Drinking behaviour in sea water and fresh water teleosts, the role of the renin-angiotensin system

Euryhaline fish larvae ingest more microplastic particles in seawater than in freshwater

Microplastic (MP) pollution is a major concern in aquatic environments. Many studies have detected MPs in fishes; however, little is known about differences of microplastic uptake by fish in freshwater (FW) and those in seawater (SW), although physiological conditions of fish differ significantly in the two media. In this study, we exposed larvae (21 days post-hatching) of Oryzias javanicus (euryhaline SW) and Oryzias latipes (euryhaline FW), to 1-µm polystyrene microspheres in SW and FW for 1, 3, or 7 days, after which, microscopic observation was conducted. MPs were detected in the gastrointestinal tracts in both FW and SW groups, and MP numbers were higher in the SW group in both species. Vertical distribution of MPs in the water, and body sizes of both species exhibited no significant difference between SW and FW. Detection of water containing a fluorescent dye revealed that O. javanicus larvae swallowed more water in SW than in FW, as has also been reported for O. latipes. Therefore, MPs are thought to be ingested with water for osmoregulation. These results imply that SW fish ingest more MPs than FW fish when exposed to the same concentration of MPs.

Microcystin-LR modulates multixenobiotic resistance proteins in the middle intestine of rainbow trout, Oncorhynchus mykiss

Global climate change favors explosive population growth events (blooms) of phytoplanktonic species, often producing toxic products, e.g., several genera of cyanobacteria synthesize a family of cyanotoxins called microcystins (MCs). Freshwater fish such as the rainbow trout Oncorhynchus mykiss can uptake MCs accumulated in the food chain. We studied the toxic effects and modulation of the activity and expression of multixenobiotic resistance proteins (ABCC transporters and the enzyme glutathione S-transferase (GST) in the O. mykiss middle intestine by microcystin-LR (MCLR). Juvenile fish were fed with MCLR incorporated in the food every 12 h and euthanized at 12, 24, or 48 h. We estimated the ABCC-mediated transport in ex vivo intestinal strips to estimate ABCC-mediated transport activity. We measured total and reduced (GSH) glutathione contents and GST and glutathione reductase (GR) activities. We studied MCLR cytotoxicity by measuring protein phosphatase 1 (PP1) activity and lysosomal membrane stability. Finally, we examined the relationship between ROS production and lysosomal membrane stability through in vitro experiments. Dietary MCLR had a time-dependent effect on ABCC-mediated transport, from inhibition at 12 h to a significant increase after 48 h. GST activity decreased only at 12 h, and GR activity only increased at 48 h. There were no effects on GSH or total glutathione contents. MCLR inhibited PP1 activity and diminished the lysosomal membrane stability at the three experimental times. In the in vitro study, the lysosomal membrane stability decreased in a concentration-dependent fashion from 0 to 5 µmol L ^- 1 MCLR, while ROS production increased only at 5 µmol L ^- 1 MCLR. MCLR did not affect mRNA expression of abcc2 or gst-π. We conclude that MCLR modulates ABCC-mediated transport activity in O. mykiss's middle intestine in a time-dependent manner. The transport rate increase does not impair MCLR cytotoxic effects.

Repeated Genetic Targets of Natural Selection Underlying Adaptation of Fishes to Changing Salinity

Ecological transitions across salinity boundaries have led to some of the most important diversification events in the animal kingdom, especially among fishes. Adaptations accompanying such transitions include changes in morphology, diet, whole-organism performance, and osmoregulatory function, which may be particularly prominent since divergent salinity regimes make opposing demands on systems that maintain ion and water balance. Research in the last decade has focused on the genetic targets underlying such adaptations, most notably by comparing populations of species that are distributed across salinity boundaries. Here, we synthesize research on the targets of natural selection using whole-genome approaches, with a particular emphasis on the osmoregulatory system. Given the complex, integrated and polygenic nature of this system, we expected that signatures of natural selection would span numerous genes across functional levels of osmoregulation, especially salinity sensing, hormonal control, and cellular ion exchange mechanisms. We find support for this prediction: genes coding for V-type, Ca2+, and Na+/K+-ATPases, which are key cellular ion exchange enzymes, are especially common targets of selection in species from six orders of fishes. This indicates that while polygenic selection contributes to adaptation across salinity boundaries, changes in ATPase enzymes may be of particular importance in supporting such transitions.

Circulating Isotocin, not Angiotensin II, is the Major Dipsogenic Hormone in Eels

Angiotensin II (AngII) is generally known as the most important dipsogenic hormone throughout vertebrates, while two other neurohypophysial hormones, vasopressin and oxytocin, are not dipsogenic in mammals. In this study, we found that systemic isotocin, but not vasotocin, is the potent dipsogenic hormone in eels. When injected intra-arterially into conscious eels, isotocin, vasotocin and AngII equally increased ventral aortic pressure dose-dependently at 0.03-1.0 nmol/kg, but only isotocin induced copious drinking. The dipsogenic effect was dose-dependent and occurred significantly at as low as 0.1 nmol/kg. By contrast, a sustained inhibition of drinking occurred after AngII, probably due to baroreflexogenic inhibition. No such inhibition was observed after isotocin despite similar concurrent hypertension. The baroreceptor may exist distal to the gill circulation because the vasopressor effect occurred at both ventral and dorsal aorta after AngII but only at ventral aorta after isotocin. By contrast, intra-cerebroventricular (i.c.v.) injection of isotocin had no effect on drinking or blood pressure, but AngII increased drinking and aortic pressure dose-dependently at 0.03-0.3 nmol/eel. Lesioning of the area postrema (AP), a sensory circumventricular organ, abolished drinking induced by peripheral isotocin, but not i.c.v. AngII. Collectively, isotocin seems to be a major circulating hormone that induces swallowing through its action on the AP, while AngII may be an intrinsic brain peptide that induces drinking through its action on a different circumventricular site, possibly a recently identified blood-brain barrier-deficient structure in the antero-ventral third ventricle of eels, as shown in birds and mammals.

The determination of thiocyanate in the blood plasma and holding water of Amphiprion clarkii after exposure to cyanide

The illegal practice of cyanide fishing continues throughout the Indo-Pacific. To combat this destructive fishing method, a reliable test to detect whether a fish has been captured using cyanide (CN) is needed. We report on the toxicokinetics of acute, pulsed CN exposure and chronic thiocyanate (SCN) exposure, the major metabolite of CN, in the clownfish species, Amphiprion clarkii. Fish were pulse exposed to 50 ppm CN for 20 or 45 s or chronically exposed to 100 ppm SCN for 12 days and blood plasma levels of SCN were measured. SCN blood plasma levels reached a maximum concentration (301–468 ppb) 0.13–0.17 days after exposure to CN and had a 0.1 to 1.2 day half-life. The half-life of blood plasma SCN after chronic exposure to SCN was found to be 0.13 days. Interestingly, we observed that when a fish, with no previous CN or SCN exposure, was placed in holding water spiked to 20 ppb SCN, there was a steady decrease in the SCN concentration in the holding water until it could no longer be detected at 24 hrs. Under chronic exposure conditions (100 ppm, 12 days), trace levels of SCN (∼40 ppb) were detected in the holding water during depuration but decreased to below detection within the first 24 hrs. Our holding water experiments demonstrate that low levels of SCN in the holding water of A. clarkii will not persist, but rather will quickly and steadily decrease to below detection limits refuting several publications. After CN exposure, A. clarkii exhibits a classic two compartment model where SCN is eliminated from the blood plasma and is likely distributed throughout the body. Similar studies of other species must be examined to continue to develop our understanding of CN metabolism in marine fish before a reliable cyanide detection test can be developed.

Osmoregulatory actions of prolactin in the gastrointestinal tract of fishes

In fishes, prolactin (Prl) signaling underlies the homeostatic regulation of hydromineral balance by controlling essential solute and water transporting functions performed by the gill, gastrointestinal tract, kidney, urinary bladder, and integument. Comparative studies spanning over 60 years have firmly established that Prl promotes physiological activities that enable euryhaline and stenohaline teleosts to reside in freshwater environments; nonetheless, the specific molecular and cellular targets of Prl in ion- and water-transporting tissues are still being resolved. In this short review, we discuss how particular targets of Prl (e.g., ion cotransporters, tight-junction proteins, and ion pumps) confer adaptive functions to the esophagus and intestine. Additionally, in some instances, Prl promotes histological and functional transformations within esophageal and intestinal epithelia by regulating cell proliferation. Collectively, the demonstrated actions of Prl in the gastrointestinal tract of teleosts indicate that Prl operates to promote phenotypes supportive of freshwater acclimation and to inhibit phenotypes associated with seawater acclimation. We conclude our review by underscoring that future investigations are warranted to determine how growth hormone/Prl-family signaling evolved in basal fishes to support the gastrointestinal processes underlying hydromineral balance.

Bioaccumulation and reproductive effects of fluorescent microplastics in medaka fish

This study was performed to elucidate the uptake and bioaccumulation of polystyrene microplastics (PS-MPs) in Japanese medaka (freshwater fish) and Java medaka (marine fish), and to assess its impacts on the survival, reproduction, and gene expression of Japanese medaka. Both species were exposed to 2-μm fluorescent PS-MPs (10⁷ beads/L) for 3 weeks. The bioaccumulation factor of PS-MPs for Java medaka was calculated at about 4 × 10², higher than that for Japanese medaka (about 1 × 10²). The exposure had no significant effects on the survival and reproduction of Japanese medaka. The mRNA sequencing analysis showed that the expression of a few genes involved in the cell adhesion, xenobiotic metabolic process, brain development, and other functions in medaka intestines significantly changed after exposure. These results suggest that virgin PS-MPs can potentially accumulate in medaka intestines, but has limited toxicity to Japanese medaka at the concentration up to 10⁷ beads/L.

The gaseous gastrointestinal tract of a seawater teleost, the English sole (Parophrys vetulus)

There has been considerable recent progress in understanding the respiratory physiology of the gastrointestinal tract (GIT) in teleosts, but the respiratory conditions inside the GIT remain largely unknown, particularly the luminal PCO₂ and PO₂ levels. The GIT of seawater teleosts is of special interest due to its additional function of water absorption linked to HCO₃^- secretion, a process that may raise luminal PCO₂ levels. Direct measurements of GIT PCO₂ and PO₂ using micro-optodes in the English sole (Parophrys vetulus; anaesthetized, artificially ventilated, 10-12 °C) revealed extreme luminal gas levels. Luminal PCO₂ was 14-17 mmHg in the stomach and intestinal segments of fasted sole, considerably higher than arterial blood levels of 5 mmHg. Moreover, feeding, which raised intestinal HCO₃^- concentration, also raised luminal PCO₂ to 34-50 mmHg. All these values were higher than comparable measurements in freshwater teleosts, and also greater than environmental CO₂ levels of concern in aquaculture or global change scenarios. The PCO₂ values in subintestinal vein blood draining the GIT of fed fish (28 mmHg) suggested some degree of equilibration with high luminal PCO₂, whereas subintestinal vein PO₂ levels were relatively low (9 mmHg). All luminal sections of the GIT were virtually anoxic (PO₂ ≤ 0.3 mmHg), in both fasted and fed animals, a novel finding in teleosts.

Drinking and Water Handling in the Medaka Intestine: A Possible Role of Claudin-15 in Paracellular Absorption?

When euryhaline fish move between fresh water (FW) and seawater (SW), the intestine undergoes functional changes to handle imbibed SW. In Japanese medaka, the potential transcellular aquaporin-mediated conduits for water are paradoxically downregulated during SW acclimation, suggesting paracellular transport to be of principal importance in hyperosmotic conditions. In mammals, intestinal claudin-15 (CLDN15) forms paracellular channels for small cations and water, which may participate in water transport. Since two cldn15 paralogs, cldn15a and cldn15b, have previously been identified in medaka, we examined the salinity effects on their mRNA expression and immunolocalization in the intestine. In addition, we analyzed the drinking rate and intestinal water handling by adding non-absorbable radiotracers, 51-Cr-EDTA or 99-Tc-DTPA, to the water. The drinking rate was >2-fold higher in SW than FW-acclimated fish, and radiotracer experiments showed anterior accumulation in FW and posterior buildup in SW intestines. Salinity had no effect on expression of cldn15a, while cldn15b was approximately 100-fold higher in FW than SW. Despite differences in transcript dynamics, Cldn15a and Cldn15b proteins were both similarly localized in the apical tight junctions of enterocytes, co-localizing with occludin and with no apparent difference in localization and abundance between FW and SW. The stability of the Cldn15 protein suggests a physiological role in water transport in the medaka intestine.

In vitro and in vivo studies of oxidative stress responses against acrylamide toxicity in zebrafish

Acrylamide (AA) is widely used in soil stabilization, water treatment, and industrial products and found in certain foods; however, its toxicity is an expanding global concern. Thus, to reveal the mechanisms involved in the development of, or protection from AA-induced toxicity has important significance. For this purpose, here we explored the intracellular stress response signaling pathways activated by AA exposure in zebrafish model. BRF41 cells derived from zebrafish were exposed to AA, and changes in the expression levels of 31 genes, including endoplasmic reticulum stress response-, oxidative stress response-, osmotic stress response-, and DNA damage and repair-related genes, were analyzed by PCR array. 12 genes upregulated in AA-exposed BRF41 cells were analyzed in zebrafish larvae by quantitative real time PCR, and the expression of all tested oxidative stress response-related genes was upregulated. Spatial expression patterns of these genes were visualized and found that their expression was upregulated and ectopically induced. In addition, AA-induced toxicity in BRF41 cells and the expression of glutathione S-transferase pi 1 (gstp1) in zebrafish larvae were reduced by N-acetylcysteine. Furthermore, inhibition of Gst activity enhanced AA toxicity. From these results, we concluded that the elicited oxidative stress response critically contributes to the protection from AA-induced toxicity.

The Amphibious Mudskipper: A Unique Model Bridging the Gap of Central Actions of Osmoregulatory Hormones Between Terrestrial and Aquatic Vertebrates

Body fluid regulation, or osmoregulation, continues to be a major topic in comparative physiology, and teleost fishes have been the subject of intensive research. Great progress has been made in understanding the osmoregulatory mechanisms including drinking behavior in teleosts and mammals. Mudskipper gobies can bridge the gap from aquatic to terrestrial habitats by their amphibious behavior, but the studies are yet emerging. In this review, we introduce this unique teleost as a model to study osmoregulatory behaviors, particularly amphibious behaviors regulated by the central action of hormones. Regarding drinking behavior of mammals, a thirst sensation is aroused by angiotensin II (Ang II) through direct actions on the forebrain circumventricular structures, which predominantly motivates them to search for water and take it into the mouth for drinking. By contrast, aquatic teleosts can drink water that is constantly present in their mouth only by reflex swallowing, and Ang II induces swallowing by acting on the hindbrain circumventricular organ without inducing thirst. In mudskippers, however, through the loss of buccal water by swallowing, which appears to induce buccal drying on land, Ang II motivates these fishes to move to water for drinking. Thus, mudskippers revealed a unique thirst regulation by sensory detection in the buccal cavity. In addition, the neurohypophysial hormones, isotocin (IT) and vasotocin (VT), promote migration to water via IT receptors in mudskippers. VT is also dipsogenic and the neurons in the forebrain may mediate their thirst. VT regulates social behaviors as well as osmoregulation. The VT-induced migration appears to be a submissive response of subordinate mudskippers to escape from competitive and dehydrating land. Together with implications of VT in aggression, mudskippers may bridge the multiple functions of neurohypophysial hormones. Interestingly, cortisol, an important hormone for seawater adaptation and stress response in teleosts, also stimulates the migration toward water, mediated possibly via the mineralocorticoid receptor. The corticosteroid system that is responsive to external stressors can accelerate emergence of migration to alternative habitats. In this review, we suggest this unique teleost as an important model to deepen insights into the behavioral roles of these hormones in relation to osmoregulation.

Intestinal ammonia transport in freshwater and seawater acclimated rainbow trout (Oncorhynchus mykiss): evidence for a Na+ coupled uptake mechanism

In vitro gut sac experiments were performed on freshwater and 60% seawater acclimated trout (Oncorhynchus mykiss) under treatments designed to discern possible mechanisms of intestinal ammonia transport. Seawater acclimation increased ammonia flux rate into the serosal saline (Jsamm) in the anterior intestine, however it did not alter Jsamm in the mid- or posterior intestine suggesting similar mechanisms of ammonia handling in freshwater and seawater fish. Both fluid transport rate (FTR) and Jsamm were inhibited in response to basolateral ouabain treatment, suggesting a linkage of ammonia uptake to active transport, possibly coupled to fluid transport processes via solvent drag. Furthermore, decreases in FTR and Jsamm caused by low Na(+) treatment indicated a Na(+) linked transport mechanism. Mucosal bumetanide (10(-4) M) had no impact on FTR, yet decreased Jsamm in the anterior and mid-intestine, suggesting NH4(+) substitution for K(+) on an apical NKCC, and at least a partial uncoupling of ammonia transport from fluid transport. Additional treatments (amiloride, 5-(N-ethyl-N-isopropyl)amiloride (EIPA), phenamil, bafilomycin, 4',6-diamidino-2-phenylindole (DAPI), high sodium) intended to disrupt alternative routes of Na(+) uptake yielded no change in FTR or Jsamm, suggesting the absence of direct competition between Na(+) and ammonia for transport. Finally, [(14)C]methylamine permeability (PMA) measurements indicated the likely presence of an intestinal Rh-mediated ammonia transport system, as increasing NH4Cl (0, 1, 5 mmol l(-1)) concentrations reduced PMA, suggesting competition for transport through Rh proteins. Overall, the data presented in this paper provide some of the first insights into mechanisms of teleost intestinal ammonia transport.

Hormonal control of drinking behavior in teleost fishes; insights from studies using eels

Marine teleost fishes drink environmental seawater to compensate for osmotic water loss, and the amount of water intake is precisely regulated to prevent dehydration or hypernatremia. Unlike terrestrial animals in which thirst motivates a series of drinking behaviors, aquatic fishes can drink environmental water by reflex swallowing without searching for water. Hormones are key effectors for the regulation of drinking. In particular, angiotensin II and atrial natriuretic peptide are likely candidates for physiological regulators because of their potent dipsogenic and antidipsogenic activities, respectively. In the eel, these hormones act on the area postrema in the medulla oblongata, a circumventricular structure without blood-brain barrier, which then regulates the activity of the glossopharyngeal-vagal motor complex. These motor neurons in the hindbrain innervate the upper esophageal sphincter muscle and other swallowing-related muscles in the pharynx and esophagus for regulation of drinking. Thus, the neural circuitry for drinking in fishes appears to be confined within the hindbrain. This simple mechanism is much different from that of terrestrial animals in which thirst sensation is induced through hormonal actions on the subfornical organ and organum vasculosum of the lamina terminalis that are located in the forebrain. It seems that the neural and hormonal mechanism that regulates drinking behavior has evolved from fishes depending on the availability of water in their natural habitats.

Intestinal fluid absorption in anadromous salmonids: importance of tight junctions and aquaporins

The anadromous salmonid life cycle includes both fresh water (FW) and seawater (SW) stages. The parr-smolt transformation (smoltification) pre-adapt the fish to SW while still in FW. The osmoregulatory organs change their mode of action from a role of preventing water inflow in FW, to absorb ions to replace water lost by osmosis in SW. During smoltification, the drinking rate increases, in the intestine the ion and fluid transport increases and is further elevated after SW entry. In SW, the intestine absorbs ions to create an inwardly directed water flow which is accomplished by increased Na⁺, K⁺-ATPase (NKA) activity in the basolateral membrane, driving ion absorption via ion channels and/or co-transporters. This review will aim at discussing the expression patterns of the ion transporting proteins involved in intestinal fluid absorption in the FW stage, during smoltification and after SW entry. Of equal importance for intestinal fluid absorption as the active absorption of ions is the permeability of the epithelium to ions and water. During the smoltification the increase in NKA activity and water uptake in SW is accompanied by decreased paracellular permeability suggesting a redirection of the fluid movement from a paracellular route in FW, to a transcellular route in SW. Increased transcellular fluid absorption could be achieved by incorporation of aquaporins (AQPs) into the enterocyte membranes and/or by a change in fatty acid profile of the enterocyte lipid bilayer. An increased incorporation of unsaturated fatty acids into the membrane phospholipids will increase water permeability by enhancing the fluidity of the membrane. A second aim of the present review is therefore to discuss the presence and regulation of expression of AQPs in the enterocyte membrane as well as to discuss the profile of fatty acids present in the membrane phospholipids during different stages of the salmonid lifecycle.

Compensatory regulation of acid-base balance during salinity transfer in rainbow trout (Oncorhynchus mykiss)

In seawater-acclimated rainbow trout (Oncorhynchus mykiss), base secretion into the intestine is a key component of the intestinal water absorption that offsets osmotic water loss to the marine environment. Acid-base balance is maintained by the matched excretion of acid equivalents via other routes, presumably the gill and/or kidney. The goal of the present study was to examine acid-base balance in rainbow trout upon transfer to more dilute environments, conditions under which base excretion into the intestine is predicted to fall, requiring compensatory adjustments of acid excretion at the gill and/or kidney if acid-base balance is to be maintained. Net acid excretion via the gill/kidney and rectal fluid, and blood acid-base status were monitored in seawater-acclimated rainbow trout maintained in seawater or transferred to iso-osmotic conditions. As predicted, transfer to iso-osmotic conditions significantly reduced base excretion into the rectal fluid (by ~48%). Transfer to iso-osmotic conditions also significantly reduced the excretion of titratable acidity via extra-intestinal routes from 183.4 ± 71.3 to -217.5 ± 42.7 μmol kg(-1) h(-1) (N = 7). At the same time, however, ammonia excretion increased significantly during iso-osmotic transfer (by ~72%) so that the apparent overall reduction in net acid excretion (from 419.7 ± 92.9 to 189.2 ± 76.5 μmol kg(-1 )h(-1); N = 7) was not significant. Trout maintained blood acid-base status during iso-osmotic transfer, although arterial pH was significantly higher in transferred fish than in those maintained in seawater. To explore the mechanisms underlying these adjustments of acid-base regulation, the relative mRNA expression and where possible, activity of a suite of proteins involved in acid-base balance were examined in intestine, gill and kidney. At the kidney, reduced mRNA expression of carbonic anhydrase (CA; cytosolic and membrane-associated CA IV), V-type H(+)-ATPase, and Na(+)/HCO(3) (-) co-transporter were consistent with a reduced role in net acid excretion following iso-osmotic transfer. Changes in relative mRNA expression and/or activity at the intestine and gill were consistent with the roles of these organs in osmotic rather than acid-base regulation. Overall, the data emphasize the coordination of acid-base, osmoregulatory and ionoregulatory processes that occur with salinity transfer in a euryhaline fish.

Functional characterization of water transport and cellular localization of three aquaporin paralogs in the salmonid intestine

Intestinal water absorption is greatly enhanced in salmonids upon acclimation from freshwater (FW) to seawater (SW); however, the molecular mechanism for water transport is unknown. We conducted a pharmacological characterization of water absorption in the rainbow trout intestine along with an investigation of the distribution and cellular localization of three aquaporins (Aqp1aa, -1ab, and -8ab) in pyloric caeca, middle (M), and posterior (P) intestine of the Atlantic salmon. In vitro iso-osmotic water absorption (J(v)) was higher in SW than FW-trout and was inhibited by (mmol L(-1)): 0.1 KCN (41%), 0.1 ouabain (72%), and 0.1 bumetanide (82%) suggesting that active transport, Na(+), K(+)-ATPase and Na(+), K(+), 2Cl(-)-co-transport are involved in establishing the driving gradient for water transport. J(v) was also inhibited by 1 mmol L(-1) HgCl(2), serosally (23% in M and 44% in P), mucosally (27% in M), or both (61% in M and 58% in P), suggesting involvement of both apical and basolateral aquaporins in water transport. The inhibition was antagonized by 5 mmol L(-1) mercaptoethanol. By comparison, 10 mmol L(-1) mucosal tetraethylammonium, an inhibitor of certain aquaporins, inhibited J(v) by 20%. In the presence of glucose, mucosal addition of phloridzin inhibited water transport by 20%, suggesting that water transport is partially linked to the Na(+)-glucose co-transporter. Using polyclonal antibodies against salmon Aqp1aa, -1ab, and -8ab, we detected Aqp1aa, and -1ab immunoreactivity in the brush border and sub-apical region of enterocytes in all intestinal segments. The Aqp8ab antibody showed a particularly strong immunoreaction in the brush border and sub-apical region of enterocytes throughout the intestine and also stained lateral membranes and peri-nuclear regions though at lower intensity. The present localization of three aquaporins in both apical and lateral membranes of salmonid enterocytes facilitates a model for transcellular water transport in the intestine of SW-acclimated salmonids.

Intestinal anion exchange in marine teleosts is involved in osmoregulation and contributes to the oceanic inorganic carbon cycle

Marine teleost fish osmoregulation involves seawater ingestion and intestinal fluid absorption. Solute coupled fluid absorption by the marine teleost fish intestine has long been believed to be the product of Na(+) and Cl(-) absorption via the Na(+) :K(+) :2Cl(-) co-transporter (NKCC2). However, the past decade has revealed that intestinal anion exchange contributes significantly to Cl(-) absorption, in exchange for HCO(3) (-) secretion, and that this process is important for intestinal water absorption. In addition to contributing to solute coupled water absorption intestinal anion exchange results in luminal precipitation of CaCO(3) which acts to reduce luminal osmotic pressure and thus assist water absorption. Most recently, activity of apical H(+) -pumps, especially in distal segments of the intestine have been suggested to not only promote anion exchange, but also to reduce luminal osmotic pressure by preventing excess HCO(3)(-) concentrations from accumulating in intestinal fluids, thereby aiding water absorption. The present review summarizes and synthesizes the most recent advances in our view of marine teleosts osmoregulation, including our emerging understanding of epithelial transport of acid-base equivalents in the intestine, the consequences for whole organism acid-base balance and finally the impact of piscine CaCO(3) formation on the global oceanic carbon cycle.

The area postrema in hindbrain is a central player for regulation of drinking behavior in Japanese eels

It is recognized that fish will drink the surrounding water by reflex swallowing without a thirst sensation. We evaluated the role of the area postrema (AP), a sensory circumventricular organ (CVO) in the medulla oblongata, in the regulation of drinking behavior of seawater (SW) eels. The antidipsogenic effects of ghrelin and atrial natriuretic peptide and hypervolemia and hyperosmolemia (1 M sucrose or 10% NaCl) as well as the dipsogenic effects of angiotensin II and hypovolemia (hemorrhage) were profoundly diminished after AP lesion (APx) in eels compared with sham controls. However, the antidipsogenic effect of urotensin II was not influenced by APx, possibly due to the direct baroreflex inhibition on the swallowing center in eels. When ingested water was drained via an esophageal fistula, water intake increased 30-fold in sham controls but only fivefold in APx eels, suggesting a role for the AP in continuous regulation of drinking by SW eels. After transfer from freshwater to SW, APx eels responded normally with an immediate burst of drinking, but after 4 wk these animals showed a much greater increase in plasma osmolality than controls, suggesting that the AP is involved in acclimation to SW by fine tuning of the drinking rate. Taken together, the AP in the hindbrain of eels plays an integral role in SW acclimation, acting as a conduit of information from plasma for the regulation of drinking, probably without a thirst sensation. This differs from mammals in which sensory CVOs in the forebrain play pivotal roles in thirst regulation.

Gastric dilation and air sacculitis in Chinook salmon, Oncorhynchus tshawytscha (Walbaum); correlation of macroscopic and microscopic lesions, and relationship of the syndrome to glomerulonephritis and serum biochemistry

Macroscopic and microscopic assessment procedures were developed to evaluate the severity and enable diagnosis from histological samples, of gastric dilation and air sacculitis (GDAS) in Chinook salmon, Oncorhynchus tshawytscha. Stomachs and swim bladders were examined from young fish with experimentally induced GDAS and from larger fish with the syndrome held in commercial saltwater net-pens. Fish fed a diet previously known to induce GDAS had significantly wider stomachs with decreased prominence of longitudinal stomach folds that contained larger amounts of fluid (P < 0.001), and thinner stomach walls with greater inter-nerve distances (P < 0.001), than fish fed an alternative commercial diet not associated with the syndrome. These fish also had swim bladders that were more likely to be opaque and contain more fluid (P < 0.001). These observations correlated well with selected criteria for stomach tissue (P < 0.002) and swim bladder (P < 0.04) that could be evaluated microscopically. Four stomach measurements, primarily measurements of wall or partial wall thickness and inter-nerve distances, were suitable for discriminating between affected and non-affected fish. A stomach width ratio, that was independent of fish weight and highly correlated with macroscopic stomach measurements (P < 0.0001), was particularly useful; this ratio was derived from the distances between both the outer border of the muscularis mucosa and mesothelium of the serosal surface to the stratum compactum. Serum biochemistry parameters (osmolality, calcium and magnesium) did not differ between fish fed different diets, but serum creatinine concentration was correlated with the microscopic thickness of the muscularis externa of the stomach wall and the total stomach thickness (P < 0.001 and P < 0.003, respectively). A glomerulonephritis was also noted in these fish. The severity of the lesion was not significantly related to GDAS nor to any serum biochemistry parameter assayed however.

Roles of intestinal glutamate dehydrogenase and glutamine synthetase in environmental ammonia detoxification in the euryhaline four-eyed sleeper, Bostrychus sinensis

This study aimed to examine the hypothesis that intestinal glutamate dehydrogenase (GDH) and glutamine synthetase (GS) could be involved in ammonia detoxification in the euryhaline Bostrychus sinensis exposed to ammonia in a hyperosmotic environment, whereby drinking was essential for osmoregulation. Our results indicate that there was a significant increase in ammonia content in the intestine of B. sinensis exposed to 15 mmol l(-1) NH(4)Cl in seawater (pH 7.0) for 6 days. There were also significant increases in the amination and deamination activities and protein abundance of intestinal GDH. The GDH amination/deamination ratio remained unchanged, indicating that there could be increases in the turnover of glutamate. However, the difference between the amination and deamination activities increased 2-fold, implying that there could be an increase in glutamate formation in the intestine. Since the intestinal glutamate content remained unchanged, excess glutamate formed might have been channeled into other amino acids and/or transported to other organs. Indeed, the intestinal glutamine content increased significantly by 2-fold, with a significant increase in the activity and protein abundance of intestinal GS. Since the magnitude of glutamine accumulation in the intestine was lower than those in liver and muscle, which lacked changes in GDH activities, intestinal glutamate could have been shuttled to liver and muscle to facilitate increased synthesis of glutamine therein. By contrast, when fish were exposed to a much higher concentration (30 mmol l(-1)) of NH(4)Cl in 5 per thousand water (pH. 7.0), the magnitude of increase in ammonia content in the intestine was less prominent, and there were no changes in activities and kinetic properties of intestinal GDH. Therefore, it can be concluded that the intestine of B. sinensis was involved in the defense against ammonia toxicity during exposure to ammonia in a hyperosmotic medium.

An immune-complex glomerulonephritis of Chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Chinook salmon from New Zealand were shown to have a generalized membranous glomerulonephritis that was most severe in large fish. Marked thickening of the glomerular basement membrane was the most consistent lesion, with the presence of an electron-dense deposit beneath the capillary endothelium.Severely affected glomeruli also had expansion of the mesangium and loss of capillaries,synechiae of the visceral and parietal epithelium and mild fibrosis of Bowmans capsule. Chinook salmon from British Columbia, Canada with bacterial kidney disease caused by Renibacterium salmoninarum had similar histological lesions. They also had thickened glomerular basement membranes that were recognized by rabbit antiserum to rainbow trout immunoglobulin. This was true only when frozen sections of kidney were used and not formalin-fixed tissue. An attempt to experimentally produce a glomerulopathy in rainbow trout by repeated immunization with killed R. salmoninarum was not successful. Case records from the Fish Pathology Laboratory at the University of Guelph over a 10-year period revealed that a range of species were diagnosed with glomerulopathies similar to those seen in Chinook salmon. The majority of these cases were determined to have chronic inflammatory disease. This report has identified the presence of immunoglobulin within thickened basement membranes of Chinook salmon with glomerulonephritis and supports the existence of type III hypersensitivity in fish.

Comparative accumulation of (109)Cd and (75)Se from water and food by an estuarine fish (Tetractenos glaber)

Few data are available on the comparative accumulation of metal(loid)s from water and food in estuarine/marine fish. Smooth toadfish (Tetractenos glaber), commonly found in estuaries in south-eastern Australia, were separately exposed to radio-labelled seawater (14kBqL(-1) of (109)Cd and 24kBqL(-1) of (75)Se) and food (ghost shrimps; Trypaea australiensis: 875Bqg(-1)(109)Cd and 1130Bqg(-1)(75)Se) for 25 days (uptake phase), followed by exposure to radionuclide-free water or food for 30 days (loss phase). Toadfish accumulated (109)Cd predominantly from water (85%) and (75)Se predominantly from food (62%), although the latter was lower than expected. For both the water and food exposures, (109)Cd was predominantly located in the gut lining (60-75%) at the end of the uptake phase, suggesting that the gut may be the primary pathway of (109)Cd uptake. This may be attributed to toadfish drinking large volumes of water to maintain osmoregulation. By the end of the loss phase, (109)Cd had predominantly shifted to the excretory organs - the liver (81%) in toadfish exposed to radio-labelled food, and in the liver, gills and kidney (82%) of toadfish exposed to radio-labelled water. In contrast, (75)Se was predominantly located in the excretory organs (gills, kidneys and liver; 66-76%) at the end of the uptake phase, irrespective of the exposure pathway, with minimal change in percentage distribution (76-83%) after the loss phase. This study emphasises the importance of differentiating accumulation pathways to better understand metal(loid) transfer dynamics and subsequent toxicity, in aquatic biota.

Drinking in juvenile Atlantic salmon (Salmo salar L.) in response to feeding and activation of the endogenous renin–angiotensin system

Drinking rate and rectal fluid production of juvenile Atlantic salmon (1-2 g) in freshwater were investigated in unfed fish and recently fed fish. Drinking was also investigated following activation of the renin-angiotensin system (RAS) by two hypotensive agents, a nitric oxide (NO) donor sodium nitroprusside (SNP) and bacterial lipopolysaccharide (LPS). In unfed fish the basal drinking rate was 0.13 microL g(-1) h(-1) and rectal fluid production was 0.076 microL g(-1) h(-1). In recently fed fish both drinking rate and rectal fluid production increased significantly by about fivefold compared to unfed fish, and similar values were obtained for fish exposed to PS for 24 h. Exposure to SNP resulted in about a tenfold elevation of drinking rate and rectal fluid production, compared to unfed fish. Absorption of water by the gut was in the range 35-60% for all treatments. Drinking may have a role in processing food in the gut and the fluid in the gut may subjected to absorptive and secretory processes. The most likely route for removal of water absorbed by the gut is excretion via the kidney and this would result in an increased osmoregulatory burden on the fish. In polluted waters drinking could be increased through stimulation of the endogenous RAS by vasodilators, e.g., LPS and the gut could be a significant target for toxin exposure.

Role of nitric oxide in larval and juvenile fish

Fish are known to express the three isoforms of nitric oxide synthase (NOS), the constitutive forms endothelial or eNOS, neuronal or nNOS and the inducible form iNOS. Most studies in fish have focussed on possible roles for NO in cardiovascular physiology although there has been recent attention on the role of nNOS in embryonic development. However compared to mammalian studies there have been relatively few studies on effects of nitric oxide (NO) on fish. Studies on heart and blood vessel preparations from various fish species appear to show results specific to the species or to the particular preparation. Possible roles of NO in the in vivo biology of adult fish or larval fish have received little attention. This article reviews effects of nitric oxide on cardiovascular physiology in fish with special emphasis on larval fish. It introduces some experimental work on possible signaling pathways in larval fish and introduces the possibility that NO could be an important environmental influence for some aquatic organisms. In higher vertebrates LPS (lipopolysaccharide) is known to activate the cytokine signaling system and stimulate increased expression of iNOS and increased production of NO, but this remains less investigated in fish. The effects of LPS on cardiovascular and osmoregulatory physiology of larval and juvenile salmonids are discussed and a possible role of NO in stress-induced drinking is suggested.

Effects of prolonged copper exposure in the marine gulf toadfish (Opsanus beta) II: copper accumulation, drinking rate and Na+/K+ -ATPase activity in osmoregulatory tissues

Gulf toadfish were exposed to sublethal levels of copper (12.8 or 55.2 microM) for 30 days. Drinking in control fish averaged 1 ml kg(-1)h(-1) but exposure to 55.2 microM copper resulted in a complex biophasic pattern with initial (3 h and 1 day) inhibition of drinking rate, followed by an elevation of drinking rate from day 3 onwards. Drinking led to copper accumulation in the intestinal fluids at levels three to five times higher than the ambient copper concentrations, which in turn resulted in intestinal copper accumulation. The gill exhibited more rapid accumulation of copper than the intestine and contributed to early copper uptake leading to accumulation in internal organs. Muscle, spleen and plasma exhibited little if any disturbance of copper homeostasis while renal copper accumulation was evident at both ambient copper concentrations. The liver exhibited the highest copper concentrations and the greatest copper accumulation of all examined internal organs during exposure to 55.2 microM. Elevated biliary copper excretion was evident from measurements of gall bladder bile copper concentrations and appeared to protect partially against internal accumulation in fish exposed to 12.8 microM copper. No inhibition of Na+/K+ -ATPase activity in either gills or intestine was seen despite copper accumulation in these organs. Calculations of inorganic copper speciation suggest that Cu(CO3)(2)2- complexes which dominate in seawater and intestinal fluids are of limited availability for uptake while the low levels of ionic Cu2+, CuOH+ and CuCO3 may be the forms taken up by the gill and the intestinal epithelium.

Physiological adaptations of the gut in the Lake Magadi tilapia, Alcolapia grahami, an alkaline- and saline-adapted teleost fish

We describe the gut physiology of the Lake Magadi tilapia (Alcolapia grahami), specifically those aspects associated with feeding and drinking while living in water of unusually high carbonate alkalinity (titratable base=245 mequiv l(-1)) and pH (9.85). Drinking of this highly alkaline lake water occurs at rates comparable to or higher than those seen in marine teleosts. Eating and drinking take place throughout the day, although drinking predominates during hours of darkness. The intestine directly intersects the esophagus at the anterior end of the stomach forming a 'T', and the pyloric sphincter, which comprises both smooth and striated muscle, is open when the stomach is empty and closed when the stomach is full. This unique configuration (a functional trifurcation) allows imbibed alkaline water to bypass the empty stomach, thereby avoiding a reactive mixing with acidic gastric fluids, and minimizes interference with a full stomach. No titratable base was present in the stomach, where the mean pH was 3.55, but the intestine was progressively more alkaline (foregut 6.96, midgut 7.74, hindgut 8.12, rectum 8.42); base levels in the intestinal fluid were comparable to those in lake water. The gut was highly efficient at absorbing water (76.6%), which accompanied the absorption of Na(+) (78.5%), titratable base (80.8%), and Cl(-) (71.8%). The majority of Na(+), base and water absorption occurred in the foregut by an apparent Na(+) plus base co-transport system. Overall, more than 70% of the intestinal flux occurred via Na(+) plus base co-transport, and less than 30% by Na(+) plus Cl(-) co-transport, a very different situation from the processes in the intestine of a typical marine teleost.

Effects of Microcystis cells, cell extracts and lipopolysaccharide on drinking and liver function in rainbow trout Oncorhynchus mykiss Walbaum

Liver mass (hepatosomatic index, HSI) increased by approximately 18% and water content in the gut by approximately 13 ml kg(-1) in freshwater rainbow trout exposed for 24 h to intact cells of a microcystin-producing cyanobacterium (Microcystis PCC 7813) together with administration of heterotrophic bacterial LPS. Exposure to broken (ultrasonicated) cyanobacterial cells together with administration of bacterial LPS increased HSI by approximately 50% and water content in the gut by almost 30 ml kg(-1). Exposure to broken or unbroken Microcystis cells without administration of bacterial LPS resulted in increased water content of the gut (by approximately 13 ml kg(-1)) with insignificant changes in HIS. Drinking rate increased with increasing dosage of bacterial LPS alone. The increased volume of water in the gut potentially increases the opportunity for uptake of waterborne toxins, including microcystins, and increased liver mass is a symptom consistent with the toxic effects of microcystins. It is concluded that exposure of fish to the cell contents of cyanobacteria (e.g. Microcystis PCC 7813) promotes osmoregulatory imbalance resulting from stimulation of the drinking response, increased volume of fluid in the gut and inability to remove excess water.

The dipsogenic effect of the renin-angiotensin system in elasmobranch fish

This study investigated the control of drinking in elasmobranch fish through manipulation of the homologous renin-angiotensin system (RAS). The smooth muscle relaxant papaverine was found to increase basal drinking levels in the European lesser-spotted dogfish, Scyliorhinus canicula, almost 20-fold. However, this response was significantly reduced with the coadministration of the angiotensin-converting enzyme inhibitor captopril which had no effect when administered alone. Captopril was also found to block a 7-fold increase in drinking rate following administration of homologous angiotensin I in S. canicula. Finally, administration of homologous angiotensin II produced a dose-dependent response in drinking rate in two species of elasmobranchs, S. canicula and the Japanese dogfish, Triakis scyllia. These results demonstrate a central role of the RAS in the control of drinking in elasmobranch fish.

How should salinity influence fish growth?

Development and growth (continuous in fish) are controlled by 'internal factors' including CNS, endocrinological and neuroendocrinological systems. Among vertebrates, they also are highly dependent on environmental conditions. Among other factors, many studies have reported an influence of water salinity on fish development and growth. In most species, egg fertilization and incubation, yolk sac resorption, early embryogenesis, swimbladder inflation, larval growth are dependent on salinity. In larger fish, salinity is also a key factor in controlling growth. Do the changes in growth rate, that depend on salinity, result from an action on: (1) standard metabolic rate; (2) food intake; (3) food conversion; and/or (4) hormonal stimulation? Better growth at intermediate salinities (8-20 psu) is very often, but not systematically, correlated to a lower standard metabolic rate. Numerous studies have shown that 20 to >50% of the total fish energy budget are dedicated to osmoregulation. However, recent ones indicate that the osmotic cost is not as high (roughly 10%) as this. Data are also available in terms of food intake and stimulation of food conversion, which are both dependent on the environmental salinity. Temperature and salinity have complex interactions. Many hormones are known to be active in both osmoregulation and growth regulation, e.g. in the control of food intake. All of these factors are reviewed. As often, multiple causality is likely to be at work and the interactive effects of salinity on physiology and behaviour must also be taken into account.

Sediment-associated tri-n-butyltin chloride and its effects on osmoregulation of freshwater-adapted 0-group European flounder, Platichthys flesus (L.)

The disruption of osmoregulatory processes was examined in European flounders exposed to environmental concentrations (150 ng TBTCl g(-1) dry weight sediment) of sediment-associated tri-n-butyltin chloride (TBTCl), by using radiotracers to measure changes in hydromineral fluxes and water balance. The water permeabilities of TBTCl-exposed fish varied during the course of the experiment and were significantly lower than those of the corresponding controls that did not change significantly with time. It was found that the maximum decrease in water permeability of TBTCl-exposed fish occurred after 14 days; thereafter there was an increase towards control values. However, there was a differential reduction of the diffusional (P(d)) and osmotic (P(os)) permeability coefficients, where the former decreased more rapidly than the latter, reflecting the reduction of diffusional membrane permeability and the increasing importance of osmotic permeability. In fish exposed to TBTCl sodium efflux and drinking rates were significantly increased but Na(+)/K(+)-ATPase activities and urine production rates were not affected. The effects of TBTCl exposure are also manifested at the level of the whole organism by a reduction in the increase of the body length of exposed fish, when compared to controls. It was concluded that tributyltin-n-chloride in sediments is capable of significantly disrupting the osmoregulatory functions of a benthic estuarine fish, at concentrations found in the sediments of Southampton Water and the River Itchen.

Circulating angiotensins in the river lamprey, Lampetra fluviatilis, acclimated to freshwater and seawater: possible involvement in the regulation of drinking

Plasma angiotensin levels were measured for the first time in a cyclostome, the river lamprey. With the demonstration that angiotensins are present in the circulation, the possibility of a physiological role in the regulation of drinking was re-examined. Angiotensin II and III concentrations and plasma osmolalities were significantly higher in lampreys acclimated to 28 ppt seawater than in those acclimated to freshwater. No changes were found in angiotensin II and III levels 4 h after transfer from freshwater to 50% seawater, although plasma osmolality had started to rise by this time. There was a suggestion that plasma angiotensin II levels might be related to osmolality in the transfer experiment. Injection of Asp(1)Val(5)- or Asn(1)Val(5)-angiotensin II (40-169 microg/kg body wt.) did not stimulate drinking in freshwater-acclimated lampreys, even when they were still capable of drinking. The angiotensin-converting enzyme inhibitor captopril and the smooth muscle relaxant papaverine both reduced drinking rate in 50% seawater-acclimated lampreys. The data do not provide direct evidence for the involvement of the renin-angiotensin system in the control of drinking behaviour in the lamprey. Indirect evidence from the captopril effect is suggestive, but could have other explanations.

Regulation of drinking rate in euryhaline tilapia larvae (Oreochromis mossambicus) during salinity challenges

Euryhaline tilapia larvae are capable of adapting to environmental salinity changes even when transferred from freshwater (FW) to seawater (SW) or vice versa. In this study, the water balance of developing tilapia larvae (Oreochromis mossambicus) adapted to FW or SW was compared, and the short-term regulation of drinking rate of the larvae during salinity adaptation was also examined. Following development, wet weight and water content of both SW- and FW-adapted larvae increased gradually, while the dry weight of both group larvae showed a slow but significant decline. On the other hand, the drinking rate of SW-adapted larvae was four- to ninefold higher than that of FW-adapted larvae from day 2 to day 5 after hatching. During acute salinity challenges, tilapia larvae reacted profoundly in drinking rate, that is, increased or decreased drinking rate within several hours while facing hypertonic or hypotonic challenges, to maintain their constancy of body fluid. This rapid regulation in water balance upon salinity challenges may be critical for the development and survival of developing larvae.

Role of the renin-angiotensin system in drinking of seawater-adapted eels Anguilla japonica: a reevaluation

The role of ANG II, a potent dipsogenic hormone, in copious drinking of seawater eels was examined. SQ-14225 (SQ), an angiotensin-converting enzyme inhibitor, infused intra-arterially at 0.01-1 microgram. kg(-1). min(-1), depressed drinking and arterial blood pressure in a dose-dependent manner. The inhibition was accompanied by a small decrease in plasma ANG II concentration, which became significant at 1 microgram. kg(-1). min(-1). After the infusate was changed back to the vehicle, the depression of drinking and arterial pressure continued for >2 h, although plasma ANG II concentration rebounded above the level before SQ infusion. By contrast, infusion of anti-ANG II serum (0.01-1 microgram. kg(-1). min(-1)) did not suppress drinking and arterial pressure, although plasma ANG II concentration decreased to undetectable levels. Plasma atrial natriuretic peptide and plasma osmolality, which influence drinking rate in eels, did not change during SQ or antiserum infusions. These results suggest that the renin-angiotensin system plays only a minor role in the vigorous drinking observed in seawater eels. The results also suggest that the antidipsogenic and vasodepressor effects of SQ in seawater eels are not due solely to the inhibition of ANG II formation in plasma.

Uptake and effects of nitrite in the marine teleost fish Platichthys flesus

The route of NO(2)(-) uptake and subsequent physiological effects were examined in the marine teleost, European flounder (Platichthys flesus), during exposure to 1 mM ambient NO(2)(-) for up to 11 days. Drinking of seawater resulted in a similar nitrite concentration in the anterior part of the intestine as in the ambient water. The NO(2)(-) concentration decreased along the gastro-intestinal tract, suggesting NO(2)(-) uptake across the intestinal epithelium. Comparison of NO(2)(-) uptake in fish that drank NO(2)(-)-contaminated seawater with fish that did not (i.e. had the intestine perfused with a NO(2)(-)-free saline during NO(2)(-) exposure) revealed that the intestinal route contributed some 66% of whole-body NO(2)(-) uptake. Plasma [NO(2)(-)] stayed below the ambient level. It reached a maximum of 0.35-0.4 mM on days 3-6 and then declined to 0.2 mM on day 11. The physiological effects of NO(2)(-) exposure were relatively minor compared with those reported in freshwater fish. Blood methemoglobin levels increased from approximately 4% in non-exposed fish to a maximum of 18% of total hemoglobin in exposed fish. An extracellular hyperkalemia was observed from day 3 of NO(2)(-) exposure, with a maximal increase in plasma K(+) concentrations of 38%. No mortality occurred during the 11 days of NO(2)(-) exposure. The lack of mortality can be related to the relatively low NO(2)(-) accumulation in the plasma and the relatively minor physiological disturbances.

Effects of cortisol and salinity challenge on water balance in developing larvae of tilapia (Oreochromis mossambicus)

Effects of exogenous cortisol on drinking rate and water content in developing larvae of tilapia (Oreochromis mossambicus) were examined. Both freshwater- and seawater-adapted larvae showed increases in drinking rates with development. Drinking rates of seawater-adapted larvae were about four- to ninefold higher than those of freshwater-adapted larvae from day 2 to day 5 after hatching. Seawater-adapted larvae showed declines in drinking rate and water content at 4 and 14 h, respectively, after immersion in 10 mg L(-1) cortisol. In the case of freshwater-adapted larvae, the drinking rate decreased after 8 h of cortisol immersion, while the water content did not show a significant change even after 32 h of cortisol immersion. In a subsequent experiment of transfer from freshwater to 20 ppt (parts per thousand, salinity) seawater, immersion in 10 mg L(-1) cortisol for 8-24 h enhanced the drinking rate in larvae at 4 h after transfer, but no significant difference was found in water contents between cortisol-treated and control groups following transfer. These results suggest that cortisol is involved in the regulation of drinking activity in developing tilapia larvae.

Effects of dehydration on plasma osmolality, thirst-related behavior, and plasma and brain angiotensin concentrations in Couch's spadefoot toad, Scaphiopus couchii

Under dehydrating conditions, many terrestrial vertebrates species exhibit increases in plasma osmolality and their drinking behavior. Under some circumstances, this behavioral change is accompanied by changes in plasma and central angiotensin concentrations, and it has been proposed that these changes in angiotensin levels induce the thirst-related behaviors. In response to dehydration, the spadefoot toad, Scaphiopus couchii, exhibits thirst-related behavior in the form of cutaneous drinking. This behavior has been termed water absorption response (WR) behavior. Spadefoot toads live in harsh desert environments and are subject annually to dehydrating conditions that may induce thirst-related behavior. We tested the hypothesis that an increase in WR behavior is associated with both an increase in plasma osmolality and an increase in plasma and brain angiotensin concentrations. First, we determined the degree of dehydration that was necessary to initiate WR behavior. Animals dehydrated to 85% of their standard bladder-empty weight via deprivation of water exhibited WR behavior more frequently than control toads left in home containers with water available. Next, using the same dehydration methods, we determined the plasma osmolality and sodium concentrations of dehydrated toads. Toads dehydrated to 85% standard weight also had a significant increase in plasma osmolality, but exhibited no overall change in plasma sodium concentrations, indicating that while an overall increase in plasma osmolality appears to be associated with WR behavior in S. couchii, changes in sodium concentrations alone are not sufficient to induce the behavior. Finally, plasma and brain angiotensin concentrations were measured in control toads and toads dehydrated to 85% standard weight. Plasma and brain angiotensin concentrations did not increase in dehydrated toads, indicating that dehydration-induced WR behavior that is associated with changes in plasma osmolality may not be induced by changes in endogenous angiotensin concentrations in S. couchii.

The effects of silver on intestinal ion and acid-base regulation in the marine teleost fish, Parophrys vetulus

Exposure to elevated silver (as AgNO3) concentrations (6-9 microM) in seawater was associated with comparably high silver concentrations in the intestinal fluids of the lemon sole (Parophrys vetulus), and a tendency for reduced drinking rate. The effects of silver on intestinal ion and acid-base regulation were studied using in situ perfusion of the intestine. Intestinal net Cl- uptake was reduced from 0.4 to 0.1 and intestinal net Na+ uptake from 0.2 to 0 mmol kg(-1) x h(-1) during silver exposure (9 microM). At the same time, intestinal HCO3- net efflux was reduced from 0.2 to 0.1 mmol kg(-1) x h(-1). Both intestinal Na+ and Cl- uptake and Cl-/HCO3- exchange are thus sensitive to silver, but to different extents. None of the observed effects were reversible during 24 h of recovery. Intestinal water transport was highly variable in vivo in the perfused preparation, and no significant effect of silver exposure was observed. However, in vitro intestine preparations exhibited reduction of intestinal net water flux from 4 to 1 microl cm(-2) x h(-1) during silver exposure together with reduced unidirectional Cl- influx. Reduced water intake and transepithelial water transport in silver-exposed fish resulted in moderate hemoconcentration evident from higher hematocrit values, but not in increased plasma ion levels. The latter could reflect a compensatory response via increased branchial Na+/K+-ATPase levels, observed in silver-exposed fish, indicative of increased branchial ion transport capacity. Impairment of intestinal ion and water transport as a result of silver intake via drinking could be an important part of the fatal cascade of physiological effects observed in marine fish during acute silver exposure.

Drinking in Atlantic salmon presmolts and smolts in response to growth hormone and salinity

Drinking rate in freshwater Atlantic salmon presmolts (about 0.1 ml/kg/h) was unaffected by daily injections of ovine GH (50 micrograms/fish) for a week but upon transfer to sea water an immediate and full drinking response was developed compared to saline treated fish (3.34 +/- 0.16 vs. 2.23 +/- 0.27 ml/kg/h). Smolting did not affect drinking rates in freshwater but after 7 days in sea water, salmon smolts imbibed 3.88 +/- 0.25 ml/ kg/h, significantly higher than the rate for saline injected presmolts (p < 0.05, one-way ANOVA), but not significantly different from oGH treated presmolts. Smolting and oGH treatment were without effect on plasma Na+ levels in freshwater fish and 7 days after transfer to sea water both groups showed a better regulation of plasma Na+ levels compared to saline treated presmolts. Atlantic salmon smolts showed higher levels of plasma Cl- than presmolts in freshwater, and after 7 days in sea water, both oGH presmolts and smolts showed significantly lower levels of plasma Cl- than saline injected presmolts. GH treatment in freshwater presmolts improved hypoosmoregulatory capacity following transfer to seawater and these results are discussed in relation to the physiology of smolting, and control of drinking.

Drinking rate in juvenile Atlantic salmon,Salmo salar L fry in response to a nitric oxide donor, sodium nitroprusside and an inhibitor of angiotensin converting enzyme, enalapril

Drinking in freshwater juvenile salmon was investigated in response to vasodilation by sodium nitroprusside (SNP), a nitric oxide donor, which significantly increased blood vessel diameter in Atlantic salmon alevins. Atlantic salmon fry (1-3 g), as previously shown, drank at a significant rate in fresh water which doubled to about 1.2 ml kg(-1) h(-1) following injection of SNP (100 μmol kg(-1)), through dilation of body vasculature and activation of a vasoconstrictive mechanism, the endogenous renin angiotensin system (RAS). This response was 50% inhibited by injection of about 100 mg kg(-1) enalapril. Fry increased drinking in response to SNP administered in the water, though the concentration required for maximal response, 1.6 mmol l(-1), was much greater than for injected SNP; this response was also inhibited by enalapril injection. Possible involvement of the gill vasculature and branchial osmoreceptors or baroreceptors in control of the drinking response is discussed.