Intestinal base excretion in the seawater-adapted rainbow trout: a role in acid-base balance?

Carbonic anhydrase activity in tissues of the icefish Chionodraco hamatus and of the red-blooded teleosts Trematomus bernacchii and Anguilla anguilla

Carbonic anhydrase (CA) activity was measured in blood, intestine, kidney and gill of two Antarctic teleosts, the haemoglobinless Chionodraco hamatus and the red-blooded Trematomus bernacchii, and of the temperate teleost Anguilla anguilla. In all species, the highest CA activity was in the gills, with the greatest activity in C. hamatus. CA activity in the blood was highest in A. anguilla, but none was detected in the blood of C. hamatus despite the presence of plasma CA inhibitors. The enzyme was present but its activity was low in the intestine and kidney of all three species.

The existence of very high CA activity in C. hamatus gills compared with the red-blooded species was investigated further by isolating and characterising the branchial cytosolic CA isoforms. The turnover rate of the C. hamatus isoform was significantly higher than that of T. bernacchii and A. anguilla. The isoforms from both the Antarctic species exhibited lower apparent Km (Km,app) and heat stability than those from A. anguilla. Sensitivity to sulphonamides was similar in all species and was within the range of the mammalian CA II isoform. The branchial CA isoforms of C. hamatus, T. bernacchii and A. anguilla displayed relative molecular masses of 28.9, 29.9 and 31.2 kDa, respectively.

The results suggest that the hemoglobinless teleost possesses a different branchial cytosolic CA isoform from that of red-blooded teleosts.

Intestinal iron uptake in the European flounder (Platichthys flesus)

Iron is an essential element because it is a key constituent of the metalloproteins involved in cellular respiration and oxygen transport. There is no known regulated excretory mechanism for iron, and homeostasis is tightly controlled via its uptake from the diet. This study assessed in vivo intestinal iron uptake and in vitro iron absorption in a marine teleost, the European flounder Platichthys flesus. Ferric iron, in the form 59FeCl3, was reduced to Fe2+ by ascorbate, and the bioavailability of Fe3+ and Fe2+ were compared. In vivo Fe2+ uptake was significantly greater than Fe3+ uptake and was reduced by the iron chelator desferrioxamine. Fe2+ was also more bioavailable than Fe3+ in in vitro studies that assessed the temporal pattern and concentration-dependency of iron absorption. The posterior region, when compared with the anterior and mid regions of the intestine, was the preferential site for Fe2+ uptake in vivo. In vitro iron absorption was upregulated in the posterior intestine in response to prior haemoglobin depletion of the fish, and the transport showed a Q10 value of 1.94. Iron absorption in the other segments of the intestine did not correlate with haematocrit, and Q10 values were lower. Manipulation of the luminal pH had no effect on in vitro iron absorption. The present study demonstrates that a marine teleost absorbs Fe2+ preferentially in the posterior intestine. This occurs in spite of extremely high luminal bicarbonate concentrations recorded in vivo, which may be expected to reduce the bioavailability of divalent cations as a result of the precipitation as carbonates (e.g. FeCO3).

Acute and chronic physiological effects of silver exposure in three marine teleosts

This study evaluated the physiological effects of waterborne silver (added as AgNO(3)) on seawater fish, using acute (48-72 h) high level exposures (250-650 microg/l Ag) on tidepool sculpins (Oligocottus maculosus), and chronic (up to 21 day) low level exposures (1.5-50 microg/l Ag) on tidepool sculpins, plainfin midshipmen (Porichthys notatus), and rainbow trout (Oncorhynchus mykiss). Sculpins were tested at different salinities. Acclimation to lower salinity (18 vs 30 ppt) led to altered physiology, with higher ammonia excretion (J(Amm)), lower oxygen consumption, and lower branchial and intestinal Na(+)/K(+)-ATPase activities, but no difference in drinking rate. Short-term exposure to high silver levels tended to stimulate M(O(2)), J(Amm), and drinking rate. However, long-term exposure to low levels of silver depressed both J(Amm) and M(O(2)), and also led to decreased drinking rates. Both inhibition and stimulation of Na(+)/K(+)-ATPase activity occurred, dependent upon length and concentration of exposure, salinity (18 vs 30 ppt), tissue (gill vs intestine), and fish species (sculpin vs midshipmen vs rainbow trout). While the effects were variable, due to differing balances between inhibitory and compensatory responses, chronic silver exposure significantly altered Na(+)/K(+)-ATPase activity levels in almost all tests. In total, these findings reinforce the view that intestinal osmoregulatory function (drinking, Na(+)/K(+)-ATPase activity) is an important site of toxic impact for waterborne silver, that gill Na(+)/K(+)-ATPase activity is also a site of impact, and that chronic exposures at silver concentrations (1.5, 14.5 microg/l Ag) close to current or proposed water quality guidelines (albeit much higher than normal environmental levels), exert a variety of sublethal effects on marine teleosts.

Sensitivity of the spiny dogfish (Squalus acanthias) to waterborne silver exposure

The physiological effects of waterborne silver exposure (added as AgNO(3)) on spiny dogfish, Squalus acanthias, were evaluated at 30, 200 and 685 microg silver per l in 30 per thousand seawater. These concentrations cover the toxic range observed for freshwater teleosts, where silver is extremely toxic, to seawater teleosts which tolerate higher silver concentrations. However, these levels are considerably higher than those that occur in the normal environment. At 685 microg l(-1), dogfish died within 24 h. Causes of death were respiratory as well as osmoregulatory failure. Arterial P(a)O(2) rapidly declined below 20 Torr, and blood acidosis (both respiratory and metabolic) occurred. Urea excretion increased dramatically and plasma urea dropped from 340 to 225 mM. There were pronounced increases in plasma Na(+), Cl(-), and Mg(2+), indicative of ionoregulatory failure due to increased diffusive permeability as well as inhibited NaCl excretion. At 200 microg l(-1), fish died between 24 and 72 h of silver exposure. The same physiological events occurred with a small time delay. At 30 microg l(-1), effects were much less severe, although slight mortality (12.5%) still occurred. Respiratory alkalosis occurred, together with moderate elevations in plasma Na(+) and Cl(-) levels. Silver accumulated to the highest concentrations on gills, with only low levels in the intestine, in accord with the virtual absence of drinking. Na(+)/K(+)-ATP-ase activities of gill and rectal gland tissue were impaired at the highest silver concentration. Normal gill function was impaired due to swelling and fusion of lamellae, lamellar aneurism and lifting of the lamellar epithelium. Our results clearly indicate that this elasmobranch is much more sensitive (about 10-fold) to silver than marine teleosts, with silver's toxic action exerted on the gill rather than on the intestine, in contrast to the latter.

Cloning and expression of two isoforms of guanylate cyclase C (GC-C) from the European eel (Anguilla anguilla)

Complementary DNA fragments for two isoforms of particulate guanylate cyclase C (GC-C) were cloned from the intestine of the European eel (Anguilla anguilla). Both isoforms exhibited higher nucleotide and amino acid sequence homologies to members of the GC-C family from other species than the related guanylate cyclase A or B (GC-A or GC-B) isoforms from the eel. Northern blots indicated that probes for both isoforms, termed GC-C1 and GC-C2, selectively hybridised to 4.8-kb transcripts in the intestine and the kidney. Expression of the GC-C2 transcript in the intestine was increased by 100% following the transfer of yellow FW-acclimated eels to SW. Likewise developmental maturation of yellow eels into pre-migratory silver eels resulted in a significant increase (60%) in the intestinal expression of GC-C2. No changes in expression of GC-C2 were seen in the kidney under any condition. RT-PCR indicated that the GC-C2 isoform is only expressed in anterior and mid-gut segments in FW-acclimated yellow eels. However, expression is also extended to the posterior gut segment when yellow eels are acclimated to SW or following developmental transformation into silver eels.

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.

Time-course changes in the expression of Na+, K+-ATPase in gills and pyloric caeca of brown trout (Salmo trutta) during acclimation to seawater

Changes in protein and mRNA expression of Na(+),K(+)-ATPase in gills and pyloric caeca of brown trout were investigated on a detailed time course after transfer from freshwater to 25 ppt seawater (SW). A transient deflection in plasma osmolality and muscle water content lasting from 4 h until day 3 was followed by restoration of hydromineral balance from day 5 onward. Gills and pyloric caeca responded to SW transfer by increasing Na(+),K(+)-ATPase activity from days 5 and 3, respectively, onward. In both tissues, this response was preceded by an increase in alpha-subunit Na(+), K(+)-ATPase mRNA as early as 12 h posttransfer. The similarity of the response in these two organs suggests that they both play significant physiological roles in restoring hydromineral balance after abrupt increase in salinity. Further, SW transfer induced a slight, though significant, increase in primary gill filament Na(+), K(+)-ATPase immunoreactive (NKIR) cell abundance. This was paralleled by a marked (50%) decrease in secondary lamellar NKIR cell abundance after less than 1 d in SW. Thus, SW acclimation in brown trout is characterised by a lasting decrease in overall NKIR cell abundance in the gill. We propose that SW transfer stimulates Na(+),K(+)-ATPase enzymatic activity within individual chloride cells long before (<1 d) it becomes apparent in measurements of whole-gill homogenate enzymatic activity. This is supported by the early stabilisation (12 h) of hydromineral balance.

Bioaccumulation and distribution of silver in four marine teleosts and two marine elasmobranchs: influence of exposure duration, concentration, and salinity

The bioaccumulation of waterborne silver (added as AgNO(3)) was compared amongst drinking (teleosts: rainbow trout, tidepool sculpin, plainfin midshipmen, and English sole) and non-drinking marine fish (elasmobranchs: Pacific spiny dogfish and long nose skate) exposed to 14.5 µg/l Ag for 21 days in 30-ppt seawater. In addition, 21-day exposures were performed on trout, midshipmen, and sculpin at 0 (control), 1.5, 14.5, and 50 µg/l Ag to evaluate the effect of silver concentration, and on sculpins acclimated to 18 and 30 ppt salinity and sampled periodically up to 21 days to evaluate the effects of salinity and exposure duration. A 48-h acute exposure (250 µg/l Ag) was also carried out on sculpins at 10, 18, 24, and 30 ppt. The 1.5-and 14.5-µg/l Ag levels are of regulatory importance, but are several orders of magnitude higher than normal environmental levels. Silver uptake occurred in all exposures, but internal accumulations were less than proportional to exposure concentration (1.5-50.0 µg/l Ag), and tended to saturate over time, suggesting that physiological regulation occurred. Control (non-exposed) fish exhibited measurable levels of silver in all tissues (10-200 µg Ag/kg wet weight), suggesting that they accumulate silver from the natural environment throughout their lifetimes. After 21-day exposure to 14.5 µg/l Ag, silver levels increased 2-20-fold in most tissues of all species, with the greatest concentrations occurring in the livers of teleosts (order: liver>gills>/=intestines>white muscle) and the gills of elasmobranchs (order: gills>liver>white muscle>intestines). Rainbow trout accumulated more silver than the other teleosts, and were the only species to suffer significant mortality, effects likely associated with added salinity stress. Accumulations were fairly uniform amongst the other teleosts. Similar concentrations in gills and intestines suggested that both branchial and intestinal uptake occurred, with the latter potentially dominant; indeed sole exhibited no silver build-up in the gills. The two elasmobranchs exhibited no silver build-up in intestines but much higher levels in gills, indicating that in the absence of drinking, only branchial uptake occurs. Nevertheless, based on whole liver content, the elasmobranchs accumulated silver 5-15-fold faster than the three teleosts. Over 21-day exposures (1.5-50.0 µg/l Ag) in sculpin, salinity markedly affected silver accumulation, with tissue-specific levels approximately 6-fold higher at 18, than at 30 ppt. However, there was negligible effect of salinity on silver accumulation during 48 h at 250 µg/l Ag. Silver bioaccumulation appears to be markedly affected by speciation. At lower salinities, or higher [Ag], a neutral charged AgCl(aq) complex exists in the water, allowing for increased bioaccumulation to occur. At higher salinity, only less bioavailable, negatively-charged AgCl(n)(1-n) complexes are present (AgCl(2)(-), AgCl(3)(2-), AgCl(4)(3-)).

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.

NO2- uptake and HCO3- excretion in the intestine of the European flounder (Platichthys flesus)

Ion transport across isolated intestinal segments from the European flounder (Platichthys flesus) was studied with the primary aim of evaluating the mechanisms of nitrite (NO2-) uptake and HCO3- excretion. A double-radiolabelling technique was applied to monitor unidirectional Cl- and Na+ influx. Furthermore, net fluxes of NO2-, HCO3-, Cl-, Na+ and water were recorded. NO2- uptake was inhibited by mucosal application of bumetanide (10(−)4 mol l-1) but not DIDS (10(−)3 mol l-1), suggesting that NO2- is transported across the intestine via the Na+/K+/2Cl- cotransporter rather than via a Cl-/HCO3- exchanger. In addition to transport via the Na+/K+/2Cl- cotransporter, NO2- uptake may also occur through the Na+/Cl- cotransporter and by conductive transport. NO2- and Cl- influx rates seemed to reflect their mucosal concentrations, and NO2- did not influence unidirectional influx or net flux of Cl-. HCO3- efflux was significantly reduced in the presence of 10(−)3 mol l-1 DIDS in the mucosal solution. This may indicate the presence of an apical Cl-/HCO3- exchanger in the intestinal epithelium, which would not comply with the current model of HCO3- excretion in the intestine of marine teleost fish. An alternative model of HCO3- excretion across the intestinal epithelium is proposed.

Effects of insulin-like growth factor-I and cortisol on Na+, K+-ATPase expression in osmoregulatory tissues of brown trout (Salmo trutta)

The effect of recombinant bovine IGF-I (rbIGF-I) on hypo-osmoregulatory ability and the effect of rbIGF-I and cortisol (F) alone and in combination on Na+,K+-ATPase expression in fresh water (FW) acclimated brown trout (Salmo trutta) were examined in two experiments. In Experiment 1, fish were given three injections of saline or 0.01 or 0.1 microgram rbIGF-I/g, respectively, and subjected to a 24-h 25 ppt seawater (SW) challenge test 24 h after the last injection. Fish treated with 0.01 and 0.1 microgram rbIGF-I/g had better hypo-osmoregulatory ability than control fish as judged by their higher level of muscle water content and lower plasma osmolality after 24 h exposure to 25 ppt SW. Compared with control fish, gill Na+,K+-ATPase activity was unchanged 24 h after the first injection at either dose but significantly stimulated after three injections of either dose of rbIGF-I. In Experiment 2, fish were given three injections of saline, 0.1 microgram rbIGF-I/g, 4 microgram F/g, or 0.1 microgram rbIGF-I + 4 microgram F/g and sampled in FW 24 h after the last injection. IGF-I and F had additive stimulatory effects on Na+,K+-ATPase activity and alpha-subunit Na+,K+-ATPase mRNA levels in the gill. Injections of IGF-I and F alone and in combination increased Na+,K+-ATPase-immunoreactive (NKIR) cell number in the primary gill filament but had no effect on secondary lamellar NKIR cell number. NKIR cells were abundant in kidney tubules, pyloric ceca, and posterior intestine, but Na+,K+-ATPase enzyme activity was unaffected by treatment with F and/or IGF-I in these tissues. F but not rbIGF-I increased in vitro fluid transport capacity in the posterior intestine. In addition to confirming an overall SW-adaptive effect of rbIGF-I and F in FW-acclimated S. trutta, the study suggests the effect to be associated with stimulation of chloride cell development and Na+,K+-ATPase expression in the gill. The study indicates that the stimulatory effects of the two hormones on Na+,K+-ATPase expression are additive, highly organ specific, and restricted to the primary filament epithelium of the gill.

A mechanism for branchial acid excretion in marine fish: identification of multiple Na+/H+ antiporter (NHE) isoforms in gills of two seawater teleosts

Both Na+/H+ exchange and the electrogenic extrusion of H+ via an H+-ATPase have been postulated to drive acid excretion across the branchial epithelium of fishes. While the H+-ATPase/Na+ channel system appears to be the predominant mechanism in some freshwater species, it may play a reduced role in seawater and brackish-water animals, where high external Na+ concentrations may thermodynamically favor Na+/H+ exchange driven by a Na+/H+ antiporter (NHE). In this study, we used molecular and immunological methods to assess the role of NHE isoforms in the branchial epithelium of the marine long-horned sculpin (Myoxocephalus octodecimspinosus) and the euryhaline killifish (Fundulus heteroclitus).Northern blot analysis of RNA probed with the human NHE-1 BamHI fragment suggested the presence of homologous gill NHE mRNA in sculpin. RT-PCR on gill RNA isolated from sculpin recovering from metabolic acidosis provided evidence for two distinct NHE isoforms; one with 76 % amino acid homology to mammalian NHE-2, and another 92 % homologous to trout erythrocytic beta-NHE. Killifish also have transcripts with 91 % homology to beta-NHE. Immunological detection using monoclonal antibodies for mammalian NHE-1 revealed a protein antigenically similar to this isoform in the gills of both species. Metabolic acidosis caused an approximately 30-fold decrease in expression of the NHE-1-like protein in sculpin. We speculate that beta-NHE in the gills plays the intracellular ‘housekeeping’ roles described for mammalian NHE-1. During systemic acidosis, apical gill NHE-2 (which is sensitive to external amiloride and low [Na+]) in parallel with a dramatic suppression of basolateral NHE-1 activity enhances net capdelta H+ transfers to the water.