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ABSTRACT
Specific binding sites for chum salmon growth hormone (sGH) were identified in the membranes obtained from tissues of rainbow trout. Specific binding of 125I-labelled sGH (% per mg protein) was found in the liver (37%), ovary (6%), brain (6%), gill (4%), intestine (4%) and posterior body kidney (4%). Specific binding was not significant in head kidney, anterior body kidney, spleen, heart, skeletal muscle or skin. Scatchard analyses demonstrated the presence of a single class of high-affinity low-capacity receptors in the liver, gill, intestine and kidney. The association constants for the membranes from liver, gill, intestine and kidney were of the same order (1 litre/nmol). Chum salmon prolactin did not inhibit the binding of 125I-labelled sGH to receptors in the liver, gill, intestine and kidney.
Transfer of rainbow trout from fresh water to 80% seawater evoked a rise in plasma concentration of GH and a significant decrease in the GH binding to the liver membranes after 1 day. Binding in the gill and kidney was not altered significantly. Membranes were treated with 4 mol MgCl2/l to remove bound GH from the receptors, and the results indicated that the reduction in binding in the liver after transfer to sea-water was probably due to receptor occupancy by increased endogenous GH. The occupancy of liver GH-binding sites was maximal 4 days after transfer. Total (MgCl2-treated) binding sites in the liver increased significantly 14 days after transfer. Scatchard analysis indicated that receptors were altered in capacity without changes in binding affinity. Although GH may also directly affect osmoregulatory organs through their GH receptors, the present results indicate the likelihood of at least partial mediation by the liver of the seawater-adapting action of GH in the rainbow trout.
Journal of Endocrinology (1991) 130, 425–433
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ABSTRACT
The possible roles of insulin-like growth factor-I (IGF-I) and insulin in regulating cartilage growth were studied in the teleost Anguilla japonica. Significant sulphation activity was found in the extracts of pancreas, liver and muscle, but not in those of kidney, intestine or spleen. The hepatic sulphation activity was significantly decreased by hypophysectomy or by fasting for 14 days, suggesting that this activity is regulated by pituitary function and nutritional status. Northern blot analysis revealed that the hepatic IGF-I mRNA in the eel consists of a major 4·0 kb band. This mRNA was GH-dependent and was significantly decreased by fasting for 14 days.
On the other hand, fasting for 14 days had no significant effect on pancreatic sulphation activity. Pancreatic extracts from both intact and hypophysectomized eels exhibited equally significant stimulating activity. Addition of bovine or human insulin (1–250 ng/ml) to the culture medium significantly stimulated sulphate uptake in a dose-dependent manner. Teleost (coho salmon) insulin was as effective as bovine insulin. Bovine insulin was more effective than IGF-I at lower concentrations (1–4 ng/ml) but less effective at higher concentrations (10–250 ng/ml). These results indicate that not only IGF-I but also insulin are likely to be involved in the regulation of cartilage growth in the eel.
Journal of Endocrinology (1992) 133, 211–219
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ABSTRACT
Our previous studies have shown that mammalian and salmon insulins stimulate sulphate uptake by cultured eel cartilage, suggesting the possible involvement of insulin in the regulation of cartilage matrix synthesis. In the present study, homologous eel insulin was isolated and characterized, and its effects on cartilage matrix synthesis and DNA synthesis were examined in vitro. Insulin was extracted from eel pancreas with acid–ethanol, and subsequently purified by isoelectric precipitation at pH 5·3, gel filtration on Sephadex G-50, and reversed-phase high-performance liquid chromatography. The amino acid composition and complete sequence (50 residues) of eel insulin revealed high homology to teleostean and mammalian insulins. The isolated eel insulin produced a more pronounced and longer lasting hypoglycaemic effect than bovine insulin in the eel. Homologous eel insulin, like bovine insulin-like growth factor (IGF-I) and insulin, stimulated sulphate uptake by cultured eel cartilage in a dose-dependent manner (16–1000 ng/ml). Combination experiments using maximal concentrations of bovine IGF-I (250 ng/ml) and increasing amounts of eel insulin (10–250 ng/ml) showed no additive effects of insulin on sulphate uptake, suggesting that insulin and IGF-I may share a common mechanism(s) of action. Eel insulin and bovine IGF-I also enhanced thymidine incorporation by eel cartilage in a dose-dependent manner (4–1000 ng/ml); eel insulin was equipotent with bovine IGF-I. These results suggest that insulin, like IGF-I, may exert direct growth-promoting actions in branchial cartilage of the eel.
Journal of Endocrinology (1992) 133, 221–230
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ABSTRACT
The ability of insulin-like growth factor-I (IGF-I), insulin and GH to promote hypoosmoregulatory ability was examined in juvenile rainbow trout (Oncorhynchus mykiss). Following adaptation to 12 parts per thousand (p.p.t.) seawater for 5 days, fish were given a single injection of hormone or vehicle, then exposed to 29 p.p.t. for 24 h and examined for changes in plasma osmolarity, ions and glucose. Ovine GH (oGH; 0·2 μg/g) significantly improved the ability of rainbow trout to maintain plasma osmolarity and sodium levels following transfer to 29 p.p.t. seawater. Recombinant bovine IGF-I (0·01, 0·05 and 0·2 μg/g) also improved the hypoosmoregulatory ability of trout; the effect being dose-dependent and greater than that of oGH. Bovine insulin (0·01, 0·05 and 0·2 μg/g) had no statistically significant effect on plasma ions. The results indicate that IGF-I is a potential mediator of the action of GH in seawater adaptation of salmonids.
Journal of Endocrinology (1991) 130, 87–92
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To clarify the roles of prolactin (PRL) and GH in the control of the immune system, the effects of environmental salinity, hypophysectomy, and PRL and GH administration on several immune functions were examined in tilapia (Oreochromis mossambicus). Transfer from fresh water (FW) to seawater (SW) did not alter plasma levels of immunoglobulin M (IgM) and lysozyme. The superoxide anion (O(2)(-)) production in head kidney leucocytes accompanied by phagocytosis was elevated in SW-acclimated fish over the levels observed in FW fish. Hypophysectomy of the fish in FW resulted in a reduction in O(2)(-) production in leucocytes isolated from the head kidney, whereas there was no significant change in plasma levels of IgM or lysozyme. Treatment with tilapia GH and PRLs (PRL(177) and PRL(188)) enhanced O(2)(-) production in vitro in head kidney leucocytes in a dose-related manner. Extrapituitary expression of two PRLs, GH and IGF-I mRNA was detected in lymphoid tissues and cells such as head kidney, spleen, intestine and leucocytes from peripheral blood and head kidney. PRL-receptor mRNA was detected in head kidney leucocytes, and the level of expression was higher in SW-acclimated fish than that in FW fish. Treatment with PRL(177) caused higher production of O(2)(-) in the head kidney leucocytes isolated from SW tilapia than that from FW fish. In view of the fact that PRL acts antagonistically to osmoregulation in SW, its immunomodulatory actions in this euryhaline fish would appear to be independent of its osmoregulatory action.
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ABSTRACT
The seawater-adapting actions of GH, which are independent of growth, were studied in juvenile rainbow trout (Salmo gairdneri). Hormones examined were chum salmon (Oncorhynchus keta) GH (sGH) and prolactin (sPRL), and ovine GH (oGH). Plasma Na levels of freshwater-adapted fish peaked 24 h after transfer to 67% seawater and remained high for at least 48 h. Twenty-four hours after transfer, plasma Na levels were inversely correlated to body weight. In order to limit size and growth effects in all subsequent experiments, fish having a narrow range of body weights, fed a fixed diet, and injected with hormones over a short time-period were used. Plasma Na levels 24 h after transfer to 80% seawater were reduced significantly by sGH (0·25 and 2·5 μg/g) and oGH (2·5 μg/g) compared with saline injections, whereas sPRL (2·5 μg/g) had no significant effect. All the GH-treated fish had lower plasma Mg levels than controls; Ca levels were significantly reduced by the high dose of sGH. Salmon prolactin had no effect on concentrations of divalent ions. When the effects of a range of doses (0·01–1·25 μg/g) of sGH on plasma ion levels was tested, 0·25 μg/g was the most potent in reducing Na and Mg levels, while 1·25 μg/g alone reduced plasma Ca concentrations significantly. These studies show that the seawater-adapting actions of GH in trout are specific to that hormone and are not consequent to an increase in size.
J. Endocr. (1987) 112, 63–68
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There is considerable evidence that the GH/IGF-I axis plays an important role in female reproduction. We report the isolation and characterization of the GH receptor (GH-R) and its gene expression profile during oogenesis in the tilapia, Oreochromis mossambicus. cDNA encoding GH-R was cloned and sequenced from the tilapia liver. The predicted GH-R preprotein consisted of 635 amino acids and contained a putative signal peptide, an extracellular region with a characteristic motif, a single transmembrane region, and a cytoplasmic region with conserved box 1 and 2 domains. The tilapia GH-R shared 34-74% identities with known GH-Rs in vertebrates. A binding assay using COS-7 cells showed that the cloned GH-R bound specifically to tilapia GH. Northern blot analysis showed a single mRNA transcript in the liver and ovary. In situ hybridization revealed intense signals of GH-R in the cytoplasm and nucleus of immature oocytes. The granulosa and theca cells surrounding vitellogenic oocytes also contained the GH-R mRNA signals. About a tenfold greater level of GH-R mRNA was found in the immature oocytes versus the mature oocytes, along with high levels of IGF-I mRNA. There were no significant changes in mRNA levels of GH-R and IGF-I in the liver or in plasma IGF-I levels during oocyte development. No correlation was found between hepatic GH-R mRNA and ovarian GH-R mRNA. These results suggest that the GH/IGF-I axis in the ovary may be involved in the early phases of oogenesis, under a different regulatory mechanism of GH-R gene expression from that of the liver.
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Abstract Studies in mammals have shown that synthetic Met-enkephalin derivatives, called growth hormone-releasing peptides (GHRPs), stimulate growth hormone (GH) release. The present study was conducted to determine whether the GHRP, KP-102, specifically stimulates GH release in a teleost. Tilapia (Oreochromis mossambicus) were given a single intraperitoneal injection of KP-102 (D-Ala-D-beta;-Nal-Ala-Trp-D-Phe-Lys-NH(2)) or bovine GHRH(1-29)-amide or vehicle and blood was sampled at 1, 6 and 12 h after injection. KP-102 was administered at two doses of 1 ng/g and 10 ng/g body weight, whereas GHRH (positive control) was administered at a single dose of 10 ng/g body weight. Plasma levels of tilapia GH and prolactins (tPRL(177) and tPRL(188)) were determined by radioimmunoassay. As expected, GHRH injection significantly (P<0.001) elevated plasma GH levels (ng/ml) in tilapia at 6 h post-injection. KP-102 also significantly elevated GH levels (at the low dose) at 6 (P<0.05) and 12 (P<0.01) hours post-injection. There were no significant effects on plasma PRL(s) levels, although mean levels of both PRLs were elevated at 6 h post-injection. These results show for the first time that GHRPs stimulate GH release in teleosts and suggest that the GHRP receptor and possibly a "Ghrelin-like" ligand are also present in lower vertebrates.
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We purified ghrelin from stomach extracts of a teleost fish, the Japanese eel (Anguilla japonica) and found that it contained an amide structure at the C-terminal end. Two molecular forms of ghrelin with 21 amino acids were identified by cDNA and mass spectrometric analyses: eel ghrelin-21, GSS(O-n-octanoyl)FLSPSQRPQGKDKKPP RV-amide and eel ghrelin-21-C10, GSS(O-n-decanoyl) FLSPSQRPQGKDKKPPRV-amide. Northern blot and RT-PCR analyses revealed high gene expression in the stomach. Low levels of expression were found only in the brain, intestines, kidney and head kidney by RT-PCR analysis. Eel ghrelin-21 increased plasma growth hormone (GH) concentrations in rats after intravenous injection; the potency was similar to that of rat ghrelin. We also examined the effect of eel ghrelin on the secretion of GH and prolactin (PRL) from organ-cultured tilapia pituitary. Eel ghrelin-21 at a dose of 0.1 nM stimulated the release of GH and PRL, indicating that ghrelin acts directly on the pituitary. The present study revealed that ghrelin is present in fish stomach and has the ability to stimulate the secretion of GH from fish pituitary. A novel regulatory pathway of GH secretion by gastric ghrelin seems to be conserved from fish to human.
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Glucocorticoids are known to impede somatic growth in a wide range of vertebrates. In order to clarify the mechanisms through which they may act in an advanced teleost fish, we examined the effects of cortisol administration on the growth hormone (GH)/insulin-like growth factor-I (IGF-I)/IGF-binding protein (IGFBP) system in the tilapia (Oreochromis mossambicus). In a short-term experiment, fish were injected intraperitoneally with cortisol (2 or 10 microg/g), and killed at 2, 4, 8 and 24 h after the injection. In a longer-term experiment, fish were killed 24 and 48 h after cortisol injection (2, 10 and 50 microg/g). Cortisol at doses of 2 and 10 microg/g significantly increased IGFBPs of four different sizes (24, 28, 30, and 32 kDa) in the plasma within 2 h without altering plasma levels of IGF-I or GH. On the other hand, cortisol at doses of 10 and 50 microg/g significantly reduced plasma IGF-I levels after 24 and 48 h. IGF-I mRNA levels in the liver were also significantly reduced by cortisol at doses of 10 and 50 microg/g after 48 h, suggesting that a decrease in plasma IGF-I levels is mediated through the attenuation of IGF-I gene expression in the liver. In contrast, no significant change was observed in plasma or pituitary contents of GH at any time point examined, which would appear to indicate that cortisol reduces IGF sensitivity to GH (GH-resistance). These results clearly indicate that cortisol induces a rapid increase in plasma IGFBPs and a more delayed decrease in IGF-I production. The dual mode of cortisol action may contribute to the inhibitory influence of cortisol on somatic growth in teleosts.
