The dynamics of branchial Na(+),K(+),2Cl(-) cotransporter (NKCC) and Na(+),K(+)-ATPase (NKA) expression were investigated in brown trout and Atlantic salmon during salinity shifts and the parr-smolt transformation, respectively. In the brown trout, Western blotting revealed that NKCC and NKA abundance increased gradually and in parallel (30- and ten-fold, respectively) after transfer to seawater (SW). The NKA hydrolytic activity increased ten-fold after SW-transfer. Following back-transfer to fresh water (FW), the levels of both proteins and NKA activity decreased. The NKCC immunostaining in the gill of SW-acclimated trout was strong, and mainly localized in large cells in the filament and around the bases of the lamellae. In FW-acclimated trout, immunostaining was less intense and more diffuse. Partial cDNAs of the secretory NKCC1 isoform were cloned and sequenced from both brown trout and Atlantic salmon gills. Two differently sized transcripts were detected by Northern blotting in the gill but not in other osmoregulatory tissues (kidney, pyloric caeca, intestine). The abundance in the gill of these transcripts and of the associated NKCC protein increased four- and 30-fold, respectively, during parr-smolt transformation. The abundance of NKA alpha-subunit protein also increased in the gill during parr-smolt transformation though to a lesser extent than enzymatic activity (2.5- and eight-fold, respectively). In separate series of in vitro experiments, cortisol directly stimulated the expression of NKCC mRNA in gill tissue of both salmonids. The study demonstrates the coordinated regulation of NKCC and NKA proteins in the gill during salinity shifts and parr-smolt transformation of salmonids.

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.

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Atlantic salmon (Salmo salar) presmolts, smolts, and postsmolts compensate for a respiratory acidosis associated with 96 h of exposure to hyperoxia (100% O 2 ; hO 2 ), hypercapnia (2% CO 2 and 98% air; hCO 2 ), and combined hO 2 /hCO 2 in freshwater (FW) by increasing strong ion difference, predominantly through a reduction in plasma [Cl -] (presumably via branchial Cl -/HCO 3 -exchange). In smolts, compensation during hO 2 or hCO 2 occurred within 24 h, whereas that in combined hO 2 /hCO 2 was much slower, resulting in 33% mortality by 96 h. FW hO 2 and combined hO 2 /hCO 2 appeared to impair gill function, likely through oxidative cell damage. This resulted in reduced hypoosmoregulatory ability following subsequent transfer to seawater (SW), as indicated by changes in plasma ion levels, osmolality, and muscle water content, resulting in considerable mortalities. Interestingly, FW hCO 2 appeared to enhance hypoosmoregulatory ability during subsequent SW transfer. Smolts are often transported from FW to a subsequent SW release site, and these data indicate that care should be taken to minimize the degree of hyperoxia experienced by the smolts. Hypercapnia, which results from metabolic CO 2 production and inadequate water aeration, does not impair SW transfer, provided it does not occur in conjunction with hyperoxia.Résumé : Les pré-smolts, smolts et post-smolts de saumon atlantique (Salmo salar) compensent l'acidose respiratoire associée à une exposition de 96 h à une hyperoxie (100% O 2 ; hO 2 ), une hypercapnie (2% de CO 2 et 98% d'air; hCO 2 ) et une combinaison de hO 2 /hCO 2 en eau douce en augmentant la différence des ions forts, principalement par une ré-duction du [Cl -] plasmique (vraisemblablement par échange branchial de Cl -/HCO 3 -). Chez les smolts, la compensation pendant hO 2 ou hCO 2 se produisait dans les 24 h, tandis qu'elle était beaucoup plus lente pour la combinaison hO 2 /hCO 2 , qui causait une mortalité de 33% en 96 h. Des conditions de hO 2 en eau douce et la combinaison hO 2 /hCO 2 semblaient entraver la fonction branchiale, vraisemblablement par dégradation oxydative des cellules. Cela provoquait une réduction de la capacité d'hypoosmorégulation suite au transfert dans l'eau de mer, comme le montraient des changements dans les concentrations plasmiques d'ions, l'osmolalité et la teneur en eau des muscles, qui causaient des mortalités considérables. Il est intéressant de constater que la condition de hCO 2 en eau douce semblait améliorer la capacité d'hypoosmorégulation pendant le transfert subséquent en eau salée. Les smolts sont souvent transportés de l'eau douce à un point de lâcher en eau salée, et nos données indiquent qu'il faut prendre soin de minimiser le degré d'hyperoxie qu'ils peuvent connaître. L'hypercapnie, qui provient de la production métabolique de CO 2 et d'une aéra-tion insuffisante de l'eau, ne nuit pas au transfert en eau salée à condition qu'elle ne survienne pas en même temps que l'hyperoxie.[Traduit par la Rédaction] Brauner et al. 2064

A 2-factorial (3 3) injection experiment was used to investigate the effect and interaction between different hormones on the initial phase of seawater (SW) acclimation in brown trout (Salmo trutta). Each fish was given 4 injections on alternate days in freshwater (FW). Factor 1 was either saline, 2 µg ovine prolactin (oPRL)/g, or 2 µg ovine growth hormone (oGH)/g. Factor 2 was either 0, 0·01, or 0·1 µg recombinant human insulin-like growth factor-I (rhIGF-I)/g. In each of the 9 treatment groups, half of the fish were subjected to a 48-h SW-challenge test, and the remaining fish were sham-transferred to FW one day after the last injection. Hypo-osmoregulatory performance was increased by GH and impaired by PRL treatment as judged by changes in plasma osmolality, [Na + ], [Cl ], total [Mg] and muscle water content (MWC) after SW transfer. IGF-I reduced plasma osmolality after transfer to SW but had no effect on plasma total [Mg] or MWC. The effects of the two factors on plasma osmolality, [Na +