The nature and properties of the inducible sodium-plus-potassium ion-dependent adenosine triphosphatase in the gills of eels (Anguilla anguilla) adapted to fresh water and sea water

Regulation of membrane lipid bilayer structure during salinity adaptation: a study with the gill epithelial cell membranes of Oreochromis niloticus

A significant variation in the membrane fluidity (as assessed by DPH-fluorescence polarisation) and membrane lipid bilayer composition is noticed in the subcellular membranes of the gill epithelial cells of Oreochromis niloticus due to exposure of the fish to 1% saline water for 1 month. Also, a 70% enhanced activity of Na(+)-K(+)-ATPase in plasma membranes and a 2.5-fold increase of glucose-6-phosphate dehydrogenase in microsomal membranes are recorded in the treated fish. The changed membrane structure and fluidity along with the changed enzymatic activity of Na(+)-K(+)-ATPase help the influx the Na(+) rather than the efflux of K(+) through the gill epithelial cells during salinity adaptation.

Synthesis of gill Na(+)-K(+)-ATPase in Atlantic salmon smolts: differences in alpha-mRNA and alpha-protein levels

Several parameters were analyzed to determine the mechanisms responsible for the enhancement of the gill Na(+)-K(+)-ATPase activity of Atlantic salmon smolts. A major alpha-subunit transcript of 3.7 kb was revealed by Northern blot in both parr and smolt gills when hybridized with two distinct cDNA probes. The alpha-mRNA abundance demonstrated an increase to maximal levels in smolts at an early stage of the parr-smolt transformation. This was followed by a gradual rise in alpha-protein levels, revealed by Western blots with specific antibodies and by an increase in gill Na(+)-K(+)-ATPase hydrolytic activity, both only reaching maximum levels a month later, at the peak of the transformation process. Parr fish experienced a decrease in alpha-mRNA abundance and had basal levels of alpha-protein and enzyme activity. Measurement of the binding of [(3)H]ouabain to Na(+)-K(+)-ATPase was characterized in smolts and parr gill membranes showing more than a twofold elevation in smolts and was of high affinity in both groups (dissociation constant = 20-23 nM). Modulation of the enzyme due to increased salinity was also observed in seawater-transferred smolts, as demonstrated by an increase in alpha-mRNA levels after 24 h with a rise in Na(+)-K(+)-ATPase activity occurring only after 11 days. No qualitative change in alpha-expression was revealed at either the mRNA or protein level. Immunological identification of the alpha-protein was performed with polyclonal antibodies directed against the rat alpha-specific isoforms, revealing that parr, freshwater, and seawater smolts have an alpha(3)-like isoform. This study shows that the increase in Na(+)-K(+)-ATPase activity in smolt gills depends first on an increase in the alpha-mRNA expression and is followed by a slower rise in alpha-protein abundance that eventually leads to a higher synthesis of Na(+)-K(+) pumps.

Lipid restructuring does not contribute to elevated activities of Na(+)/K(+)-ATPase in basolateral membranes from the gill of seawater-acclimated eel (Anguilla rostrata)

In teleost fishes, increases in gill Na(+)/K(+)-ATPase activity accompanying the transition from fresh water to sea water may be attributed to changes in either the numbers of enzyme molecules present or to turnover number (k(cat)). The sensitivity of Na(+)/K(+)-ATPase to its chemical/physical environment in the membrane makes it plausible that modulation of enzyme activity may be driven, in part, by changes in membrane properties. In the current study, I test the hypothesis that lipid compositional changes (restructuring) contribute to the modulation of gill Na(+)/K(+)-ATPase activity. An enriched preparation of basolateral membranes was prepared from the gills of freshwater- and seawater-acclimated American eel (Anguilla rostrata). Phospholipid class distribution, fatty acyl chain compositions and cholesterol contents were determined. Phosphatidylcholine, the most abundant phospholipid present in gill basolateral membranes, makes up more than 60 % of the total phospholipid content in both freshwater- and seawater-acclimated animals. The contents of other phospholipids and major fatty acyl chains are also similar for the two acclimation groups. Cholesterol/phospholipid molar ratios are 0.28 for freshwater and 0.29 for seawater animals. The similarity between lipid compositions in membranes from freshwater- and seawater-acclimated eels indicates that lipid restructuring is not a mechanism for modulation of gill Na(+)/K(+)-ATPase activity in Anguilla rostrata, at least during the acclimation time course used in the present study.

Transport mechanisms of seawater teleost chloride cells: an inclusive model of a multifunctional cell

This review assembles recent information on seawater-type chloride cells of marine teleost fish and evaluates the secretion of Na+, Cl-, K+, H+ and NH4+ and the absorption of Ca2+. The evidence for the distribution (apical vs basolateral) and the abundance of the various ion pumps, cotransporters, channels and exchangers is assessed and an inclusive model is constructed. Relationships among the transport systems are presented to suggest that many, if not all, of these systems may be operating simultaneously in individual, multifunctional chloride cells.

Oxygen consumption and Na+,K+-ATPase activity of rectal gland and gill tissue in the spiny dogfish,Squalus acanthias

Oxygen consumption was measured in rectal gland and gill tissue of the spiny dogfish (Squalus acanthias) to estimate the energy cost of salt (NaCl) transport in these osmoregulatory organs. Ouabain (0.5 mM) was used to specifically inhibit Na+,K+-ATPase activity and thus the portion of tissue respiration required by the Na+/K+pump. The total mass-specific oxygen consumption of rectal gland tissue (14.2 ± 1.2 μmol O2/(g wet mass∙h)) was significantly higher than measured for the gills (9.6 ± 1.4 μmol O2/(g wet mass∙h)), and ouabain significantly reduced oxygen consumption in both tissues. Ouabain-sensitive oxygen consumption of the rectal gland accounted for 55% of total mass-specific oxygen consumption, compared with 22% for the gill. The higher ion transport capacity of the rectal gland was also evident in Na+,K+-ATPase specific activity measurements of fresh tissue samples, which were sixfold higher in the rectal gland than in the gill. Ouabain-sensitive oxygen consumption was also calculated on the basis of total organ mass to determine the portion of whole-animal oxygen uptake related to organismal NaCl transport. The cost of NaCl secretion was estimated to be 0.5% of standard metabolic rate for the rectal gland compared with 0.14% for the gills, suggesting that this process constitutes a relatively small portion of the total energy budget in the spiny dogfish.

Comparison between parr and smolt Atlantic salmon (Salmo salar) α subunit gene expression of Na(+)/K (+) ATPase in gill tissue

Increases in branchial Na(+)/K(+) ATPase activity during seawater adaptation of euryhaline fish species, have been well documented. During the parr-smolt transformation of salmonids this activity increases two to five fold and is used as an indicator of the transformation. In order to improve the understanding of differences in enzyme activity found between Atlantic salmonSalmo salar parr and smolt fish, we investigated the gene expression of the Na(+)/K(+) ATPase α-subunit(s) in gill tissue. Gill mRNAs were analyzed and quantified at distinct time points using Northern and Dot blot techniques. We amplified by PCR, a conserved region of the cDNA encoding the Na(+)/K(+) ATPase α-subunit of the rainbow troutOncorhynchus mykiss. The PCR products (670 bp) were cloned and all independent clones showed a sequence corresponding to the α subunit of the Na(+)/K(+) ATPase. The fragments obtained appeared as a heterogenous population of three sequences showing, when compared between each other, 86 to 93% identity. This suggests that different allelic forms of the α-subunit are expressed in gill tissue. Hybridization studies performed with these PCR probes revealed two mRNA species, a major 3.7 kb transcript and a minor transcript of 1.8 kb. Enhanced 3.7 kb transcript levels are concurrent with elevated enzyme activity in smolts during the March and April parrsmolt transformation of Atlantic salmon. Interestingly, our study disclosed that smolt fish only displayed a two-fold increase in transcript levels when compared to parr whereas enzyme activity showed a 4 to 5 fold increase. This suggests that the increase in the 3.7 kb mRNA content of gill tissue is probably not the only mediator leading to the rise in enzyme activity during parr-smolt transformation.

Effects of salinity on chloride cells and Na+, K(+)-ATPase activity in the teleost Gillichthys mirabilis

1. Longjawed mudsuckers, Gillichthys mirabilis, in 30 ppt seawater (SW) were transferred to 1.5, 30 and 60 ppt SW. 2. In the first 1-3 days after transfer, plasma chloride level and plasma osmolarity rose in the 60 ppt SW fish, and decreased in the 1.5 ppt SW fish. 3. By day 21, however, plasma chloride and osmolarity were at or near the levels seen in the controls (30 ppt). 4. Branchial and jawskin Na+, K(+)-ATPase activities were high in all salinities, and did not differ significantly among treatments. 5. The vital fluorescent stains DASPEI and anthroylouabain were used to detect mitochondria and Na+, K(+)-ATPase, respectively, in chloride cells. 6. Both stains indicated that jawskin chloride cell density did not differ among treatment groups. 7. In contrast, chloride cell size increased significantly with increasing salinity. 8. The chloride cells of fish in 60 ppt SW were noticeably angular in outline, whereas those of both the 1.5 and 30 ppt SW fish were circular. 9. The results are discussed in relation to the ion transport requirements encountered in the intertidal habitat of the mudsucker.

Gill (Na+ + K+)-ATPase involvement and regulation during salmonid adaptation to salt water

1. The involvement of gill (Na+ +K+)-ATPase in salmonid adaptation to salt water (SW) is discussed. 2. Gill (Na+ +K+)-ATPase increase during SW adaptation is mainly related to the increased number and complexity of chloride cells deputed to salt extrusion. 3. The temporal relationships between serum peaks of thyroid hormones, cortisol, growth hormone, prolactin and gill (Na+ +K+)-ATPase rise during salmonid smoltification, suggest a hormonal involvement in the enzyme stimulation and thus in the acquirement of SW tolerance. 4. Literature on gill (Na+ +K+)-ATPase response to hormonal treatment is reviewed. The effects produced on gill (Na+ +K+)-ATPase and chloride cells by exogenous hormones point out a complex inter-relationship between the hormones considered. The mechanisms involved in hormonal regulation of the enzyme remain a matter of debate.

Gill ultrastructure of the Pacific hagfish Eptatretus stouti

At the gross anatomical level, hagfish gills show unusual features not seen in any other fish gills. Our study was undertaken to determine if peculiarities also characterize the microscopic anatomy and ultrastructure of hagfish gills. To the contrary, branchial respiratory lamellae of Pacific hagfish were found to resemble the lamellae of lampreys, elasmobranchs, and teleosts, often down to the finest subcellular details. As in other fish, hagfish lamellae are lined by epithelium containing pavement cells with organelles indicative of a secretory function, basal cells showing undifferentiated cell features, and branchial ionocytes. The ionocytes are identical to chloride cells of teleosts in cytostructure, distribution, and abundance. There are pillar and marginal capillaries in hagfish gill lamellae. Pillar cells contain bundles of 5-nm microfilaments, and they associate with collagen columns as in other fish. Hagfish pillar cells do exhibit odd features, however: They cluster (groups of up to nine were seen), and their extracellular collagen columns are rarer than in other fish gills (averaging only two columns per three pillar cells). Other special features of hagfish gills are the following: lipid droplets and smooth endoplasmic reticulum are well developed in all cell types; pavement cells secrete a lipomucous product (stains with periodic acid-Schiff, Alcian blue, and Sudan black B); and goblet cells are absent. The presence of "chloride cells" in hagfish is puzzling, as hagfish body fluids are iso-osmotic to seawater and there is no need to osmoregulate for sodium chloride; the ionocytes contain carbonic anhydrase, suggesting a function in acid/base regulation.

Glucocorticoid receptors in the gill tissue of fish

The presence of glucocorticoid-binding macromolecular receptors was demonstrated in the Na2MO4 (10 mM)-stabilized gill cytosol of the American eel, Anguilla rostrata and in that of the trout, Salmo gairdneri. In all experiments, tritiated triamcinolone acetonide [( 3H]TA) was used as ligand. In the eel, the steroid was bound with a KD of 2.84 +/- 0.4 nM and an Nmax of 188 +/- 34 fmol/mg protein. The binding parameters for the trout cytosol were KD = 1.43 +/- 0.13 nM; Nmax = 271 +/- 113 fmol/mg protein. Competition studies with [3H]TA-labeled eel gill cytosol and radioinert steroids gave the following binding hierarchy: TA greater than dexamethasone greater than cortisol greater than 11-deoxycortisol greater than 21-deoxycortisol. Aldosterone, estrogens, or androgens did not complete. The eel gill receptor was deactivated by prior treatment with trypsin or mersalyl. RNase was without effect, but DNase degraded the receptor except when used in the presence of trypsin inhibitor. The eel gill TA-receptor complex sedimented on a linear (10-30%) sucrose gradient with a single peak at 7.0 S or 3.5 S, in hypotonic or hypertonic (0.4 M KCl) gradients, respectively. The eel ligand-receptor complex did not bind, following heat activation, to DNA-cellulose or phospho-cellulose, though it bound to DEAE-cellulose. In this respect, it behaved similarly to the eel intestinal mucosal TA-receptor complex, described previously. The initiation of dissociation of the eel receptor-[3H]TA complex with excess TA yielded pseudo-first-order dissociation kinetics (k-1 at 0 degree C: 2.39 X 10(-5) S-1), while the association kinetics of the receptor with the ligand was of second order (k + 1: 2.51 X 10(4) M-1 S-1). Sepharose column chromatography indicated a molecular weight of 334,690 Da. Calculation of the Stokes radius gave a value of 84.5 A and the frictional ratio, calculated from the molecular weight, was 1.84. From these data it was concluded that the gills of these two euryhaline teleosts contain tetrapod-type glucocorticoid receptors. These studies are the first to demonstrate these steroid recognition molecules in fish gill. The presence of receptors in the fish gill tissue are in agreement with the physiological action of corticosteroids in allowing adaptation of the animals to habitats of different salinity.

Two anatomical pathways for the renewal of surface glycoproteins in chloride cells of fish gills

The cytoplasm of chloride cells found in the epithelium lining the gills of guppies (Lebistes reticulatus) contains, in addition to the Golgi apparatus and cisternae of endoplasmic reticulum, two distinct membranous components, the vesiculotubular and the tubular systems. While the latter is connected to the laterobasal plasma membrane, the former, made up of small vesicles and short membranous tubules, is seen mainly between the Golgi apparatus and the apical cavity which invaginates the apex of the cell. The role of these two systems in the transport of glycoproteins from the Golgi apparatus to the cell surface was investigated in fishes maintained in fresh and salt water, injected with 3H-fucose, and sacrificed at various intervals thereafter (10 and 30 min; 2.5, 8, 15.5, 24, and 48 hours). The distribution of the label was analyzed by quantitative radioautography in sections examined with the light and electron microscopes. The light microscopic data suggested that the label incorporated in the supranuclear region, where the Golgi apparatus is located, migrated toward the apical and the laterobasal regions of the chloride cells. The relative concentration of the tracer over the various components of the cytoplasm of these cells was calculated from data collected on electron microscope radioautographs at various intervals after 3H-fucose injection. The curves obtained supported the view that glycoproteins synthesized in the Golgi apparatus were transported to the apical surface via the vesiculotubular system, and to the laterobasal membrane via the tubular system.

A carrier enzyme basis for ammonium excretion in teleost gill. NH+4-stimulated Na-dependent ATPase activity in Opsanus beta

1. Branchial Na+K+-ATPase specific activity is some 20% greater in hyposaline adapted Opsanus beta than in SW specimens. 2. Ouabain insensitive ATPase (Mg2+-ATPase) specific activities were similar, while whole body activity differences in low salinity and SW adapted fish could be accounted for by the 30% difference in extractable gill protein. 3. NH+4 ion was 15% more effective at dephosphorylation of the microsomal Na-dependent phosphoenzyme than either Rb+ or K+, and revealed a maximal ATPase affinity (Km = 0.2 mM) within the physiological range of blood [K+]. 4. Similar properties as pH optima, ATP and Mg2+ Km's, ouabain sensitivity, percent recoveries and subcell distribution indicated that the NH+4-stimulation acts through the Na+ K+-ATPase carrier enzyme and may be responsible for the Na+/NH+4 exchange in Opsanus beta.

Relationship between gill Na+,K+-activated ATPase activity and osmotic stress in the plecopteran nymph, Paragnetina media

A relationship between osmotic stress and gill Na+,K+-activated ATPase was observed in plecopteran nymphs acclimated to diluted creek water and a hypertonic medium. An increase of 84% in diluted creek water is presumably related to an active uptake of sodium ions from the hypotonic medium. Whereas a 21% decrease in the enzyme activity may be related to the morphological changes in the specialized cells in the gills. The Na+,K+-activated ATPase activity was also compared with the Malpighian tubules and the rectum. The highest ATPase activity of 32.6 +/- 2 mumoles Pi mg protein-1 30 min-1 was observed in the Malpighian tubules. The activity in the gills (19 +/- 1.2 mumoles Pi mg-1 30 min-1) was slightly lower than the rectum. Since the ATPase activity in the gills is quite high, the gills can be considered to play an active role in hyperosmotic regulation in plecopteran nymphs.

The partial purification of sodium-plus-potassium ion-dependent adenosine triphosphatase from the gills of Anguilla anguilla and its inhibition by orthovanadate

1. (Na+ +K+)-dependent ATPase was partially purified from eel gills by a procedure in which the microsomal fraction of crude preparations of chloride cells was selectively extracted with sodium dodecyl sulphate. 2. The microsomal specific activity was increased 2-fold during optimal treatment with detergent. 3. The final preparation (56% pure) had a specific activity of 341 mumol of ATP hydrolysed/h per mg of protein and a turnover number of 3560 min-1. The number of ouabain-binding sties equalled the number of sites phosphorylated by ATP. 4. Both sodium orthovanadate and ouabain inhibited the purified preparation more than the microsomal fraction, vanadate being more effective on an equimolar basis than ouabain. 5. Inhibition by orthovanadate was not enhanced at 28 mM-as compared with 1mM-MgCl2 and was not reversed by beta-adrenergic agonists (cf. Josephson & Cantley (1977) Biochemistry 16, 4572--4578). 6. Of various other metallic oxyanions tested only niobate proved an effective inhibitor of the enzyme although this anion was less effective than orthovanadate. 7. Orthovanadate partially inhibited phosphorylation of the enzyme by ATP in the presence of 28 mM-MgCl2.

Ultracytochemical localization of Na+,K+-activated ATPase in chloride cells from the gills of a euryhaline teleost

The activity of the electrolyte transport enzyme, sodium, potassium-activated adenosine triphosphatase (Na+,K+-ATPase), in the gills of the pinfish, Lagodon rhomboides, increased markedly following transfer of fish from brackish water to seawater. Cytochemical localization of Na+,K+-ATPase via its potassium-dependent phosphatase (K+-NPPase) activity in the branchial epithelium of pinfish adapted to seawater demonstrated that chloride cells are the major sites for the enzyme. Subcellularly, the heaviest depositions of reaction product were observed lining the cytoplasmic membrane surfaces of the labyrinth of anastomosing plasma membrane tubules that ramifies throughout the chloride cell cytoplasm. Enzyme activity was demonstrated also on the cytoplasmic surface of the apical crypt membrane and on the cytoplasmic surfaces of vesicles in the cytoplasm subjacent to the crypt. Deletion of potassium from the cytochemical incubation medium or inclusion of 10 mM ouabain abolished the reaction products associated with these membranes. The significance of these cytochemical results is discussed with reference to current hypotheses of chloride cell function.

Rapid isolation of chloride cells from pinfish gill

Enriched fractions of chloride cells with good ultrastructural integrity have been obtained from gill filaments of the euyhaline teleost, Lagodon rhomboides. The branchial epithelium from seawater-adapted fish was dissociated by gentle mechanical means in a Ca++, Mg++-free balanced salt solution. Density gradient centrifugation of the mixed cell suspensions through a Ficoll gradient yielded a fraction containing between 50 and 70% chloride cells. This fraction showed a 3- to 4-fold enrichment over comparable gill homogenate values for sodium plus potassium-activated adenosinetriphosphatase, (Na+, K+ ATPase), an enzyme concentrated in chloride cells. Isolation of chloride cells from fish adapted to one-third seawater was less successful, due to the smaller size and reduced number of these cells, although fractions with at least a 2-fold enrichment of the enzyme were obtained. These results continue to support the belief that chloride cells are responsible for osmoregulatory activity associated with the branchial epithelium of teleosts and that this vital function is mediated through the activity of the transport associated enzyme, Na+, K+-ATPase, the specific activity of which increases with osmotic stress.

Rapid modulation of gill Na+ + K+-dependent ATPase activity during acclimation of the killifish Fundulus heteroclitus to salinity change

The enzymatic properties of membrane-bound Na+ + K+-ATPase from gills of killifish acclimated to fresh water, to 16% sea water, or to 30% sea water appear to be identical, indicating that the same enzyme may function to absorb Na+ in low salinities and excrete Na+ at the gills in high salinities. Ammonium ion is an effective substitute for K+: in the ATPase reaction itself, in blocking phosphorylation of the ATPase protein, and in inhibiting the binding of ouabain to the enzyme. The specific activities of the Na+ + K+-ATPase in the three different salinities are consistent with the expected Na+ pumping rates: higher in fresh water and 30% sea water than in 16% sea water. Within one-half hour after transfer of killifish from one salinity to another, gill Na+ + K+-ATPase activities reach equilibrium levels. The rapid increase in Na+ + K+-ATPase activity in gill microsomes of fish acclimating from fresh water to 30% sea water is accompanied by a slow decrease in the number of binding sites for ouabain, supporting the idea that acclimation to short-term salinity changes may involve modifications in the catalytic rate rather than the number of Na+ + K+-ATPase molecules.

The activation of sodium-plus-potassium ion-dependent adenosine triphosphatase from marine teleost gills by univalent cations

The apparent affinity constants for the binding of Cs+, Rb+, K+, Li+, Tl+ and NH4+ to (Na+ + K+)-dependent adenosine triphosphatase from teleost gills were measured and the values discussed in terms of the ion-selectivity isotherm described by Eisenman & Krasne (1975) [in MTP International Review of Science: Biochemistry Series One (Fox, C.F., ed.), vol. 2, pp. 27--59, Butterworths University Park Press, Baltimore]. The ion selectivity of the present enzyme is remarkably similar to that from nerve and brain.

Changes in the levels of chloride cells and (Na+ + K+)-dependent ATPase in the gills of yellow and silver eels adapting to seawater

Changes were measured in the numbers of chloride cells and the levels of (Na+ + K+)-DEPENDENT ATPase in the gills of immature, yellow eels and mature, silver eels during adaptation from freshwater to seawater. The percentage of chloride cells in yellow eels more than doubled after six days in seawater; at this time the specific activity and concentration of (Na+ + K+)-dependent ATPase in gills start to increase in parallel to reach maxima after two weeks that are 2.5 times the starting values. It is concluded that adaptation of yellow eels to seawater involves an increase in the numbers of chloride cells in gills as well as an increased amount of (Na+ + K+)-dependent ATPase per chloride cell. Mature silver eels in freshwater had essentially the same numbers of chloride cells and the same specific activity of the enzyme in the gills as yellow eels fully adapted to seawater. Transferring silver eels to seawater did not alter the percentage of chloride cells in gills although the level of (Na+ + K+)-dependent ATPase and its specific activity increased slightly. Thus, although the silver eel is better prepared for life in seawater than the yellow eel, it still has to attain an increased level of (Na+ + K+)-dependent ATPase in its chloride cells to be fully adapted to seawater.

Characterization of gill (Na + K)-activated adenosine triphosphatase from chinook salmon, Oncorhynchus tshawytscha

(Na+K)-activated ATPase activity from gills of yearling spring chinook was examined using a new rapid assay method. Characterization of the enzyme activity was performed. Optimal activity was obtained at pH 7.2 in the presence of 240 mM NaCl, 120 mM KCl, 20 mM MgCl2 and 10 mM Na2ATP. Maximal inhibition of the enzyme was observed in the presence of 0.5 mM ouabain. Differential centrifugation indicated that 75% of the enzymatic activity was sedimented at 1000 x g. Only 8% of the activity was found in the microsomal pellet. Treatment with 0.1% sodium deoxycholate liberated activity from the 1000 x g pellet and elevated the activity. This treatment caused a loss of 20% of the original activity of the preparation. Statistical analysis of the sampling procedure for gill (Na+K)-activated ATPase activity indicated that there was small variation in the technique itself when compared to variation between the individual gill arches and between individual fish. Results indicate that for meaningful comparisons of groups of fish, the sampling of the gill arches must be standardized and a large number of individual fish must be sampled.

Teleost chloride cell. II. Autoradiographic localization of gill Na,K-ATPase in killifish Fundulus heteroclitus adapted to low and high salinity environments

The specific binding and inhibitory action of (3H)ouabain were employed to localize transport Na,K-ATPase in the euryhaline teleost gill, a NaCl-transporting osmoregulatory tissue in which both enzyme activity and transepithelial transport vary with environmental salinity. In killifish fully adapted to 10%, 100%, or 200% seawater, the gills were internally perfused and externally irrigated in situ. After suitable internal or external exposure to (3H)ouabain, individual gill arches were excised for Na,K-ATPase assay, measurement of radiolabel binding, or quantitative high-resolution autoradiography. Internal exposure to 50 muM ouabain resulted in essentially complete enzyme inhibition, and binding paralleled the increases in enzyme activity at higher salinities; in contrast, external exposure gave minimal and erratic results consistent with leakage of external ouabain into interstitial fluid. (3H)Ouabain autoradiographs demonstrated that, irrespective of exposure or salinity, most of the gill binding was associated with chloride cell. These cells increased in size and number with salinity and, at the subcellular level, the distribution pattern for bound ouabain was always identical to that for the amplified basal-lateral (tubular system) membrane. The combined physiologicmorphologic results constitute final direct proof that chloride cells are the primary site of gill Na,K-ATPase. More important, they provide convincing evidence for unexpected increases in basal-lateral enzyme at higher salinities and thus raise a fundamental objection to the long-postulated role of the Na pump in secretory NaCl transport.

Induction of the catalytic protein of (Na+ plus K+)-ATPase in the salt gland of the duck

The (Na+ plus K+)-ATPase activities in salt gland homogenates increased 3- to 4-fold after saline treatment of ducks for 3 weeks. The ATPase was purified to a specific activity of 460 and 1015 mumol Pi/mg protein per h, respectively, in control and saline-treated ducks. The catalytic protein was identified on polyacrylamide electrophoresis gels by phosphorylating the enzyme with (32P)ATP. The molecular weight of the protein was estimated to be 98 000. The amount of catalytic unit increased commensurately with the enzyme activity after saline treatment. It is therefore concluded that the increased enzyme activity is due to a de novo enzyme synthesis and is not an activation effect. Phospholipid concentration in the salt gland tissue increased 1.7-fold after the saline treatment. Significant increases occurred in the percentage of the total phospholipids as phosphatidylserine and sphingomyelin. In the partially purified (Na+ plus K+)-ATPase preparation, the percentage composition of phosphatidylserine and phosphatidylethanolamine increased after saline treatment.