Cell volume regulation following hypotonic stress in the intestine of the eel, Anguilla anguilla, is Ca2+-dependent

The involvement of Ca2+ in the regulatory volume decrease (RVD) mechanism was studied in both isolated enterocytes and intestine of the eel, Anguilla anguilla. Videometric methods and electrophysiological techniques were respectively employed. The isolated enterocytes rapidly swelled following a change from isotonic (315 mOsm/kg) to hypotonic (180 mOsm/kg) saline solutions. Afterwards, they tended to recover their original size. This homeostatic response was inhibited both in the absence of extracellular Ca2+ and in the presence of TMB8, an inhibitor of Ca2+ release from intracellular stores. It is likely that Ca2+ entry through verapamil-sensitive Ca2+ channels is responsible for RVD since the blocker impaired the ability of the cell to recover its volume after the hypotonic shock. The observation that a 10-fold increase of K+ concentration as well as the presence of quinine in the hypotonic solution completely abolished RVD indicated the involvement of K+ in this response. Experiments performed with the isolated intestine suggested that the opening of basolateral K+ channels facilitates K+ loss (and hence water efflux) from the cell during RVD and that this opening is probably due to Ca2+ entry into the cell through both the mucosal and the serosal membranes.     

Nitric oxide modulates ionic transport in the isolated intestine of the eel, Anguilla anguilla

We investigated the role of NO (nitric oxide) in the isolated intestine of the sea water adapted eel, by testing the effect of various donors on I(sc) (short-circuit current), due to net Cl(-) absorption in the control conditions. We found that the endogenous NO-synthase substrate l-arginine as well as two different NO donors, SNP (sodium nitroprusside) and SIN-1 (3-morpholinosydnonimine), produced a slow and gradual decrease of I(sc). The effect of SNP was reduced by the pretreatment with ODQ (1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one), a specific inhibitor of the soluble guanylyl cyclase, suggesting the involvement of cGMP (cyclic GMP) in some physiological actions of NO. The effect of the NO donors on I(sc) was similar to that observed when the tissues were perfused with solution in which the HCO(3)(-) buffer was substituted with Hepes buffer. In addition the NO donors produced a negligible effect on I(sc) when the tissues were perfused with Hepes buffer or in the presence of bilateral SITS(4-Acetoamido-4'-iso-thiocyanatostilbene-2,2'disulphonic acid), an inhibitor of the HCO(3)(-) transport mechanisms, operating on both cell membranes of the eel enterocyte and responsible for HCO(3)(-) uptake by the cell. Based on these observations we suggest that NO regulates I(sc) and hence the transepithelial ion transport indirectly by modulating the endocellular concentration of HCO(3)(-) and/or H(+). In addition it is likely that NO modulates the permeability of the paracellular pathway since SNP produced also an increase of the tissue conductance and a decrease of the magnitude of the dilution potential.     

Different effects of cGMP and cAMP in the intestine of the European eel,Anguilla anguilla

The regulation of salt absorption in the sea water cell intestine was studied by evaluating the effects of theophylline, 8 Br cyclic adenosine monophosphate, 8 Br cyclic guanosine monophosphate, atriopeptin III, porcine vasoactive intestinal peptide and prostaglandin E ₁ on the short-circuit current, the transepithelial voltage difference and conductance and on the dilution potentials. It was shown that theophylline increased the transepithelial conductance and reduced the magnitude of the dilution potentials, indicating that the drug increase the anion conductance of the tight junctions. In addition its inhibitory effect on short-circuit current and transepithelial voltage difference suggests that theophylline also affects the transcellular transport mechanisms. It was shown that 8 Br cyclic guanosine monophosphate and 8 Br cyclic adenosine monophosphate affect transcellular mechanisms underlying Cl^– transport since both compounds reduced short-circuit current and transepithelial voltage difference; however, cyclic adenosine monophosphate is less effective since unlike cyclic guanosine monophosphate, even at maximal concentration, it was not able to completely abolish transepithelial voltage difference and short-circuit current. The effects of cyclic guanosine monophosphate and cyclic adenosine monophosphate were not additive even if cyclic guanosine monophosphate may produce further inhibition of ion transport in 8 Br cyclic adenosine monophosphate-treated tissues. In addition, cyclic guanosine monophosphate but not cyclic adenosine monophosphate reduced the magnitude of the dilution potentials, suggesting that cyclic guanosine monophosphate acts also on the paracellular pathway. Rat atriopeptin III, a peptide known to increase cyclic guanosine monophosphate cellular levels, behaved like 8 Br cyclic guanosine monophosphate since it lowered the dilution potentials and reduced short-circuit current and transepithelial voltage difference to near zero values, suggesting that the hormone modulates both paracellular and transcellular transport mechanisms, probably acting on the Na-K-2Cl cotransport. Agents acting via cyclic adenosine monophosphate, like porcine vasoactive intenstinal peptide and prostaglandin, behaved like 8 Br cyclic adenosine monophosphate. They were less effective in inhibiting ion transport and did not interfere with the paracellular pathway.Abbreviations AP III rat artriopeptin III - 8 Br cAMP 8 Br cyclic adenosine monophosphate - 8 Br cGMP 8 Br cyclic guanosine monophosphate - g _t transepithelial conductance - I _sc short circuit current - IC ₅₀ half-maximal inhibitory concentration - NaK ATPase Na-K-adenosine monophosphate - NPPB 5-nitro-2-(3-phenylpropylamino)-benzoic acid - PGE prostaglandin E ₁ - R _t tissue resistance - SITS 4-acetamide-4-isothiocyano-stilbene-2,2-disulfonic acid - V _t transepithelial voltage difference - VIP porcine vasoactive intestinal peptide     

Gossypol affects ion transport in the isolated intestine of the seawater adapted eel, Anguilla anguilla

Cottonseed (Gossypium sp.) meals are protein rich and inexpensive, but the presence of the polyphenolic dialdehyde, gossypol, is responsible of many toxic effects in animals including fishes. Recently an effect on the transepithelial ion transport in rat colon has been demonstrated. In this study we investigated the effect of gossypol on the transepithelial electrical parameters of the isolated intestine of seawater adapted eel, Anguilla anguilla, by employing a Ussing chamber technique. We showed that the addition of gossypol to the perfusion media reduced short circuit current (I(sc)), a measure of Cl- active absorption in this tissue, and increased tissue conductance (g(t)). The observation that the effect of gossypol on both I(sc) and g(t) was modified by the pretreatment with TFP, a calmodulin inhibitor, suggests that the substance acts via a Ca2+ calmodulin pathway and excludes the possibility that the observed effects were due to a cytotoxic action. In addition, experiments performed in the presence of verapamil suggest that the polyphenolic pigment increases Ca2+ influx. It is likely that gossypol stimulates a basolateral quinine sensitive K+ conductance producing a K+ flux in absorptive direction that explains the reduction of I(sc). In addition dilution potential experiments showed that the polyphenolic aldehyde increases the anion conductance of the paracellular pathway. In conclusion our study suggests that gossypol alters ion transport in eel intestine by acting on both transcellular and paracellular pathways. Since the intestine is an important organ for maintaining the water and ion balance in seawater adapted fish, it is conceivable that gossypol could impair the ability of the animals to adapt to the environment.     

Hearing in the eel (Anguilla anguilla)

Summary The auditory sensitivity in the European eel (Anguilla anguilla) was measured using an acoustic tube producing sound stimuli with different ratios between sound pressure and particle motion. The upper audible frequency limit in the eel was about 300 Hz. At low frequencies the relevant stimulus parameter was particle motion, excluding involvement of the swimbladder. At the higher frequencies within the audible range the swimbladder conveyed an auditory advantage for stimuli with a high ratio between pressure and particle motion. The eel has an extremely long distance between the swimbladder and the ear. An auditory function of the swimbladder in this species therefore indicates an efficient transmission channel for the reradiated swimbladder pulsations between the bladder and the ear, although specialized anatomical adaptations for this purpose are lacking.     

Effect of the flavonol quercetin on ion transport in the isolated intestine of the eel, Anguilla anguilla

Flavonoids are phenolic compounds used in fish diet formulations for the control of sex differentiation. Consequently it is of interest to know their effects on fish gastrointestinal mucosa that is the first target of these substances after food intake. We studied the effects of the flavonol quercetin on the transepithelial electrical parameters of the isolated intestine of the eel, Anguilla anguilla, by employing Ussing chamber technique. We showed that luminal quercetin (10(-4) M) reduced the serosa negative I(sc) (short circuit current) due to a net Cl- absorption in the control conditions. The reduction of I(sc) was blocked by luminal glybenclamide (10(-4) M). The I(sc) response to the flavonoid was also observed when HCO3- was omitted from the perfusion solutions. The effect of quercetin was nullified by TMB8, a blocker of Ca2+ release from intracellular stores, and was strongly reduced by trifluoroperazine, an inhibitor of calmodulin actions, while it was unmodified by experimental manoeuvres able to alter cytosolic cAMP concentrations. These results suggest that mucosal quercetin stimulated Cl- but not HCO3- secretion; the secretory activity appears to be dependent on the Ca2+-calmodulin system, independent of the cAMP pathway. In addition, this study showed that flavonol mimics the effect of carbachol on the tissue. This conclusion is supported by the following observations: 1) noradrenalin reversed the reduction of I(sc) produced by both carbachol and quercetin; 2) the flavonol was ineffective in tissues in which the muscarinic agonist had already elicited its maximal reduction of I(sc).     

Long-term numerical changes and regulation in a river stock of European eel Anguilla anguilla

1. The objective was to explore the regulatory mechanisms underlying the numerical dynamics of a river stock of European eel Anguilla anguilla based on a purely observational study. We used a 21-year data set on the Rio Esva (Asturias, northwestern Spain) stock of eels, encompassing eel density at an estuary site and at nine sites distributed among three tributaries selected to differ in the distance from the tidal limit and fishery data on glass eels from the nearby Rio Nalón estuary.
2. The abundance of glass eels and estuarine and in-stream densities declined simultaneously over a 15-year period. However, following a historical minimum in the abundance of glass eels around the year 2000, the density continued to decline in the estuary whereas density in the streams began an increase that has continued to the present.
3. In-stream cohort mortality rate ( Z ) varied from 0.0016 day⁻¹ to only 0.0003 day⁻¹ (on an annual basis from 0.57 to 0.099 year⁻¹). The negative effects of intra-cohort and inter-cohort densities on mortality rate emphasized the responses of the riverine stock to compensatory density-dependence. As a consequence, the number of eels surviving to commence the silvering stage was similar in magnitude in the strongest and weakest cohorts.
4. These results suggest that, even if the low abundance of glass eels persists, in-stream density may increase towards a stable value regulated by in-stream density-dependence.     

Morphometric analysis of the gills of the European eel, Anguilla anguilla

The gills of the European eel, Anguilla unguilla L. were analysed morphometrically. Fresh (unfixed) gills and resin-replica casts of the branchial vascular system were examined. The total gill surface area was found to be proportional to (body mas) 0-715 for fish of between 60 and 1 160 g. This relationship between gill surfxe area and body mass was maintained irrespective of a reduction in body mass produced by fasting. Vessel dimensions were obtained from the vascular casts. Calculations made using these values suggest that the major sites of gill vascular resistance lie at the level of the afferent lamellar arterioles and the secondary lamellae.     

Post-larval growth in the lateral white muscle of the eel, Anguilla anguilla

The post-larval growth of lateral white muscle was studied in eels at different stages of post-larval development (glass, yellow and silver eels) by means of histochemical methods for myosin-ATPase (mATPase) activity, immunohistochemistry (for myosin isoforms) and electron microscopy.
Morphological, histo- and immunohistochemical data reveal a uniform appearance of white muscle in glass eels, whereas in following stages the typical mosaic appearance is present. Small-diameter fibres show a more acid-labile mATPase activity than large fibres and react with anti-F, anti-FHC and anti-S sera, but not with anti-SHC serum. In the silver stage, the small fibres tend to decrease in number as the size of the eels increases.
Electron microscopy reveals the presence of satellite cells at every stage: in glass eels there are also 'activated' elements showing scarce myofilaments in their cytoplasm; in yellow eels very small fibres are present, enveloped within the basal lamina of well-differentiated muscle fibres; in silver eels there are no fibres showing signs of immaturity.
Presumably the post-larval development of white muscle involves in juvenile eels a substantial recruitment of fibres from the satellite cell population; later the hyperplasia decreases or ceases and hypertrophy remains the only mechanism for muscle growth.     

Colonisation of freshwater habitats by the European eel Anguilla anguilla

1. The spatial distribution of European eels in 18 U.K. rivers was related to distance from tidal limit using a negative exponential model. This function accounted for between 19 and 90% of the variation in eel density where quantitative data was available. For semiquantitative data the negative exponential function was a significant predictor of eel densities in only six out of 10 cases, although all rivers showed a consistent decline in abundance with distance upstream from the tidal limit. 2. The spatial distribution of different age groups of European eel in River Severn showed an initial rapid dispersion into freshwater followed by a much slower dispersion rate. Movement of the population upstream by a wave‐form migration process does not occur in this system. Instead colonisation of freshwaters can be seen as a two‐phase dispersion. Phase‐1 is a rapid dispersion upstream driven by density at the point source. Phase‐2 commences once the eels become yellow eels and is equivalent to random diffusion of particles. 3. These processes have important implications for the penetration of freshwaters with reduced numbers of eel larvae arriving on the coast of Europe and North America. Eel abundance will decrease more in freshwaters in an upstream direction whilst it may remain stable or decrease to a lesser extent in estuaries. They are also able to explain the demography of eels migrating upstream over weirs and the observations of varying sex ratios within catchments. We conclude that a dispersion model dependent on age, temperature, difficulty of migration, habitat quality and density of eels should be an important part of freshwater eel management.     

Effect of environmental salinity change on osmotic permeability of the isolated gill of the eel, Anguilla anguilla L.

Net water fluxes in the isolated gills of Anguilla anguilla were studied during incubation in fresh water (FW) and in sea water (SW). When incubated in FW, water entry was greater in SW-adapted eels than in FW-adapted eels. In contrast, water loss in SW was less in SW-adapted eels than in FW-adapted eels. Rectification of osmotic water fluxes was observed for both FW and SW-adapted eels, net water fluxes in the mucosal-serosal (m-s) direction being greater than those in the opposite (s-m) direction. These results indicate that adaptation to a given external medium brings about a decrease in the osmotic permeability so that water gain in FW or water loss in SW is minimal.     

A pseudocystic lipoma in the European eel (Anguilla anguilla)

This is the first report of lipoma in the European eel Anguilla anguilla. In a single eel that was obtained from a polyculture fish farm in northern Egypt, a yellowish swelling (10 mm in diameter) was observed near the tip of the lower lip. The tumor was composed of mature lipocytes that occasionally fused into an unlined cystic space.     

Development of the swimbladder in the European eel (Anguilla anguilla)

The swimbladder of the adult eel, Anguilla anguilla, with its bipolar countercurrent system, the rete mirabile, is a widely used model for swimbladder function, but very little is known about the development of this swimbladder. Our histological studies on the developing swimbladder revealed that during metamorphosis the swimbladder becomes present as a dorsal outgrowth of the esophagus. It is filled with surfactant, and gas was not detected in the swimbladder. In the young glass-eel, the epithelial (gas gland) cells of the swimbladder are columnar, but do not yet have the typical basolateral labyrinth established in adult animals. Few blood vessels are found in the swimbladder tissue, and the submucosa is present as a thick layer of connective tissue, giving a large diffusion distance between blood vessel and swimbladder lumen. Within the next 2 or 3 months of development, gas gland cells develop their typical basolateral labyrinth, and the thickness of the submucosa is significantly reduced, resulting in a short diffusion distance between blood vessels and the swimbladder lumen. The first filling of the swimbladder with gas is observed while the gas gland cells are still in a poorly differentiated status and it appears unlikely that these cells can accomplish their typical role in gas deposition. The presence of small gas bubbles in the swimbladder as well as in the ductus pneumaticus at the time of initial swimbladder inflation suggests that the swimbladder is filled by air gulping or possibly by taking up gas bubbles from the water.     

Iron-rich particles in European eel (Anguilla anguilla L.)

Magnetic material in the European eel (Anguilla anguilla L.) was investigated by a combination of magnetic susceptibility measurements, energy dispersive X-ray fluorescence analysis and transmission electron microscopy. It was shown that the magnetic material is associated with iron. The main part of the iron is present in the form of iron-rich particles with irregular shapes about 100-3000 A large. The structures of magnetite (Fe3O4), hematite (alpha-Fe2O3) and alpha-iron (bcc structure) were identified. The particles are composed of more than one of these phases with magnetite being a minority phase when present. The iron-rich particles found in the eel are different from the materials reported for bacteria or bees.     

Persistence of herpesvirus of eel Herpesvirus anguillae in farmed European eel Anguilla anguilla.

Herpesvirus of eel Herpesvirus anguillae (HVA) was isolated repeatedly from farmed eel of an outwardly healthy stock, but virus isolation was much greater in an experimental group of fish that were injected with dexamethasone. The results suggest that HVA can establish a latent infection in eel. Previous exposure of these eels to HVA virus was shown by detection of HVA-specific antibodies. These eels did not show clinical signs after a secondary infection with HVA. Tracing of seropositive eel stocks, which had previous contact with HVA, and of HVA carrier fish can be useful to control disease outbreaks due to HVA infection.