Development and validation of a salmon prolactin radioimmunoassay

Na+/HCO3- cotransporter 1 (nbce1) isoform gene expression during smoltification and seawater acclimation of Atlantic salmon

The life history of Atlantic salmon (Salmo salar) includes an initial freshwater phase (parr) that precedes a springtime migration to marine environments as smolts. The development of osmoregulatory systems that will ultimately support the survival of juveniles upon entry into marine habitats is a key aspect of smoltification. While the acquisition of seawater tolerance in all euryhaline species demands the concerted activity of specific ion pumps, transporters, and channels, the contributions of Na⁺/HCO₃^- cotransporter 1 (Nbce1) to salinity acclimation remain unresolved. Here, we investigated the branchial and intestinal expression of three Na⁺/HCO₃^- cotransporter 1 isoforms, denoted nbce1.1, -1.2a, and -1.2b. Given the proposed role of Nbce1 in supporting the absorption of environmental Na⁺ by ionocytes, we first hypothesized that expression of a branchial nbce1 transcript (nbce1.2a) would be attenuated in salmon undergoing smoltification and following seawater exposure. In two separate years, we observed spring increases in branchial Na⁺/K⁺-ATPase activity, Na⁺/K⁺/2Cl^- cotransporter 1, and cystic fibrosis transmembrane regulator 1 expression characteristic of smoltification, whereas there were no attendant changes in nbce1.2a expression. Nonetheless, branchial nbce1.2a levels were reduced in parr and smolts within 2 days of seawater exposure. In the intestine, gene transcript abundance for nbce1.1 increased from spring to summer in the anterior intestine, but not in the posterior intestine or pyloric caeca, and nbce1.1 and -1.2b expression in the intestine showed season-dependent transcriptional regulation by seawater exposure. Collectively, our data indicate that tissue-specific modulation of all three nbce1 isoforms underlies adaptive responses to seawater.

Changes in PRL Gene Expression During Upstream Movement of the Japanese Eel, Anguilla japonica

Changes in mRNA levels of prolactin (PRL) during upstream movement were examined in juvenile Japanese eels, Anguilla japonica. Glass eels and elvers were collected from 2007 to 2009 near the entrance of Hamana Lake, and in a small inflowing stream, the Egawa River. Quantification of mRNA was performed by real-time PCR and expressed as whole-body content. PRL mRNA levels of glass eels caught in the coastal zone and tidal area were low. Eels that moved downward in the tidal zone and migrated upstream to enter into freshwater showed increased levels of PRL mRNA. These changes suggest the importance of up-regulation of PRL gene expression in juvenile eels during their upstream movement from seawater to fresh water, particularly in relation to hyperosmoregulation.

Prolactin and upstream migration of the amphidromous teleost, ayu Plecoglossus altivelis

Changes in mRNA levels of prolactin (PRL) during the upstream migration were examined in fry of the amphidromous fish, ayu Plecoglossus altivelis. Quantification of mRNA has been done with real-time PCR and expressed as whole body or pituitary contents depending the body size of fry. PRL mRNA levels of ayu caught in seawater of the coastal area remained low during early spring. Prior to the start of the upstream migration, the fish caught in the coastal area in mid spring showed increased levels of PRL mRNA. There were further increases in PRL levels in the fish caught in the river. Analysis of proportions revealed that there were significant differences among PRL mRNA in the fish caught in different environmental salinities. Body weight showed a positive relation with PRL mRNA in ayu caught in seawater. A landlocked population of ayu, which migrates from lake to river, showed no significant change in PRL mRNA levels before and after upstream migration. Results in this study indicate the importance of up-regulation of PRL gene expression of ayu during the upstream migration from seawater to fresh water. There is a possible relationship between body size and PRL in the early developmental stage of ayu in seawater, but not in the fish in fresh water.

Physiological, molecular, and cellular mechanisms of impaired seawater tolerance following exposure of Atlantic salmon, Salmo salar, smolts to acid and aluminum

We examined the physiological, molecular, and cellular mechanisms of impaired ion regulation in Atlantic salmon, Salmo salar, smolts following acute acid and aluminum (Al) exposure. Smolts were exposed to: control (pH 6.5, 3.4 micrpg l(-1) Al), acid and low Al (LAl: pH 5.4, 11 microg l(-1) Al), acid and moderate Al (MAl: pH 5.3, 42 microg l(-1) Al), and acid and high Al (HAl: pH 5.4, 56 microg l(-1) Al) for two and six days. At each time-point, smolts were sampled directly from freshwater treatment tanks and after a 24h seawater challenge. Exposure to acid/MAl and acid/HAl led to accumulation of gill Al, substantial alterations in gill morphology, reduced gill Na(+)/K(+)-ATPase (NKA) activity, and impaired ion regulation in both freshwater and seawater. Exposure to acid/MAl for six days also led to a decrease in gill mRNA expression of the apical Cl(-) channel (cystic fibrosis transmembrane conductance regulator I), increased apoptosis upon seawater exposure, an increase in the surface expression of mitochondria-rich cells (MRCs) within the filament epithelium of the gill, but reduced abundance of gill NKA-positive MRCs. By contrast, smolts exposed to acid and the lowest Al concentration exhibited minor gill Al accumulation, slight morphological modifications in the gill, and impaired seawater tolerance in the absence of a detectable effect on freshwater ion regulation. These impacts were accompanied by decreased cell proliferation, a slight increase in the surface expression of MRCs within the filament epithelium, but no impact on gill apoptosis or total MRC abundance was observed. However, MRCs in the gills of smolts exposed to acid/LAl exhibited morphological alterations including decreased size, staining intensity, and shape factor. We demonstrate that the seawater tolerance of Atlantic salmon smolts is extremely sensitive to acute exposure to acid and low levels of Al, and that the mechanisms underlying this depend on the time-course and severity of Al exposure. We propose that when smolts are exposed to acid and moderate to high Al concentrations, impaired seawater tolerance results from extensive gill Al accumulation, damage to the epithelium, reduced MRC and transport protein abundance, and a synergistic stimulation of apoptosis in the gill upon seawater exposure. When smolts are exposed to acid and low levels of Al, loss of seawater tolerance appears to be independent of these mechanisms and may result instead from a shift in the phenotype of MRCs present in the gill epithelium.

Peptide purification, complementary deoxyribonucleic acid (DNA) and genomic DNA cloning, and functional characterization of ghrelin in rainbow trout

We have identified ghrelin from the stomach of rainbow trout. Four isoforms of ghrelin peptide were isolated: the C-terminal amidated type of rainbow trout ghrelin (rt ghrelin) composed of 24 amino acids (GSSFLSPSQKPQVRQGKGKPPRV-amide) is a basic form; des-VRQ-rt ghrelin, which deleted three amino acids (V13R14Q15) from rt ghrelin; and further two types of rt ghrelin that retained the glycine residue at the C terminus, rt ghrelin-Gly, and des-VRQ-rt ghrelin-Gly. The third serine residue was modified by octanoic acid, decanoic acid, or the unsaturated form of those fatty acids. In agreement with the isolated peptides, two cDNAs of different lengths were isolated. The rt ghrelin gene has five exons and four introns, and two different mRNA molecules are predicted to be produced by alternative splicing of the gene. A high level of ghrelin mRNA expression was detected in the stomach, and moderate levels were detected in the brain, hypothalamus, and intestinal tracts. Des-VRQ-rt ghrelin stimulated the release of GH in the rat in vivo. Furthermore, des-VRQ-rt ghrelin stimulated the release of GH, but not the release of prolactin and somatolactin in rainbow trout in vivo and in vitro. These results indicate that ghrelin is a novel GH secretagogue in rainbow trout that may affect somatic growth or osmoregulation through GH. Because ghrelin is expressed in various tissues other than stomach, it may play important role(s) in cellular function as a local regulator.

Structure and tissue distribution of prolactin receptor mRNA in Japanese flounder (Paralichtys olivaceus): conserved and preferential expression in osmoregulatory organs

In diadromous and euryhaline teleosts, it has been established that prolactin (PRL) is a major hormone regulating the maintenance of water and electrolyte homeostasis by acting on its receptor (PRLR) expressed in the osmoregulatory organs. To investigate the major physiological role of PRL in a marine teleost, cDNA for the Japanese flounder (Paralichtys olivaceus) prolactin receptor (fPRLR) has been cloned and characterized. The predicted fPRLR is composed of 636 amino acids conserving common structural features, such as the WSXWS motif and box 1, that are observed in the members of the cytokine receptor superfamily. By Northern blot analysis, 3.5-kb transcripts for fPRLR were clearly detected in the gill, kidney, and intestine. By RNase protection assay, similarly high levels of mRNA expression were detected in these osmoregulatory organs and lower expression levels were seen in the brain for both males and females. Interestingly, a distinct expression level of fPRLR mRNA was observed in the testis, but not in the ovary. The present results suggest that PRL may play an important role in the control of water and electrolyte balance through PRLR expressed in the osmoregulatory organs in the marine teleost the Japanese flounder as well as in other teleosts. Furthermore, PRL may differentially regulate gonadal functions in males and females of Japanese flounder.

Purification, characterization, and bioassay of prolactin and growth hormone from temperate basses, genus Morone

Prolactin (PRL) and two variants of growth hormone (GH), purified from pituitaries of striped bass (Morone saxatilis) and its hybrid with white bass (M. saxatilis x M. chrysops) by gel filtration chromatography under alkaline conditions followed by reversed-phase high pressure liquid chromatography, appear similar between species. Both the minor (GH I) and the major (GH II) forms of purified GH appeared as single bands (M(r) approximately 23,000) after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, as did the purified PRL (M(r) approximately 24,000). The molecular weights of GH II and PRL determined by MALDI TOF mass spectroscopy were 21.2 and 21.3 kDa, respectively. In Western blotting experiments, an antiserum against tilapia (Oreochromis mossambicus) 24K PRL specifically recognized Morone PRL, while an antiserum against tilapia GH specifically recognized Morone GH I and II. Chemical identities of the putative PRL and GH I were further confirmed by N-terminal peptide sequencing, while internal sequence analysis was performed on GH II because it was blocked at its N-terminus. Over a stretch of 29 amino acids, Morone PRL was found to be 76% identical to tilapia 24K PRL, 72% identical to tilapia 20K PRL, 72% identical to chum salmon (Oncorhynchus keta) PRL I, and 69% identical to eel (Anguilla japonica) PRL I. Alignment of the hybrid striped bass GH sequences with those of several other advanced marine teleosts indicated 75-85% sequence identity for GH I (40 amino acids) and 95-98% identity for GH II (45 amino acids). Biological activity of striped bass GH II was confirmed using a heterologous in vitro assay of insulin-like growth factor I mRNA production by coho salmon (On. kisutch) hepatocytes. An in vivo bioassay, involving hypophysectomy of hybrid striped bass and treatment of the fish maintained in fresh water with homologous PRL, confirmed that the purified striped bass PRL was also bioactive.

Effects of prolactin and growth hormone on plasma immunoglobulin M levels of hypophysectomized rainbow trout, Oncorhynchus mykiss

Immunoglobulin M (IgM) is a major component of the humoral immune system of teleosts. This study examines the effects of hypophysectomy and subsequent replacement with prolactin (PRL) or growth hormone (GH) upon the plasma IgM levels of the rainbow trout (Oncorhynchus mykiss). Plasma IgM levels of the hypophysectomized fish were decreased to 30% of those in sham-operated fish 1 or 4 weeks after operation. Implantation of a cholesterol pellet containing salmon PRL or GH restored plasma IgM levels of the hypophysectomized fish, suggesting important roles for PRL and GH in the regulation of circulating IgM level in trout.

Changes in plasma somatolactin levels during spawning migration of chum salmon (Oncorhynchus keta)

Plasma somatolactin (SL) concentrations were examined in chum salmon in relation to gonadal maturation; immature salmon in the Bering Sea at various stages of maturation, and mature salmon during upstream migration caught at the ocean, bay and river. Plasma SL concentrations as well as plasma prolactin (PRL) and growth hormone (GH) levels in the immature fish caught in the Bering Sea were maintained essentially at similar levels. Plasma SL in mature salmon increased significantly from the fish in the ocean to the fish in the river in both sexes. Although all the fish had fully developed gonads, females completed ovulation while still in the bay, whereas final spermeation in males was achieved after entry into the river. Thus, no clear correlation was seen between plasma SL levels and final gonadal maturation. On the other hand, plasma PRL concentrations in both male and female fish were higher in the fish in the river than those in the ocean and bay, and plasma GH levels were higher in both sexes in the fish in the bay and river than those in the ocean. Plasma levels of triglycerides, glucose, free fatty acids and ionized sodium and calcium were also examined. Significant-negative correlations were seen between plasma SL and plasma ionized calcium in mature male salmon, and between plasma SL and plasma triglycerides in mature female salmon. Although our findings do not rule out the possibility of the involvement of SL in final maturation, the results indicate that SL seems to be involved at least in energy and/or calcium metabolism during the spawning migration.

Osmoregulation in the stenohaline freshwater catfish, Heteropneustes fossilis (Bloch) in deionized water

Transfer of the stenohaline catfish, Heteropneustes fossilis from tap water (TW) to deionized water (DW) resulted in an increase in the glomerular filtration rate, urine volume and osmolar and free water clearance. In a closed system, where the DW was renewed only once a day, no change in the plasma osmolality was evident for up to 14 days. When DW was renewed four times a day for 25 days, a significant reduction in the plasma osmolality was observed within 24h. When the fish were transferred back to TW, plasma osmolality increased to normal freshwater level within 24h. These observations suggest the existence of highly efficient branchial mechanisms for active uptake of salts from an exceedingly dilute ambient medium. The fact that prolactin-secreting cells as well as corticotrophs in the pituitary of the fish in DW were highly stimulated suggests the involvement of the hormones in the adaptive responses of the catfish to DW.

The effects of confinement stress on circulating prolactin levels in rainbow trout (Oncorhynchus mykiss) in fresh water

Rainbow trout were confined for 48 hr, during which time water quality either was allowed to deteriorate (resulting in elevated NH3, elevated free CO2, and reduced dissolved O2) or was maintained at preconfinement levels. Fish were removed and blood samples taken at 0, 2, 4, 8, 24, and 48 hr after the onset of confinement from both stressed (confined) and unstressed (unconfined) fish. Plasma cortisol and plasma prolactin (PRL) levels were determined using specific RIAs. Chronic confinement of rainbow trout, accompanied by a decline in water quality, resulted in significant elevation of plasma cortisol, maintained for the period of confinement. Plasma PRL levels were significantly lower in stressed fish, by up to 60% relative to control fish, during the first 24 hr of confinement. The stress of confinement alone, in the absence of deterioration in water quality, produced similar results, with the change in prolactin levels being less rapid but more prolonged under these conditions.

Changes in growth hormone and prolactin messenger ribonucleic acid levels during seawater adaptation of amago salmon (Oncorhynchus rhodurus)

To examine the changes in secretion of growth hormone (GH) and prolactin (PRL) with reference to their osmoregulatory roles, changes in pituitary mRNA levels and plasma concentrations of these hormones were examined during seawater adaptation in silvery juveniles (smolts) and precociously mature males (dark parr) of amago salmon (Oncorhynchus rhodurus). Transfer to seawater increased plasma sodium levels in both smolts and dark parr. Smolts adjusted their plasma sodium to the level associated with seawater-adaptation (165 mEq/liter) within 3 days, whereas no adjustment was seen in dark parr; the latter failed to survive in seawater for more than 3 days. In smolts, plasma GH levels increased significantly 1 day after transfer, whereas there was no significant change in dark parr. An increase in GH mRNA levels was observed in smolts in association with increased plasma GH, whereas there was no change in dark parr. In contrast, a reduction in plasma PRL levels was consistently observed in both smolts and dark parr after transfer to seawater. However, there was no significant change in PRL mRNA levels in either smolts or dark parr. These results suggest that both gene expression and release of GH are activated by seawater transfer only in smolts with adequate seawater adaptability, whereas PRL gene expression is decreased after seawater transfer regardless of seawater adaptability.

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.

Kinetic studies of growth hormone and prolactin during adaptation of coho salmon, Oncorhynchus kisutch, to different salinities

The kinetics of growth hormone (GH) and prolactin (PRL) in coho salmon (Oncorhynchus kisutch) transferred from fresh water (FW) to seawater (SW) and vice versa were examined to help clarify the osmoregulatory roles of the two hormones during periods of migration to different salinities. Chum salmon GH or PRL was administered by a single injection intraarterially, and metabolic clearance rate (MCR) and secretion rate (SR) of injected hormones were calculated from the disappearance of the hormones from the plasma. When coho salmon smolts were acclimated to SW, MCR, SR, and plasma level of GH in SW-adapted (2-3 weeks) fish were twice as great as those in fish in FW. On the other hand, there was no difference in the kinetics of GH between the adult coho salmon in SW and those adapted to FW (2-3 weeks). The transfer of the adult coho salmon from SW to FW was followed after 2 days by a rise in plasma level and SR of PRL, which tended to stay at high levels after 2-3 weeks. The MCR of PRL increased significantly after 2-3 weeks in FW. These results support the likelihood of an important role of GH in SW adaptation and of PRL in FW adaptation in coho salmon.

Seasonal changes in seawater adaptability and plasma levels of prolactin and growth hormone in landlocked sockeye salmon (Oncorhynchus nerka) and amago salmon (O. rhodurus)

In order to clarify the roles of prolactin (PRL) and growth hormone (GH) in the development of seawater adaptability in salmonids, seasonal changes in salinity tolerance, plasma PRL, and plasma GH were examined in juvenile landlocked sockeye salmon (Oncorhynchus nerka) and amago salmon (O. rhodurus). Assessed by the 24-hr seawater-challenge test, the landlocked sockeye salmon possessed seawater adaptability as underyearlings in spring, which was maintained throughout the year, and a further increase was observed as yearlings the next spring. An increase in seawater adaptability was observed in silvery juvenile amago salmon as underyearlings from autumn to winter, when some of the wild population migrate to the sea. Precociously mature amago salmon, which did not develop a silvery body color but maintained distinct parr marks, also showed an improvement in seawater adaptability during autumn to winter, although plasma sodium levels after transfer to seawater were still higher than those of the silvery juveniles. In both sockeye and amago salmon, seasonal changes in plasma levels of PRL and GH were not correlated with development of seawater adaptability. In both species, acclimation to seawater resulted in decreased plasma levels of PRL irrespective of their adaptability to seawater, in agreement with the inhibitory effects of PRL in seawater adaptation. On the other hand, plasma GH levels increased only when seawater adaptability was high, in agreement with previous observations indicating an important role of GH in seawater adaptation of salmonids.

Development of a homologous radioimmunoassay for eel prolactin

A highly specific and homologous radioimmunoassay (RIA) for the measurement of prolactin (PRL) in the plasma and the pituitary of the eel was developed using a rabbit antiserum to eel PRL. PRL was purified from the pituitary of Japanese eel (Anguilla japonica). Pituitary extracts and plasma from the Japanese and European eels exhibited displacement curves parallel to the eel PRL standard. Plasma and pituitary extracts from chum salmon, rainbow trout, Japanese charr, tilapia, goldfish, and carp, as well as plasma from hypophysectomized eel, showed negligible cross-reactivity. PRL and growth hormone (GH) preparations from chum salmon, tilapia, and sheep, carp PRL, and eel GH did not cross-react with the antibody. The RIA sensitivity was less than 0.1 ng eel PRL per milliliter. Intra- and interassay coefficients of variations were 2.4 and 11.8%, respectively. The immunoreactive PRL levels in plasma and pituitary of the eel adapted to 50% seawater were significantly lower than those of the eel in fresh water. Plasma PRL levels increased maximally 2 days after transfer from seawater to fresh water, as would be expected from the well-established role of PRL in freshwater adaptation in several euryhaline teleosts.

Isolation and characterization of Japanese eel prolactins

A highly purified prolactin (PRL) was isolated from the pituitary of the Japanese eel (Anguilla japonica) by extraction with acid-acetone, gel filtration on Sephadex G-75, and reversed-phase HPLC on TSK-gel ODS 120T and on TSK-gel TMS 250. Eel PRL is comprised of two variants (ePRL I and II), which were separated by HPLC on an ODS column. The two PRLs were also secreted by organ-cultured pituitaries in a defined medium. After being dialyzed against distilled water and lyophilized, the medium was dissolved in 0.01 M ammonium acetate (pH 9.0), and then the insoluble fraction was subjected to HPLC on an ODS column to isolate the secreted PRLs. The ePRL I and II were equipotent in retaining plasma Na in the hypophysectomized killifish, Fundulus heteroclitus, transferred from sea-water to fresh water. The putative PRL-producing cells in the rostral pars distalis of the eel pituitary were specifically stained with the antiserum against the mixture of ePRL I and II. Both PRLs had a molecular weight of 22 kDa in SDS-PAGE, an isoelectric point of 10.1 by gel electrofocusing, and an N-terminal residue of valine. Amino acid compositions and the partial amino acid sequences of ePRL I and II show that they are highly homologous with a limited number of substitutions, and that they are more closely related to those of teleostean PRLs than to those of mammalian PRLs.

Plasma prolactin and cortisol concentrations of stressed coho salmon, Oncorhynchus kisutch, in fresh water or salt water

Juvenile coho salmon, Oncorhynchus kisutch, adapted to fresh water or seawater were either acutely handled or continuously stressed by severe confinement. Chronic stress, independent of external salinity, caused a gradual increase in the concentration of circulating prolactin that persisted for 1 to 5 days but lagged behind the cortisol response which peaked much more rapidly and remained elevated. Acutely stressed fish showed a rapid, more transient increase in plasma cortisol titer with no apparent effect on prolactin. Confinement appeared to be more stressful to fish in salt water than to those in fresh water, as judged by their sodium regulatory ability, hormone profiles, and mortality. Stress always elevated plasma prolactin concentrations, regardless of medium or developmental stage.

Development and validation of a competitive enzyme immunoassay for chum salmon prolactin: a comparison to radioimmunoassay

An enzyme immunoassay (EIA), based on a competitive assay system, for the measurement of prolactin (PRL) in the pituitary of salmonid fishes and of hormone released in medium from incubated pituitary was developed using a rabbit antiserum to chum salmon PRL (PRL, a combination of PRL I and PRL II). Chum salmon PRL was coupled to horseradish peroxidase (HRP). The incubation procedure for the antigen-antibody reaction was analogous to that in the radioimmunoassay (RIA) for PRL. The antibody-bound HRP-PRL was separated by a double antibody method. The enzyme activity in the precipitate was followed by a colorimetric method, in which 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) and o-phenylenediamine were used as substrates. PRL, PRL I, and PRL II showed exactly the same competitive curves in the EIA system. PRL (127-158) showed the highest cross-reactivity among the fragments of PRL examined. Low cross-reactivity was seen with other hormones isolated from chum salmon pituitary. The displacement curves for pituitary extracts from several salmonids, including chum salmon, coho salmon, and rainbow trout, were parallel to that of the PRL standard, whereas those from the carp and tilapia showed negligible cross-reactivity. A parallel displacement curve to the PRL standard was also seen with incubation medium of the pars distalis of the chum salmon pituitary. Plasma from chum salmon, coho salmon, and rainbow trout gave nonspecific HRP activity in the EIA. The values of PRL-EIA were significantly correlated (y = 0.99x + 1.06, r = 0.942, P less than 0.05, n = 24) with those obtained in PRL-RIA.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Differential response of plasma prolactin to freshwater transfer of smolts and postsmolts of seawater-adapted coho salmon (Oncorhynchus kisutch)

Prolactin (PRL) was purified from chum salmon, Oncorhynchus keta, pituitary glands and was used to develop a homologous radioimmunoassay for the measurement of PRL from salmon. The plasma PRL response to freshwater (FW) transfer differed in seawater (SW)-adapted postsmolt (250 g) and smolts (15 g) of coho salmon. Postsmolts had a pronounced and prolonged elevation of plasma titers of PRL with hypercalcemia and stable plasma sodium levels. The FW-transferred postsmolts had significantly lower pituitary gland PRL only at 0.5 and 2 hr post-transfer as compared to SW-SW. Smaller smolts showed stable plasma PRL levels after FW transfer, hypocalcemia 48 post-transfer, depressed plasma sodium concentrations, and lowered plasma osmotic pressure. This different response may be due to an increased osmoionic regulatory challenge encountered by the smaller smolts or possibly due to some other developmental change between the two different age classes.

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.

Changes in plasma prolactin and growth hormone concentrations during freshwater adaptation of juvenile chum salmon (Oncorhynchus keta) reared in seawater for a prolonged period

Freshwater adaptability of chum salmon was examined in juvenile fish reared in seawater for 4 months. The fish, weighing about 40g, were transferred directly to fresh water in October, when their cohorts are migrating in the North Pacific Ocean. Plasma sodium concentration decreased from 167 mM in seawater to about 130 mM during the first 24h, and increased gradually during 2-7 days after the transfer. No immunoreactive prolactin (PRL) was detected in the plasma of the seawater-adapted fish nor during the first 24h in fresh water. Significant levels of PRL were detected after 2-3 days. The maximal level (2.6 ng/ml) was observed after 5 days and became undetectable again after 7 days; no significant correlation was seen between the changes in plasma sodium and PRL levels during the transfer. Plasma growth hormone levels were relatively constant, except for a significant decrease 12h after the transfer. Although plasma thyroxine levels were highly variable during the experiment, a significant decrease and an increase were observed 12h and 5 days after the transfer, respectively. The present study indicates that juvenile chum salmon retain hyperosmoregulatory ability even after prolonged rearing in seawater. Examination of turnover rates, rather than changes in plasma levels, seems to be essential to clarify the osmoregulatory roles of the hormones.

Hypothalamic control of prolactin release in the rainbow trout,Salmo gairdneri: in vitro studies

Hypothalamic control of prolactin (PRL) release in immature rainbow troutSalmo gairdneri was investigated using anin vitro perifusion system of the rostral pars distalis. Hypothalamic extract of trout induced a dose-dependent stimulation of PRL release. A similar effect was observed when infusing the medium from a 24h static incubation of the hypothalamus. Extracts from different control tissues (muscle, liver, gut) did not changein vitro release, thus confirming the specificity of this stimulatory effect. Hypothalamic extract from adult male rat, known to contain PRL release inhibiting factors, stimulatedin vitro PRL secretion in rainbow trout. This suggests that PRL cells are predominantly influenced by PRL releasing factors. Measurement of TRH and serotonin content in trout hypothalamus indicated consistent physiological levels of these two factors. HPLC studies of hypothalamic extract showed that immunoreactive - TRH eluted at the same place as labelled TRH standard. Moreover, pizotifen, a serotonin antagonist, partially inhibited the stimulation observed with trout hypothalamic extract. These results suggest that, in immature rainbow trout, PRL release is under stimulatory hypothalamic control and that serotonin and probably TRH play a major role in this control.

Smoltification and seawater adaptation in Atlantic salmon (Salmo salar): plasma prolactin, growth hormone, and thyroid hormones

To obtain more information on the role of prolactin and growth hormone during the parr-smolt transformation of Atlantic salmon, a population of fish in fresh water was sampled from January to June during two consecutive years. Gill Na+,K+-ATPase activity increased steadily during smoltification and a plasma thyroxine peak was observed 2-3 weeks before the gill Na+,K+-ATPase peak. On the basis of these two parameters, smoltification was considered complete in our populations in April 1985 and May 1986. Two peaks in plasma growth hormone levels occurred in 1986, one in mid-April and the second in mid-May. In both cases, these peaks coincided with a peak in plasma triiodothyronine and preceded the thyroxine peak by 1-2 weeks. Moreover, the second peak which lasted for 1 month coincided with maximal gill Na+,K+-ATPase activity. A decrease in plasma prolactin levels was observed during smoltification of Atlantic salmon in 2 consecutive years. During this period of decreasing and low plasma prolactin levels, gill Na+,K+-ATPase activity increased to its highest values. Atlantic salmon smolts were also directly transferred into seawater. After 2 days or more in seawater, plasma prolactin levels were not significantly different from those on Day 0, whereas in fresh water they showed large fluctuations. All these data indicate that growth hormone may play an important role in the development of hypoosmoregulatory activity. Increased hypoosmoregulatory ability also appears to be associated with low prolactin levels.

Purification of chinook salmon (Oncorhynchus tshawytscha) GH for receptor study

A method for the purification of chinook Salmon (Oncorhynchus tshawytscha) GH, which retains its biological activity, is described. The biological activity was investigated with an established radioreceptor assay using liver membranes from pregnant rabbits and bovine GH as standard and labelled hormone. The enrichment of the preparation was checked with electrophoresis (SDS-PAGE). Extraction and further steps were carried out using low molarity alkaline buffer (pH 8-10, M = 100 mM). Three chromatography steps were performed (Concanavalin-A sepharose, Bio-gel P60, DEAE). Ion exchange chromatography was performed under isocratic conditions (using a 50 cm column). Two isoforms (sGH1 and sGH2) were isolated. The purification yield is 0.7% compared to lyophilized pituitaries. The molecule is homogeneous in SDS-PAGE. Contamination by prolactin, gonadotrophin and corticotrophin is negligible (< 0.5%). It could be demonstrated that the biological activity of the preparation is maintained since this preparation stimulates the growth of juvenile trout (Salmo gairdneri) and binds specifically (35%) to trout liver membranes.

Purification and characterization of Atlantic salmon prolactin

Prolactin was isolated from the Atlantic salmon (Salmo salar) pituitary gland by extraction with acid acetone, gel filtration, ion exchange-chromatography, and reversed-phase high-performance liquid chromatography. The yield was 0.6 mg/g wet tissue. The hormone had a molecular weight of 23.5 kDa as determined by SDS-polyacrylamide gel electrophoresis. Isoelectric focusing gave an isoelectric point of 9.2. The N-terminal sequence and the amino acid composition indicated extensive homology between Atlantic and Pacific salmon prolactin. Antiserum against Atlantic salmon prolactin cross-reacted with chum salmon prolactin, but not with human, rat, or sheep prolactin.

Localization and characterization of prolactin-like immunoreactivity in the pituitary of the frog Rana ridibunda

Distribution and quantification of PRL in the pituitary gland of the frog Rana ridibunda were investigated using a high-affinity antiserum raised against bullfrog prolactin (PRL). The immunoreactive PRL-producing cells were distributed throughout the pars distalis, the highest density of cells being observed in the rostral region of the adenohypophysis facing the neurointermediate lobe. The dorsal region of the pars distalis contained only a few scattered PRL-immunoreactive cells. At the electron microscopic level, PRL-containing cells were visualized using the immunogold procedure. PRL-immunoreactive material was exclusively stored in secretory granules (size ranging from 200 to 700 nm in diameter). Neither the rough endoplasmic reticulum nor the Golgi apparatus were immunolabeled. Using a radioimmunoassay method we have compared the displacement curves obtained with bullfrog PRL and acetic extracts from Rana ridibunda pituitary. The two binding curves were not completely parallel, suggesting the existence of slight variations of the amino acid sequences of PRL in the two species. The concentration of PRL in the green frog adenohypophysis appeared somewhat higher (35.3 +/- 8.8 micrograms/mg protein) than that in the bullfrog pituitary. These results validate the use of an antiserum to bullfrog PRL to investigate the regulation of PRL secretion in Rana ridibunda.

Gonadotropic hormone (GtH) receptors in the testis of the troutSalmo gairdneri: in vitro studies

A particulate fraction obtained from trout testis at the time of spermiation shows saturable binding sites for(125)I-labeled salmon gonadotropin ((125)I-GtH). Non-gonadal tissues (liver, muscle and spleen) did not demonstrate specific(125)I-GtH binding. The tracer's specific activity was determined by the self-displacement method (18 to 30 μCi/μg). Maximal specific binding ability of(125)I-GtH varied from 20 to 30% of the labelled ligand added, depending on the hormone preparation. Specific binding of(125)I-GtH to 20 mg of the testis membrane varied from 40 to 85% of the total binding depending on the method of membrane prepratation, and was competitively inhibited by concentrations of unlabelled GtH ranging from ca 1 to 1000 ng/ml of incubate. Gonadotropin of mammalian origin, ovine TSH or salmon prolactin competed only weakly, or not at all, for testicular gonadotropin binding sites (relative potencies s-GtH>>FSH=hCG>s-PRL>bTSH). Scatchard analysis of equilibrium binding studies shows that saturable gonadotropin binding was due to a class of high affinity binding sites (sites I Ka≊3×10(10) M(-1)) and possibly to a second class of lower affinity binding sites (sites II Ka=5 to 14×10(8) M(-1)). The binding capacity of sites I, as measured in enriched membrane preparations, was 45±18 fmoles/g of testis during the period of spermiation. The concentration of GtH required to obtain half maximal displacement of(125)I-GtH in the binding studies was of the same order of magnitude as the apparent ED50 for GtH stimulation of 11-Cetotestosterone (11KT) secretion by trout testesin vitro. Mammalian LH and FSH were 100 to 1000 folds less potent than salmor GtH to increase 11 KT secretion.

Immunohistochemical investigation of the pituitary of the sturgeon (Acipenser baeri, Chondrostei)

An immunohistochemical study of the sturgeon (Acipenser baeri) pituitary was undertaken using antisera directed against hormones from various classes of vertebrates, including the only pituitary hormone available from sturgeon, gonadotrophin. A positive reaction was obtained after application of antisera towards the following hormones 1-24 synthetic ACTH (1-24 ACTH), melanophore stimulating hormone (MSH), ovine prolactin (oPRL), ovine growth hormone (oGH), salmon growth hormone (sGH), carp gonadotrophin (cGTH) and its beta subunit (βcGTH), sturgeon gonadotrophin (aciGTH), carp thyrotrophin (cTSH) and β subunit of the human thyrotrophin (βhTSH). The results demonstrate that, in general, the sturgeon pituitary resembles that of teleosts as regards the distribution of the different cell types: ACTH and PRL cells in the rostral pars distalis, GTH, TSH and GH cells in the proximal pars distalis and MSH and PAS-cells in pars intermedia. In addition to the topographical organization of the sturgeon pituitary, this study provides data on the immunological relationships between sturgeon pituitary hormones and those of other vertebrates.

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.

Effect of osmotic pressure on prolactin release in rainbow trout: in vitro studies

To investigate a possible effect of osmotic pressure on prolactin (PRL) release in rainbow trout, we developed a technique for in vitro perifusion of trout pituitaries. Changes in osmotic pressure similar to those observed in fish plasma during transfer experiments did not induce significant modifications of PRL release. In contrast, high-amplitude variation of osmotic pressure resulted in clear modifications of PRL secretion: hyperosmotic medium caused a reduction in PRL release, while infusion of hyposmotic medium induced a transitory increase in PRL release. By using different concentrations of mannitol, we found that the modifications of prolactin secretion could not be ascribed to alterations of the ionic composition of the medium but actually resulted from variations in the osmotic pressure of the incubation medium. In further experiments osmotic pressure was decreased from 300 to 220 mOsm/kg or from 400 to 300 mOsm/kg; a similar transitory increase in PRL release was observed. Measurement of gonadotropin (GtH) in the perifusion effluent medium showed that PRL and GtH secretion followed similar patterns. Thus, our results suggest a possible mechanical effect of wide changes in osmotic pressure on pituitary cell membranes. These data indicate that the rainbow trout differs notably from nonsalmonid teleost species thus far studied in the lack of sensitivity of its PRL cells to osmotic pressure.

Prolactin and growth hormone secretion during long-term incubation of the pituitary pars distalis of mature chum salmon, Oncorhynchus keta

The effects of hypotonicity of the medium on prolactin (PRL) and growth hormone (GH) release from the pars distalis (PD) of mature chum salmon pituitary were examined during culture for 4 days. Large amounts of PRL and GH were released during the first 6 hr and secretion decreased rapidly thereafter. The amounts of GH released as well as the residual content in PD were 10 times greater than those of PRL. There was no difference in PRL or GH release between the PD incubated in isotonic medium (325 mOsm) and those in hypotonic medium (250 mOsm) even during the first 6 hr. Female PD secreted more PRL than male PD during the first 6 hr, whereas a sex difference was not observed in GH release. Considerable amounts of both PRL and GH remained in the PD at the end of the culture period. The amount of PRL released during 4 days of culture from female PD (37-39%) was greater than that from male PD (23-26%), and the amount of GH released was less in females (28-32%) than that in males (53-54%). Release of PRL and GH appears to be mainly under stimulatory hypothalamic control in the mature chum salmon.

Osmoregulatory ability of chum salmon,Oncorhynchus keta, reared in fresh water for prolonged periods

The osmoregulatory ability of chum salmon (Oncorhynchus keta), reared in fresh water for a prolonged period, was examined by transferring them directly to seawater and then back to fresh water. When fry and juveniles weighing 0.3-125g, reared in fresh water for 1.5-13 months, were transferred directly to seawater, they adjusted their plasma Na(+) concentration to the seawater-adapted level within 12-24h. When they were transferred back to fresh water after having been adapted to seawater for 2 weeks, the plasma Na(+) level gradually decreased during the first 12-24h, and then increased to reattain the initial freshwater level after 5-7 days. No mortality was observed during the experiment except among the smallest fry weighing about 0.3g after transfer to seawater (2.1%). The maintenance of good osmoregulatory ability of the chum salmon for a prolonged period in fresh water seems to be unique among Pacific salmon, with the possible exception of the pink salmon.Changes in plasma levels of hormones during the transfer experiments were recorded in juveniles reared in fresh water for 13 months. Prolactin levels increased maximally 3 days after transfer from seawater to fresh water, as would be expected from its well-established role in freshwater adaptation in several euryhaline teleosts. In addition, an increase in plasma growth hormone was observed during the first 12h after seawater transfer, along with a tendency towards a decrease during freshwater transfer, suggesting an important role for this hormone in seawater adaptation. There were no consistent changes in plasma levels of thyroxine and cortisol during freshwater to seawater or seawater to freshwater transfer.

Primary structure of common carp prolactins

The complete amino acid sequence of common carp (Cyprinus carpio) prolactin (PRL) has been determined. One milligram of carp PRL was reduced, carboxymethylated, and then cleaved with cyanogen bromide and several enzymes. Another milligram of carp PRL was digested with lysyl endopeptidase. The resulting fragments were separated by reverse-phase high-performance liquid chromatography and subjected to sequence analysis by the automatic Edman method with a gas-liquid sequencer and the manual DNS-Edman method. The carp PRL consists of 186 amino acid residues with two disulfide linkages formed between residues 46-160 and 177-186. In addition, it lacks the linkage in the amino-terminal portion of mammalian prolactins, as do salmon and tilapia PRLs, and shows 77% sequence identity with salmon PRL and 36% identity with mammalian PRLs. The sequence comparison has been performed with 190 alignment positions. Four conserved segments of the alignment positions 1-32, 46-66, 71-94, and 155-182 that were found between salmon and mammalian PRLs are further confirmed in carp PRL. Variant residues are clustered in the region of residues 98-143 which has 48% sequence identity with salmon PRL and only 2% with mammalian PRL.

A radioimmunoassay for N-terminal peptide of chum salmon proopiocortin

A highly specific radioimmunoassay was developed for N-terminal peptide of salmonid proopiocortin using a guinea pig antiserum to the chum salmon peptide (sNPP 1). Since sNPP I has no tyrosine residue nor free N-terminal amino group, a mixture of minor components of sNPP 1, which have extensions of H-Val-LysGly- and H-Lys-Gly- at the N-terminus, were iodinated by the lactoperoxidase method after incorporation of 3-(phydroxyphenyl)-propionate to the terminal amino groups. Plasma and pituitary extracts of several salmonid species showed parallel displacement to the standard hormone. Samples from carp, goldfish, tilapia, and eel, as well as the plasma of hypophysectomized rainbow trout, showed no crossreactivity. Proopiocortin-related hormones isolated from the chum salmon pituitary, including melanotropins, endorphins, corticotropin-like intermediate lobe peptides, and gonadotropin and prolactin showed negligible cross-reactivity. NPP contents in the pars intermedia of rainbow trout and chum salmon were 10 to 15 times greater than those in the pars distalis. Plasma levels of NPP in the mature chum salmon caught in the bay were about 50ng/ml. Plasma NPP levels in the mature chum salmon of both sexes decreased after transfer from seawater to fresh water. Plasma cortisol showed a concomitant change with NPP, thus supporting previous findings that NPP modulates corticotropin action on the trout interrenal.

Gonadotropin hormone (GtH) receptors in the ovary of the brown trout Salmo trutta L. in vitro studies

A method has been developed suitable for the study of the binding of salmon (Oncorhynchus tsawytscha) gonadotropic hormone (GtH) to brown trout (Salmo trutta) ovarian particulate fractions. Several proteolytic enzyme inhibitors were tested to preserve the binding capacity of particulate fractions (1000, 20,000, and 110,000g) from yolky oocytes. The best preservation was obtained with trypsin inhibitor. The effects on binding of the pH, time, and temperature of the incubation were investigated. Binding increased linearly with amount of particulate fraction, and is saturable. In both the 20,000 and 110,000g fractions, there are two classes of receptors: one with a high-affinity constant (3.14 and 0.95 10(10) M-1 for the 20,000 and 110,000g fractions, respectively) and a low capacity (1.76 and 0.63 fmol/g of ovary), the other with a higher-affinity constant (3.44 and 1.78 10(9) M-1) but a higher capacity (7.37 and 7.42 fmol/g of ovary). GtH binding was not affected by ovine luteinizing hormone (oHL), ovine follice-stimulating hormone (oFSH), and human chorionic gonadotropin (hCG), but was partially inhibited by 1 micrograms of bovine thyroid-stimulating hormone (bTSH) and salmon prolactin (sPRL). The results are discussed in relation to the biological specificity of fish GtH and to the unsolved problem concerning the number of gonadotropins in fish.

Development of a salmon growth hormone radioimmunoassay

This study describes the development of a growth hormone (GH) radioimmunoassay (RIA) using chum salmon (Oncorhynchus keta) GH and an antiserum raised against this preparation. The assay does not cross-react with salmon prolactin and is valid for the genera Salmo and Oncorhynchus. Hypophysectomy of coho salmon (O. kisutch) reduced plasma immunoreactivity to nondetectable levels in seven of eight individuals. Handling stress had no effect upon GH levels in the rainbow trout (Salmo gairdneri) whereas starvation (3 weeks) induced a ninefold increase in plasma immunoreactivity. Plasma GH levels in trout were positively correlated, following a lag phase of 1 week, with the weekly changes in growth rate displayed by this species.

Development and validation of a salmon growth hormone radioimmunoassay

A highly specific and sensitive radioimmunoassay (RIA) for the measurement of plasma and pituitary growth hormone (GH) levels in salmonid fishes was developed using an anti-serum raised in rabbit against chum salmon (Oncorhynchus keta) GH (sGH). Pituitary extracts and plasma from chum, coho, masu, and amago salmon, and from rainbow trout and Japanese charr, all exhibited displacement curves parallel to the sGH standard. Samples from the eel, carp, goldfish, and tilapia, as well as plasma from hypophysectomized chum salmon and rainbow trout, all showed negligible cross-reactivity. None of the mammalian or teleostean GH or prolactin preparations tested cross-reacted with the antibody in the assay system. RIA sensitivity was 0.6 ng sGH/ml of plasma when 100 microliter of plasma was employed. Intra- and interassay coefficients of variation were 3.9 and 4.1%, respectively. Plasma GH levels of the mature chum salmon caught in Otsuchi Bay were highly variable, especially in females (20.2 +/- 8.2 ng/ml) as compared with males (16.0 +/- 1.1 ng/ml), and there was no significant change after transfer to fresh water. Whereas there was no change in plasma GH levels in males kept in seawater, the levels in females increased with time in close correlation with the increase in plasma chloride.