A strong foundation of basic and applied research documents that the estuarine fish Fundulus heteroclitus and related species are unique laboratory and field models for understanding how individuals and populations interact with their environment. In this paper we summarize an extensive body of work examining the adaptive responses of Fundulus species to environmental conditions, and describe how this research has contributed importantly to our understanding of physiology, gene regulation, toxicology, and ecological and evolutionary genetics of teleosts and other vertebrates. These explorations have reached a critical juncture at which advancement is hindered by the lack of genomic resources for these species. We suggest that a more complete genomics toolbox for F. heteroclitus and related species will permit researchers to exploit the power of this model organism to rapidly advance our understanding of fundamental biological and pathological mechanisms among vertebrates, as well as ecological strategies and evolutionary processes common to all living organisms.

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K-Cl cotransporters (KCC) play fundamental roles in ionic and osmotic homeostasis. To date, four mammalian KCC genes have been identified. KCC2 is expressed exclusively in neurons. Injection of Xenopus oocytes with KCC2 cRNA induced a 20-fold increase in Cl(-)-dependent, furosemide-sensitive K(+) uptake. Oocyte swelling increased KCC2 activity 2-3 fold. A canonical tyrosine phosphorylation site is located in the carboxy termini of KCC2 (R1081-Y1087) and KCC4, but not in other KCC isoforms. Pharmacological studies, however, revealed no regulatory role for phosphorylation of KCC2 tyrosine residues. Replacement of Y1087 with aspartate or arginine dramatically reduced K(+) uptake under isotonic and hypotonic conditions. Normal or near-normal cotransporter activity was observed when Y1087 was mutated to phenylalanine, alanine, or isoleucine. A tyrosine residue equivalent to Y1087 is conserved in all identified KCCs from nematodes to humans. Mutation of the Y1087 congener in KCC1 to aspartate also dramatically inhibited cotransporter activity. Taken together, these results suggest that replacement of Y1087 and its congeners with charged residues disrupts the conformational state of the carboxy terminus. We postulate that the carboxy terminus plays an essential role in maintaining the functional conformation of KCC cotransporters and/or is involved in essential regulatory protein-protein interactions.

. A divergent CFTR homologue: highly regulated salt transport in the euryhaline teleost F. heteroclitus. Am. J. Physiol. 274 (Cell Physiol. 43): C715-C723, 1998.-The killifish, Fundulus heteroclitus, is a euryhaline teleost fish capable of adapting rapidly to transfer from freshwater (FW) to four times seawater (SW). To investigate osmoregulation at a molecular level, a 5.7-kilobase cDNA homologous to human cystic fibrosis transmembrane conductance regulator (hCFTR) was isolated from a gill cDNA library from SW-adapted killifish. This cDNA encodes a protein product (kfCFTR) that is 59% identical to hCFTR, the most divergent form of CFTR characterized to date. Expression of kfCFTR in Xenopus oocytes generated adenosine 3Ј,5Ј-cyclic monophosphate-activated, Cl Ϫ -selective currents similar to those generated by hCFTR. In SW-adapted killifish, kfCFTR was expressed at high levels in the gill, opercular epithelium, and intestine. After abrupt exposure of FWadapted killifish to SW, kfCFTR expression in the gill increased severalfold, suggesting a role for kfCFTR in salinity adaptation. Under similar conditions, plasma Na ϩ levels rose significantly after 8 h and then fell, although it is not known whether these changes are directly responsible for the changes in kfCFTR expression. The killifish provides a unique opportunity to understand teleost osmoregulation and the role of CFTR.

The seawater tolerance of Atlantic salmon (Salmo salar) smolts reared under identical hatchery conditions was assessed in two Norwegian strains: AquaGen and Imsa. Plasma ion levels were disrupted in both strains following seawater exposure, but these disruptions were more profound in the AquaGen fish. To investigate the mechanisms underlying these differences, we measured gill Na+,K+-adenosine triphosphatase (ATPase) activity and mRNA levels of Na+,K+-ATPase α-subunit and two isoforms of the cystic fibrosis transmembrane conductance regulator (CFTR). Gill Na+,K+-ATPase activity rose significantly in both strains following seawater exposure. Both Na+,K+-ATPase α-subunit and CFTR I mRNA levels were significantly elevated for the entire 2-week period following seawater exposure, whereas CFTR II levels were transiently elevated during the first 24 h only. There were no differences in enzyme activity or gene expression between strains, with the exception of CFTR II, which was significantly lower in the Imsa strain 2 weeks following seawater exposure. This suggests that although changes in mRNA and protein expression for these genes are associated with seawater transfer, they are not the basis of observed physiological differences between strains.

The gills and intestinal epithelia of teleost fish express cystic fibrosis transmembrane conductance regulator (CFTR), and utilize this low conductance anion channel in the apical membrane for ion secretion in seawater gill and in the basolateral membrane for ion absorption in freshwater gill. Similarly, in the intestine CFTR is present in the basolateral membrane for intestinal absorption and also in the apical membrane of secreting intestine. The expression of CFTR and the directed trafficking of the protein to the apical or basolateral membrane is salinity-dependent. The CFTR gene has been cloned and sequenced from several teleost species and although all the major elements in the human gene are present, including two nucleotide binding domains that are common to all ATP binding cassette (ABC) transporters, the sequences are divergent compared to shark or human. In euryhaline fish adapting to seawater, CFTR, localized immunocytochemically, redistributes slowly from a basolateral location to the apical membrane while ion secretory capacity increases. The facility with which teleosts regulate CFTR expression and activation during salinity adaptation make this system an appealing model for the expression and trafficking operation of this labile gene product.

19963. Aspects of the energy metabolism of lake sturgeon, Acipenses fulvescens, with special emphasis on lipid and ketone body metabolism. Can. J. Fish. Aquat. Sci. 47: 873-88'1.Key enzymes in several metabolic pathways in five tissues were measured in a primitive osteichtkyan, the lake sturgeon (Acipenses fdvescens). bevels of nonesterified fatty acids (NEFAs) were measured in the plasma as an indicator of fatty acid mobilization and differential utilization of individual NEFAs as substrates for lipid oxidation. The metabolism of lake sturgeon differs from that of most teleosts studied; it has detectable levels of beta-hydroxybutyrate dehydrogenase in all tissues, possibly a primitive metabolic feature of vertebrates, subsequently lost in the telecasts. Based on HOAD and CPT activities lipid oxidation in extrahepatic tissues of sturgeon is intermediate between elasmobranch and telegast models. Sturgeon plasma NEFA concentrations are clearly higher than those detected in any elasmobranch, indicating that the acipenserid chondrosteans may be among the first jawed dish to mobilize and transport IBIEFAs. Oleic acid ('1 8:1 ,n9) which amounts to 45% sf total NEFA content of the plasma may be a preferred substrate of carnitine dependent oxidation. High levels of long chain fatty acid in the plasma may represent a high turnover of eicosanoid precursors. The low levels sf LDH in the anoxia tolerant sturgeon may indicate that alternative anaerobic end products, perhaps ethanol, are used to survive under anoxic conditions. bes principales enzymes de plusieurs voies metaboliques dans cinq tissus ont kte rnesurks dans un osteichthyen primitif, l'esturgeon de lac (Acipewses fdvescens). bes concentrations d'acide gras nonesterifiks (NEFA) ont kt@ mesurkes dans le plasma 3 titre d'indicateur de la rnobilisatican de I'acide gras et de I'utiiisation diffkrentielle de chaque NEFA comme substrats pour I'oxydation des lipides, he mktabolisrne de I'esturgeon de lac difiere de celui de la plupart des td6ostkens etudiks; ce poisson adiiche en effet des concentrations detectables de beta-hydroxybutyratedkhydrog6nase dans tous ses tissus, ce qui etait probablement une caracteristique mktabolique primitive des vertebrks, disparue par la suite chez les tkl6ostkens. D'aprGs les activites de HOAD et de CPT' l'oxydatican des lipides dans les tissus extrahktape'ques de I'esturgeon se situe h un niveau intermkdiaire entre celle des modeles elasmobranches et tklesstkens. Les concentrations plasmatiques de NEFA de 'esturgeon sont de toute kvidence plus klevkes que celles relevkes chez tous les 6/asmobranches, ce qui r6v&le que les chondrosteens acipenseridks pourraient avoir kt4 parmi les premiers pissons A m2chsires h mobiliser et h transporter les NEFA. L'acide olkique (1 $:I, n9), qui constitue jusqu'h 45 % de la teneur totale en NEFA du plasma, pourrait Stre un des substrats prkfer6s de I'oxydation like 21 la carnitine. Les fortes concentrations plasmatiques d'acides gras & langue chaine pourraient temoigner d'un fort taux de rensuvellement ...

In this study, we set out to examine the role of the somatotropic axis in the ion-regulation process in rainbow trout. Specifically, our objective was to examine whether plasma insulin-like growth factor-binding proteins (IGFBPs) are modulated by gradual salinity exposure. To this end, freshwater (FW)-adapted rainbow trout were subjected to gradual salinity increases, up to 66% seawater, over a period of 5 days. During this acclimation process, minimal elevations in plasma Ca 2ϩ and Cl Ϫ were seen in the salinity-acclimated groups compared with FW controls. There were no changes in plasma Na ϩ levels, and only a minor transient change in plasma cortisol levels was seen with salinity exposure. The salinity challenged animals responded with elevations in plasma growth hormone (GH) and IGF-I levels and gill Na ϩ -K ϩ -ATPase activity. We identified IGFBPs of 21, 32, 42, and 50 kDa in size in the plasma of these animals, and they were consistently higher with salinity. Despite the overall increase in IGFBPs with salinity, transient changes in individual BPs over the 5-day period were noted in the FW and salinity-exposed fish. Specifically, the transient changes in plasma levels of the 21-, 42-, and 50-kDa IGFBPs were different between the FW and salinity groups, while the 32-kDa IGFBP showed a similar trend (increases with sampling time) in both groups. Considered together, the elevated plasma IGFBPs suggest a key role for these binding proteins in the regulation of IGF-I during salinity acclimation in salmonids.insulin-like growth factor binding-proteins; insulin-like growth factor-I; growth hormone; rainbow trout; salinity EURYHALINE TELEOSTS, such as killifish, tilapia, and eels, survive abrupt full-strength seawater (SW) transfer without prior physiological preparation (14,42,92), whereas salmonids undergo a preacclimation process that is essential to SW survival (46). The preacclimation process involves physiological and morphological changes that improve salinity tolerance of developing/migrating salmonids (16,31,48). These physiological changes, such as salinity tolerance, are driven by changes in circulating hormones such as growth hormone (GH), prolactin (PRL), somatolactin (SL), insulin-like growth factor-I (IGF-I), and cortisol (8,24,45).Efforts to separate the endocrine and physiological response to salinity exposure, from the acquisition of salinity tolerance by migrating salmonids, have been made difficult by species differences (48) and aspects of the preacclimation process such as developmental and sex-specific differences (8,10,30,48,50,55), changes in temperature and photoperiod (5, 8, 9, 27), diet and rearing conditions (64, 97), growth, genetic lineage and domestication (24,27,57), and the method and duration used for salinity challenge (9,72,73,75,93).Aside from the aforementioned factors that influence salinity tolerance and hormone levels in salmonids, there is good evidence to show that the preacclimation process, SW tolerance, and enhanced growth of SW-acclimated salmonids are controlled by ...