subsequent influence of melatonin on growth and insulin-like growth factor-I (IGF-I) were assessed in juvenile rainbow trout. In Experiment 1, triplicate groups of all female underyearling rainbow trout were exposed to one of three photoperiods; simulated natural photoperiod (SNP), constant short-days (LD 8:16), or constant long-days (LD 18:6) from June to December 2000 under ambient water temperatures. Fish exposed to LD 18:6 grew to a significantly heavier mean weight than the other treatments. Regression analysis showed a strong correlation between circulating plasma IGF-I, growth rate and temperature. Furthermore, it was apparent that fish exposed to LD 18:6 expressed significantly higher circulating levels of IGF-I. In a second experiment, duplicate groups of all female yearling trout were exposed to one of three photoperiods; SNP, LD 8:16, or constant light (LL), with sub groups receiving either a slow-release melatonin implant (18 mg), sham implant or left intact (control). LL increased growth rate in controls, reaching a significantly greater weight than SNP or LD 8:16 photoperiods but did not affect circulating IGF-I levels. Melatonin implants reduced growth rate in all photoperiod treatments below that of their respective controls but again did not affect circulating IGF-I levels. No differences in growth rate were found in implanted fish between photoperiods suggesting that a diel cycle of melatonin is necessary for the perception of daylength. These results would indicate that extended photoperiods (LD 18:6) may cause direct photostimulation of growth through up-regulation of IGF-I production. In contrast, in the absence of a changing diel melatonin signal, growth appeared to be maintained by a possible underlying endogenous rhythm, which was phase advanced under LL, as such plasma IGF-I levels simply reflected growth rate rather than photostimulation of the somatotropic axis. Overall, these findings indicate that measuring plasma IGF-I may be a useful tool for studying environmental influences on growth in rainbow trout.

Effects of exogenous cortisol on the GH/IGF-I/IGFBP network in channel catfish

Glucocorticoids are known to hinder somatic growth in a number of vertebrate species. In order to better understand the mechanisms through which they may act in channel catfish, we examined the effects of feeding cortisol on the growth hormone (GH)/insulin-like growth factor-I (IGF-I)/IGF-binding protein (IGFBP) network. Fish (30.6+/-3.0 g) were fed once daily for 4 weeks and treatments included: (1) High-cortisol (dietary cortisol provided at 400 mg/kg feed), (2) Low-cortisol (dietary cortisol provided at 200 mg/kg feed), and (3) Control (commercial catfish feed). Fish fed diets with cortisol weighed approximately 50% less than Controls. Feed intake was reduced by approximately 30% in both treatments of cortisol fed fish compared to Controls. A approximately 20-kDa IGFBP was observed in plasma from High- and Low-treated fish while it was not detected in Control fish plasma. High-cortisol treatment increased pituitary GH mRNA expression approximately 10-fold while liver IGF-I mRNA expression was not different between cortisol-treated fish and Controls. Cortisol treatments decreased plasma levels of IGF-I. These data indicate that feeding cortisol for 4 weeks reduces weight gain, feed intake, and plasma levels of IGF-I and induces a approximately 20-kDa IGFBP. One mechanism through which cortisol may impede growth of catfish is through an increase in a low molecular weight IGFBP which may lead to inhibitory effects on the action of IGF-I.

Insulin-like growth factor signaling in fish

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

Growth hormone secretion from the Arctic charr (Salvelinus alpinus) pituitary gland in vitro: effects of somatostatin-14, insulin-like growth factor-I, and nutritional status

This study investigated the influence of nutritional status on the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis in Arctic charr (Salvelinus alpinus). The objectives were to study the regulation of GH secretion in vitro by somatostatin-14 (SRIF) and hIGF-I, and to determine whether pituitary sensitivity to these factors is dependent upon nutritional status. Arctic charr were fed at three different ration levels (0, 0.35, and 0.70% BWd(-1)), and pituitary glands were harvested at 1, 2, and 5 weeks for in vitro study. Both SRIF and hIGF-I inhibited GH secretion from Arctic charr pituitary tissue in long-term (18 h) static hemipituitary culture, as well as after acute exposure in a pituitary fragment perifusion system. This response appeared to be dose-dependent for SRIF in static culture over the range of 0.01-1 nM, but not for hIGF-I. The acute inhibitory action of hIGF-I on GH release in the perifusion system suggests an action that is initially independent of any effects on GH gene expression or protein synthesis. Nutritional status did not affect the sensitivity of Arctic charr pituitary tissue to either SRIF or hIGF-I in vitro, indicating that changes in abundance of pituitary SRIF or IGF-I receptors may not explain the alterations in plasma GH levels found during dietary restriction.

Time course of the GH/IGF axis response to fasting and increased ration in chinook salmon (Oncorhynchus tshawytscha)

Body growth in vertebrates is chiefly regulated by the GH/IGF axis. Pituitary growth hormone (GH) stimulates liver insulin-like growth factor-I (IGF-I) production. During fasting, plasma IGF-I levels decline due to the development of liver GH resistance, while GH levels generally increase. In mammals, decreased insulin during fasting is thought to cause liver GH resistance. However, the sequence of events in the GH/IGF axis response to fasting is not well characterized, especially in non-mammalian vertebrates. We assessed the time course of the GH/IGF axis response to fasting and increased ration in chinook salmon. Fish were placed on Fasting, Increased, or Control rations, and sampled daily for 4 days and at more widely spaced intervals through 29 days. Plasma IGF-I, GH, insulin, and 41 kDa IGF binding protein (putative salmon IGFBP-3), and liver IGF-I gene expression were measured. Control and Increased ration fish did not differ strongly. Plasma IGF-I and 41 kDa IGFBP were significantly lower in Fasted versus Control fish from day 4 onward, and liver IGF-I gene expression was significantly lower from day 6 onward. Liver IGF-I gene expression and plasma IGF-I levels were correlated. Plasma insulin was lower in Fasted fish from day 6 onward. There was a trend toward increased GH in Fasted fish on days 1-2, and GH was significantly increased Fasted fish from day 3 onward. Fasted GH first increased (days 1-3) to a plateau of 10-20 ng/ml (days 4-12) and then increased dramatically (days 15-29), suggesting that the GH response to fasting had three phases. The early increase in GH, followed by the decrease in plasma IGF-I after 4 days, suggests that GH resistance developed within 4 days.

Salinity acclimation affects the somatotropic axis in rainbow trout

In this study, we set out to examine the role of the somatotropic axis in the ion-regulation process in rainbow trout. Specifically, our objective was to examine whether plasma insulin-like growth factor-binding proteins (IGFBPs) are modulated by gradual salinity exposure. To this end, freshwater (FW)-adapted rainbow trout were subjected to gradual salinity increases, up to 66% seawater, over a period of 5 days. During this acclimation process, minimal elevations in plasma Ca2+ and Cl- were seen in the salinity-acclimated groups compared with FW controls. There were no changes in plasma Na+ levels, and only a minor transient change in plasma cortisol levels was seen with salinity exposure. The salinity challenged animals responded with elevations in plasma growth hormone (GH) and IGF-I levels and gill Na+-K+-ATPase activity. We identified IGFBPs of 21, 32, 42, and 50 kDa in size in the plasma of these animals, and they were consistently higher with salinity. Despite the overall increase in IGFBPs with salinity, transient changes in individual BPs over the 5-day period were noted in the FW and salinity-exposed fish. Specifically, the transient changes in plasma levels of the 21-, 42-, and 50-kDa IGFBPs were different between the FW and salinity groups, while the 32-kDa IGFBP showed a similar trend (increases with sampling time) in both groups. Considered together, the elevated plasma IGFBPs suggest a key role for these binding proteins in the regulation of IGF-I during salinity acclimation in salmonids.

Development and characterization of a competitive polyclonal antibody enzyme-immunoassay for salmon insulin-like growth factor-II

This paper describes the development and validation of a competitive, polyclonal antibody enzyme-immunoassay (EIA) for the measurement of salmon and trout insulin-like growth factor-II (IGF-II). A polyclonal antiserum was raised against a synthetic peptide epitope, corresponding to amino acid residues 1-9 of the N-terminus of mature Atlantic salmon (Salmo salar) IGF-II. The antiserum was purified by hydrophobic charge induction chromatography (HCIC). The partially purified immunoglobulins were used in an enzyme-immunoassay system (EIA) resulting in a highly specific assay for salmon IGF-II with cross-reactivity of less than 0.01% for recombinant salmon IGF-I and recombinant salmon growth hormone (GH), and 5.57% for salmon insulin (sIns). The recombinant salmon IGF-II (rsIGF-II) standard curve limit of detection was 1.37 ng/ml with an EC(50) of 44.97+/-0.82 ng/ml. Intra- and interassay coefficients of variation were determined at 7.47% (n=15) and 7.42% (n=15), respectively. Added rsIGF-II was adequately recovered from acid-treated Atlantic salmon and rainbow trout (Oncorhynchus mykiss) plasma samples. Parallel dose-response inhibition curves were demonstrated for the plasma of both fish species tested. Circulating IGF-II levels of 22.26+/-2.66 and 18.24+/-1.43 ng/ml were determined for acid-treated plasma of normal adult Atlantic salmon and rainbow trout, respectively. This EIA should prove to be useful in the study of factors which influence circulating plasma levels of IGF-II in these fish species.

Androgen effects on plasma GH, IGF-I, and 41-kDa IGFBP in coho salmon (Oncorhynchus kisutch)

Among many species of salmonids, fast growing fish mature earlier than slow growing fish, and maturing males grow faster than non-maturing ones. To study the potential endocrine basis for this reciprocal relationship we examined the in vivo effects of the androgens, testosterone (T) and 11-ketotestosterone (11-KT), on plasma growth hormone (GH), insulin-like growth factor-I (IGF-I) and 41-kDa IGF binding protein (41-kDa IGFBP) (putative IGFBP-3) in coho salmon, Oncorhynchus kisutch. Immature male and female, two-year old fish (avg. wt. 31.7 +/- 0.63 g) were injected with coconut oil containing T or 11-KT at a dose of 0.1, 0.25, or 1 microg/g body weight. Blood samples were taken 1 and 2 weeks postinjection, and analyzed by immunoassay for T, 11-KT, GH, IGF-I, and 41-kDa IGFBP. Steroid treatments elevated the plasma T and 11-KT levels to physiological ranges typical of maturing fish. Plasma IGF-I and 41-kDa IGFBP levels increased in response to both T and 11-KT in a significant and dose-dependent manner after 1 and 2 weeks, but GH levels were not altered. These data suggest that during reproductive maturation, in addition to the previously demonstrated effects of the IGFs on steroidogenesis, the gonadal steroids may in turn play a significant role in regulating IGF-I and its binding proteins in fish. The interaction between the reproductive and growth axes may provide a regulatory mechanism for bringing about the dimorphic growth patterns observed between maturing and non-maturing salmonids and other species of fish.

Effects of fasting on circulating IGF-binding proteins, glucose, and cortisol in channel catfish (Ictalurus punctatus)

The effects of fasting on IGF-binding proteins (IGFBPs), glucose, and cortisol in channel catfish were examined. Fed fish (controls) were compared to 14-, 30-, and 45-day fasted fish and 45-day fasted fish refed for 15 additional days. Body length and weight changes, condition factor (CF), hepatosomatic index (HSI), and plasma glucose and cortisol were assessed to determine growth and metabolic status. Body length and growth rates were inhibited (P<0.05) after 14, 30, and 45 days of fasting. The 14-, 30-, and 45-day fasted fish exhibited hypoglycemia and reduced CF and HSI. Cortisol levels were increased (22.8 +/- 15.2 ng/ml versus 4.7 +/- 3.9 ng/ml) in 30-day fasted fish compared to fed controls (P<0.05). Associated with the increase in cortisol in fasted fish was a concomitant increase in plasma levels of a 20-kDa IGFBP through day 45. A 35- and a 45-kDa IGFBP were also identified but were similar between fed and unfed fish throughout the experiment. At the end of 15 days of refeeding, 20-kDa IGFBP, glucose, and cortisol levels were similar to fed controls. Refeeding also caused an increase in growth rates. These results suggest the existence of a catfish counter part to mammalian IGFBP-1, similar to lower molecular mass IGFBPs reported in other species of fish. These results also suggest that a 20-kDa IGFBP is upregulated during fasting-induced growth inhibition of channel catfish and provide additional evidence of the conserved nature of the IGF-IGFBP-growth axis in fish.

Response of the somatotropic axis of juvenile coho salmon to alterations in plane of nutrition with an analysis of the relationships among growth rate and circulating IGF-I and 41 kDa IGFBP

The effect of different feeding levels on plasma levels of insulin-like growth factor-I (IGF-I), 41 kDa insulin-like growth factor binding protein (41 kDa IGFBP), and growth hormone (GH) were assessed in post-smolt coho salmon. Fish were fed at either stable (1 and 2% body weight/day) or varying (1-0.5-1%, 2-0.5-2% body weight/day) feeding rates and plasma was sampled from 10 fish/treatment at 2-3 week intervals over five dates from June to September, resulting in a total of 200 samples. Fish fed at higher rates grew faster and had higher plasma IGF-I and 41 kDa IGFBP levels. Plasma GH levels were variable but generally showed an inverse relationship to feeding rate. Both plasma IGF-I and 41 kDa IGFBP increased seasonally, average IGF-I levels doubled from June to September, regardless of feeding rate. On any one date both IGF-I and 41 kDa IGFBP were highly related to growth rate with regression coefficients ranging from 0.36 to 0.68 (IGF-I) and from 0.33 to 0.70 (41 kDa IGFBP). No relationship was found between IGF-I:41 kDa IGFBP ratio and individual growth rate. Overall, both feeding rate and date were important in explaining variation in IGF-I and 41 kDa IGFBP levels.

Endocrine changes during onset of puberty in male spring Chinook salmon, Oncorhynchus tshawytscha

In male salmonids, the age of maturation varies from 1 to 6 years and is influenced by growth during critical periods of the life cycle. The endocrine mechanisms controlling spermatogenesis and how growth affects this process are poorly understood. Recent research has indicated that gonadotropins, 11-ketotestosterone, and insulin-like growth factor I play roles in spermatogenesis in fish. To expand our understanding of the roles of these endocrine factors in onset of puberty, male spring chinook salmon (Oncorhynchus tshawytscha) were sampled at monthly intervals 14 mo prior to spermiation. This sampling regime encompassed two hypothesized critical periods when growth influences the initiation and completion of puberty for this species. Approximately 80% of the males matured during the experimental period, at age 2 in September 1999. An initial decline in the ratio of primary A to transitional spermatogonia was observed from July to December 1998, and during this period plasma levels of 11-ketotestosterone and pituitary levels of FSH increased. From January 1999 onward, males with low plasma 11-ketotestosterone levels (<1 ng/ml) had low pituitary and plasma FSH levels and no advanced development of germ cells. Conversely, from January through September 1999, males with high plasma 11-ketotestosterone levels (>1 ng/ml) had testes with progressively more advanced germ cell stages along with elevated pituitary and plasma FSH. Plasma levels of insulin-like growth factor I increased during maturation. These data provide the first physiological evidence for activation of the pituitary-testis axis during the fall critical period when maturation is initiated for the following year.

Purification of a 41-kDa insulin-like growth factor binding protein from serum of chinook salmon, Oncorhynchus tshawytscha

In salmon, at least three insulin-like growth factor binding proteins (IGFBPs) with molecular masses of 41, 28, and 22kDa exist in serum. The 41-kDa IGFBP is up-regulated by growth hormone treatment and down-regulated by fasting, suggesting that it is a homolog of IGFBP-3. We purified the 41-kDa IGFBP from chinook salmon serum by IGF-I affinity chromatography followed by reversed-phase high pressure liquid chromatography. Purified IGFBP appeared as doublet bands on electrophoresis and was N-glycosylated. Analysis of partial N-terminal amino acid sequence revealed that salmon 41-kDa IGFBP has the cysteine rich domain conserved among IGFBP family. In a binding assay using 125I-salmon IGF-I, purified 41-kDa IGFBP specifically bound salmon IGF-I, human IGF-I and human IGF-II, but neither Long R(3)IGF-I nor salmon insulin, showing that binding characteristics of the salmon IGFBP are similar to those of mammalian IGFBPs. Although the partial amino acid sequence of 41-kDa IGFBP showed highest homologies with zebrafish and seabream IGFBP-2, the highly conserved nature of the N-terminus makes it impossible to identify the type of IGFBP from partial sequence data. However, based on physiological responses, molecular weight and type of glycosylation, the 41-kDa IGFBP is most similar to mammalian IGFBP-3.

Effects of fasting on growth hormone/insulin-like growth factor I axis in the tilapia, Oreochromis mossambicus

Effects of fasting on the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis were examined in the tilapia (Oreochromis mossambicus) acclimated to fresh water. Fasting for 2 weeks resulted in significant reductions in body weight, specific growth rate and hepatosomatic index in both males and females. Significant reductions in specific growth rates were observed after 1 and 2 weeks in both sexes, although the decrease in body weight was not significant in the female. A significant reduction was also seen in the condition factor of females after 2 weeks. No change was seen in the gonadosomatic index in either sex. Two weeks of fasting also produced a significant reduction in plasma IGF-I but not in plasma GH, prolactin (PRL(188)) or cortisol. Significant reductions in the hepatic IGF-I mRNA were seen in both sexes. On the other hand, a significant increase was observed in cortisol receptor mRNA in the female liver. Plasma IGF-I levels were correlated significantly with specific growth rate, condition factor and hepatosomatic index, indicating that plasma IGF-I is a good indicator of growth in the tilapia. No change was seen in plasma glucose or osmolality after 2 weeks of fasting. During fasting, tilapia appears to convert metabolic energy from growth to basal metabolism including maintenance of ion and water balance.

Structure, developmental expression, and physiological regulation of zebrafish IGF binding protein-1

The biological activity and availability of IGFs are regulated by a group of secreted proteins that belong to the IGF-binding protein (IGFBP) gene family. Although six IGFBPs have been identified and studied in mammals, their nonmammalian orthologs remain poorly defined. In this study, we cloned and characterized the full-length zebrafish IGFBP-1. Sequence analysis indicated that its structure is homologous to mammalian IGFBP-1. Using in situ RNA hybridization and RT-PCR, we discovered that IGFBP-1 mRNA was present in all early embryonic stages albeit at very low levels. IGFBP-1 mRNA was initially expressed in multiple embryonic tissues but became restricted to the liver shortly after hatching. In the adult stage, IGFBP-1 mRNA was found only in the liver at low levels. Prolonged food deprivation caused a significant increase in the hepatic IGFBP-1 mRNA levels, and refeeding restored the IGFBP-1 mRNA to the basal levels. When adult fish or embryos were subjected to hypoxic conditions, the IGFBP-1 mRNA expression increased dramatically. Intriguingly, the hypoxia-induced IGFBP-1 expression operated in different embryonic tissues in a developmental-stage-dependent manner. In early embryos, hypoxia-stimulated IGFBP-1 mRNA expression in the pharyngeal arches, ventricle, atrium, and brain. After hatching, the hypoxia-induced IGFBP-1 expression became liver specific. These results not only provide new information about the structural conservation, developmental expression, and physiological regulation of the IGFBP-1 gene but also present the opportunity to elucidate the developmental role of IGFBP-1 using a unique vertebrate model organism.

Effects of insulin-like growth factors (IGF-I and -II) on growth hormone and prolactin release and gene expression in euryhaline tilapia, Oreochromis mossambicus

We investigated in vitro effects of insulin-like growth factors (IGF-I and -II) on growth hormone (GH) and prolactin (PRL) release and gene expression in euryhaline tilapia, Oreochromis mossambicus. Pituitaries were removed from freshwater-acclimated adult males and incubated for 2-24h in the presence of human IGF-I or -II at doses ranging from 1-1000 ng/ml (0.13-130 nM). IGF-I at concentrations higher than 10 ng/ml and IGF-II higher than 100 ng/ml significantly inhibited GH release after 8, 16, and 24h. No effect of IGFs was seen during the first 4h of incubation. IGFs at the same concentrations also significantly attenuated GH gene expression after 24h, although no effect was seen at 2h. By contrast, PRL(188) release was stimulated significantly and in a dose-related manner by IGF-I at concentrations higher than 10 ng/ml and by IGF-II at concentrations higher than 100 ng/ml within 2h. No stimulation was observed after 4h. Similarly, both IGFs at concentrations higher than 10 ng/ml increased PRL(177) release within 2h. However, no significant effect of IGF-I or -II was observed on mRNA levels of both PRLs after 2 and 24h at all concentrations examined. These results clearly indicate differential regulation of GH and PRL release and synthesis by IGFs in the tilapia pituitary, i.e., rapid-acting, stimulatory effects of IGFs on PRL release and slow-acting, inhibitory effects on GH release and synthesis.

Activation of the growth hormone/insulin-like growth factor axis by treatment with 17 alpha-methyltestosterone and seawater rearing in the tilapia, Oreochromis mossambicus

Effects of 17 alpha-methyltestosterone (MT) treatment and environmental salinity on the growth hormone (GH)/insulin-like growth factor (IGF) axis were examined in the euryhaline tilapia, Oreochromis mossambicus. Yolk-sac fry were collected from brood stock in fresh water (FW). After yolk-sac absorption, they were assigned randomly to 1 of 4 groups: FW, MT treatment in FW, SW, and MT treatment in seawater (SW). After 147 days, FW controls had the lowest levels of GH mRNA followed by FW fish treated with MT and SW control fish. Seawater fish fed with a diet containing MT, which grew the fastest, had significantly higher levels of GH mRNA than all the other groups. A significant correlation was observed between GH mRNA and the size of the individual fish. By contrast, plasma GH levels did not vary significantly among the groups. Pituitary GH mRNA levels, plasma IGF-I levels, and fish size varied in a correlated pattern, i.e., SW+MT>FW+MT=SW control>FW control. The tilapia pituitary produces two prolactins (PRLs), PRL(177) and PRL(188). Prolactin(177), but not PRL(188), exhibits growth-promoting actions in FW tilapia. Pituitary mRNA levels of both PRLs were significantly higher in fish reared in FW than those reared in SW. Treatment with MT significantly increased mRNA levels of both PRLs in FW, but had no effect on SW fish. No correlation was seen between plasma PRL levels and growth or between PRL mRNA levels and growth. These results indicate that SW rearing and MT treatment stimulate the GH/IGF-I axis, and suggest that pituitary GH mRNA at this stage of development is a better indicator of growth than plasma levels of GH and IGF-I.

Somatotropic regulation of fish growth and adiposity: growth hormone (GH) and somatolactin (SL) relationship

Growth hormone (GH) and insulin-like growth factors (IGFs) play a major role in fish development and metabolism, and several studies have allowed discernment of a complex and tissue-specific collection of salmonid IGF-I transcripts (Ea-4, Ea-3, Ea-2, Ea-1), which are the result of the alternative splicing of the E-domain region. However, the pattern of IGF-I expression is different in non-salmonid fish, and only one or two transcripts (Ea-4, Ea-2) have been detected in hepatic and extrahepatic tissues of common carp, barramundi, black sea bream and gilthead sea bream. Despite this, when comparisons are made within Mediterranean fish species (European sea bass, common dentex and gilthead sea bream), plasma IGF-I levels are consistent with fish species differences in growth rates. Changes of growth rates, and plasma IGF-I and GH levels are also found in response to changes in diet composition and ration size, which may serve to assess the suitability of feeding regimes in aquaculture practice. Regulation of plasma somatolactin (SL) levels is also examined in gilthead sea bream, and the resulting plasma SL profile differs from that of GH. Thus, in contrast to GH, plasma SL levels augment with the increase of ration size and fish size (advancement of age). A transient increase in plasma SL levels is also found in short-term fasted fish, and this fish peptide may act as an anti-obesity hormone helping to expedite growth-reproductive processes following replenishment of fat stores, and/or mediate the adaptation to fasting until the lipolytic action of GH and/or other endocrine factors is fully accomplished. This agrees with the known increase of plasma SL levels during acute stress and exhaustive exercise. However, a causal link between SL and energy mobilisation (lipid metabolism) remains to be established, and further research is needed to determine the extent to which SL and GH act in a complementary manner to make available metabolic fuels and to regulate body fat mass and feeding behaviour.

Production, in vitro characterisation, in vivo clearance, and tissue localisation of recombinant barramundi (Lates calcarifer) insulin-like growth factor II

Recombinant barramundi insulin-like growth-factor-II (bIGF-II) has been produced in Escherichia coli after modification of an expression plasmid that coded for a chicken IGF-II fusion protein. The bIGF-II fusion protein, deposited in bacterial inclusion bodies, was dissolved under reducing conditions, desalted, and refolded. The protein was then released from the fusion protein by cleavage with subtilisin BPN'. Finally the protein was purified to homogeneity with a number of HPLC steps. In vitro analysis of recombinant bIGF-II demonstrated decreased potency in stimulating protein synthesis when compared to human and barramundi IGF-I (bIGF-I). The in vivo distribution of radiolabeled bIGF-II and bIGF-I in the circulation and tissue uptake of radiolabeled bIGF-II was also compared in juvenile barramundi (Lates calcarifer). Analysis of trichloroacetic acid-precipitable radioactivity in sequential samples following bolus injection of radiolabeled IGFs revealed that bIGF-II was degraded faster than bIGF-I. Moreover, neutral gel chromatography of these samples suggested this difference may be due to reduced affinity of bIGF-II, compared to blGF-I, for the IGF-binding proteins (IGFBPs) present in the barramundi circulation. Based on these results, it would appear that elements important in the function of IGFs have been well conserved during vertebrate evolution. However, to clearly define the IGF system in fish it will be necessary to characterise the IGFBPs present and to determine how they influence the biological actions of native IGFs.

Serum insulin-like growth factor binding proteins (IGFBPs) as markers for anabolic/catabolic condition in fishes

In fishes as well as in all vertebrates in which it has been assessed, physiological shifts toward catabolism (e.g. such as during food deprivation) are consistently associated with elevations in the serum levels of at least one (often two in fishes) IGFBP in the < or =31-kDa size range. In mammals, 30-kDa IGFBP-1 is strongly up-regulated under catabolic circumstances, and it plays an important physiological role by sequestering IGF peptides to inhibit energy-expensive growth until conditions are more favorable (e.g. with resumed feeding). Similarly in fishes, it has been found that when the < or =31-kDa IGFBPs are elevated in serum, somatic growth is inhibited, suggesting a similar growth-inhibitory role of these proteins to that of mammalian IGFBP-1. Three different experimentally-induced catabolic states in fishes are compared in this paper: fasting; insulin-dependent diabetes mellitus (IDDM); and stress. A strong relationship between elevated serum cortisol concentrations and the presence of IGFBPs in each case is noted, and the utility of serum IGFBP measurement to serve as an effective indicator (marker) of catabolic condition in fishes is discussed.

Paradigms of growth in fish

Most fish are indeterminate growers with white muscle making up the majority of the acquired bulk. Within the muscle, the myofibrillar fraction accounts for almost two-thirds of the protein synthetic activity, implying that it is accretion of myofibrillar proteins that makes the single most important contribution to fish growth. Fish muscle growth itself is not linear and occurs through a combination of hyperplasia and hypertrophy in post-juvenile stages. Superimposed on periodicity of growth in length and mass can be other phases governed by lunar, reproductive or circannual cycles. Data on fish growth are discussed in the framework of site-specific muscle abundance, metabolic and functional zonation of muscle, proliferation and differentiation of satellite cells and the contribution of myofibrillar proteins. Hormonal control of muscle growth is described against the backdrop of plasma availability of myogens (insulin, IGF-I, growth hormone), distribution and dynamics of their respective receptors, and their interactions. Important contributions of the 'supply side' are discussed with hormones regulating amino acid resorption from the intestine, intestinal growth, liver processing and amino acid uptake by the muscle. Data are also interpreted from metabolic angles, to explain lipolytic and nitrogen-sparing effects of growth hormones, and lipogenic effects of insulin and high protein diets. Finally, special attention is devoted to the multifaceted roles of arginine in fish growth, as precursor, intermediate and hormone secretagogue.

Relationship of insulin-like growth factor-I and insulin to size and adiposity of under-yearling chinook salmon

Sub-yearling spring chinook salmon were fed either a LoFat or HiFat diet from February to November. Fish were sampled over 2 days in November, following 24- and 48-h fasts. Length vs. weight relationships between fish fed the two diets were similar; however, fish fed the HiFat diet had roughly twice the body lipid as fish fed the LoFat diet (9% vs. 4.5%, respectively). Plasma IGF-I vs. length relations between fish fed the two diets were similar; overall, there was a strong relation between plasma IGF-I and length (r(2)=0.53). Similarly, plasma log (insulin) vs. length relations did not vary between the two diets; however, the relationship of log (insulin) vs. length was weak (r(2)=0.2). There was little or no relationship between plasma IGF-I or log (insulin) and body adiposity. Finally, there was a weak relationship between plasma IGF-I and log (insulin) (r(2)=0.23).

Comparison of extraction methods and assay validation for salmon insulin-like growth factor-I using commercially available components

Insulin-like growth factor-binding proteins (IGFBPs) may interfere with accurate measurement of plasma IGFs in radioimmunoassay (RIA). Although several simplified extraction methods for IGFs have been developed, these methods are not always validated for differing physiological states, developmental stages, and animal species. For teleost fish, neither the necessity of plasma extraction nor the validity of extraction methods for IGF RIA is widely established. We systematically examined the validity of acid-ethanol (AE) extraction, AE extraction followed by cryoprecipitation (AEC extraction), and SP-Sephadex extraction in RIA for salmon IGF-I using commercially available components (GroPep Pty Ltd). Displacement curves of plasma extracted by AE, AEC, and SP-Sephadex were parallel to those of the standard. Measured IGF-I levels in plasma from several developmental stages and under different physiological and experimental conditions were significantly increased by the extractions and comparable to those after acid-size exclusion chromatography (SEC). On Western ligand blotting using digoxigenin-labeled human IGF-I, the intensity of IGFBP bands remaining in plasma were reduced after extraction, although some IGFBPs remained. However, these residual IGFBPs did not interfere measurably with the RIA based on quantitative comparison of IGF-I levels with acid-SEC. We conclude that with this RIA extraction is necessary for measurement of salmon IGF-I in plasma since measured values were routinely lower in unextracted samples, and AE, AEC, and SP-Sephadex extractions are applicable to the IGF-I RIA using the commercially available components. Using the validated RIA for IGF-I, plasma IGF-I levels in nonmaturing and precociously maturing chinook salmon in spring were measured after AE extraction. During spring, nonmaturing and maturing fish fed and grew well, and plasma IGF-I level was significantly correlated with body weight in both fish. This result indicates that circulating IGF-I plays a key role in controlling growth in precociously maturing chinook salmon in spring as in nonmaturing fish.

Long-term peripheral treatment of immature coho salmon (Oncorhynchus kisutch) with human leptin has no clear physiologic effect

The effects of long-term peripheral exposure to recombinant human leptin were tested in immature coho salmon under both fed and fasted conditions. We found that high circulating levels of human leptin did not alter growth, energy stores, gonad weight, pituitary content of follicle-stimulating hormone, or plasma levels of insulin-like growth factor-I, insulin, growth hormone, or thyroxine.