Effect of teleost pituitary growth hormone on growth of Tilapia mossambica and on growth and seawater adaptation of sockeye salmon (Oncorhynchus nerka)

The GH/IGF axis in the sea lamprey during metamorphosis and seawater acclimation

How the growth hormone (GH)/insulin-like growth factor (IGF) system affects osmoregulation in basal vertebrates remains unknown. We examined changes in the expression of components of the GH/IGF axis and gill ion transporters during metamorphosis and following seawater (SW) exposure of sea lamprey. During metamorphosis, increases in gill nka and nkcc1 and salinity tolerance were accompanied by increases in pituitary gh, liver igf1, gill ghr and igf1, but not liver ghr. SW exposure of fully metamorphosed sea lamprey resulted in slight increases in plasma chloride concentrations after SW exposure, indicating a high level of SW tolerance, but no major changes in mRNA levels of gill ion transporters or components of the GH/IGF axis. Our results indicate that metamorphosis is a critical point in the lifecycle of sea lamprey for stimulation of the GH/IGF axis and is temporally associated with and likely promotes metamorphosis and SW tolerance.

Effects of recombinant salmon type II growth hormone and bovine growth hormone on growth of coho salmon (Oncorhynchus kisutch)

A comparison of the efficacy of salmon and bovine growth hormone to stimulate growth of coho salmon juveniles was performed. Oncorhynchus nerka (sockeye salmon) type II growth hormone (nGH2) was produced using a bacterial expression system, yielding approximately 25 mg of refolded recombinant protein per litre of cells. The purified nGH2 and bovine growth hormone (bGH) were tested in juvenile O. kisutch (coho salmon) over 24 weeks. Weekly intraperitoneal injections of 0.1 and 0.5 μg/g nGH2 resulted in a dose-dependent increase in weight and fork length compared to control fish injected with bovine serum albumin (BSA). Application of 0.5 μg/g bGH resulted in the same stimulation of growth as did 0.5 μg/g nGH2, indicating these proteins were equipotent. Following 6 weeks of treatment and a subsequent rest period of 7 weeks, coho salmon were further treated with bGH at 0.5 μg/g. A prior treatment with bGH did not reduce growth-promoting activity of bGH in subsequent treatments. Throughout the experiment, condition factor decreased at similar rates for all treatment groups. These data show that bGH, which is widely available, can be used to elevate growth rate in juvenile salmon comparably to homologous GH, and validate the use of bGH in physiological or ecological experiments where rapid growth is desired compared to that seen in wild type.

Salinity acclimation and advanced parr–smolt transformation in growth-hormone transgenic coho salmon (Oncorhynchus kisutch)

Growth hormone (GH) is involved in the parr–smolt transformation of salmonid fishes and is known to improve salinity tolerance. This study compared the capacity for seawater acclimation of GH transgenic coho salmon (Oncorhynchus kisutch (Walbaum, 1792)) to that of wild-type fish, allowing examination of responses to sustained (chronic) exposure to elevated GH. GH transgenic fish (GH TG) smolted 1 year in advance of wild-type salmon and showed a greater capacity to hypo-osmoregulate in seawater. As GH TG fish were much larger than the wild-type fish, a second experiment was conducted with three size-matched groups of coho salmon (a 1+-year-old wild-type group, a 1+-year-old ration-restricted GH TG group, and a 0+-year-old fully fed GH TG group). When size-matched, the effect of GH transgenesis was not as dramatic, but the feed-rationed TG1+ group exhibited smaller deviations in plasma ion and osmolality levels following seawater exposure than did the other groups, suggesting a somewhat improved hypo-osmoregulatory ability. These results support a role for GH in the development of seawater tolerance by salmonid fishes independent of fish size.

Effect of parental mate choice and semi-natural early rearing environment on the growth performance and seawater tolerance of Chinook salmon Oncorhynchus tshawytscha

To assess whether parental mate choice and early rearing in a semi-natural spawning channel may benefit the culture of Chinook salmon Oncorhynchus tshawytscha, 90 day growth trials were conducted using hatchery O. tshawytscha (hatchery), mate choice O. tshawytscha (i.e. the offspring of parents allowed to choose their own mate) that spent 6 months in a spawning channel prior to hatchery rearing (channel) and mate choice O. tshawytscha transferred to the hatchery as fertilized eggs (transfer). During the growth trials, all O. tshawytscha stocks were reared separately or in either mixed channel and hatchery or transfer and hatchery groups for comparison of performance to traditional practices. After 60 days in fresh water, all O. tshawytscha were transferred to seawater for an additional 30 days. Reared separately, all stocks grew c. 4.5 fold over 90 days but specific growth rate (G) and food conversion efficiency were higher in fresh water than after seawater transfer on day 60. In contrast, hatchery O. tshawytscha from mixed hatchery and channel and hatchery and transfer growth trials had a larger mass and length gain than their counterparts on day 60, but reduced G in seawater. In general, plasma levels of growth hormone, insulin-like growth factor I and cortisol did not differ among any O. tshawytscha groups in either the separate or mixed growth trials. Despite some differences in gill Na(+),K(+)-ATPase activity, all O. tshawytscha had a high degree of seawater tolerance and experienced virtually no perturbation in plasma chloride following seawater transfer. Overall, all O. tshawytscha exhibited similar growth and seawater performance under traditional hatchery conditions and any benefit derived from either parental mate choice or semi-natural early rearing environment was only observed in the presence of mutual competition with hatchery O. tshawytscha.

Study of goldfish (Carassius auratus) growth hormone structure–function relationship by domain swapping

Using goldfish as a model, the structure-function relationship of goldfish growth hormone was studied using the strategy of homologous domain swapping. Chimeric mutants were constructed by exchanging homologous regions between goldfish growth hormone (gfGH II) and goldfish prolactin (gfPRL) with their cloned complementary DNAs. Six mutants, with their domain-swapped, were generated to have different combinations of three target regions, including the helix a, helix d and the large section in between these helices (possess the helices b, c and other random coiled regions). After expression in E. coli and refolding, these mutants were characterized by using competitive receptor binding assay (RRA) and growth hormone responding promoter activation assay. The different activity profiles of mutants in Spi 2.1 gene promoter assays from that in RRA shows that, for gfGH, receptor binding dose not confer receptor signal activations. When either helices a or d of gfGH was maintained with other helices replaced by their gfPRL counterparts, both receptor binding and hence gene activation activities are reduced. In mutants with helices b and c in gfGH maintained, containing the gfGH middle section, and helices a and d swapped with gfPRL, the had reduced RRA activities but the promoter activation activities retained. In conclusion, as in the case of human GH, the gfGH molecule possesses two functional sites: one of them is composed of discontinuous epitopes located on the target regions of this study and is for receptor binding; another site is located on the middle section of the molecule that helices a and d are not involved, and it is for activation of GH receptor and intracellular signals.

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.

Differential expression of Gh1 and Gh 2 genes by competitive RT-PCR in rainbow trout pituitary

The expressions of the GH1 and GH2 genes were examined by competitive RT-PCR and whole-mount in situ hybridization of pituitary of starved rainbow trout (Oncorhynchus mykiss). lambda RNA having GH primers at both 5' and 3' sites of the gene was used for the competitive RT-PCR, and thermostable reverse transcriptase produced a reasonable band when authentic RNA was examined. The amount of GH1 gene expression was significantly (P < 0.05) greater than that of GH2. Although almost the same amount of GH1 gene expression was obtained during the day, it was significantly (P < 0.05) decreased at midnight. However, there was no significant change in GH2 gene expression in the daily cycle. There were also some differences in gene expression in the pituitary: GH2 gene was more widely expressed than GH1. However, the signal intensity of the GH1 gene was greater than that of GH2.

Distribution, characterization, and growth hormone-releasing activity of pituitary adenylate cyclase-activating polypeptide in the European eel, Anguilla anguilla

The complementary DNA encoding pituitary adenylate cyclase-activating polypeptide (PACAP) has been cloned from two species of teleost fishes, the Sockeye salmon and the Thai catfish, and the amino acid sequence of PACAP has been determined in another teleost, the stargazer. However, to date, the detailed distribution of PACAP immunoreactivity has never been investigated in the fish brain. In the present study, we have determined the localization of PACAP-immunoreactive neurons in the central nervous system of a primitive teleost fish, the European eel Anguilla anguilla, using an antiserum raised against PACAP27. PACAP-positive perikarya were exclusively observed in the diencephalon, i.e. in the preoptic nucleus of the hypothalamus and in the dorsal and ventral nuclei of the thalamus. PACAP-immunoreactive fibers were detected in various areas of the brain, notably in the ventral telencephalon, the diencephalon, the mesencephalon, the cerebellar valvula, and the medulla oblongata. In addition, a dense accumulation of PACAP-containing nerve terminals was found in the pars distalis of the pituitary. The PACAP-like immunoreactivity contained in the eel brain was characterized by HPLC analysis combined with RIA quantification. The major form of PACAP-immunoreactive material coeluted with mammalian PACAP38. Molecular cloning of the PACAP precursor has previously shown that in fish, PACAP and GH-releasing hormone (GHRH) originate from the same precursor. We have thus investigated the effects of PACAP and GHRH on GH secretion from eel pituitary cells in primary culture. Dose-response experiments revealed that PACAP27 and PACAP38 possessed the same efficacy, but PACAP38 was 12 times more potent than PACAP27 in stimulating GH release (ED50 = 4.3 x 10(-10) and 3.5 x 10(-9) M, respectively). In contrast, GHRH, even at a high concentration (10(-6) M), had no effect on GH release. Taken together, these data indicate that in the eel, PACAP may play a significant role in the regulation of somatotrope cells: 1) PACAP-immunoreactive neurons are exclusively located in the diencephalon and send numerous projections in the pars distalis; and 2) PACAP, but not GHRH, dose dependently stimulates GH secretion from cultured eel pituitary cells.

Identification and modulation of a growth hormone-binding protein in rainbow trout (Oncorhynchus mykiss) plasma during seawater adaptation

A soluble protein that specifically bound 125I-human growth hormone (hGH) was identified in rainbow trout plasma, using HPLC-gel filtration. The binding affinity of the protein for hGH was 1.2 x 10(9)M-1. 125I-rainbow trout GH (tGH) was also able to bind to the protein albeit with a lower affinity (6.6 x 10(7)M-1) than hGH. Crosslinking experiments using 125I-hGH revealed two specific bands of 150 and 130 kDa. The complex 125I-hGH-BP could be precipitated by a monoclonal anti-GH receptor antibody, suggesting a close relationship between the plasma GH-BP and the GH receptor. A fourfold increase in the hGH binding to the GH-BP was shown 48 h after transfer of the fishes from freshwater to seawater. The increase in binding was related to a high binding capacity without significant changes in binding affinity. These results suggest a potential role of this related GH-BP as an index of GH effects during seawater adaptation in salmonids.

Production of a biologically active novel goldfish growth hormone in Escherichia coli

Goldfish pituitary contains two types of growth hormones. One with five cysteine residues (type-I) similar to other Cyprinid GHs, and the other with four Cys residues (type-II) similar to those of other fish and tertapod species. Recombinant goldfish type II GH (gfGH-II) was produced in Escherichia coli using the pRSETB expression vector. The gfGH-II was produced fused to a leader sequence, which sequestered into inclusion bodies after expression. The inclusion bodies were solubilized using sodium hydroxide and the fusion protein purified by chelating affinity chromatography. Subsequently, gfGH-II was cleaved and analyzed by Western blotting, using a specific antiserum. For comparison we also produced recombinant common carp GH (cGH) which has 95% similarity to gfGH-II, and tested their growth promoting activity in goldfish. Both forms of GH significantly increased the growth rate of goldfish (P < 0.05), although cGH was found to have a somewhat higher potency than gfGH-II.

Insulin-like growth factor-I and environmental modulation of growth during smoltification of spring chinook salmon (Oncorhynchus tshawystscha)

The relations among rearing environment, fish size, insulin-like growth factor-I, and smoltification were examined in yearling spring chinook salmon (Oncorhynchus tshawytscha). Juvenile chinook salmon were size-graded into small and large categories. Half of the fish in each group were reared at an increased temperature and feeding rate beginning in mid-February, resulting in four distinct treatment groups: large warm-water (LW), large cool-water (LC), small warm-water (SW), and small cool-water (SC). Increased temperature and feeding rate resulted in overall higher growth rates for the LW and SW groups. Temporal increases in insulin-like growth factor-I (IGF-I) were found in all groups through the spring. Plasma IGF-I levels were significantly higher in warm-water groups than in cool-water groups from late March through May. Size itself appeared to have little relation to plasma IGF-I levels. Simple regression showed a significant relation between plasma IGF-I and growth (P < 0. 001, R2 = 0.50). No differences were found between treatment groups in other physiological parameters assessed (plasma thyroxine, gill Na+-K+-ATPase, liver glycogen, body lipid).

Somatotropic actions of the homologous growth hormone and prolactins in the euryhaline teleost, the tilapia, Oreochromis mossambicus

It is increasingly clear that growth hormone (GH) has growth-promoting effects in fishes, which are mediated in part by the insulin-like growth factor (IGF)-I. Growth-promoting actions of prolactin (PRL) have been reported in higher vertebrates, but are less well established in teleosts. We examined the effects of injecting homologous GH or the two homologous tilapia PRLs (tPRL177 and tPRL188) on the in vitro incorporation of [35S] sulfate (extracellular matrix synthesis) and [3H]thymidine (DNA synthesis) by ceratobranchial cartilage explants and on IGF-I mRNA levels in tilapia liver. Tilapia GH (tGH) and tPRL177 stimulated sulfate uptake at the highest doses examined. Thymidine incorporation was stimulated by tPRL177. tPRL188 was without these effects. Consistent with its somatotropic actions, tGH elevated IGF-I mRNA levels in the liver. tPRL177 also elevated liver IGF-I levels. Consistent with the previously described osmoregulatory actions of GH and PRL in teleosts, we observed that tGH elevated and tPRL177 and tPRL188 lowered levels of gill Na+,K+-ATPase activity. High-affinity, low-capacity binding sites for tGH in the tilapia liver were identified. tPRL177 binds with lower affinity than tGH to these sites but can displace 125I-labeled tGH from its receptor. The ability of tPRL177 to displace tGH was similar to that of ovine GH. tPRL188 did not displace 125I-labeled tGH binding. Collectively, this work suggests that tPRL177 may possess somatotropic actions similar to tGH, but only in freshwater tilapia where tPRL177 levels are sufficiently high for it to act as a competitive ligand for GH receptors.

The role of growth hormone in the adaptability of Atlantic salmon (Salmo salar L.) to seawater: effects on the morphology of the mucosa of the middle intestine

Ultrastructural modifications of the middle intestine of the salmon, Salmo salar, induced by transfer to seawater have been studied in two groups of fish: the first group received sham treatment and the second was treated with ovine growth hormone (oGH). In sham-treated fish during the first 2 days in seawater, significant distension of the intercellular spaces was observed between the apical tight junction and the basement membrane. In the basal part of the enterocytes, tubular invaginations in the intercellular spaces were closely associated with mitochondria. In oGH-implanted fish, we observed no signs of modification of the ultrastructure of the mucosa. There were no dilatations of the intercellular spaces and no infoldings in the basal part of the enterocytes. After 7 days in seawater, the mucosa of the intestine of sham- and oGH-treated fish was quite similar. The effects of oGH treatment were clear, and treatment seemed to provoke "pre-adaptation" of the intestinal mucosa before exposure to high salinity to maintain the morphology of the middle intestine of Atlantic salmon abruptly transferred to seawater.

Gill epithelial cells kinetics in a freshwater teleost, Oncorhynchus mykiss during adaptation to ion-poor water and hormonal treatments

The aim of this work was to determine the kinetics of the dramatic development of the gill chloride cells (CCs) during adaptation of the salmonid Oncorhynchus mykiss to an ion-poor environment.To monitor cell division, the incorporation in the mitotic cell DNA of bromo-deoxyuridine (BrdUrd) was visualized with a monoclonal antibody. The density of labelled nuclei was used as an index of cellular division (proliferation), concomitantly with morphometry of phenotypic changes monitored with SEM.In the filament epithelium, a phase of CC differentiation occurred within 12h after the transfer, followed by a delayed phase of cell proliferation (48h). In the lamellar epithelium, the present study demonstrates the absence of cell proliferation after ion-poor water transfer. The conclusion is that proliferation (mitosis) is important in the primary filament whereas differentiation and migration (from the filament) is the main mechanism for the appearance of CCs on the secondary lamellae.The present study suggests that cortisol promoted differentiation, but not division, of cells. CCs, presumably premature, were stained by anti-cortisol monoclonal antibody indicating the presence of cortisol. No mature CCs were stained.Growth hormone (oGH, ratGH) increased the rate of cell division both in lamellar and filament epithelium.

Hormonal replacement therapy in fish:human growth hormone gene function in hypophysectomized carp

Transgenic common carp,Cyprinus carpio, produced by the microinjection of fertilized eggs with a linearized chimeric plasmid pMThGH, a human growth hormone (hGH) gene with a mouse metallothionein-1 (MT) gene promoter in pBR322, were used to produce F1 and F2 transgenics. Following hypophysectomy of the transgenic F2 common carp, non-transgenic common carp and non-transgenic crucian carp, growth was monitored for up to 110 days. In addition, recombinant hGH was injected subcutaenously into a group of the non-transgenic crucian carp. Growth rate analyses indicated that (1) hypophysectomy of non-transgenic common carp and crucian carp results in the cessation of growth, (2) hGH administration can stimulate the growth of hypophysectomized crucian carp and (3) hypophysectomized hGH-transgenic common carp continue to grow in the absence of their own growth hormone, suggesting that the hGH-transgene is being expressed in tissues other than the pituitary.

Osmoregulatory actions of growth hormone and its mode of action in salmonids: A review

Osmoregulatory actions of growth hormone (GH) and its mode of action in salmonids are reviewed. We present evidence suggesting that insulin-like growth factor I (IGF-I) mediates some of the actions of GH on seawater acclimation. Plasma concentration and turnover of GH rise following exposure to seawater. Exogenous GH (in vivo) increases gill Na(+),K(+)-ATPase activity and the number of gill chloride cells, and inhibits an increase in plasma osmolarity and ions following transfer of fish to seawater. A single class of high affinity GH receptors is present in the liver, gill, intestine, and kidney. The levels of IGF-I mRNA in the liver, gill and kidney increased after GH-injection. After transfer to seawater, IGF-I mRNA increased in the gill and kidney following the rise in plasma GH, although no significant change was seen in the liver. Injection of IGF-I improved the ability of the fish to maintain plasma sodium levels after transfer to seawater. GH treatment also sensitizes the interrenal to adrenocorticotropin (ACTH), increasing cortisol secretion. Both cortisol and IGF-I may be involved in mediating the action of GH in seawater adaptation, although studies on the effect of GH on osmoregulatory physiology of non-salmonid species are limited. An integrated model of the osmoregulatory actions of GH is presented, and areas in need of research are outlined.

Sequence analysis suggests a recent duplication of the growth hormone-encoding gene in Tilapia nilotica

The sequence of two growth hormone(GH)-encoding genes from tilapia fish (Tilapia nilotica) is reported. Our data indicate that the presence of two GH in the tilapia genome is a consequence of a relatively recent duplication event. The two genes are highly homologous, having a similar intron (five)/exon (six) arrangement, and both encode an identical polypeptide. Sequence similarity extends up to bp -628 upstream to the transcription start point, after which the sequences of the two genes are not related to each other. The presence of two GH in the tilapia genome is supported both by the nucleotide sequence and by genomic DNA blot hybridization analysis. Tilapias, like salmonids, contain an extra intron compared with the mammalian GH structure. We suggest that within the superorder Teleostei, the insertion of intron 5 into GH took place after the evolutionary separation of Cyprinoidea, but before Isospondyli (salmonids) and Acanthopterygii (tilapias) were separated. Thus, the additional intron which is probably present in many teleost fish GH may provide an excellent natural marker for evolution and classification studies.

Expression of insulin-like growth factor I gene in osmoregulatory organs during seawater adaptation of the salmonid fish: possible mode of osmoregulatory action of growth hormone

Growth hormone has been shown to contribute to seawater adaptation of salmonid fishes. The growth influence of growth hormone is mediated largely by hepatic production of insulin-like growth factor I (IGF-I). To study the growth hormone-IGF-I axis in osmoregulation, we measured IGF-I mRNA in the liver, gill, and body kidney from rainbow trout by Northern analysis. The levels of IGF-I mRNA in all tissues increased significantly after injection of growth hormone. Transfer of trout from fresh water to 80% seawater evoked an increase in plasma growth hormone after 1 day. IGF-I mRNA was not altered significantly in the liver, but it was increased in the gill and body kidney after 1 and 8 days, respectively. These observations indicate that the IGF-I gene is expressed differently among these organs during seawater adaptation. Growth hormone may stimulate hypoosmoregulatory ability by inducing local expression of IGF-I in osmoregulatory organs, although the possibility that IGF-I expression might occur in part independently of growth hormone during seawater adaptation cannot be excluded.

Primary-structural and evolutionary analyses of the growth-hormone gene from grass carp (Ctenopharyngodon idellus)

The growth-hormone (GH) gene of grass carp, one of the fastest-growing species of farmed fish, was isolated and the DNA sequenced. Only one GH gene is found in this species. This gene, which is 2.5 kb in length, has five exons and four introns, in common with all of the mammalian and the recently published common-carp GH genes. In the course of vertebrate evolution, the total lengths of the intron and the non-coding region of exon 5 of the GH gene have been shortened by 40-70%, whereas the encoding exons of the gene have been slightly increased. The more closely related species exhibit the closest sequence similarity in their GH genes. For example, the similarity of the exons is 84.1-93.2% between grass carp and common carp (within the same family of Syprinedae), 43.5-82.1% between grass carp and rainbow trout (in different orders of Teleostei) and 45.8-58.6% between grass carp and rat (in different grades of Vertebrata). In addition, similar DNA domains, such as thyroid-hormone-receptor-complex-binding site and cell-type-specific cis elements involved in regulation of expression of rat and human GH genes, have been localized in the corresponding regions of the grass-carp GH gene.

The complete amino acid sequence of growth hormone from sturgeon (Acipencer guldenstadti)

The complete amino acid sequence of growth hormone (GH) from a chondrostean species, the sturgeon (Acipencer gludenstaditi), has been determined. Two variants of GH, termed GH I and GH II, were isolated from the pituitary by alkaline extraction, gel filtration on a Sephadex G-100 column, and reversed-phase high-performance liquid chromatography (rpHPLC) on a TSK gel ODS-120T column. The purified proteins were confirmed to be GHs by immunoblotting using bovine and chum salmon GH antisera. For determining of the primary structures, these GHs were digested with lysyl endopeptidase and cleaved with cyanogen bromide. The resulting fragments were separated by rpHPLC and subjected to sequence analysis on an automated gas-phase sequencer employing an Edman method. Both GHs consist of 190 amino acid residues, and contain two disulfide linkages at positions 52-163 and 180-188. The GHs differ from each other at only three positions. Sequence comparison with GHs from other vertebrates revealed that sturgeon GHs have greater sequence homology with tetrapod GHs (63-76%) than with teleost GHs (42-63%).

Growth hormone heterogeneity in American plaice pituitaries: isolation, characterization, and partial amino acid sequence

Growth hormones (GHs) have been isolated from pituitary glands of American plaice (Hippoglossoides platessoides), a marine flatfish, using affinity and gel filtration chromatography, followed by preparative polyacrylamide gel electrophoresis (PAGE). A bioassay based on serum triiodothyronine elevation in immature rainbow trout was used to monitor biological activity. These GHs originate from two molecular mass regions, 42K and less than 33K relative molecular mass (Mr), in their native state. The 42K Mr region yielded two forms of GH, which differ in terms of quantity and net charge as evidenced by native PAGE, a major variant with a relative mobility of (Rf) 0.22 and a lesser variant with Rf 0.28. The less than 33 Mr region has a single GH species with Rf 0.22. Upon sodium dodecyl sulfate-PAGE, without reduction, both GH variants from the 42K Mr region gave Mrs of 21K, while the GH from the less than 33K Mr region was 20K Mr, typical of monomeric vertebrate GHs. The proteins composing the 42K Mr region are proposed as GH dimers since they yield 21K Mr peptides. The less than 33K Mr region contains a GH monomer (20K Mr) in its native state. An amino-terminal amino acid sequence, identical for both the 42K and the 20K Mr Rf 0.22 forms, has good homology with other complete fish GH sequences near their carboxyl-terminal regions (between amino acids 130 and 196). The GH dimers (42K Mr) predominate in the plaice pituitary, contributing 93% of the total, of which 86% gives rise to the Rf 0.22 variant.

Development and validation of a highly sensitive radioimmunoassay for chinook salmon (Oncorhynchus tshawytscha) growth hormone

This study describes the development of a highly specific and very sensitive radioimmunoassay for salmonid growth hormone. Antiserum raised against chinook (Oncorhynchus tshawytscha) GH2, which did not recognize 125I-sPRL and 125I-sGTH (at 1:1000 initial dilution), was able to inhibit growth when injected into rainbow trout (Oncorhynchus mykiss). 125I-sGH2, used as tracer, was not recognized by anti-sGTH or by anti-sPRL. Mammalian GH and ACTH and salmonid GTH, TSH, and PRL did not cross-react in the sGH assay. The inhibition curves for pituitary extracts and plasma from salmonids were parallel to the salmon GH standard, whereas those from carp, tilapia, and catfish showed no significant cross reactivity. The RIA ED90 and ED50 values were 0.2 and 1.5 ng/ml, respectively. Using this RIA for measuring GH release by cultured pituitary cell we observed a strong inhibiting effect of SRIF (10(-6) M) and a stimulatory effect of hGRF (10(-6) M). This RIA allowed us also to detect daily fluctuations in the plasma GH concentration in cannulated rainbow trout.

Amino acid sequence of growth hormone isolated from medium of incubated pituitary glands of tilapia (Oreochromis mossambicus)

The amino acid sequence of tilapia (Oreochromis mossambicus) growth hormone (GH) was determined directly by Edman degradation of peptide fragments generated by lysyl endopeptidase and trypsin digestion. The N-terminal residue was deduced to be pyroglutamic acid through the use of pyroglutamyl aminopeptidase; its removal allowed amino acid sequence determination of the remainder of the N-terminal trypsin peptide by Edman degradation. Tilapia GH is composed of 187 amino acid residues and shows high similarity to other perciform GHs. Sequence identities are: 89% with tuna GH, 83% with bonito GH, 85% with yellowtail GH, 89% with red sea bream GH, and 34% with bovine GH. The two asparagine residues (Asn-148 and Asn-184) were recovered by Edman degradation, suggesting the absence of N-glycosylation.

Presence of specific growth hormone binding sites in rainbow trout (Oncorhynchus mykiss) tissues: characterization of the hepatic receptor

The present work outlines the presence of specific binding for chinook salmon growth hormone (sGH) in different tissue preparations of rainbow trout. Optimal incubation conditions (pH, Tris, MgCl2) were determined. Specific binding was very sensitive to salt concentration during incubation. The specific binding reached a plateau after 15 and 25 hr of incubation at 12 and 4 degrees. At 20 degrees, specific and nonspecific binding were not stable. Specific binding dissociation was slower than association and was only partial. The binding was saturable (Bmax = 187 +/- 167 pmol), of high affinity (Ka = 2.4 +/- 0.8 10(9) M-1), and very specific for GH, properties which are in agreement with the characteristics of hormonal receptors. Sea bream and mammalian GH appeared 2- and 30-fold, respectively, less potent than cold sGH2 for displacing 125I-sGH2. Tissue preparations from ovary, testis, fat, skin, cartilage, gill, blood pellet, brain, spleen, kidney, and muscle showed significant saturable binding.

Radioreceptor assay for growth hormone in coho salmon (Oncorhynchus kisutch) and its application to the study of stunting

Binding sites for native chum salmon growth hormone (sGH) in coho salmon (Oncorhynchus kisutch) hepatic membranes were analyzed by radioreceptor assay. Displaceable (specific) binding represented up to 25% of total radiolabeled sGH added. Binding was dependent on buffer pH and membrane protein concentration, and was complete by 24 hours at 15 degrees C. Specific binding was greatest in liver membranes, and was also detected in muscle, ovary, gill, kidney, and brain. Scatchard analyses indicated a single class of hepatic binding sites that were specific for sGH. In stunts, abnormal seawater salmon with elevated plasma GH levels and inhibited growth, specific binding of sGH to liver membranes was three times lower than in normal seawater smolts. The concentration of salmon GH binding sites was decreased in stunt livers by 60%, while their affinity for sGH was unchanged. Down-regulation of hepatic GH receptors by high plasma GH levels may explain in part the low sGH binding in stunts.

Plasma growth hormone levels increase during seawater exposure of sexually mature Atlantic salmon parr (Salmo salar L.)

At the time of smoltification in May, smolts and sexually mature male parr were transferred to seawater (25% salinity) and sampled after 6 and 24 hr. Plasma levels of growth hormone (GH) were measured by radioimmunoassay. There was no difference in GH levels between smolts and mature parr in fresh water. GH levels did not change during exposure of smolts to seawater. In the mature male parr, plasma GH levels increased after 24 hr, when the levels were almost five times those of the freshwater controls. In the mature male parr, there was an increase in plasma osmolality, sodium, and magnesium after 24 hr in seawater; magnesium also increased after 6 hr. The levels of potassium and calcium did not change in either immature parr or mature male parr. The increase in plasma GH levels in the mature parr in seawater may be part of a mechanism to increase hypoosmoregulatory ability in fish not ready for seawater entry.

Stimulation of thyroid function by several pituitary hormones results in an increase in plasma thyroxine and reverse triiodothyronine in tilapia (Tilapia nilotica)

In this study, intravenous injection of several doses of porcine follicle stimulating hormone (pFSH: 0.002, 0.01, 0.05, and 0.5 micrograms/g body wt), bovine TSH (bTSH: 0.5 micrograms/g body wt), and ovine growth hormone (oGH: 0.04, 0.02, and 0.4 microgram/g body wt) stimulated an increase in plasma thyroxine (T4) and reverse triiodothyronine (rT3) in tilapia. This effect occurred in a dose-dependent manner. pFSH was the most potent in stimulating thyroid function. The dose of 0.002 microgram pFSH/g body wt increased plasma levels of T4 over control levels (2.59 +/- 0.16 ng/ml) about 2.5-fold within 4 hr, whereas a concentration of 0.5 micrograms/g body wt caused a great and prolonged increase of T4 and rT3 levels. Control levels (2.59 +/- 0.16 ng/ml for T4 and 40.37 +/- 8.60 pg/ml for rT3) were increased 19- and 22-fold respectively, over 24 hr. An increase of T4 and rT3 levels occurred also after injection of total hypophyseal extract and Con A II glycoprotein fraction of a tilapia pituitary homogenate, whereas the protein fraction failed to alter plasma concentrations of T4 and rT3. rT3 levels were also significantly increased at 2 hr, but not at 1 hr, after injection of T4. Basal T3 levels (1.90 +/- 0.22 ng/ml) were reduced by half over 24 hr in all experiments. These results suggest the existence, in tilapia, of a 5-D pathway deiodination of T4 which is pituitary independent. Stimulation of T4 release is always followed by an increase in plasma rT3 levels.

The role of cortisol and growth hormone in seawater adaptation and development of hypoosmoregulatory mechanisms in sea trout parr (Salmo trutta trutta)

The role of growth hormone (GH) and cortisol in the development of hypoosmoregulatory mechanisms in sea trout parr, Salmo trutta trutta, was investigated by injecting freshwater (FW) yearlings every second day with saline, ovine growth hormone (oGH, 2.0 micrograms/g), cortisol (hydrocortisone hemisuccinate, 8.0 micrograms/g), or oGH + cortisol for a maximum of 14 days. Subgroups of the treated fish were transferred to three-fourths seawater (SW) after 7 or 15 days of treatment and the effects on plasma Na+, Cl-, muscle water content, gill Na+/K(+)-ATPase activity, and gill interlamellar chloride cell density were examined. In FW, gill Na+/K(+)-ATPase chloride cell density, and chloride cell apical to basal length increased by all hormone treatments, most significant by oGH + cortisol treatment. Plasma ions and muscle water content were unaffected in FW. Both SW transfers resulted in considerable mortality (50%) in control fish, whereas few cortisol-treated and no GH-treated or GH + cortisol-treated fish died. Plasma Na+ and Cl- levels increased dramatically (greater than 50%) in control fish and muscle water content decreased (8%) on Day 2 after both transfers. All hormone-treated groups regulated plasma ions and muscle water significantly better than controls in SW, indicating the physiological significance of the treatment. Notably, the oGH + cortisol-treated fish showed only insignificant changes in ion-osmotic homeostasis after SW transfer, suggesting a synergistic effect of the two hormones. It is concluded that treatment with the two hormones increases the salinity tolerance of sea trout parr at a developmental stage where FW life is obligatory.

Enhanced hypoosmoregulatory response to growth hormone after cortisol treatment in immature rainbow trout, Salmo gairdneri

The growth-independent effect of ovine growth hormone (oGH) and oGH + cortisol treatment on seawater (SW) adaptation in immature rainbow trout, Salmo gairdneri was investigated. Fish were injected every second day with saline, 2.0 μg oGH/g or 2.0 μg oGH + 8.0 μg cortisol/g for a maximum of 8 injections in freshwater (FW). Subgroups were transferred to 28‰ SW after 4 or 8 injections, and changes in plasma Na(+) and Cl(-), muscle water content and gill Na(+)/K(+)-ATPase activity were measured. In both of the hormone-treated groups retained in FW, gill Na(+)/K(+)-ATPase activity and interlamellar chloride cell density increased. The effects were most pronounced in the oGH + cortisol group after 2 weeks of treatment. After transfer to SW most of the control fish died due to the osmotic stress, whereas in the hormone-treated groups, mortality was low and there was a positive correlation between pretransfer gill Na(+)/K(+)-ATPase and the ability to maintain ionic-osmotic homeostasis after SW transfer. After two weeks of oGH + cortisol treatment, gill Na(+)/K(+)-ATPase activity was maximal. In contrast, after SW transfer, Na(+)/K(+)-ATPase activity increased further in the oGH-treated group. This group regulated ionic-osmotic parameters less effectively than the oGH + cortisol-treated group. The data indicate that GH and cortisol are important hormones in the regulation of hypoosmoregulatory mechanisms in S. gairdneri.

Changes in plasma hormone levels during loss of hypoosmoregulatory capacity in mature chum salmon (Oncorhynchus keta) kept in seawater

Returning chum salmon (Oncorhynchus keta) in northern Honshu Island, Japan, complete gonadal maturation while in the bay. Mature fish caught in the bay failed to survive in seawater for more than a week, whereas they adapted to fresh water efficiently. Mortality in seawater seems to be due primarily to an increased plasma osmolality. Maladaptation to seawater was more pronounced in the fish caught deep in the bay than those caught outside the bay, and also greater in females than in males. In close correlation with the increased plasma osmolality and electrolyte concentrations, plasma levels of cortisol and growth hormone increased in the fish kept in seawater. Cortisol and growth hormone may be secreted in response to the increased plasma osmolality and would not be the direct cause of the maladaptation to seawater. Plasma prolactin remained low in the seawater fish, indicating that the increased secretion of prolactin, a freshwater-adapting hormone, is not the cause of maladaptation to seawater either. On the other hand, when the fish caught in the river were kept in fresh water, an increase in plasma prolactin concentrations was seen, particularly in females, whereas no significant change was seen in plasma cortisol and growth hormone. Concentrations of 17 alpha,20 beta-dihydroxy-4-pregnen-3-one in the female and of testosterone in both the male and female were extremely high in the bay fish and decreased slightly but significantly after 7 days in fresh water. The gonadal steroids may have inhibitory effects on osmoregulation in the mature salmon in seawater.

Growth hormone secretion during longterm incubation of the pituitary of the Japanese eel, Anguilla japonica

Growth hormone (GH) secretion from organ-cultured pituitaries of the eel (Anguilla japonica) was studied during incubation in a defined medium for 2 weeks, using a homologous radioimmunoassay which does not distinguish between the two molecular forms of eel GH. The total amount of GH secreted increased gradually during the incubation period; so that the amount of GH released on day 14 was about 30 times greater than that on day 1. On day 14, the proportion of GH released relative to the total amount of GH present (the sum of GH released into the medium and residual content in the pituitary) was 96% and the amount produced on day 14 was 4 times greater than the content in the unincubated pituitary. Somatostatin (SRIF, 1.8 × 10(-7) M) inhibited the increase in GH release. On day 7, the proportion of GH released by pituitaries treated with SRIF (28%) was less than that released by the control pituitary (91%). There was no significant difference in GH release between the pituitaries incubated in isotonic medium (300 mOsm) and those in hypotonic medium (240 mOsm) for 2 weeks except for the first 3 days, when the pituitaries in hypotonic medium secreted significantly greater amounts of GH than those incubated under isotonic condition. Hypertonic medium (350 mOsm) had no effect on GH release except for significant inhibition on days 6 and 14. When secretion of the two forms of GH (GH I and II) was examined after separation by polyacrylamide gel electrophoresis followed by densitometry, slightly more GH I tended to be secreted than GH II during the culture period, although the effects of SRIF and osmolality of the media on GH I release were similar to those on GH II. It is concluded that GH secretion and production in the eel is mainly under the inhibitory control of hypothalamus, and that osmolality has a minimum influence on the GH release.

Fish as model systems

Fish represent the largest and most diverse group of vertebrates. Their evolutionary position relative to other vertebrates and their ability to adapt to a wide variety of environments make them ideal for studying both organismic and molecular evolution. A number of other characteristics make them excellent experimental models for studies in embryology, neurobiology, endocrinology, environmental biology, and other areas. In fact, they have played a critical role in the development of several of these disciplines. Research techniques that enable scientists to make isogenic lines in a single generation, create and maintain mutants, culture cells, and transfer cloned genes into embryos signal an increasing role for fish as experimental models.

Smoltification and seawater adaptation in coho salmon (Oncorhynchus kisutch): plasma prolactin, growth hormone, thyroid hormones, and cortisol

The status of circulating growth hormone and prolactin during the parr-smolt transformation and during seawater adaptation of coho salmon (Oncorhynchus kisutch) was investigated in relation to changes in plasma levels of thyroxine, triiodothyronine, and cortisol, and in hypoosmoregulatory ability. Sampling (biweekly or monthly) occurred between early February and October. When peak hypoosmoregulatory ability was achieved (mid-April), one group of fish was acclimated to seawater over a period of 18 hr and was sampled 1, 3, and 7 days after the introduction of fish to seawater and biweekly thereafter. Plasma prolactin levels rose steadily from the first sampling date to a peak of 15 ng/ml in early April, declined rapidly, and remained low until June when a second increase occurred. Prolactin declined to 2 ng/ml within 1 day of the beginning of seawater adaptation. Growth hormone increased twofold from February to late March, and achieved plateau levels of 20 ng/ml in the period from mid-April to July and then gradually declined to 10 ng/ml in September and October. Plasma levels of growth hormone in seawater-acclimated fish were similar to those of freshwater coho, but with larger fluctuations; no increase was apparent during the first week of seawater acclimation. Plasma cortisol and plasma triiodothyronine increased at the same time as plasma growth hormone; increases in plasma thyroxine occurred later. In general, both growth hormone and cortisol levels were elevated when hypoosmoregulatory ability was high. Conversely, prolactin levels generally showed a negative relationship with hypoosmoregulatory ability.

Survival of salmonids in seawater and the time-frame of growth hormone action

In salmonids, growth hormone (GH) effectively promotes adaptation of freshwater (FW) fish to seawater (SW), but it has been unclear whether GH has osmoregulatory actions apart from those consequent to an increase in body size. Our objectives were first, to examine the minimum time and dose required for GH to enhance SW adaptation; and second, to optimize the conditions for the acute GH response in developing a convenient GH bioassay based on its plasma ion lowering effect. Trout showed markedly improved SW survival when transferred from fresh water 6, 24, or 48h after a single chum salmon GH injection (0.25 μg/g). Preadapting trout to 1/3 SW enhanced the plasma ion lowering effect of ovine GH (oGH) injected 48h before transfer of the fish to 80% SW. Endogenous plasma GH levels were elevated in control trout switched from low salinities to 80% SW but were depressed in oGH-injected fish after transfer. Under optimal test conditions (1/3 SW preadaptation, 48h pre-transfer injection, and 100% SW final challenge), the reduction in plasma Na(+), Ca(++), and Mg(++) levels of oGH-injected fish was dose-dependent. The oGH doses giving minimum and maximum responses were 50 and 200 ng/g, respectively. In short, GH exerts acute osmoregulatory actions that promote SW adaptation in the absence of changes in body size. Compared with growth GH bioassays, the osmoregulatory assay is superior in economy of time, animal costs, and hormone quantity required and potentially in specificity.

Tilapia growth hormone: molecular cloning of cDNA and expression in Escherichia coli

A cDNA library was prepared from poly(A)+RNA extracted from tilapia Oreochromis niloticus anterior pituitaries. The recombinant clones carrying the cDNA sequence of tilapia growth hormone (tiGH) were selected using a fragment of the trout growth hormone (tGH) cDNA as hybridization probe. The nucleotide sequence of the full-length tiGH cDNA was determined. This cDNA encodes a protein of 204 amino acids, including the putative signal peptide of 17 amino acids. Mature tiGH cDNA was inserted in an Escherichia coli expression vector which led to the production of tiGH protein with a yield estimated to be 20% of the total bacterial proteins.

Molecular cloning and characterization of two forms of trout growth hormone cDNA: expression and secretion of tGH-II by Escherichia coli

We constructed a cDNA library using mRNA isolated from rainbow trout pituitaries. Two types of cDNA clones encoding growth hormone (GH) were isolated and their complete nucleotide sequences determined. Twenty seven nucleotide substitutions in the coding region and 108 in the noncoding region distinguish the cDNAs of trout GH-I and II. Both cDNAs encode polypeptides of 210 amino acids, including a putative signal peptide of 22 amino acids, which differ by 12 residues. In both trout and salmon, GH-I mRNA is predominant, which suggests that the variation in the amount of secreted GH originates from a transcriptional event. Moreover, comparison of rainbow trout and chum salmon GH reveals that, in both cases, the predominant GH-I has mutated less than its GH-II counterpart. Mature tGH-II was expressed in Escherichia coli using the pIN-III-ompA-Hind secretion vector.

Effects of vertebrate prolactins and growth hormones on thyroxine 5'-monodeiodination in the eel (Anguilla anguilla): a potential bioassay for growth hormone

Growth hormones (GHs) and prolactins (Prls) purified from representatives of each vertebrate class from bony fish onwards were tested for their ability to stimulate in vivo peripheral deiodination of labeled thyroxine (T4*) into triiodo-L-thyronine (T3*) in the eel. Plasma T3*/T4* ratio was used as parameter. All GHs significantly increased T3*/T4*, the magnitude of the response being unrelated to the phylogenic position of species. No significant stimulation was shown with the various Prl, with the exception of ovine Prl, suggesting a heterosomatotropic effect of this preparation in the eel. Furthermore, both tilapia and ovine GH produced a dose-related effect on plasma T3*, T4*, and T3*/T4*. The stimulation of the peripheral deiodination of T4* into T3* estimated in vivo in the eel could become a specific, sensitive, and rapid fish bioassay for GH.

Enhanced response of the interrenal of coho salmon (Oncorhynchus kisutch) to ACTH after growth hormone treatment in vivo and in vitro

The effects of ovine growth hormone (GH) on the response of the interrenal of yearling coho salmon (Oncorhynchus kisutch) to ACTH were examined in two series of experiments. In the first series, GH was administered intraperitoneally (2 micrograms/g body wt, three daily injections) to coho salmon which had not undergone the parr-smolt transformation. Animals were sacrificed 30 hr after the final injection and head kidney tissue was prepared for in vitro incubation. GH had no effect on plasma cortisol levels but tissue taken from treated fish had an enhanced sensitivity to ACTH in vitro and its steroidogenic capacity was increased. In the second series of experiments, interrenal tissue was preincubated with GH in the dose range of 10-1000 ng/ml and was then challenged with ACTH or was incubated with 1-10 microM pregnenolone. GH-pretreated tissue displayed an enhanced response to ACTH. Cortisol production by tissue pretreated with 500 ng/ml GH was significantly enhanced after incubation with pregnenolone. No significant effect of GH was found after simultaneous exposure of interrenal tissue to combinations of ACTH and GH for 3 hr. Pretreatment of interrenal tissue with salmon prolactin had no significant effect on the subsequent response of tissue to ACTH. These results indicate that GH regulates interrenal activity in coho salmon and that one probable site of action is on the biosynthetic pathway between pregnenolone and cortisol.

Isolation and characterization of carp prolactin

Prolactin (PRL) was extracted with acid-acetone from common carp (Cyprinus carpio) pituitary glands and purified by gel filtration on Sephadex G-75, ion-exchange chromatography on DEAE-cellulose, and reversed-phase high-performance liquid chromatography (HPLC) on TSK-gel TMS 250 with a yield of 0.7 mg/g wet tissue. At each stage of purification, fractions were monitored by HPLC on TSK-gel ODS 120T and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Carp PRL was almost equipotent with ovine PRL in retaining plasma Na concentrations in the hypophysectomized killifish, Fundulus heteroclitus. Immunocytochemistry at both the light and electron microscopic levels revealed that carp PRL antiserum specifically stained cells in the goldfish rostral pars distalis. No cross reaction with putative growth hormone (GH) cells in the proximal pars distalis was observed. The specificity of the carp PRL antiserum was confirmed by immunoblot studies. Although immunostaining of both carp and salmon PRL was observed, there was no cross reaction to GHs from these species. Carp PRL had a sole N-terminal residue of valine, a molecular weight of 23 kDa in SDS-PAGE, and an isoelectric point of 7.3 by gel electrofocusing. Based on these results, together with the knowledge of physicochemical properties of salmon and tilapia PRLs, we propose a standard procedure for isolation of fish PRLs.