Responsiveness of gill Na+/K+-ATPase to cortisol is related to gill corticosteroid receptor concentration in juvenile rainbow trout

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Temporal changes in intestinal Na+, K+-ATPase activity and in vitro responsiveness to cortisol in juvenile chinook salmon

Seasonal changes in endogenous Na+, K(+)-ATPase activity were measured in pyloric ceca and posterior intestine of juvenile chinook salmon (Oncorhynchus tshawytscha) maintained in fresh water over 18 months. In tissues from these same fish, the in vitro responsiveness of Na+, K(+)-ATPase activity to 10 microg cortisol/ml was assessed. There were pronounced increases in endogenous Na+, K(+)-ATPase activity in summer for both intestinal regions, in underyearlings and yearlings. In pyloric ceca, a significant positive response of Na+, K(+)-ATPase activity to cortisol, in vitro, was restricted to the months preceding increases in endogenous Na+, K(+)-ATPase and the month afterward. Na+, K(+)-ATPase activity of the posterior intestine was only responsive to cortisol in underyearlings in the period before the peak in endogenous enzyme activity. At a time when explants were responsive to cortisol, in vitro exposure to 0.1-10 microg cortisol/ml resulted in dose-dependent elevations of Na+, K(+)-ATPase activity over controls (0 microg cortisol/ml). The results show that the intestine exhibits increased enzymatic potential for water absorption that is indicative of parr-smolt transformation. Alterations in tissue responsiveness to cortisol may contribute to these changes in Na+, K(+)-ATPase activity of pyloric ceca.

Changes in gene expression in gills of the euryhaline killifish Fundulus heteroclitus after abrupt salinity transfer

Maintenance of ion balance requires that ionoregulatory epithelia modulate ion flux in response to internal or environmental osmotic challenges. We have explored the basis of this functional plasticity in the gills of the euryhaline killifish Fundulus heteroclitus. The expression patterns of several genes encoding ion transport proteins were quantified after transfer from near-isosmotic brackish water [10 parts/thousand (ppt)] to either freshwater (FW) or seawater (SW). Many changes in response to SW transfer were transient. Increased mRNA expression occurred 1 day after transfer for Na(+)-K(+)-ATPase-alpha(1a) (3-fold), Na(+)-K(+)-2Cl(-)-cotransporter 1 (NKCC1) (3-fold), and glucocorticoid receptor (1.3-fold) and was paralleled by elevated Na(+)-K(+)-ATPase activity (2-fold). The transient increase in NKCC1 mRNA expression was followed by a later 2-fold rise in NKCC protein abundance. In contrast to the other genes studied in the present work, mRNA expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel generally remained elevated (2-fold) in SW. No change in protein abundance was detected, however, suggesting posttranscriptional regulation. The responses to FW transfer were quite different from those to SW transfer. In particular, FW transfer increased Na(+)-K(+)-ATPase-alpha(1a) mRNA expression and Na(+)-K(+)-ATPase activity to a greater extent than did SW transfer but had no effect on V-type H(+)-ATPase expression, supporting the current suggestion that killifish gills transport Na(+) via Na(+)/H(+) exchange. These findings demonstrate unique patterns of ion transporter expression in killifish gills after salinity transfer and illustrate important mechanisms of functional plasticity in ion-transporting epithelia.

Influence of cortisol on osmoregulation and energy metabolism in gilthead seabream Sparus aurata

Gilthead seabream Sparus aurata were injected intraperitoneally with slow-release implants of coconut oil alone or containing cortisol (50 and 100 microg x g(-1) body weight), and sampled after two, five, and seven days to assess the simultaneous effects of cortisol on both osmoregulation and energy metabolism. Plasma cortisol levels increased in treated fish to 50-70 ng x ml(-1). An enhanced hypoosmoregulatory capacity of cortisol-implanted fish is suggested by the increase observed in gill Na+, K+-ATPase activity, and the decrease observed in plasma ion concentration (Na+ and Cl-) and osmolality. Cortisol also elicited metabolic changes in liver (increased gluconeogenic potential suggested by elevated FBPase activity, and decreased potential of glycolysis and pentose-phosphate shunt, suggested by the decreased activities of both PK and G6PDH) supporting changes in levels of plasma metabolites suitable for use in other tissues. Thus in this study, we demonstrate for the first time in fish that cortisol treatments elicit changes in the use of exogenous glucose in gills (decreased HK activity) and an increased glycolytic and glycogenic potential in brain (increased GPase, PK and PFK activities).

Cortisol treatment affects glucocorticoid receptor and glucocorticoid-responsive genes in the liver of rainbow trout

We investigated whether longer-term cortisol exposure modified hepatic glucocorticoid receptor (GR) status and tissue responsiveness to cortisol stimulation in rainbow trout. Fish were given intraperitoneal implants of cortisol (50mg/kg body mass) and this led to elevated plasma cortisol levels mimicking chronically stressed salmonids. There was significantly higher hepatic GR mRNA abundance, despite a drop in GR protein content in the liver of cortisol-treated fish. The tissue responsiveness to cortisol stimulation was apparent from the higher plasma glucose concentration and liver glycogen content. Also, the higher phosphoenolpyruvate carboxykinase (PEPCK) mRNA abundance, a key glucocorticoid-responsive gene, by cortisol suggests activation of the GR signalling pathway. There was no significant effect of cortisol treatment on liver PEPCK, alanine aminotransferase, aspartate aminotransferase, and lactate dehydrogenase activities compared to the sham fish. The higher heat shock protein (hsp) 90 mRNA abundance and a corresponding elevation in this protein and constitutive hsp70 (hsc70) protein content in the cortisol-treated fish reflects a role for glucocorticoids in the hepatic stress response process. Taken together, the molecular and biochemical responses evident in the liver of trout imply changes favouring tissue responsiveness to glucocorticoids and may be a mechanism to offset GR protein downregulation evident with chronic cortisol stimulation in rainbow trout.

Glucocorticoid receptor upregulation during seawater adaptation in a euryhaline teleost, the tilapia (Oreochromis mossambicus)

Cortisol is an important seawater (SW) osmoregulatory hormone in the Mozambique tilapia (Oreochromis mossambicus), a highly euryhaline cichlid able to live in environments ranging from fresh water (FW) to salinities well in excess of full-strength seawater. Previous studies indicate that cortisol may promote SW adaptation by increasing gill chloride cell differentiation, Na(+)/K(+)-ATPase activity and subsequent excretion of excess salt following seawater acclimation. Despite cortisol's widely accepted role as a SW-adapting hormone, cortisol receptor regulation during SW acclimation is not well understood. The purpose of these studies was to determine whether the intracellular glucocorticoid receptor (GR) might be regulated in a manner consistent with cortisol's actions in SW adaptation. Saturation radioligand binding assays were conducted on gill cytoplasm preparations from fish sampled 4 and 24h and 4 and 14 days after transfer from FW to 2/3 SW or FW (control). Affinity (K(d)) of the gill GR remained constant over the timecourse, while numbers of receptors (B(max)) in SW fish were significantly elevated compared with controls at 24h and 4 days after transfer. Plasma osmolality was higher in fish transferred to SW for 24h, 4 days, and 14 days compared with those animals moved to FW. Plasma cortisol levels and hepatic cortisol binding remained constant between SW and FW fish throughout the timecourse of the salinity challenge. These studies indicate that seawater acclimation is accompanied by a specific upregulation of intracellular GR numbers in gill tissue. The lack of increase in circulating cortisol following SW adaptation may reflect enhancement of clearance of the steroid. It appears that an increase in cortisol receptors, which is closely associated with the rise in blood osmotic pressure that accompanies SW exposure, is an important component of cortisol's ability to promote SW adaptation in the tilapia.

Na(+), Cl(-), Ca(2+) and Zn(2+) transport by fish gills: retrospective review and prospective synthesis

The secondary active Cl(-) secretion in seawater (SW) teleost fish gills and elasmobranch rectal gland involves basolateral Na(+),K(+)-ATPase and NKCC, apical membrane CFTR anion channels, and a paracellular Na(+)-selective conductance. In freshwater (FW) teleost gill, the mechanism of NaCl uptake is more controversial and involves apical V-type H(+)-ATPase linked to an apical Na(+) channel, apical Cl(-)-HCO-3 exchange and basolateral Na(+),K(+)-ATPase. Ca(2+) uptake (in FW and SW) is via Ca(2+) channels in the apical membrane and Ca(2+)-ATPase in the basolateral membrane. Mainly this transport occurs in mitochondria rich (MR) chloride cells, but there is a role for the pavement cells also. Future research will likely expand in two major directions, molded by methodology: first in physiological genomics of all the transporters, including their expression, trafficking, operation, and regulation at the molecular level, and second in biotelemetry to examine multivariable components in behavioral physiological ecology, thus widening the integration of physiology from the molecular to the environmental levels while deepening understanding at all levels.

Cell signaling and ion transport across the fish gill epithelium

A large array of circulating and local signaling agents modulate transport of ions across the gill epithelium of fishes by either affecting transport directly or by altering the size and distribution of transporting cells in the epithelium. In some cases, these transport effects are in addition to cardiovascular effects of the same agents, which may affect the perfusion pathways in the gill vasculature and, in turn, affect epithelial transport indirectly. Prolactin is generally considered to function in freshwater, because it is the only agent that allows survival of some hypophysectomized fish species in freshwater. It appears to function by either reducing branchial permeability, Na,K-activated ATPase activity, or reducing the density of chloride cells. Cortisol was initially considered to produce virtually opposite effects (e.g., stimulation of Na,K-activated ATPase and of chloride cell size and density), but more recent studies have found that this steroid stimulates ionic uptake in freshwater fishes, as well as the activity of H-ATPase, an enzyme thought to be central to ionic uptake. Thus, cortisol may function in both high and low salinities. Growth hormone and insulin-like growth factor appear to act synergistically to affect ion regulation in seawater fishes, stimulating both Na,K-activated ATPase and Na-K-2Cl co-transporter activity, and chloride cell size, independent of their effects on growth. Some of the effects of the GH-IGF axis may be via stimulation of the number of cortisol receptors. Thyroid hormones appear to affect seawater ion regulation indirectly, by stimulating the GH-IGF axis. Natriuretic peptides were initially thought to stimulate gill ionic extrusion, but recent studies have not corroborated this finding, so it appears that the major mode of action of these peptides may be reduction of salt loading by inhibition of oral ingestion and intestinal ionic uptake. Receptors for both arginine vasotocin and angiotensin have been described in the gill epithelium, but their respective roles and importance in fish ion regulation remains unknown. The gill epithelium may be affected by both circulating and local adrenergic agents, and a variety of studies have demonstrated that stimulation of alpha-adrenergic versus beta-adrenergic receptors produces inhibition or stimulation of active salt extrusion, respectively. Local effectors, such as prostaglandins, nitric oxide, and endothelin, may affect active salt extrusion as well as gill perfusion. Recent studies have suggested that the endothelin inhibition of salt extrusion is actually mediated by the release of both NO and prostaglandins. It is hoped that modern molecular techniques, combined with physiological measurements, will allow the dissection of the relative roles in ion transport across the fish gill epithelium of this surprisingly large array of putative signaling agents.

Evidence for a mineralocorticoid-like receptor linked to branchial chloride cell proliferation in freshwater rainbow trout

Fish acclimated to ion-deficient water exhibit proliferation of branchial chloride cells. The objective of the present study was to investigate the role of cortisol in this response using the corticosteroid receptor antagonists RU486 and spironolactone. RU486 is a potent antagonist of the glucocorticoid actions of cortisol, whereas spironolactone exhibits high-affinity binding to mineralocorticoid receptors, with a resulting blockade of mineralocorticoid properties in mammals. Untreated rainbow trout, as well as rainbow trout given a single intraperitoneal implant of coconut oil alone, coconut oil containing RU486 (0.5 mg g–1) or coconut oil containing spironolactone (0.1 mg g–1), were exposed to either dechlorinated city-of-Ottawa tapwater or artificial softwater for 7 days. Neither corticosteroid antagonist nor acclimation condition affected circulating plasma cortisol levels, plasma ion concentrations or gill Na+-K+-ATPase activity. Kidney Na+-K+-ATPase activity was significantly higher in softwater-acclimated fish than in fish held in dechlorinated tapwater. In addition, whereas RU486 treatment was found to be without effect on gill morphometrics, treatment with spironolactone inhibited the proliferation of chloride cells normally associated with acclimation to ion-deficient water. The results of the present study provide further evidence for the mineralocorticoid actions of cortisol in freshwater fish, specifically in eliciting chloride cell proliferation. Furthermore, these results support the hypothesis that distinct glucocorticoid and mineralocorticoid receptor populations are present in teleost fish, despite the apparent absence of the classic mineralocorticoid hormone, aldosterone.

Metallothionein and cortisol receptor expression in gills of Atlantic salmon, Salmo salar, exposed to dietary cadmium

Commercial fish feeds may contain significant levels of cadmium (Cd). However, little is known about the effects of dietary cadmium on fish organs, especially gills, the key osmoregulatory organ. We therefore studied the effects of dietary cadmium on metallothionein (MT) and cortisol receptor (GR) immunoreactivity in the branchial epithelium of the Atlantic salmon (Salmo salar). Cadmium was daily administered via food at 0.2mg (control), 5mg (low dose) and 125 mg (high dose) Cd per kilogram dry pellet weight. Fish were sampled after four and eight weeks. After both four and eight weeks, plasma cadmium concentration had increased significantly only in fish fed the high cadmium dose. Plasma calcium, sodium, chloride and cortisol levels were not affected. In the controls, most MT was colocated with the chloride cell marker, Na(+)/K(+)-ATPase, but some MT was present in pavement and respiratory cells. GR expression was found in chloride, pavement, respiratory and undifferentiated cells in all fish groups, but cadmium accumulation and a marked stimulation of MT expression were seen only in the chloride cells in the gills of fish fed the high cadmium dose. Cadmium treatment did not alter GR expression. When the double staining technique for MT and GR was applied, a marked heterogeneity became apparent in the chloride, pavement and respiratory cells of both groups of cadmium-treated fish and in the control fish. Some fish showed double staining, others stained only for one of the antibodies, whereas other cells were negative for both. We conclude that cadmium entering the gut also enters the gills, where it accumulates in chloride cells and stimulates MT expression.

Cortisol modulates HSP90 mRNA expression in primary cultures of trout hepatocytes

The objective of the present study was to understand the role of cortisol in the cellular stress response process in fish. Specifically, our studies addressed whether cortisol exposure modified heat shock protein 90 (HSP90) mRNA expression in rainbow trout (Oncorhynchus mykiss) hepatocytes maintained in primary culture. We also subjected these hepatocytes to heat shock (HS) in order to examine the role of cortisol on HS-induced HSP90 mRNA expression. A cDNA fragment of 500 bp was cloned from trout liver by reverse transcriptase- polymerase chain reaction (RT-PCR) with primers designed from the conserved regions of chinook salmon and zebrafish HSP90 cDNAs. The PCR product showed very high homology to chinook salmon (98%), zebrafish (84%) and human (77%) HSP90. Heat shock (+6 degrees C) induced transient elevation in HSP90 mRNA in trout hepatocytes, peaking within 10-h post-HS, and remained elevated over a 24-h period. Cortisol did not modify the unstimulated expression of HSP90 mRNA, whereas the HS-induced HSP90 mRNA expression was attenuated in trout hepatocytes. Our results suggest that elevated plasma cortisol levels modulate the cellular stress response by affecting the transcription of HSP90 in fish.

Effects of copper on cortisol receptor and metallothionein expression in gills of Oncorhynchus mykiss

Effects of waterborne Cu (2.4 microM) on the expression of glucocorticoid receptor (GR) and metallothionein (MT) in the branchial epithelium of freshwater rainbow trout (Oncorhynchus mykiss) was studied by immunocytochemistry. After 5 days of Cu exposure, the number of GR-immunoreactive (GR-ir) cells in the gill epithelium had decreased, whereas the number of MT-ir cells had increased. Localization of GR in chloride cells was achieved by double staining for Na(+)/K(+)-ATPase; other cell types were identified on the basis of their topology. GRs were present in the chloride cells in both the filaments and lamellae, in respiratory cells in the lamellae, in pavement cells, basal layer cells and undifferentiated cells in the filaments. Co-localization of Na(+)/K(+)-ATPase and MT revealed chat MT was expressed in chloride cells, both in filaments and lamellae. Occasionally, MT immunoreactivity was found in pavement cells and in undifferentiated cells. By double staining for Na(+)/K(+)-ATPase and GR, for Na(+)/K(+)-ATPase and MT and for GR and MT, we can conclude that after 5 days of Cu stress there are chloride cells that express GR and MT, GR or MT alone or neither of the two proteins. This apparent functional heterogeneity of branchial chloride cells may reflect a limited window when chloride cell subpopulations show an adaptive response to Cu.

High blood cortisol levels and low cortisol receptor affinity: is the chub, Leuciscus cephalus, a cortisol-resistant teleost?

In contrast to the relatively minor intra- and interspecies differences in blood cortisol levels reported for salmonid species, there is a more pronounced distinction between cortisol levels among the Salmonidae and the Cyprinidae, with both basal and stress-induced cortisol levels markedly higher in the latter. This study shows that in the chub, Leuciscus cephalus, a widely distributed European cyprinid, mean blood cortisol levels during stress (1500 ng mL(-1)) exceeded those reported for most other species of fish and, even in unstressed chub, cortisol levels (50-100 ng mL(-1)) were within the range known to cause immunosuppression, growth retardation, and reproductive dysfunction in salmonid fish. The chub appears to be atypical only with respect to plasma cortisol levels; the levels of plasma glucose and plasma lactate in unstressed and stressed chub are similar to those reported for other species. Plasma levels of 11-ketotestosterone in males and 17beta-estradiol in females are lower than those reported for salmonids but similar to those reported for other cyprinid species and display clear stress-induced reduction. Comparative analysis of the binding characteristics of the trout and chub gill cortisol receptor revealed that the total number of binding sites in gill tissue for each species was similar (B(max); approximately 50-100 fmol mg(-1) protein). However, the affinity of the binding site for cortisol displayed an eightfold difference between the species (rainbow trout: K(d) approximately 6 nM; chub: K(d) approximately 50 nM). Therefore, the potentially adverse effects of high circulating levels of cortisol found both at rest and under conditions of stress in chub may be offset by the lower affinity of the cortisol receptor, rather than the abundance of target-tissue receptor sites. This strategy is similar to that reported for some glucocorticoid-resistant rodents and New World primates.