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

Insulin-Like Growth Factor Binding Protein-5 in Physiology and Disease

Insulin-like growth factor (IGF) signaling is regulated by a conserved family of IGF binding proteins (IGFBPs) in vertebrates. Among the six distinct types of IGFBPs, IGFBP-5 is the most highly conserved across species and has the broadest range of biological activities. IGFBP-5 is expressed in diverse cell types, and its expression level is regulated by a variety of signaling pathways in different contexts. IGFBP-5 can exert a range of biological actions including prolonging the half-life of IGFs in the circulation, inhibition of IGF signaling by competing with the IGF-1 receptor for ligand binding, concentrating IGFs in certain cells and tissues, and potentiation of IGF signaling by delivery of IGFs to the IGF-1 receptor. IGFBP-5 also has IGF-independent activities and is even detected in the nucleus. Its broad biological activities make IGFBP-5 an excellent representative for understanding IGFBP functions. Despite its evolutionary conservation and numerous biological activities, knockout of IGFBP-5 in mice produced only a negligible phenotype. Recent research has begun to explain this paradox by demonstrating cell type-specific and physiological/pathological context-dependent roles for IGFBP-5. In this review, we survey and discuss what is currently known about IGFBP-5 in normal physiology and human disease. Based on recent in vivo genetic evidence, we suggest that IGFBP-5 is a multifunctional protein with the ability to act as a molecular switch to conditionally regulate IGF signaling.

Type II Na+-phosphate Cotransporters and Phosphate Balance in Teleost Fish

Teleost fish are excellent models to study the phylogeny of the slc34 gene family, Slc34-mediated phosphate (P_i) transport and how Slc34 transporters contribute P_i homeostasis. Fish need to accumulate P_i from the diet to sustain growth. Much alike in mammals, intestinal uptake in fish is partly a paracellular and partly a Slc34-mediated transcellular process. Acute regulation of P_i balance is achieved in the kidney via a combination of Slc34-mediated secretion and/or reabsorption. A great plasticity is observed in how various species perform and combine the different processes of secretion and reabsorption. A reason for this diversity is found in one or two whole genome duplication events followed by potential gene loss; consequently, teleosts exhibit distinctly different repertoires of Slc34 transporters. Moreover, due to habitats with vastly different salinity, teleosts face the challenge of either preserving water in a hyperosmotic environment (seawater) or excreting water in hypoosmotic freshwater. An additional challenge in understanding teleost P_i homeostasis are the genome duplication and retention events that diversified peptide hormones such as parathyroid hormone and stanniocalcin. Dietary P_i and non-coding RNAs also regulate the expression of piscine Slc34 transporters. The adaptive responses of teleost Slc34 transporters to e.g. P_i diets and vitamin D are informative in the context of comparative physiology, but also relevant in applied physiology and aquaculture. In fact, P_i is essential for teleost fish growth but it also exerts significant adverse consequences if over-supplied. Thus, investigating Slc34 transporters helps tuning the physiology of commercially valuable teleost fish in a confined environment.

Molecular Physiology of the Hypocalcemic Action of Fibroblast Growth Factor 23 in Zebrafish (Danio rerio)

Fibroblast growth factor 23 (FGF23), a hormone required for phosphorus metabolism, was recently proposed to act on Ca2+ uptake; however, the available evidence of how FGF23 controls the body fluid Ca2+ homeostasis needs to be further clarified. The use of zebrafish as a model system revealed that FGF23 is specifically expressed in the corpuscles of Stannius (CS), an organ involved in Ca2+ homeostasis in fish, and that its expression is stimulated by ambient water with a high Ca2+ level. The overexpression of FGF23 inhibited Ca2+ uptake by downregulating the messenger RNA (mRNA) expression of epithelium calcium channel. Calcium-sensing receptor (CaSR), which senses changes in extracellular Ca2+ levels and modulates calciotropic hormones in organs controlling Ca2+ homeostasis in vertebrates, was found to be coexpressed with FGF23 in the CS. In addition, upregulated expression of FGF23 mRNA was detected in morphants of stanniocalcin 1 (stc1, another hypocalcemic factor synthesized in the CS), and knockdown of CaSR suppressed such upregulation and enhanced Ca2+ uptake. Taken together, our data indicate that FGF23 functions as a hypocalcemic hormone in zebrafish and that the CaSR/STC1-FGF23 axis is involved in body fluid Ca2+ homeostasis in vertebrates.

Characterization of stanniocalcin 1 binding and signaling in gill cells of Japanese eels

Stanniocalcin 1 (STC1) is a hypocalcemic hormone that is known to play an important role in calcium metabolism in teleost fish. An increase in blood Ca(2) (+) levels stimulates its synthesis and release. The biological action of STC1 inhibits gill Ca(2) (+) transport (GCAT), but we as yet have no clear understanding of how STC1 inhibits GCAT. In the present study, we characterized the binding, signaling, and action of STC1 on gill cells. Treatment of gill cell cultures with the extracts of corpuscles of Stannius or recombinant STC1 proteins (STC1-V5) led to an increase in cytosolic cAMP levels. Using in situ ligand-binding assays, we demonstrated that STC1-V5 binds to both lamellar and inter-lamellar regions of gill sections. The binding sites were significantly increased in gill sections obtained from fish adapted to high-Ca(2) (+) (2 mM) freshwater (FW) as compared with those from fish adapted to low-Ca(2) (+) (0.2 mM) FW. Receptor-binding assays illustrated specific binding of STC1-alkaline phosphatase to plasma membrane (Kd of 0.36 nM), mitochondria (Kd of 0.41 nM), and nuclear (Kd of 0.71 nM) preparations from gill cells. STC1 binding capacity was significantly greater in the plasma membrane preparations of gills obtained from fish adapted to high-Ca(2) (+) FW. Using isolated pavement cells and mitochondria-rich cells in cAMP assays, we obtained results indicating that both cell types responded to STC1. To illustrate the biological action of STC1, we conducted Ca(2) (+) imaging experiments to demonstrate the effects of STC1 on thapsigargin-induced elevation of cytosolic Ca(2) (+). Our results indicated that STC1 exerted its inhibitory action via a cAMP pathway to lower intracellular Ca(2) (+) levels. Intriguingly, we were able to block the action of STC1 using an inhibitor, NS-398, of cyclooxygenase-2 (COX-2), which is known to stimulate the activity of sarcoplasmic and endoplasmic reticulum Ca(2) (+)-ATPase (SERCA). A follow-up experiment in which gill cells were incubated with STC1 revealed a downregulation of the epithelial Ca(2) (+) channel (ecacl) but an upregulation of cox-2 expression. The ECaCl is a gatekeeper for Ca(2) (+) entry, whereas COX-2 mediates an activation of SERCA. Taking these results together, the present study is, to our knowledge, the first to provide evidence of STC1 binding and signaling as well as the first to decipher the mechanism of the effect of STC1 on fish gills.

Synergistic effect of p53 on TSA-induced stanniocalcin 1 expression in human nasopharyngeal carcinoma cells, CNE2

Human stanniocalcin 1 (STC1) has recently been identified as a putative protein factor involved in cellular apoptosis. The use of histone deacetylase inhibitor (i.e. trichostatin A (TSA)) and doxorubicin (Dox) is one of the common treatment methods to induce apoptosis in human cancer cells. A study on TSA and Dox-mediated apoptosis may shed light on the regulation and function of STC1 in cancer treatment. In this study, TSA and Dox cotreatment in human nasopharyngeal carcinoma cells (CNE2) elicited synergistic effects on STC1 gene expression and cellular apoptosis. An activation of p53 (TP53) transcriptional activity in Dox- or Dox+TSA-treated cells was revealed by the increased expression levels of p53 mRNA/protein as well as p53-driven luciferase activities. To elucidate the possible involvement of p53 in STC1 gene transcription, a vector expressing wild-type or dominant negative (DN) p53 was transiently transfected into the cells. Both STC1 promoter luciferase constructs and chromatin immunoprecipitation assays did not support the direct role of p53 in STC1 gene transactivation. However, the synergistic effects of p53 on the induction of NF-κB phosphorylation and the recruitment of acetylated histone H3 in STC1 promoter were observed in TSA-cotreated cells. The overexpression of exogenous STC1 sensitized apoptosis in Dox-treated cells. Taken together, this study provides data to show the cross talk of NF-κB, p53, and histone protein in the regulation of STC1 expression and function.

Evolution and roles of stanniocalcin

In fish, stanniocalcin-1 (STC1) is a key endocrine factor that acts on gill, intestine and kidney to regulate serum calcium and phosphate homeostasis. The recent identification and study of mammalian STCs (STC1 and STC2) revealed that the hormones are made in virtually all tissues and they act primarily as paracrine/autocrine factors to regulate various biological functions. Based on their ubiquitous expression patterns and generally undetectable levels in blood serum, it is unlikely that the mammalian STCs play important roles in serum Ca(2+)/P(i) homeostasis. However current evidences still support the local action of STCs in Ca(2+) and P(i) transport, probably via their action on Ca(2+)-channels and Na(+)/P(i) co-transporter. At present, information about the sequence, expression and distribution of the STC receptor(s) is lacking. However, recent emerging evidence hints the involvement of STC1 and STC2 in the sub-cellular functions of mitochondria and endoplasmic reticulum respectively, particularly responding to oxidative stress and unfolded protein response. With increasing evidence that demonstrates the local actions of STCs, the focus of the research has been moved to cellular inflammation and carcinogenesis. This review integrates the information available on STCs in fish and mammals, focusing mainly on their embryonic origin, tissue distribution, their potential regulatory mechanisms and the modes of action, and their physiological and pathophysiological functions, particularly in cancer biology.

Characterization of stanniocalcin-1 receptors in the rainbow trout

Mammalian stanniocalcin-1 (STC-1) is one of several ligands targeted to mitochondria. High affinity STC-1 receptors are present on the mitochondrial membranes of nephron cells, myocytes, and hepatocytes, to enable ligand sequestration within the matrix. However, STC-1 receptors have not been characterized in fish. Nor is it known if mitochondrial targeting occurs in fish. The aim of the study was to address these questions. Saturation binding assays were carried out to obtain estimates of K_D and B_max. They revealed the presence of saturable, high-affinity receptors on both membranes and mitochondria of liver, muscle, and gill filament. In situ ligand binding (ISLB) was used to localize receptors at the histological level and revealed some unexpected findings. In cranium, for instance, receptors were found mainly in the cartilage matrix, as opposed to the chondrocytes. In brain, the majority of receptors were located on neuropil areas as opposed to neuronal cell bodies. In skeletal muscle, receptors were confined to periodic striations, tentatively identified as the Z lines. Receptors were even found on STC-1 producing corpuscles of Stannius cells, raising the possibility of there being an autocrine feedback loop or, perhaps, a soluble binding protein that is released with the ligand to regulate its bioavailability.

Effects of stanniocalcin 1 on calcium uptake in zebrafish (Danio rerio) embryo

Stanniocalcin (STC) formerly called hypocalcin or teleocalcin, is a 50-kDa disulfide-linked homodimeric glycoprotein that was originally identified in fish and secreted from the corpuscles of Stannius (CS). One of the main functions of STC-1 is Ca(2+) uptake inhibition; however, the mechanisms remain unknown. In the present study, we provide molecular evidence to elucidate how zebrafish STC-1 regulates Ca(2+) uptake in zebrafish embryos. In a wide variety of tissues including the kidney, brain, gill, muscle, and skin, zstc-1 was expressed. Incubating zebrafish embryos in low-Ca(2+) (0.02 mM) freshwater stimulated whole body Ca(2+) influx and zebrafish epithelial Ca(2+) channel (zECaC) mRNA expression, while downregulated zstc-1 expression. A morpholino microinjection approach was used to knockdown the zSTC-1 protein, and the results showed that the Ca(2+) content, Ca(2+) influx, and zECaC mRNA expression all increased in morphants. These data suggest that zSTC-1 negatively regulates ECaC gene expression to reduce Ca(2+) uptake in zebrafish embryos.

Interactions of waterborne and dietary cadmium on the expression of calcium transporters in the gills of rainbow trout: influence of dietary calcium supplementation

Recent studies have shown that dietary Ca(2+) supplementation strongly inhibits uptake of Ca(2+) and Cd at the fish gill. To better understand the influence of dietary Ca(2+) on branchial Ca(2+) transport, we examined the expression of two trout gill calcium transporters during waterborne and dietary Cd exposure, at two different levels of dietary Ca(2+). Quantitative polymerase chain reaction (PCR) was used to monitor epithelial calcium channel (ECaC) and sodium-calcium exchange (NCX) mRNA levels following 7-28 days of exposure to these treatments. In brief, juvenile rainbow trout (Oncorhynchus mykiss) were exposed to control, 3 microg/L waterborne Cd, 500 mg/kg dietary Cd, or a combined 3 microg/L waterborne plus 500 mg/kg dietary Cd exposure, supplemented with either 20 mg/g or 60 mg/g dietary calcium (Ca(2+)). Two-way analysis of variance was used to discern the main effects of Cd exposure and dietary Ca(2+) supplementation on ECaC and NCX mRNA levels. We found that dietary Ca(2+) supplementation decreased significantly ECaC mRNA expression on days 14 and 21. In comparison, NCX mRNA levels were not influenced by dietary Ca(2+) supplementation, but rather were significantly inhibited in the combined waterborne and dietary Cd exposure on day 7 alone. Statistical analysis found no interactive effects between Cd exposure and dietary Ca(2+) exposure at any time point, except for day 28. This study provides evidence of the importance of nutritional status on the transcriptional regulation of ion transport at the fish gill. We discuss the importance of diet and nutritional status to the development of new regulatory approaches, such as the biotic ligand model, which currently do not account for the significance of diet on metal bioavailability in aquatic organisms.

Endothelin and calciotropic hormones share regulatory pathways in multidrug resistance protein 2-mediated transport

The kidney of vertebrates plays a key role in excretion of endogenous waste products and xenobiotics. Active secretion in the proximal nephron is at the basis of this excretion, mediated by carrier proteins including multidrug resistance protein 2 (Mrp2). We previously showed that Mrp2 function is reduced by endothelin-1 (ET-1) through a basolateral B-type receptor, nitric oxide (NO), cGMP, and PKC (Notenboom S, Miller DS, Smits P, Russel FGM, Masereeuw R. Am J Physiol Renal Physiol 282: F458-F464, 2002; Notenboom S, Miller DS, Smits P, Russel FG, Masereeuw R. Am J Physiol Renal Physiol 287: F33-F38, 2004). This pathway was rapidly activated by several nephrotoxicants and appeared to be calcium dependent. In the present study, we studied the effect of the calciotropic hormones parathyroid hormone (PTH), PTH-related protein (PTHrP), and stanniocalcin (STC) to interfere with ET-regulated Mrp2 transport. Like ET-1, PTH reduces Mrp2-mediated transport by 40% in killifish renal proximal tubules. When given in combination, an additive effect was seen, which is partially reversed by the PKC inhibitor calphostin C. Recombinant PTHrP shows a comparable inhibitory effect, which is concentration dependent and additive to the inhibition by ET. STC fully reverses PTHrP-inhibited transport as does a guanylyl cyclase inhibitor. Finally, to confirm PTHrP bioactivity in a homologous assay, we performed immunolocalization and transport studies in sea bream kidney tubules. Mrp2 immunoreactivity was observed in approximately 40% of the tubules and is associated with the brush-border and apical plasma membrane of cells. Both proximal tubules and distal (collecting) tubules express the antigen. A highly significant 40% inhibition of Mrp2-mediated transport was observed with PTHrP in sea bream tubules. In conclusion, ET-regulated Mrp2 transport is influenced by calciotropic hormones and involves PKC and cGMP signaling.

Passive immunization of lactating mice with stanniocalcin-1 antiserum reduces mammary gland development, milk fat content, and postnatal pup growth

During pregnancy and lactation in rodents, stanniocalcin-1 (STC-1) production by the ovaries is upregulated markedly and released into the circulation. The mammary glands are one target of this systemically delivered hormone. The purpose of this study was to lower serum levels of STC-1 in lactating mice through passive immunization so as to monitor the effects on mammary gland function and postnatal pup growth. Passive immunization significantly reduced circulating hormone levels, and pup growth was significantly compromised (30%), even though control and experimental litters had ingested equal amounts of milk. When mammary glands were analyzed, the alveolar area was significantly reduced in antibody-treated mothers. An analysis of milk composition revealed no changes in lactose, protein, or electrolyte levels but an approximately 40% reduction in triglyceride levels. The latter was due to a significant reduction in mammary gland lipoprotein lipase activity and led to a significant buildup of triglycerides in the serum. Body fat content was also significantly reduced in pups from antibody-treated mothers, whereas pup fecal fat content was increased. In mothers, passive immunization also caused significant behavioral effects, in particular, increased locomotor and hindleg rearing activities. Collectively, the results suggest that systemically derived STC-1 has important effects on mammary gland development and the transfer of serum-based triglycerides into milk. Locomotor effects suggest that STC-1 also has a role in maternal behavior.

The stanniocalcin family of proteins

Stannniocalcin (STC) is a polypeptide hormone that was originally identified in bony fishes as a systemic regulator of mineral metabolism, and is best known for its regulatory effects on calcium/phosphate transport by the gills, gut and kidneys. The mammalian homolog to fish STC was discovered in 1995 and has resulted in progressively growing interest ever since as to its possible role in humans. Moreover, new discoveries in the mammalian STC field are resulting in significant reappraisals as to its role in fishes. Perhaps the most significant of these has been the discovery of a second gene encoding stanniocalcin-related protein, or STC-2, first in mammals and subsequently in fish. This review covers the comparative endocrinology of the STCs in fishes and mammals from the perspectives of structure, function and regulation. It then delves into some of the newer aspects of STC-1/STC-2 biology that have been uncovered using both classical and transgenic approaches. Of these, one of the most intriguing discoveries relates to the receptor-mediated sequestration of STC by target cell organelles. The functions of other newly discovered mammalian and fish STC variants are also discussed, as is the recent discovery of STC-related homologs in invertebrates. Based on our current state of knowledge, it is apparent that STC has an ancient lineage and that the STC family of proteins is proving to have significant roles in metabolism, reproduction and development.

Stanniocalcin: no longer just a fish tale

Stanniocalcin was originally described as a hormone with calcitonin-like actions in fish. During the last decade, mammalian forms of stanniocalcin have been identified, and this discovery has led to important advances in our understanding of this enigmatic polypeptide hormone. This review briefly covers some early studies on stanniocalcin in fish and then provides a more in-depth look at some of the more intriguing, new aspects of its functions in mammals. The roles of stanniocalcin in renal function, metabolism, angiogenesis, pregnancy and lactation, bone formation, and neural protection are discussed, along with new information relating to its receptor-mediated sequestration and accumulation in target cell organelles.

Evidence for stanniocalcin and a related receptor in annelids

Stanniocalcin (STC) is a prime example of a hormone whose discovery in fish led to its subsequent discovery in mammals. STC is considered to be first and foremost a vertebrate polypeptide hormone with regulatory effects on ion transport, mitochondrial function and steroid hormone synthesis. The gene is widely expressed in both fishes and mammals, and the hormone can operate via both local and endocrine signaling pathways. In spite of the growing catalogue of vertebrate hormones and receptors with homologues in invertebrates, the notion that there might be an invertebrate STC homolog has received scant attention to date. In the present study, we have provided evidence for STC in annelid worms (freshwater leeches). Western blot analysis revealed the presence of two STC immunoreactive (STCir) proteins in leech tissue extracts of 100 and 193 kDa. These same extracts significantly lowered the rate of gill calcium transport upon injection into fish. Similarly, fish STC increased the rate of whole body calcium uptake when administered to leeches, and STC receptors of high affinity were identified on isolated leech plasma membranes. Two discrete populations of STC-positive cells were also identified in leeches using antibodies to fish STC and fish STC cRNA probes. One of the cell types was confined to the skin. The second cell type was confined to the coelomic cavity and identified as an adipose cell, which in leeches is a major repository of fat. Collectively, the data constitutes compelling evidence for the existence of STC-related proteins and receptors in annelids that share structural and functional similarities with those in vertebrates.

Primary structure of stanniocalcin in two basal Actinopterygii

The primary structure of stanniocalcin (STC), the principal product of the corpuscles of Stannius (CS) in ray-finned fishes, was deduced from STC cDNA clones for two species of holostean, the gar, Lepisosteus osseus and the bowfin, Amia calva. Overlapping partial cDNA clones were amplified by polymerase chain reaction (PCR) from single-strand cDNA of the CS. Excluding the poly(A) tail, the cDNAs of 1863 base pairs [bp] (gar) and 914 bp (bowfin) contained the 5' untranslated region followed by the coding region and the 3' untranslated region. Both the gar and bowfin STC cDNA encode a prehormone of 252 amino acids (aa) with a signal peptide of 32 aa and a mature protein of 220 aa. The deduced aa sequence of gar STC shows 87% identity with bowfin STC, 60-72% identity with most vertebrate STCs and 26% identity with mouse STC2. Phylogenetic analysis of the sequences support a view that the gar and bowfin form a monophyletic holostean clade. RT-PCR revealed in the gar and bowfin that, just as in mammals and rainbow trout, the expression of STC mRNA is widely spread in many tissues and organs. Since the gar and bowfin are representatives of the most ancient fishes known to possess CS, the corpuscular-derived STC molecule in fish has had a conserved evolution.

Calcitropic peptides: neural perspectives

In mammals and higher vertebrates, calcitropic peptides are produced by peripheral endocrine glands: the parathyroid gland (PTH), thyroid or ultimobranchial gland (calcitonin) and the anterior pituitary gland (growth hormone and prolactin). These hormones are, however, also found in the neural tissues of lower vertebrates and invertebrates that lack these endocrine organs, suggesting that neural tissue may be an ancestral site of calcitropic peptide synthesis. Indeed, the demonstration of CNS receptors for these calcitropic peptides and their induction of neurological actions suggest that these hormones arose as neuropeptides. Neural and neuroendocrine roles of some of these calcitropic hormones (calcitonin and parathyroid hormone) and related peptides (calcitonin gene related peptide, stanniocalcin and parathyroid hormone related peptide) are thus the focus of this review.

Stanniocalcin in the seawater salmon: structure, function, and regulation

Stanniocalcin (STC) is a homodimeric glycoprotein hormone that was first discovered in fish, where it is produced by unique endocrine glands known as the corpuscles of Stannius (CS). In freshwater salmon, STC plays an integral role in Ca2+ and phosphate homeostasis. High levels of extracellular Ca2+ promote the synthesis and release of STC, which on entering the bloodstream reduces the levels of gill and gut Ca2+ transport and renal phosphate excretion to restore normocalcemia. In this report, we have examined STC in seawater salmon. We have studied the distribution of STC protein and mRNA in marine Atlantic salmon CS cells, the responsiveness of these cells to Ca2+, and some physical properties of the hormone. Our results demonstrated that all Atlantic salmon CS cells expressed the STC gene. Furthermore, these cells exhibited a Ca2+ sensitivity that was remarkably similar to those in freshwater salmon in terms of its ability to stimulate STC secretion and gene expression. When Atlantic salmon glands were fractionated by concanavalin A (ConA)-Sepharose chromatography, two distinct forms of the hormone were identified, both of which were recognized by sockeye salmon STC antiserum, and designated as STC1 and STC2. STC1 was a glycosylated, 42-kDa disulfide-linked dimer, with a high affinity for ConA. STC2 did not bind to ConA, was 44 kDa in size, and had a different subunit structure. STC2 was also a less effective inhibitor of gill Ca2+ transport in fish. Collectively, the results suggest that there is a second form of STC in salmon.

Expression, purification, and bioassay of human stanniocalcin from baculovirus-infected insect cells and recombinant CHO cells

Stanniocalcin is a calcium- and phosphate-regulating glycoprotein hormone that was first described in fish where it functions in preventing hypercalcemia. Human cDNA clones encoding the homolog of stanniocalcin have been recently isolated. In this study, the full-length cDNA coding for human stanniocalcin (hSTC) was cloned into both baculovirus and CHO expression vectors. Recombinant hSTC was then produced efficiently from both baculovirus-infected insect cells and CHO cells in large-scale bioreactors. Purification protocols were developed and used to purify recombinant hSTC from both sources in four chromatography steps. The hSTCs from both expression systems were secreted as glycosylated proteins and as disulfide-linked homodimers. The results from glycosylation studies indicated that stanniocalcin from both sources contained N-linked oligosaccharides but no O-linked sugars. In an in vivo bioassay based on the inhibition of gill calcium transport in fishes, the baculovirus and CHO-expressed protein showed biological activity which is dose dependent. The inhibitory effects of hSTC produced from both systems were essentially equipotent in fishes, despite the differences in glycosylation. Consequently, the precise role of the carbohydrate moiety in recombinant hSTC remains to be determined.

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.

Calcium regulation in the freshwater-adapted mummichog

In light of recent findings of an unusual pattern of ionoregulation (high Na+ uptake and negligible Cl- uptake) in the freshwater-adapted mummichog Fundulus heteroclitus, the pattern of Ca2+ regulation was examined. Under control conditions (water Ca2+=200 μEq l-1), unidirectional Ca2+ influx was 11±4 nEq g-1 h-1. Acute variation of external Ca2+ levels revealed a saturable Ca2+ uptake system with a relatively high affinity (Km=125±36 μEq l-1) and a transport capacity (Jmax=31±4 nEq g-1 h-1) comparable to those of other teleosts. Lanthanum (equimolar to [Ca2+]) significantly blocked Ca2+ uptake by 67% whereas mag-nesium had no effect. Chronic low Ca2+ exposure (50 μEq l-1) stimulated Ca2+ uptake almost three-fold above control values, whereas chronic high Ca2+ exposure (20 000 μEq l-1) had no effect. Lanthanum and chronic low Ca2+ treatments disturbed the normally positive Ca2+ and Na+ balances of the animals whereas acid-base balance and ammonia excretion were undisturbed. The results indicate that Ca2+ regulation by the mummichog conforms to the model for freshwater Ca2+ transport whereby chloride cells on the gills take up Ca2+ actively from the water. However, the absence of extra-intestinal Cl- uptake and the recent demonstration of significant Ca2+ uptake by opercular epithelia raise questions about the relative roles of branchial and opercular epithelial chloride cells in freshwater F. heteroclitus. 1997 The Fisheries Society of the British Isles

Development of a dose-response bioassay for stanniocalcin in fish

Stanniocalcin (STC) is a polypeptide hormone that was first discovered in fish and recently identified in mammals. In fish, STC is released into the bloodstream in classical endocrine fashion and has well established regulatory effects on calcium and phosphate homeostasis. However, there are no suitable dose-response bioassays for STC and consequently no methods for assigning units of potency to preparations of the hormone. All the available in vitro bioassays are too complex from a technical standpoint to readily accommodate the large number of samples required in dose-response bioassays. Most in vivo bioassays are hampered by the fact that fish have natural rhythms governing plasma STC levels which tend to make them variably sensitive to the injected hormone. In this report we have developed a new in vivo bioassay for STC using rainbow trout. The key feature of the bioassay involves suppressing plasma STC levels to the extent that fish are always receptive to injected hormone. This has been accomplished by phosphate-loading the animals, which lowers their plasma calcium levels, removes the stimulus for STC secretion and brings about a reduction in resting plasma hormone levels. The net effect is an animal that is always responsive to injected STC. With this bioassay we have been able to obtain sensitive and reproducible, dose-related effects of salmon STC on gill calcium transport.

Human stanniocalcin inhibits renal phosphate excretion in the rat

Stanniocalcin (STC) is a glycoprotein hormone first identified in bony fishes where it counteracts hypercalcemia by inhibiting gill calcium uptake and stimulating renal inorganic phosphate (Pi) reabsorption. Human STC (hSTC) has recently been cloned and sequenced and is highly homologous to the fish hormone at the amino acid level. The objective of this study was to examine the possible effects of hSTC on electrolyte homeostasis and renal function in the rat. Recombinant hSTC was expressed in bacteria and purified by metal-ion affinity chromatography and reverse-phase high performance liquid chromatography. Anesthetized animals were given bolus infusions of 1, 5, or 10 nmol hSTC per kilogram of body weight. Control animals received solvent alone. The most effective dosage was 5 nmol/kg, which caused significant reductions in both absolute and fractional phosphate excretion in comparison with control rats. The hSTC had no effect on the renal excretion of other ions, the glomerular filtration rate, renal blood flow, blood pressure, or plasma electrolytes (Na+, K+, Ca2+, Pi, Mg/+). The maximum effect of hSTC on phosphate excretion was observed 60-80 minutes postinjection. Lesser effects were obtained with higher and lower dosages of hormone. When renal cortical brush-border membrane vesicles were isolated from control and hormone-treated animals 80 minutes postinjection, the rate of Na+/Pi cotransport was found to be 40% higher in vesicles from hormone-treated animals (p < 0.01; 5 nmol hSTC/kg). Together, the renal clearance and membrane vesicle data indicate that hSTC participates in the renal regulation of Pi homeostasis in mammals.

Immunological and biological evidence for a stanniocalcin-like hormone in human kidney

The corpuscles of Stannius are responsible for the synthesis and secretion of stanniocalcin (STC), a glycoprotein hormone that regulates calcium and phosphate homeostasis in fishes through its actions on the gills and kidneys. The corpuscles of Stannius and STC are considered to be an endocrine system that is unique to fishes. In this report, we provide evidence for the existence of STC-like proteins in vertebrates other than fishes, in particular, humans. By using a well-characterized RIA for salmon STC, sera from vertebrates as diverse as sharks and humans contained measurable levels of STC-like immunoreactivity in the concentration range commonly observed in fishes, and all of these sera exhibited parallelism in the assay. By using Western blot analysis, proteins were also identified in human kidney extracts that shared several properties with the fish hormone in addition to their cross-reactivity with salmon STC antiserum. The same antiserum was used to identify a discrete population of cells in human kidney tubules that could be the source of serum immunoreactivity. Human kidney extracts containing the STC-immunoreactive proteins also had STC-related effects when injected into fishes. Collectively, the data suggest that STC may be more widespread among the vertebrates than is currently accepted.

Calcium regulates stanniocalcin mRNA levels in primary cultured rainbow trout corpuscles of stannius

Stanniocalcin (STC) is an inhibitor of gill calcium transport produced by the corpuscles of Stannius (CS), endocrine glands in bony fishes. In previous studies we have described how STC secretion is regulated by calcium both in vitro and in vivo, using rainbow trout as a model system. In this report we have examined the effects of calcium on STC mRNA levels in primary cultured trout CS cells. The results show that message levels are positively regulated by extracellular calcium concentrations within the physiological range. The calcium response was also temporally-related as more prolonged exposures tended to have greater effects. Similar concentrations of magnesium had no effect on message levels. This represents another level at which calcium regulates the CS cell, in addition to its established effects on STC synthesis and secretion. The results are discussed in relation to the other known calciotropic hormones, calcitonin and parathyroid hormone.