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

A Stenohaline Medaka, Oryzias woworae, Increases Expression of Gill Na(+), K(+)-ATPase and Na(+), K(+), 2Cl(-) Cotransporter 1 to Tolerate Osmotic Stress

The present study aimed to evaluate the osmoregulatory mechanism of Daisy's medaka, O. woworae,as well as demonstrate the major factors affecting the hypo-osmoregulatory characteristics of euryhaline and stenohaline medaka. The medaka phylogenetic tree indicates that Daisy's medaka belongs to the celebensis species group. The salinity tolerance of Daisy's medaka was assessed. Our findings revealed that 20‰ (hypertonic) saltwater (SW) was lethal to Daisy's medaka. However, 62.5% of individuals survived 10‰ (isotonic) SW with pre-acclimation to 5‰ SW for one week. This transfer regime, "Experimental (Exp.) 10‰ SW", was used in the following experiments. After 10‰ SW-transfer, the plasma osmolality of Daisy's medaka significantly increased. The protein abundance and distribution of branchial Na(+), K(+)-ATPase (NKA) and Na(+), K(+), 2Cl(-) cotransporter 1 (NKCC1) were also examined after transfer to 10‰ SW for one week. Gill NKA activity increased significantly after transfer to 10‰ SW. Meanwhile, elevation of gill NKA αα-subunit protein-abundance was found in the 10‰ SW-acclimated fish. In gill cross-sections, more and larger NKA-immunoreactive (NKA-IR) cells were observed in the Exp. 10‰ SW medaka. The relative abundance of branchial NKCC1 protein increased significantly after transfer to 10‰ SW. NKCC1 was distributed in the basolateral membrane of NKA-IR cells of the Exp. 10‰ SW group. Furthermore, a higher abundance of NKCC1 protein was found in the gill homogenates of the euryhaline medaka, O. dancena, than in that of the stenohaline medaka, O. woworae.

Branchial ionocyte organization and ion-transport protein expression in juvenile alewives acclimated to freshwater or seawater

The alewife (Alosa pseudoharengus) is a clupeid that undergoes larval and juvenile development in freshwater preceding marine habitation. The purpose of this study was to investigate osmoregulatory mechanisms in alewives that permit homeostasis in different salinities. To this end, we measured physiological, branchial biochemical and cellular responses in juvenile alewives acclimated to freshwater (0.5 p.p.t.) or seawater (35.0 p.p.t.). Plasma chloride concentration was higher in seawater-acclimated than freshwater-acclimated individuals (141 mmol l(-1) vs 134 mmol l(-1)), but the hematocrit remained unchanged. In seawater-acclimated individuals, branchial Na(+)/K(+)-ATPase (NKA) activity was higher by 75%. Western blot analysis indicated that the abundance of the NKA α-subunit and a Na(+)/K(+)/2Cl(-) cotransporter (NKCC1) were greater in seawater-acclimated individuals by 40% and 200%, respectively. NKA and NKCC1 were localized on the basolateral surface and tubular network of ionocytes in both acclimation groups. Immunohistochemical labeling for the cystic fibrosis transmembrane conductance regulator (CFTR) was restricted to the apical crypt of ionocytes in seawater-acclimated individuals, whereas sodium/hydrogen exchanger 3 (NHE3) labeling was present on the apical surface of ionocytes in both acclimation groups. Ionocytes were concentrated on the trailing edge of the gill filament, evenly distributed along the proximal 75% of the filamental axis and reduced distally. Ionocyte size and number on the gill filament were not affected by salinity; however, the number of lamellar ionocytes was significantly lower in seawater-acclimated fish. Confocal z-series reconstructions revealed that mature ionocytes in seawater-acclimated alewives occurred in multicellular complexes. These complexes might reduce paracellular Na(+) resistance, hence facilitating Na(+) extrusion in hypo-osmoregulating juvenile alewives after seaward migration.

Glycogen phosphorylase in glycogen-rich cells is involved in the energy supply for ion regulation in fish gill epithelia

The molecular and cellular mechanisms behind glycogen metabolism and the energy metabolite translocation between mammal neurons and astrocytes have been well studied. A similar mechanism is proposed for rapid mobilization of local energy stores to support energy-dependent transepithelial ion transport in gills of the Mozambique tilapia ( Oreochromis mossambicus). A novel gill glycogen phosphorylase isoform (tGPGG), which catalyzes the initial degradation of glycogen, was identified in branchial epithelial cells of O. mossambicus. Double in situ hybridization and immunocytochemistry demonstrated that tGPGG mRNA and glycogen were colocalized in glycogen-rich cells (GRCs), which surround ionocytes (labeled with a Na+-K+-ATPase antiserum) in gill epithelia. Concanavalin-A (a marker for the apical membrane) labeling indicated that GRCs and mitochondria-rich cells share the same apical opening. Quantitative real-time PCR analyses showed that tGPGG mRNA expression levels specifically responded to environmental salinity changes. Indeed, the glycogen content, glycogen phosphorylase (GP) protein level and total activity, and the density of tGPGG-expressing cells (i.e., GRCs) in fish acclimated to seawater (SW) were significantly higher than those in freshwater controls. Short-term acclimation to SW caused an evident depletion in the glycogen content of GRCs. Taken altogether, tGPGG expression in GRCs is stimulated by hyperosmotic challenge, and this may catalyze initial glycogen degradation to provide the adjacent ionocytes with energy to carry out iono- and osmoregulatory functions.

Chloride Uptake and Base Secretion in Freshwater Fish: A Transepithelial Ion‐Transport Metabolon?

Despite all the efforts and technological advances during the last few decades, the cellular mechanisms for branchial chloride uptake in freshwater (FW) fish are still unclear. Although a tight 1 : 1 link with HCO-3 secretion has been established, not much is known about the identity of the ion-transporting proteins involved or the energizing steps that allow for the inward transport of Cl- against the concentration gradient. We propose a new model for Cl- uptake in FW fish whereby the combined action of an apical anion exchanger, cytoplasmic carbonic anhydrase, and basolateral V-type H+ -ATPase creates a local [HCO-3] high enough to energize Cl- uptake. Our model is based on analyses of structure-function relationships, reinterpretation of previous results, and novel observations about gill cell subtypes and immunolocalization of the V-H+ -ATPase.

V-H+-ATPase, Na+/K+-ATPase and NHE2 immunoreactivity in the gill epithelium of the Pacific hagfish (Epatretus stoutii)

We report the presence of the ion transporting proteins V-H(+)-ATPase, Na(+)/K(+)-ATPase and NHE2 in the gill epithelium of the Pacific hagfish Epatretus stoutii. Heterologous antibodies recognized single bands of the appropriate sizes for the three transporters in western blots. Immunohistochemical staining revealed that the distribution of labeled cells in the gill epithelium was identical for the three proteins. Immunopositive cells were most abundant in the primary filament from the afferent side of the gill pouch, and their number diminished towards the lamella. Na(+)/K(+)-ATPase-like immunoreactivity (L-IR) occurred throughout the cell cytoplasm, probably associated to the basolateral tubular system. V-H(+)-ATPase L-IR was similar to Na(+)/K(+)-ATPase, although some cells had slightly heavier staining in either the supra- or infra-nuclear region. NHE2 L-IR was also generally cytoplasmic, but a minority of the cells had stronger immunoreactivity in the apical region. In general, all three ion transporting proteins were localized in the same cells, as estimated from 4-microm immunostained consecutive sections. We hypothesize that these putative ion-transporting cells are involved in systemic acid/base regulation and discuss other possible roles. This is the first report of V-H(+)-ATPase in myxinoids, and the first NHE2 report in the Pacific hagfish.

Aquaporin-3 expressed in the basolateral membrane of gill chloride cells in Mozambique tilapia Oreochromis mossambicus adapted to freshwater and seawater

We have cloned a homologue of mammalian aquaporin-3 (AQP3) from gills of Mozambique tilapia using a reverse transcription-polymerase chain reaction (RT-PCR). The deduced amino acid sequence shared 64-75% homology with other vertebrate AQP3 homologues. RT-PCR revealed that tilapia AQP3 was expressed in the brain, pituitary, kidney, spleen, intestine, skin, eye and gill in tilapia adapted to freshwater (FW) and seawater (SW). We also examined functional characteristics of tilapia AQP3 using Xenopus oocytes as an in vitro transcribed cRNA expression system. Osmotic water permeability (Pf) of Xenopus oocytes expressing tilapia AQP3 was about 30-fold higher than that of control oocytes, and was 80% inhibited by treatment with 0.3 mmol l(-1) HgCl2. Light-microscopic immunocytochemistry of branchial epithelia revealed that tilapia AQP3 was expressed in gill chloride cells of FW- and SW-adapted tilapia. Electron-microscopic immunocytochemistry further demonstrated that tilapia AQP3 was localized in the basolateral membrane of gill chloride cells. Basolateral localization of AQP3 in gill chloride cells suggests that AQP3 is involved in regulatory volume changes and osmoreception, which could trigger functional differentiation of chloride cells.

Adaptive branchial mechanisms in the sturgeon Acipenser naccarii during acclimation to saltwater

Variations of Na(+)/K(+)-ATPase activity and fatty-acid composition in the gills of the sturgeon Acipenser naccarii subjected to progressive acclimation to full seawater (35 ppt) were determined in relation to the hypo-osmoregulatory capacity of this species in the hyperosmotic medium. Blood samples were taken and gills arches were removed at intermediate salinity levels between 0 and 35 ppt and after 20 days at constant salinity (35 ppt). Plasma osmolality and Na(+)/K(+)-ATPase activity increased significantly with growing environmental salinity. Total saturated fatty acids (SFAs) decreased, while total polyunsaturated fatty acids (PUFAs) increased significantly with increasing salinity due mainly to changes in n-3 PUFAs (20:5n-3 and 22:6n-3). The n-3/n-6 ratio increased significantly during the acclimation process. The results show a direct relationship between salinity, increased gill Na(+)/K(+)-ATPase activity and ultrastructural changes of the gill chloride cells. Changes in the fatty-acid composition in gills of A. naccarii during progressive acclimation to full seawater suggest that variations of gill fatty acids may also have a role in osmoregulatory mechanisms.

Fine structural and cytochemical mapping of enamel organ during the enameloid formation stages in gars, Lepisosteus oculatus, Actinopterygii

During cap enameloid formation in gars (Lepisosteus oculatus), the dental epithelial cells that constitute the enamel organ were observed by means of transmission electron microscopy and enzyme cytochemistry to detect the hydrolytic enzyme activities, alkaline phosphatase (ALPase), acid phosphatase (ACPase), calcium-dependent adenosine triphosphatase (Ca-ATPase) and potassium-dependent p-nitrophenylphosphatase (K-NPPase) (sodium, potassium-activated adenoshine triphosphatase (Na-K-ATPase)). The enameloid formation process in gars was divided into three stages: matrix formation, mineralisation and maturation. The enamel organ consisted of the outer dental epithelial (ODE) cells, stellate reticulum (SR), stratum intermedium (SI) and the inner dental epithelial (IDE) cells during the whole of the cap enameloid formation stages. During the matrix formation stage, many cisternae of rough endoplasmic reticulum and widely distributed Golgi apparatus, in which the procollagen granules containing cross-striations were often found, were remarkable elements in the IDE cells. During the stage of mineralisation, the IDE cells were tall columnar, and infoldings of distal plasma membrane of the IDE cells became marked. The most developed Golgi apparatus was visible at this stage, and large secretory granules containing fine granular or tubular materials were found in the distal cytoplasm that was close to the infoldings of the distal end. Many lysosomes that were ACPase positive were seen near the Golgi apparatus and in the distal cytoplasm of the IDE cells. ACPase positive granules often contained the cross-striation structure resembling procollagen, suggesting that the procollagen is degenerated in the IDE cells. During the maturation stage, the distal infoldings became unclear, and there were no large granules containing tubular materials, but many ACPase positive lysosomes were still present in the IDE cells. Non-specific ALPase was detected at the plasma membrane of the IDE cells at the mineralisation and maturation stages. K-NPPase was markedly detected at the plasma membrane of the IDE cells at the maturation stage. These results demonstrate that the IDE cells might be mainly involved in the removal of degenerated organic matrix from enameloid during the later formation stages. Strong Ca-ATPase activity was observed at the entire plasma membrane of the stratum intermedium cells, and there was slightly weak activity at the plasma membrane of the IDE cells during the mineralisation and maturation stages, implying that these cells are related to the active Ca transport to the maturing enameloid. It is likely that although the structure of the enamel organ is different, the function, especially at the mineralisation and maturation stages, is similar to other actinopterygians having well-mineralized cap enameloid.

Fine structural and cytochemical observations on the dental epithelial cells during cap enameloid formation stages in Polypterus senegalus, a bony fish (Actinopterygii)

Tooth germs during cap enameloid formation stages in Polypterus senegalus were investigated by transmission electron microscopy and enzyme histo- and cytochemistry. Enameloid formation was divided into three stages: matrix formation, mineralization, and maturation. The enamel organ consisted of the inner dental epithelial cells, stellate reticulum, and outer dental epithelial cells during cap enameloid formation stages, but no stratum intermedium was found. During the matrix formation stage, the tall inner dental epithelial cells contained well-developed Golgi apparatus, abundant cisternae of rough endoplasmic reticulum and mitochondria. Spindle-shaped vesicles containing a filamentous structure were seen in the distal cytoplasm. During mineralization and maturation stages, many ACPase positive lysosomes were present in the cytoplasm, whereas the organelles were decreased in number. The infoldings of the distal plasma membrane of the inner dental epithelial cells were visible in the mineralization stage but were not marked in the maturation stage. The activity of nonspecific ALPase, Ca-ATPase, and K-NPPase was detected at the plasma membrane of the inner dental epithelial cells during the stages of mineralization and maturation. The results of fine structure and enzyme cytochemistry suggested that the dental epithelial cells were mainly involved in the degeneration and removal of enameloid matrix and in material transportation during the enameloid mineralization and maturation stages, rather than in the enameloid matrix formation. The results also showed that the structure of the dental epithelial cells in Polypterus was different from that in teleosts and gars, but that the function of the dental epithelial cells was similar to that in teleosts possessing well-mineralized cap enameloid.

Ontogeny of the antennal glands in the crayfish Astacus leptodactylus (Crustacea, Decapoda): immunolocalization of Na+,K+-ATPase

The involvement of the antennal urinary glands in the ontogeny of osmoregulatory functions was investigated during the development of Astacus leptodactylus by measurements of hemolymph and urine osmolality in juvenile and adult crayfish and by the immunodetection of the enzyme Na+,K+-ATPase. In stage II juveniles, 1-year-old juveniles, and adults, all of which were maintained in freshwater, urine was significantly hypotonic to hemolymph. In adults, chloride and sodium concentrations were much lower in urine than in hemolymph. During embryonic development, Na+,K+-ATPase was detected by immunocytochemistry in ionocytes lining the tubule and the bladder, at an eye index (EI) of 220-250 microm, and in the labyrinth, at EI 350 microm. In all regions, immunofluorescence was mainly located at the basolateral side of the cells. No immunofluorescence was detected at any stage in the coelomosac. In late embryonic stages (EI 410-440 microm), in stage I juveniles, and in adults, strong positive immunofluorescence was found from the labyrinth up to and including the bladder. These results show that, as early as hatching, juvenile crayfish are able to produce dilute urine hypotonic to hemolymph. This ability originates from the presence of Na+,K+-ATPase in ion-transporting cells located in the labyrinth, the tubule, and the bladder of the antennal glands and constitutes one of the main adaptations of crayfish to freshwater.

17β-Estradiol affects osmoregulation in Fundulus heteroclitus

The effect of 17beta-estradiol (E(2)) on osmoregulatory performance was examined in the euryhaline killifish, Fundulus heteroclitus. Fish were injected once with 1, 2 and 5 microg g(-1) E(2) and, 6 h after injection, transferred from 1 ppt seawater (SW) to full strength SW (40 ppt) or from SW to 1 ppt SW. In another set of experiments, fish were injected four times on alternate days with 2 microg g(-1) E(2) and then, 6 h after the last injection, transferred from 1 ppt SW to SW or from SW to 1 ppt SW. Fish were sampled 18 h after transfer (i.e., 24 h post-injection), and plasma osmolality, Na(+) and Cl(-) concentration and gill K(+)-pNPPase activity (a reflection of the sodium pump) were examined. Transfer from 1 ppt SW to SW resulted in significantly increased plasma osmolality, but did not affect gill K(+)-pNPPase activity. A single dose of E(2) (1, 2 and 5 microg g(-1)) prior to transfer from 1 ppt SW to SW increased plasma osmolality and decreased gill K(+)-pNPPase activity in a dose-dependent manner. Prolonged treatment with E(2) increased plasma osmolality and decreased gill K(+)-pNPPase activity in 1 ppt SW-adapted fish. Transfer of fish thus treated from 1 ppt SW to SW increased plasma osmolality and did not alter gill K(+)-pNPPase activity. Transfer from SW to 1 ppt SW had no significant effect on plasma osmolality or gill K(+)-pNPPase activity. Only the highest single dose of E(2) (5 microg g(-1)) prior to transfer from SW to 1 ppt SW decreased gill K(+)-pNPPase activity. Prolonged treatment with 2 microg g(-1) E(2) decreased gill K(+)-pNPPase activity only following transfer from SW to 1 ppt SW. The results substantiate an inhibitory action of E(2) on hypoosmoregulatory capacity in this euryhaline teleost.

Na+ versus Cl- transport in the intact killifish after rapid salinity transfer

Much of the early research elucidating the general mechanisms of euryhalinity was performed on the common killifish. More recently, its opercular epithelium with abundant mitochondria-rich cells has proven to be a powerful model for analyzing the mechanisms of active NaCl transport under Ussing conditions in vitro (i.e., with isotonic saline on both surfaces, at short-circuit). However, it is unclear whether this preparation duplicates the gill under real world conditions-i.e., at open-circuit, with real seawater (SW) or freshwater (FW) on the mucosal surface. There have been only limited studies, mostly about 35 years ago, on ion transport in the intact killifish. Therefore, using radioisotopes (22Na, 36Cl), we developed and evaluated methods for the independent measurement of unidirectional Na(+) and Cl(-) influx and efflux rates and internal pools in intact killifish acclimated to 10% SW and abruptly transferred to either 100% SW or FW. Internal Na(+) pools were disturbed less than internal Cl(-) pools by transfer, and were corrected after 3 days in 100% SW or 7 days in FW. Influx and efflux rates in 10% SW were about 3000 micromol kg(-1) h(-1) and increased to 15,000-18,000 micromol kg(-1) h(-1) after transfer to 100% SW, remaining approximately equal and equimolar for Na(+) and Cl(-), and stable from 0.5 to 7 days post-transfer. After transfer to FW, Na(+) influx and efflux rates dropped to 1000-1500 micromol kg(-1) h(-1), with efflux slightly exceeding influx, and remained approximately stable from 0.5 to 7 days. However, while Cl(-) efflux responded similarly, Cl(-) influx rate dropped immediately to negligible values (20-50 micromol kg(-1) h(-1)) without recovery through 7 days. These results differ from early ion transport data in 100% SW, and demonstrate that fluxes stabilize quickly after salinity transfer. They also show that the intact animal responds more quickly than the epithelium, provide qualitative but not quantitative support for the opercular epithelium as a model for the gill under real world SW conditions, and no support for its use as a gill model under real world FW conditions, where branchial Cl(-) uptake is negligible.

Na(+), Cl(-), Ca(2+) and Zn(2+) transport by fish gills: retrospective review and prospective synthesis

The secondary active Cl(-) secretion in seawater (SW) teleost fish gills and elasmobranch rectal gland involves basolateral Na(+),K(+)-ATPase and NKCC, apical membrane CFTR anion channels, and a paracellular Na(+)-selective conductance. In freshwater (FW) teleost gill, the mechanism of NaCl uptake is more controversial and involves apical V-type H(+)-ATPase linked to an apical Na(+) channel, apical Cl(-)-HCO-3 exchange and basolateral Na(+),K(+)-ATPase. Ca(2+) uptake (in FW and SW) is via Ca(2+) channels in the apical membrane and Ca(2+)-ATPase in the basolateral membrane. Mainly this transport occurs in mitochondria rich (MR) chloride cells, but there is a role for the pavement cells also. Future research will likely expand in two major directions, molded by methodology: first in physiological genomics of all the transporters, including their expression, trafficking, operation, and regulation at the molecular level, and second in biotelemetry to examine multivariable components in behavioral physiological ecology, thus widening the integration of physiology from the molecular to the environmental levels while deepening understanding at all levels.

Effects of a low-Ca2+environment on branchial chloride cell morphology in chum salmon fry and immunolocalization of Ca2+-ATPase in chloride cells

To clarify the involvement of branchial chloride cells in Ca2+uptake in fresh water (FW), chloride-cell morphology was compared in chum salmon (Oncorhynchus keta) fry acclimated to defined FWs with different Ca2+concentrations (0, 0.1, and 0.5 mM). Using immunocytochemical staining with an antiserum specific for Na+,K+-ATPase, chloride cells were detected in both filament and lamellar epithelia. The numbers and sizes of chloride cells in the lamellar epithelia were greater in the low-Ca2+groups (0 and 0.1 mM Ca2+) than in the normal-Ca2+groups (0.5 mM Ca2+and normal FW), whereas filament chloride cells were not affected in number or size by the environmental Ca2+concentration. Electron-microscope observations also revealed that enlarged lamellar chloride cells were more frequently observed in the 0 mM Ca2+group than in the 0.5 mM Ca2+group. To obtain morphological evidence for Ca2+uptake through the branchial epithelia, cellular localization of Ca2+-ATPase was examined with a monoclonal antibody specific for human erythrocyte Ca2+-ATPase. Ca2+-ATPase immunoreactivity was detected in Na+,K+-ATPase-immunoreactive chloride cells in both filament and lamellar epithelia. Using electron-microscope immunocytochemistry, Ca2+-ATPase was found to be localized in the tubular system, which is continuous with the basolateral membrane of chloride cells. These findings indicate that chloride cells in the lamellar epithelia activated by a low Ca2+concentration may constitute the extra Ca2+and NaCl uptake capacity required to maintain homeostasis in soft water.

Ionoregulatory changes in the gill epithelia of coho salmon during seawater acclimation

Short-term exposure of coho salmon smolts (Oncorhynchus kisutch) to a gradual increase in salinity over 2 d (0 per thousand -32 per thousand ) resulted in a decrease in proton pump abundance, detected as changes in immunoreactivity with a polyclonal antibody against subunit A of bovine brain vacuolar H(+)-ATPase. N-ethylmaleimide (NEM)-sensitive H(+)-ATPase activities in gill homogenates remained unchanged over 8 d to coincide with a 3.5-fold increase in Na(+)/K(+)-ATPase activities. A transient increase in plasma [Na(+)] and [Cl(-)] levels over the 8-d period was preceded by a 10-fold increase in plasma cortisol levels, which peaked after 12 h. Long-term (1 mo) acclimation to seawater resulted in the loss of apical immunoreactivity for vH(+)-ATPase and band 3-like anion exchanger in the mitochondria-rich cells identified by high levels of Na(+)/K(+)-ATPase immunoreactivity. The polyclonal antibody Ab597 recognized a Na(+)/H(+) exchanger (NHE-2)-like protein in what appears to be an accessory cell (AC) type. Populations of these ACs were found associated with Na(+)/K(+)-ATPase rich chloride cells in both freshwater- and seawater-acclimated animals.

Ultrastructure and blood supply of the tegmentum vasculosum in the cochlea of the duckling

The tegmentum vasculosum of the duckling consists of a highly folded epithelium which extends over the dorsal and lateral walls of the cochlear duct, separating the scala media from the scala vestibuli. This epithelium consists of two distinct cell types, dark cells and light cells, and is well vascularized. The surface of the epithelium is formed by a mosaic of alternating dark and light cells. The goblet-shaped dark cells have an electron-dense, organelle-rich cytoplasm, and are expanded basally by extensive basolateral plasma membrane infoldings, within which are numerous mitochondria. Dark cells are isolated from each other and from the capillaries within the epithelium by intervening light cells. In contrast, columnar light cells exhibit an electron-lucent, organelle-poor cytoplasm and may extend from the underlying capillaries to the endolymphatic surface. Light cells contain abundant, coated endocytic vesicles on their apical surfaces and are bound, apically, to other light cells or to dark cells by tight junctions and desmosomes. Laterally, light cells are linked to each other either by complex, fluid-filled membrane interdigitations or by extensive gap junctions. Plasma membrane interdigitations and obvious, fluid-filled intercellular spaces characterize the lateral borders between light and dark cells. Vascular corrosion casting reveals the three-dimensional anatomy of the cochlear vasculature. A continuous arteriolar loop fed by anterior and posterior cochlear arterioles encircles the cochlear duct. The rich capillary beds of the tegmentum vasculosum are supplied by arching arterioles arising from this loop. These capillaries are the continuous type and are situated primarily within the core of the epithelium or along its border with the scala vestibuli. The structure and blood supply of the tegmentum vasculosum are characteristic of an epithelium involved in active transport.

Na+,K+-ATPase activity and ultrastructural localization in the tegmentum vasculosum in the cochlea of the duckling

The tegmentum vasculosum of the avian cochlear duct mimics the stria vascularis of the mammalian cochlear duct in both location and structure, and previous studies indicate that it may be its functional counterpart with regard to endolymph synthesis. In the present study, we report on the enzymatic activity and ultrastructural localization of the Na+,K+-ATPase in the tegmentum vasculosum of the duckling. Na+,K+-ATPase activity was determined by measuring K+-dependent, ouabain-sensitive p-nitrophenyl phosphatase (p-NPPase) activity in homogenates of dissected regions of the cochlear duct. The ultrastructural localization of the Na+,K+-ATPase was identified using K+-dependent, ouabain-sensitive, p-NPPase cytochemistry. Specific enzyme activity was localized primarily in homogenates of the tegmentum vasculosum (2.27 micromol p-nitrophenyl phosphate/mg protein/min) when compared to homogenates of the entire cochlear duct (0.69 micromol p-nitrophenyl phosphate/mg protein/min). Reaction product for p-NPPase was localized primarily along the basolateral plasma membrane folds of the dark cells. The cytochemical deposits appeared to be located exclusively on the cytoplasmic side of the plasma membrane. The light cells were devoid of reaction product. Biochemical and cytochemical localization of p-NPPase activity on the basolateral plasma membrane folds of the dark cells of the tegmentum vasculosum in conjunction with the ultrastructural morphology of these cells is compatible with a Na+,K+-ATPase-dependent ion transport function related to endolymph synthesis.

Gill Na+-K+-2Cl−cotransporter abundance and location in Atlantic salmon: effects of seawater and smolting

Na+-K+-2Cl−cotransporter abundance and location was examined in the gills of Atlantic salmon ( Salmo salar) during seawater acclimation and smolting. Western blots revealed three bands centered at 285, 160, and 120 kDa. The Na+-K+-2Cl−cotransporter was colocalized with Na+-K+-ATPase to chloride cells on both the primary filament and secondary lamellae. Parr acclimated to 30 parts per thousand seawater had increased gill Na+-K+-2Cl− cotransporter abundance, large and numerous Na+-K+-2Cl− cotransporter immunoreactive chloride cells on the primary filament, and reduced numbers on the secondary lamellae. Gill Na+-K+-2Cl− cotransporter levels were low in presmolts (February) and increased 3.3-fold in smolts (May), coincident with elevated seawater tolerance. Cotransporter levels decreased below presmolt values in postsmolts in freshwater (June). The size and number of immunoreactive chloride cells on the primary filament increased threefold during smolting and decreased in postsmolts. Gill Na+-K+-ATPase activity and Na+-K+-2Cl− cotransporter abundance increased in parallel during both seawater acclimation and smolting. These data indicate a direct role of the Na+-K+-2Cl− cotransporter in salt secretion by gill chloride cells of teleost fish.

Dynamics of Pavement Cell–Chloride Cell Interactions During Abrupt Salinity Change in FUNDULUS HETEROCLITUS

Freshwater-adapted killifish (Fundulus heteroclitus) opercular epithelia were dissected and subjected to blood-side hypertonic bathing solution in Ussing-style chambers to simulate the increase in blood osmolality during migration to sea water. Conversely, seawater-acclimated killifish opercular epithelia were subjected to hypotonic bathing solutions to simulate the initial stages of migration to fresh water. Freshwater-acclimation (hypertonic stress) induced a rapid (approximately 30min) increase in membrane conductance (Gt) from 3.10±0.56 to 7.52±1.15mScm−2 (P<0.01, N=27), whereas seawater-acclimation (hypotonic stress) induced a rapid decrease in Gt from 8.22±1.15 to 4.41±1.00mScm−2 (P<0.01, N=27; means ± s.e.m.). Control seawater-acclimated membranes had a density of apical crypts (where chloride cells are exposed to the environment; detected by scanning electron microscopy) of 1133±96.4cryptsmm−2 (N=12), whereas the hypotonically shocked specimens had a lower crypt density of 870±36.7cryptsmm−2 (P<0.01 N=10; means ± s.e.m.). Hypertonic shock of freshwater membranes increased crypt density from 383.3±73.9 (N=12) to 630±102.9cryptsmm−2 (P<0.05; N=11; means ± s.e.m.). There was no change in density of chloride cells, as detected by fluorescence microscopy; hence, osmotic stress changes the degree of exposure, not the number of chloride cells. Cytochalasin D (5.0μmoll−1) completely blocked the conductance response to hypotonic shock and the reduction in apical crypt density measured by scanning electron microscopy, while phalloidin (33μmoll−1), colchicine (3×10−4moll−1) and griseofulvin (1.0μmoll−1) were ineffective. Actin imaging by phalloidin staining and confocal microscopy revealed extensive actin cords in pavement cell microridges and a ring of actin at the apex of chloride cells. We conclude that the actin cytoskeleton of chloride cells is required to maintain crypt opening and that osmotic shock causes chloride cells to adjust their apical crypt size.

Distinct seawater and freshwater types of chloride cells in killifish, Fundulus heteroclitus

Physiological and morphological differences between killifish adapted to seawater (SW) and fresh water (FW) were examined with special reference to chloride cells. There was no difference in plasma osmolality between SW- and FW-adapted fish, reflecting their euryhalinity. A rich population of chloride cells was detected in whole-mount preparations of the gills and opercular membrane from SW- and FW-adapted fish. There was no difference between SW- and FW-adapted fish in gill Na+,K+-ATPase activity or oxygen-consumption rates. The gill chloride cells were located mostly in a flat region of the afferent-vascular edge of the filaments. In both tissues, the cells were larger in FW- than in SW-adapted fish. The apical membrane of chloride cells was invaginated to form a pit in SW-adapted fish, whereas it was flat or showed projections and was equipped with microvilli in FW-adapted fish. Chloride cells often interdigitated with neighboring accessory cells in SW-adapted fish, forming multicellular complexes. In FW-adapted fish, on the other hand, a pair of chloride cells that were similar in size was occasionally associated to form "twin cells." Thus, distinct SW and FW types of chloride cells were defined. Our findings suggest that SW- and FW-type chloride cells are equally active in the two environments, but exhibit different ion-transporting functions.

NaCl uptake by the branchial epithelium in freshwater teleost fish: an immunological approach to ion-transport protein localization

Teleost fishes, living in fresh water, engage in active ion uptake to maintain ion homeostasis. Current models for NaCl uptake involve Na(+) uptake via an apical amiloride-sensitive epithelial Na(+) channel (ENaC), energized by an apical vacuolar-type proton pump (V-ATPase) or alternatively by an amiloride-sensitive Na(+)/H(+) exchange (NHE) protein, and apical Cl(−) uptake mediated by an electroneutral, SITS-sensitive Cl(−)/HCO(3-) anion-exchange protein. Using non-homologous antibodies, we have determined the cellular distributions of these ion-transport proteins to test the predicted models. Na(+)/K(+)-ATPase was used as a cellular marker for differentiating branchial epithelium mitochondria-rich (MR) cells from pavement cells. In both the freshwater tilapia (Oreochromis mossambicus) and rainbow trout (Oncorhynchus mykiss), V-ATPase and ENaC-like immunoreactivity co-localized to pavement cells, although apical labelling was also found in MR cells in the trout. In the freshwater tilapia, apical anion-exchanger-like immunoreactivity is found in the MR cells. Thus, a freshwater-type MR chloride cell exists in teleost fishes. The NHE-like immunoreactivity is associated with the accessory cell type and with a small population of pavement cells in tilapia.

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.

Metamorphosis in the summer flounder Paralichthys dentatus: changes in gill mitochondria-rich cells

Salinity tolerance changes during larval development and metamorphosis in the summer flounder (Paralichthys dentatus) and other teleosts. The physiological mechanisms responsible for osmoregulation during these early stages of development are not well understood. This study characterized changes in ultrastructure, intracellular membranes and immunoreactive Na(+)/K(+)-ATPase of mitochondria-rich cells (MRCs) in the gills of summer flounder during metamorphosis. Gill ultrastructure at the start of metamorphosis revealed only one type of MRC, which had weak reactivity to osmium and lacked a well-defined apical pit. In juveniles, two types of MRCs were observed: light-staining MRCs (LMRCs) with weak reactivity to osmium, and dark-staining MRCs (DMRCs) with strong reactivity to osmium and positioned adjacent to LMRCs. Compared with MRCs at the start of metamorphosis, the mitochondria of juvenile MRCs appeared smaller, with more transverse cristae and electron-dense matrices. Changes in MRCs during metamorphosis were also accompanied by increased immunoreactive Na(+)/K(+)-ATPase. These findings suggest that gill MRCs develop during the metamorphosis of summer flounder as the gill takes on an increasingly important osmoregulatory role.

Chloride cell subtypes in the gill epithelium of Japanese eel Anguilla japonica

The purpose of the present study was to characterize chloride cell subtypes in the fish gill and to monitor the kinetic change of cell division in the gill epithelia during seawater adaptation. Employing a three-step Percoll gradient method, the gill chloride cells and nonchloride cell population were isolated. The isolated cells were studied using multiparameter flow cytometry, recording the changes in 1) cell size, 2) cellular granularity, and 3) cell autofluorescence. Two chloride cell subtypes were identified in the freshwater eels. Within 2-4 days after entering seawater, new subtypes of transitory chloride cell, with bigger cell size and more intense mitochondria autofluorescence, appeared. After full adaptation, two major seawater chloride cell subtypes were again discerned; their sizes were the largest and their mitochondria autofluorescence was the highest. In the second part of the experiment, cell cycle analysis demonstrated a progressive increase in the percentage of gill cells entering the DNA synthesis phase during seawater adaptation, where a small population of mitotic cells was identified in the nonchloride cell population but not in chloride cells. We hypothesize that the mitotic cells identified are stem cells, which will ultimately differentiate into seawater chloride cells. Our results confirm the existence of heterogeneity of chloride cells. Individual subtypes could be isolated in high purity for further studies to elucidate their respective function in mediating ion transport.

Time course of salinity adaptation in a strongly euryhaline estuarine teleost, fundulus heteroclitus: a multivariable approach

Freshwater-adapted killifish (Fundulus heteroclitus) were transferred directly from soft fresh water to full-strength sea water for periods of 1 h, 3 h, 8 h and 1, 2, 7, 14 and 30 days. Controls were transferred to fresh water for 24 h. Measured variables included: blood [Na+], osmolality, glucose and cortisol levels, basal and stimulated rates of ion transport and permeability of in vitro opercular epithelium, gill Na+/K+-ATPase and citrate synthase activity and chloride cell ultrastructure. These data were compared with previously published killifish cystic fibrosis transmembrane conductance regulator (kfCFTR) expression in the gills measured over a similar time course. Plasma cortisol levels peaked at 1 h, coincident with a rise in plasma [Na+]. At 8 h after transfer to sea water, a time at which previous work has shown kfCFTR expression to be elevated, blood osmolality and [Na+] were high, and cortisol levels and opercular membrane short-circuit current (Isc; a measure of Cl- secretion rate) were low. The 24 h group, which showed the highest level of kfCFTR expression, had the highest plasma [Na+] and osmolality, elevated plasma cortisol levels, significantly lower opercular membrane resistance, an increased opercular membrane ion secretion rate and collapsed tubule inclusions in mitochondria-rich cells, but no change in gill Na+/K+-ATPase and citrate synthase activity or plasma glucose levels. Apparently, killifish have a rapid (<1 h) cortisol response to salinity coupled to subsequent (8–48 h) expression of kfCFTR anion channel proteins in existing mitochondria-rich cells that convert transport from ion uptake to ion secretion.

Presence of Na-K-ATPase in mitochondria-rich cells in the yolk-sac epithelium of larvae of the teleost Oreochromis mossambicus

The purpose of this study is to provide biochemical evidence for the functions of the mitochondria-rich cell (MR cell) in the yolk-sac epithelium of the developing larvae of tilapia Oreochromis mossambicus. Western blotting with the antibody (6F) raised against avian Na-K-ATPase alpha1 subunit demonstrated the presence of Na-K-ATPase in yolk-sac epithelium of tilapia larvae and about 1. 46-fold more of the enzyme in seawater larvae than in freshwater ones. The yolk-sac MR cells were immunoreacted to the antibody (alpha5) against the alpha subunit of avian Na-K-ATPase and were double-labeled with anthroylouabain and dimethylaminostyrylethyl-pyridiniumiodine, suggesting the existence and activity of Na-K-ATPase in these cells. Binding of 3H-ouabain in the yolk sac of seawater larvae was much higher than in that of freshwater larvae (4.183+/-0.143 pmol/mg protein versus 1.610+/-0. 060 pmol/mg protein or 0.0508+/-0.0053 pmol/yolk sac versus 0. 0188+/-0.0073 pmol/yolk sac). These biochemical results are further evidence that yolk-sac MR cells are responsible for a major role in the osmoregulatory mechanism of early developmental stages before the function of gills is fully developed.

Isoform expression of Na+-K+-ATPase alpha-subunit in gills of the teleost Oreochromis mossambicus

Three isoform-specific antibodies, 6F against the alpha1-isoform of the avian sodium pump, HERED against the rat alpha2-isoform, and Ax2 against the rat alpha3-isoform, were used to detect the expression of Na+-K+-ATPase alpha-subunits in gills of a teleost, the tilapia (Oreochromis mossambicus). Tilapia gill tissue showed positive reactions to antibodies specific for alpha1- and alpha3-isoforms. The results of immunoblots were converted to numerical values (relative intensities) by image analysis for comparisons. Relative amounts of alpha1-like isoform alone and consequently the ratio of alpha1-like to alpha3-like isoforms were higher in gills of seawater-adapted tilapia than in those of freshwater-adapted ones, indicating that the two isoforms respond differently to environmental salinities. In the subsequent immunocytochemical experiments, gill mitochondria-rich cells were demonstrated to immunoreact with antibodies specific for alpha1- and alpha3-isoforms. alpha1-like and alpha3-like isoforms of gill Na+-K+-ATPase are suggested to be involved in the ion- and osmoregulation mechanisms in tilapia. Moreover, differential expressions of two isoforms may be associated with different functions, secretion and uptake of ions and acid-base regulation, in gills of seawater- and freshwater-adapted tilapia.

Cytochemical evidence for potassium-dependent p-nitrophenylphosphatase activity in pavement cells of Rana esculenta mesentery

BACKGROUND

We previously reported that during hibernation in Rana esculenta, various organs (i.e., skin, urinary bladder, kidney) change their osmoregulatory activity. Here, we considered the possible role of the frog mesentery in the ion transport, evaluating morphological and cytochemical (K+-p-nitrophenylphosphatase activity) aspects.

METHODS

Pieces of mesentery from Rana esculenta collected in their natural environment during April, June, October, and January were processed to reveal ultrastructural morphology and K+-p-NPPase activity, using cerium as capture agent.

RESULTS

The mesenteric mesothelium contained three types of cells: pavement, mitochondria-rich, and ciliated. Only the pavement cells expressed intense reactivity on the basolateral membranes and in the adjacent pinocytotic vesicles; some reaction product also was found on the apical membranes. Moreover, morphological and cytochemical characteristics of the pavement cells appeared to be very seasonal.

CONCLUSIONS

The presence of mitochondria-rich cells and ciliated cells, generally found in structures involved in the transport of liquids, as well as K+-p-NPPase activity and pinocytosis in pavement cells, is consistent with the hypothesis that frog mesentery may be involved in seasonally variable osmoregulation.

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.

Localization of Na+, K(+)-ATPase in tissues of rabbit and teleosts using an antiserum directed against a partial sequence of the alpha-subunit

A specific polyclonal antibody against Na+, K(+)-ATPase alpha-subunit was developed using a synthetic oligopeptide as antigen. By Western blot analysis under non-reducing conditions, this antibody recognized a protein band of approximately 150 kDa corresponding to the intact form (alpha beta-complex) of Na+, K(+)-ATPase in rabbit kidney. Furthermore, this antibody recognized a 150 kDa protein band corresponding to the intact form of Na+, K(+)-ATPase and some bands of about 60-65 kDa corresponding to fragments of the alpha-subunit in gill and kidney of masu salmon. This antibody did not recognize the alpha-subunit under reducing conditions. By immunohistochemical analysis, cells immunoreactive with this antibody were observed in renal tubular epithelial cells in kidney sections of rabbit, masu salmon, eel and rockfish. In addition, large spherical eosinophilic cells in gills of masu salmon, eel and rockfish were immunoreactive with the antibody. It is likely that these immunoreactive cells correspond to gill chloride cells. These data indicate that this antibody is a useful tool for studying changes in and the function of Na+, K(+)-ATPase during osmoregulatory adaptation in a variety of fish species.

Gill epithelial cells kinetics in a freshwater teleost, Oncorhynchus mykiss during adaptation to ion-poor water and hormonal treatments

The aim of this work was to determine the kinetics of the dramatic development of the gill chloride cells (CCs) during adaptation of the salmonid Oncorhynchus mykiss to an ion-poor environment.To monitor cell division, the incorporation in the mitotic cell DNA of bromo-deoxyuridine (BrdUrd) was visualized with a monoclonal antibody. The density of labelled nuclei was used as an index of cellular division (proliferation), concomitantly with morphometry of phenotypic changes monitored with SEM.In the filament epithelium, a phase of CC differentiation occurred within 12h after the transfer, followed by a delayed phase of cell proliferation (48h). In the lamellar epithelium, the present study demonstrates the absence of cell proliferation after ion-poor water transfer. The conclusion is that proliferation (mitosis) is important in the primary filament whereas differentiation and migration (from the filament) is the main mechanism for the appearance of CCs on the secondary lamellae.The present study suggests that cortisol promoted differentiation, but not division, of cells. CCs, presumably premature, were stained by anti-cortisol monoclonal antibody indicating the presence of cortisol. No mature CCs were stained.Growth hormone (oGH, ratGH) increased the rate of cell division both in lamellar and filament epithelium.

Ultrastructure and permeability of the Schwann cell layer surrounding the giant axon of the squid

The ultrastructure of the Schwann cell layer surrounding the giant axon of the squid Alloteuthis subulata is described, and the permeability of extracellular compartments assessed by exposure to electron-dense tracers. Morphometric analysis is used to deduce the number, size and shape of the Schwann cells, and the routes for ion flux across the Schwann cell layer. Axons (mean diameter 233 microns) were surrounded by a 1-2 microns thick layer of Schwann cells which were approximately 1 micron thick, approximately 70 microns long and approximately 23 microns wide. There were around 62,000 Schwann cells per cm2 axon surface. The outer (abaxonal) surface of the Schwann cells was invaginated, with evidence for a covering of fine Schwann cells processes; the inner (adaxonal) surface of the Schwann cells was less folded. The percentage area occupied by mesaxonal cleft openings to the axon and to the basal lamina was 0.02% and 1.09% respectively. A system of tubules, the glial tubular system, occupied 3.9% of the Schwann cell volume, and opened to both axonal and basal lamina surfaces, with more elaborate lattice-like clusters towards the basal side of the cell. Tubule openings accounted for 0.26% of the surface area facing the axon and 0.37% of the area facing the basal lamina (where there was greater clustering of openings). The electron dense tracers horseradish peroxidase, ionic lanthanum and tannic acid filled mesaxon clefts, glial tubular system and periaxonal space. If ion flux occurred via the mesaxonal clefts, a theoretical series resistance (Rsth) of > 20 omega cm2, would be predicted, whereas if it occurred via the tubular system, the figure would be < 2 omega cm2, closer to physiological estimates. The results presented show that the glial tubular system is likely to be the major route for ion flux into and across the Schwann cell layer, and for clearance of K+ from the periaxonal space during periods of axonal stimulation. The implications for K+ homeostasis in the axonal microenvironment are discussed.