Calcium is an equipotent stimulator of stanniocalcin secretion in freshwater and seawater salmon.

Stanniocalcin (STC) is a calcium- and phosphate-regulating hormone produced by the corpuscles of Stannius in fishes. A rise in ion calcium (Ca2+) levels is the principal stimulus for secretion, and the hormone acts on the gills, gut, and kidneys to restore normocalcemia. The STC-producing cells in marine fishes are metabolically more active and secrete more hormone than those in freshwater fishes, which has been attributed to the higher calcium content of seawater placing a greater burden on the organ systems governing Ca2+ homeostasis. In this study we have addressed the question of whether or not the STC cells in marine fishes are more sensitive to Ca2+, by comparing the secretagogic effects of Ca2+ in freshwater- and seawater-adapted coho salmon. The results showed that the STC cells were equally Ca(2+)-sensitive in the two groups. Therefore, in spite of the fact that the STC cells are more active in marine fishes this requires no apparent adjustment in cellular sensitivity to calcium.

The gill calcium transport cycle in rainbow trout is correlated with plasma levels of bioactive, not immunoreactive, stanniocalcin.

Stanniocalcin (STC) is an inhibitor of gill Ca2+ transport that is produced by the corpuscles of Stannius, endocrine glands in bony fish. In young rainbow trout (Oncorhynchus mykiss), there are cyclical changes in the rate of gill Ca2+ transport, with alternating phases of accelerated and reduced uptake every 14 days. Previous studies by our laboratory have established that the responsiveness of young trout to the inhibitory effects of exogenous STC is dependent on this cycle. Trout are highly responsive to STC at peaks of Ca2+ uptake and unresponsive at nadirs, which has led us to suggest that the gill Ca2+ transport cycle may be regulated by a reciprocal cycle in the levels of plasma STC. In this report, we have further characterized the gill Ca2+ transport cycle in salmonids and investigated the role of STC in its regulation. Our results showed that the cycle is synchronous and is likely a characteristic feature in all salmonids but that it varies in amplitude between species. Surprisingly, we observed no correlation between circulating levels of radioimmunoassayable STC and the rate of gill Ca2+ transport in trout. To address this apparent contradiction, trout fry were passively immunized with STC antiserum to determine if there were variable amounts of bioactive STC in the circulation, at times when trout were either more or less sensitive to exogenous STC. We observed that during the times when trout were responsive to STC treatment (i.e., cycle peaks), passive immunization had no effect on the rate of gill Ca2+ transport in fish from the same population, indicating that there were low levels of bioactive STC in the circulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Salmon stanniocalcin and bovine parathyroid hormone have dissimilar actions on mammalian bone.

Stanniocalcin (STC), a calcium-regulating glycoprotein hormone isolated from the corpuscles of Stannius of salmon, was tested for effects on bone and calcium metabolism in mammalian species (rats and mice). STC generally failed to alter serum calcium of parathyroidectomized rats at concentrations equimolar with effective concentrations of parathyroid hormone (PTH). STC did not increase cAMP in ROS 17/2.8 or UMR-108 osteosarcoma cells, OK kidney cells, fetal rat limb bones, or neonatal mouse calvariae, and similarly failed to increase urinary cAMP in rats. STC did not consistently stimulate resorption in any of the rodent bone culture systems, although variable resorptive responses were elicited in fetal mouse calvariae. The results indicate that this fish hormone has limited, if any, PTH-like activity on calcium metabolism in mammalian systems.

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.

Effects of synthetic peptide fragments of teleocalcin (hypocalcin) on calcium uptake in juvenile rainbow trout (Salmo gairdneri).

A rainbow trout fry bioassay based on 45Ca uptake was used to compare the effects of pure coho salmon teleocalcin (TC) and several synthetic peptide fragments of TC. Calcium uptake in the fry exhibited a cycle, with an amplitude variation of 3.3 to 48.8 mumol.kg-1.hr-1 and a periodicity of 8 to 21 days. The N-terminal 1-20 amino acid peptides of both eel and salmon TC significantly inhibited 45Ca uptake at the high point of the calcium uptake cycle (up to 75%), although the effective doses of the peptides on a molar basis were 20 to 200 times that of the intact molecule. In contrast, the C-terminal fragment of eel TC (amino acids 202-231) did not have an inhibitory effect on calcium uptake. Instead, it significantly enhanced 45Ca uptake in trout fry (up to sixfold) at the low point of the calcium uptake cycle.

The effect of prolactin on calcium homeostasis in coho salmon (Oncorhynchus kisutch).

The response of plasma prolactin (PRL) to manipulations of plasma calcium were investigated in coho salmon. Injection of 3.16 mumol EGTA/10 g body weight (to reduce the plasma calcium activity) resulted in significantly higher plasma PRL levels. Lower doses of EGTA (1.06 and 2.13 mumol/10 g body wt) had no effect on plasma PRL concentrations. Injection of 8.75 mumol CaCl2/10 g body wt (but not 4.35 mumol CaCl2/10 g body wt) resulted in significantly lower plasma PRL levels. Plasma total calcium and sodium concentrations were significantly elevated following three daily injections of 0.5 micrograms/g pure chum salmon PRL. This is the first report of a role for salmon PRL in calcium homeostasis of Oncorhynchus.

The localization of labelled fish prolactin in various tissues of rainbow trout (Salmo gairdneri).

Rostral lobes of the pars distalis from rainbow trout, Salmo gairdneri, were incubated in vitro in a medium containing 14C-labelled lysine. The labelled proteins in these lobes and medium were separated by polyacrylamide gel electrophoresis, the prolactin eluted from the appropriate band, and injected into intact trout. Following the injections, various tissues were dissected out and observed autoradiographically. There was no binding of labelled prolactin to tissues of pituitary gland, thyroid, pyloric caecum, stomach, pancreas or muscle. There was, however, significant labelling in liver, intestine, kidney, bladder, skin and gill. The binding of labelled chum salmon prolactin to these latter tissues in vitro was significantly reduced when unlabelled hormone was also added.

Further characterization of growth hormone from the chum salmon (Oncorhynchus keta).

This report describes the isolation of growth hormone (GH) from the chum salmon (Oncorhynchus keta) pituitary using gel, affinity, and ion exchange chromatography. Chum GH has an estimated molecular weight of 23,500 and an amino acid composition that is consistent with a vertebrate GH. The differentially charged forms of chum GH which are only apparent under alkaline conditions were separated by ion exchange and compared immunologically and biologically; Peak I, which consists of a single band (Rf = 0.35) under alkaline electrophoresis and Peak II which consists of two bands with Rf's of 0.41 and 0.45. Both forms were found to be immunologically identical by immunodiffusion and to have similar growth promoting properties in intact rainbow trout (Salmo gairdneri). Chum GH was also active in the rat tibia test at a daily dosage of 70 micrograms/animal. The results are discussed in relation to previous studies with chum GH and other fish GHs.