Björn Th. Björnsson

Growth hormone increases growth rate, appetite and dominance in juvenile rainbow trout, Oncorhynchus mykiss

Abstract Abstract. The hypothesis that exogenous growth hormone elevates feeding motivation, and thereby increases dominance status in juvenile rainbow trout was tested by comparing the feeding behaviour of growth hormone-treated and control (vehicle-injected) trout in two experimental series. Growth rate, food conversion and hypo-osmoregulatory ability of the two groups were also compared. During a 4-day treatment period, specific growth rate was significantly higher in growth hormone-injected trout than in control fish. Further, growth hormone-treated fish had significantly greater hypo-osmoregulatory ability than control fish. Satiation feeding experiments on individual trout revealed a considerably higher appetite in growth hormone-injected trout, which ate twice as much as control fish over the 2 days of feeding. Also, food conversion efficiency was higher in growth hormone-treated fish than in control fish. Feeding competition trials involving similar-sized matched pairs of growth hormone-treated and control trout revealed that growth hormone-treated juveniles were significantly dominant over control fish when the pairs were selected before the 4-day hormonal treatment but in a second dominance experiment, where growth hormone-treated and control fish were paired after the growth hormone treatment, no significant difference was found between the treatment groups. Overall, the results support the hypothesis that exogenous growth hormone can increase appetite and dominance in juvenile rainbow trout. It is suggested that the endogenous production of growth hormone in rainbow trout is limited by mortality costs associated with maximal growth rates.

Chum salmon (Oncorhynchus keta) stanniocalcin inhibitis in vitro intestinal calcium uptake in Atlantic cod (Gadus morhua)

SummaryChum salmon (Oncorhynchus keta) stanniocalcin was purified, partially identified and tested for bioactivity in an assay on the intestinal calcium uptake in a marine teleost (Gadus morhua). Basic ethanol extraction, ion exchange chromatography, gel filtration and reverse-phase high-performance liquid chromatography resulted in the isolation of a homogenous glycoprotein that appears as a 46-kDa product under non-reducing conditions and as a 23-kDa product under reducing conditions after sodium dodecylsulphate-polyacrylamide gel electrophoresis. The glycoprotein is likely to be a homodimer composed of two subunits of 23 kDa each. Further characterization indicates homology to Australian eel, sockeye salmon, coho salmon and rainbow trout stanniocalcin, and the glycoprotein is thus concluded to be stanniocalcin. Stanniocalcin-like immunoreactivity was demonstrated in the corpuscles of Stannius of the Atlantic cod, with a specific antiserum raised against purified chum salmon stanniocalcin. The physiological importance and the biological activity of chum salmon stanniocalcin was tested by evaluating its effect on intestinal calcium uptake by the Atlantic cod in vitro. The intestine was perfused, both vascularly and through the intestinal lumen, and the calcium mucosa-to-serosa flux was measured using 45Ca2+ as a tracer. Stanniocalcin decreased the intestinal calcium uptake in a dose-related manner by 13.5% and 22.4% at doses of 2.2 and 10.9 nM stanniocalcin, respectively. The results establish the intestine as a target organ for stanniocalcin in marine teleosts.

Dominance, Nutritional State, and Growth Hormone Levels in Rainbow Trout (Oncorhynchus mykiss)

This study addressed three questions concerning interactions between physiology and dominance in juvenile rainbow trout: (1) the validity of a model predicting a time-dependent effect of fasting on competitive ability (i.e., the ability to obtain contested food items) was tested in a series of dominance trials between fed and progressively more fasted trout, as was (2) the association between fasting and plasma growth hormone levels. (3) The relationship between plasma growth hormone levels and the competitive ability of individual trout was also studied. The main results were as follows: (1) The predictions of the time-dependent model were supported by the fasting-dominance experiment. After 3 days, fasted fish were dominant over fed fish, whereas after 6 and 9 days, the competitive ability of fed and fasted fish was similar. After 12 days, there was a tendency for fed fish to be dominant over their fasted competitors. (2) Sampling of plasma from fed and fasted trout, after 3, 6, 9, and 12 days, demonstrated that plasma growth hormone levels increases in food-deprived rainbow trout after more than 6 days of fasting, which is consistent with previous work. (3) No difference in plasma growth hormone levels was found between paired dominant and subordinate trout. Possible interactions between nutritional state, growth hormone levels, and dominance, and their implications are discussed.

Physiological and hormonal differences among Atlantic salmon parr and smolts reared in the wild, and hatchery smolts

Atlantic salmon which had been released as fry in tributaries of the Connecticut River (Northeastern USA) were captured during the migratory period (9 May to 6 June) 1 to 2 years later. Migrants (smolts) were captured above a dam on the main-stem; non-migrants (parr) were captured in two tributaries. Migrants were significantly larger than non-migrants (16.9 and 13.0 cm, respectively), but had a 20% lower condition factor. Relative to non-migrants, migrants had significantly higher gill Na+, K+-ATPase activity (5-fold), plasma glucose (1.8-fold), plasma thyroxine (8-fold), plasma cortisol (5-fold) and plasma growth hormone (100-fold). There were no significant differences in plasma [Na+], [K+] and [Ca2+] between migrants and non-migrants. Most of the differences seen between non-migrants and migrants were similar to those that occurred seasonally in a captive (hatchery) population of the same stock of Atlantic salmon. However, hatchery smolts had lower levels of plasma thyroxine and exhibited no significant change in plasma growth hormone from February to June. The large differences between migrants and non-migrants may be related to both the parr-smolt transformation and migration.

Estradiol-17β-induced calcium uptake and resorption in juvenile rainbow trout, Oncorhynchus mykiss

Juvenile rainbow trout, Oncorhynchus mykiss, were injected with estradiol-17β (E2) in order to study the source of extra calcium needed during vitellogenesis. E2-treatment increased the calcium uptake from the external medium as well as calcium mobilization from muscle and scale. Judged by the increase in plasma protein-bound calcium levels, the E2-induced increase in calcium uptake is an apparent over-mobilization of calcium, i.e., the calcium uptake of the fish is in excess of what is found bound to plasma proteins. As the calcium excretion and calcium space (calculated from free plasma calcium levels) were unaffected, the excess calcium is suggested to be incorporated into internal calcium stores. This implies that the systems regulating vitellogenesis and calcium balance are integrated on the mechanistic or endocrine level, and that E2 causes calcium mobilization of a magnitude geared to the needs of the sexually maturing female.

Effects of vitamin D3, 25(OH) vitamin D3, 24,25 (OH)2 vitamin D3, and 1,25(OH)2 vitamin D3 on the in vitro intestinal calcium absorption in the marine teleost, Atlantic cod (Gadus morhua).

The role of vitamin D3, 25(OH) vitamin D3, 24,25(OH)2 vitamin D3, and 1,25(OH)2 vitamin D3, in the regulation of calcium absorption across the intestine in the marine teleost, Gadus morhua, was investigated. The intestine was perfused, in vitro, both vascularly and through the intestinal lumen, and the calcium influx was measured using 45Ca. Vitamin D3 and its metabolites were tested in perfusate concentrations of 10 ng.ml-1.25(OH)D3 increased the intestinal calcium uptake by 65%, while 24,25(OH)2D3 decreased it by 36%. Vitamin D3 and 1,25(OH)2D3, on the other hand, did not affect the calcium influx across the intestinal mucosa. This indicates that 25(OH)D3 and 24,25(OH)2D3 may be active regulators of calcium transport across the intestine of Atlantic cod.

Wild and hatchery-reared brown trout, Salmo trutta, differ in smolt related characteristics during parr-smolt transformation

Morphological changes during parr-smolt transformation are generally less apparent in hatchery-reared than wild salmonids. The aim of the present study was to investigate possible differences between wild and hatchery-reared brown trout in regard to physiological character- istics during smoltification. Plasma growth hormone levels, hypoosmoregulatory ability, gill Na q , K q -ATPase activity and condition factor were compared between wild and hatchery-reared fish from the same river stock, in two different streams on the Swedish coast. Plasma growth hormone levels were consistently higher in wild compared with hatchery-reared trout, and the growth hormone levels increased in wild fish from one of the streams after a 24 h seawater challenge test. At the time of parr-smolt transformation, there was a peak in gill Na q ,K q -ATPase activity, which coincided with the lowest plasma sodium levels. Wild fish possessed consistently higher gill Na q ,K q -ATPase activity and lower plasma sodium levels compared with hatchery-reared fish. The condition factor of wild fish decreased throughout the smoltification period, in both river strains, whereas the hatchery-reared fish had consistently high condition factor. It is concluded that the artificial environment of hatchery-reared anadromous brown trout can depress the natural parr-smolt transformation, and that this may adversely affect the success of seawater migration and long-term survival of the fish. q 1998 Elsevier Science B.V. All rights reserved.

Effects of Hypophysectomy on the Plasma Ionic and Osmotic Balance in Rainbow Trout, Salmo gairdneri

The effects of hypophysectomy on the plasma ionic and osmotic balance in juvenile rainbow trout (Salmo gairdneri) kept in 1/3 sea water for various periods were studied. Hypophysectomy caused a decrease in the plasma levels of ionized calcium and chloride within a week after the operation. At 2-5 weeks after hypophysectomy, the plasma levels of ionized calcium and chloride were still significantly lower in hypophysectomized fish when compared to sham-operated controls. Hypophysectomy had no effects on the plasma levels of sodium, total calcium, total magnesium, inorganic phosphate, plasma proteins, or plasma osmolality. When 1/3 sea water adapted (1-4 weeks), hypophysectomized and sham-operated fish were exposed to fresh water for 2 weeks, the plasma levels of ionized calcium, chloride, sodium, and plasma osmolality were significantly lower in hypophysectomized fish when compared to sham-operated fish, while the plasma levels of total calcium, total magnesium, and proteins were higher. No effects were noted on the plasma levels of potassium and inorganic phosphate. The presented data indicate that the pituitary gland plays a significant role in the regulation of plasma calcium and chloride levels in S. gairdneri, while its role in the regulation of plasma magnesium and sodium is less clear. The pituitary gland seems to play a minimal role in the plasma regulation of inorganic phosphate and potassium.

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.

Free plasma thyroxine levels in coho salmon, Oncorhynchus kisutch, during parr-smolt transformation: comparison with total thyroxine, total triiodothyronine, and growth hormone levels

Free plasma thyroxine (FT4) levels were measured in coho salmon, Oncorhynchus kisutch, during parr-smolt transformation (smoltification) using an equilibrium dialysis system followed by a radioimmunoassay. The FT4 data were correlated to total plasma thyroxine (TT4), triiodothyronine (TT3), and growth hormone (GH). Plasma samples were taken weekly from early April to late May, when the salmon where released from the hatchery. Free thyroxine and GH levels increased gradually through smoltification. TT4 levels increased significantly in mid-April and in mid-May. TT3 levels increased in April and remained elevated until late-May after which they declined to the lowest levels. During the first increase in TT4 levels, FT4 levels remained low and TT3 levels did not increase until FT4 levels increased in late-April. In addition, after TT4 levels decreased in late May, FT4 levels remained elevated. These data show that there are differences between the plasma FT4 and TT4 profiles during smoltification. Nevertheless, regression analysis indicates that FT4 levels are highly correlated to the increases in the levels of GH (r=0.73) and TT4 (r=0.70). In addition, GH is less correlated to TT4 and TT3 (r=0.24 and r=−0.46, respectively) compared with FT4 (r=0.73), suggesting a close relationship between the increases of FT4 and GH. In addition, these data suggest that this method of measuring free plasma thyroxine may provide a new tool for studying the timing of thyroid hormone action and regulation during parr-smolt transformation in salmonids.