Accessory cells in the gill epithelium of the freshwater rainbow trout Salmo gairdneri

Two types of mitochondria-rich cells were identified in the gill epithelium of the freshwater-adapted rainbow trout, Salmo gairdneri, after selective impregnation of their tubular system with reduced osmium. A first type consisted of large cells with a poorly developed and loosely anastomosed tubular system; thus, that resembled the chloride cells commonly encountered in the gill epithelium of freshwater-adapted euryhaline fishes. A second type comprised smaller cells with an extensively developed and tightly anastomosed tubular system. These never reached the basal lamina of the gill epithelium and were adjacent to chloride cells, to which they were linked by shallow apical junctions (100-200 nm); thus, they resembled accessory cells, which are currently found in the gill epithelium of seawater-adapted fishes but are usually lacking in freshwater living fishes. Transfer of the freshwater-adapted trout into seawater induced the proliferation of the tubular system in the chloride cells and the formation of lateral plasma membrane interdigitations between accessory cells and the apical portion of the chloride cells. The length of the apical junction sealing off this extended intercellular space was reduced to 20-50 nm. The tubular system of the accessory cells was not modified. The extension of the tubular system in the chloride cells of the seawater-adapted fishes indicated that, as in most euryhaline fishes, these cells have a role in the adaptation of the rainbow trout to seawater. In contrast, the function of the presumptive accessory cells in freshwater trout remains to be established.     

Aspects of purine metabolism in the gill epithelium of rainbow trout, Salmo gairdneri Richardson.

1. Enzymes interconnecting the adenylate pool were present in high concentration. 2. AMP and adenosine were easily deaminated by the corresponding enzymes whose high levels were detected. 3. Adenylate was hydrolyzed either by deamination to yield IMP which was further dephosphorylated to inosine or by dephosphorylation to adenosine followed by deamination to inosine. 4. Incubation of gill extract with [-14C]-AMP in the presence and absence of ATP but with adenosine deaminase inhibitors allowed demonstration that ATP controlled the balance between these pathways. 5. Some biochemical properties of 5'-nucleotidase. AMP deaminase and adenosine deaminase were defined. 6. Purine salvage enzymes were also estimated.     

The effect of 3,5,3'-triiodo-L-thyronine on gill Na+/K+-ATPase of rainbow trout Salmo gairdneri, in fresh water

Gill and liver microsomal Na+/K+-adenosine triphosphatase (ATPase) activities and plasma levels of 3,5,3'-triiodo-L-thyronine (T3) were measured in rainbow trout (100-300 g) immersed in a freshwater solution of T3 for 6 or 7 days at 11 degrees C. Ambient T3 (1.25 or 2 micrograms T3/100 ml H2O) elevated plasma T3 within a physiologic range; an ambient concentration of 10 micrograms T3/100 ml produced supranormal plasma T3 levels. All T3 treatments depressed gill ATPase to a similar degree. Liver ATPase was lower than gill ATPase and was elevated by a physiologic T3 treatment.     

Glucose transport across the gill of the rainbow trout, Salmo gairdneri

Glucose fluxes across the gills were measured in freshwater-adapted trout (Salmo gairdneri) using an in vitro, perfused-head preparation. A large asymmetry was observed for the primary lamellar pathway, glucose permeability in the serosa-to-mucosa direction being up to 24 times greater than the permeability in the mucosa-to-serosa direction. Chloride cells appeared to possess a maximal rate of transport, or TMG, of 79 mumol/hr per 100 g. Phlorizin, phloretin and, to a lesser extent, harmaline caused an increase in the rate of glucose efflux. The results suggest that the tubulo-vesicular reticulum, into which plasma is introduced under low pressure, may be regarded as a reabsorption site for glucose in a way similar to the nephron proximal tubule. Thus, essential molecules such as glucose are removed while excess or non-essential substances are excreted into the external medium.     

Tolerance to gas supersaturation in fresh water and sea water by steelhead trout, Salmo gairdneri Richardson

The euryhaline status of steelhead trout, Salmo gairdneri , smolts was challenged in sea water for 2 weeks after which half of the total fish population was returned to fresh water. Acclimation continued and created two test populations in 29%osea water and fresh water. Subsequently these fish were exposed in fresh water or sea water to approximately equal hyperbaric dissolved total gas pressures (ΔP) of 190 mm Hg or about 125% of barometric pressure. Sea water was easier to supersaturate with air and required only about 10% of the entrained air which was required in fresh water at the same temperature and pressure. Mean time to first mortality was sooner in sea water. Mean times to mortality (10–50%) were not significantly different between fresh water and sea water, but there was a noticeable trend for longer survival in fresh water.     

The development of gill arches and gill blood vessels of the rainbow trout,Salmo gairdneri

Gill development begins on the sixth day of incubation at 10^°C and is complete by 31 days (hatching). Gill arches are formed by fusion and perforation of ectoderm and endoderm across the pharyngeal wall. A primary branchial artery forms within each arch and a second branchial artery forms as a branch from its ventral end. A series of filament loop vessels forms connecting the two arteries and when several are patent a unidirectional blood flow is established via afferent (second) branchial artery, filament loop vessels to efferent (primary) branchial artery. Part of the efferent branchial artery just above its junction with the afferent branchial artery constricts and occludes. It is suggested that this change in the pattern of blood flow is dependent on differences in resistance of the two branchial arteries. A later extension of the gill ventrally is thought not to be homologous with similar regions in elasmobranchs and Acipenser.     

Effects of adrenaline on the isolated heart of the trout (Salmo gairdneri R.) before and after transfer from fresh water to sea water

1. Adrenaline induces positive inotropic and chronotropic effects on the isolated heart of the trout (Salmo gairdnerii) via the stimulation of beta adrenoreceptors. 2. The effects are relatively more marked in winter than in spring. But there is no seasonal variation as in the cel. The intrinsic reactivity of the heart is not basically modified after a transfer from fresh water to sea water. 3. However, after such a transfer, adrenaline induces an increase of the membrane resting potential which suggests that adrenaline can enhance the activity of the Na/K pump and/or modify the Ca/K balance and could improve the cardiac performances during the "crisis period" following the transfer.     

An unknown structure in the chloride cells of the gill epithelium of the rainbow trout (Salmo gairdneri)

Summary An unknown membranous structure consisting of densely-packed tubules about 0.1 micron in diameter has been found in a few chloride cells of a rainbow trout, S. gairdneri. The structure is not normally found in this situation, and has not been previously reported, although it bears a superficial resemblance to one recently found in the Jacobson's organ of the cat.     

The structure of the gill of the trout,Salmo gairdneri (Richardson)

Summary A light and electron microscopic study was made of the structure of the gill arch, filament and secondary lamella of Salmo gairdneri R. Blood pathways through the gill were traced from serial histological sections, and from the examination of ink perfused tissue and perspex casts formed following resin injection of the circulatory system.The epithelium covering the gill consists of unspecialized, dark, chloride and mucous cells. The distribution of specialized cells appears to be related to gill function. The basement membrane underlying the epithelium consists of three layers, the inner collagen layer being continuous with the connective tissue core of the gills.Blood supply to the secondary lamellar respiratory surface is via branchial, filament and secondary lamellar arteries. Blood spaces of the secondary lamellae are delimited by pillar cells containing what appears to be contractile material. The marginal channel of each lamella is bounded distally by cells of endothelial origin. A network of lymph spaces within the filaments connects with efferent branchial arteries. Nutritionary capillaries within the filaments connect with afferent branchial arteries. No shunts between afferent and efferent filament arteries were found.Data from this study and previous physiological and histopathological studies suggest a mechanism for the control of blood flow to suit the respiratory requirements of the fish. This mechanism involves a system of recruitment of additional respiratory units and changes in overall blood flow patterns.This work formed part of a thesis submitted for the degree of Doctor of Philosophy in 1971 and for which M. M. was in receipt of a studentship from the Natural Environmental Research Council. The authors are grateful for the support given by research grants from the M.R.C (P.T.) and the N.E.R.C. (M.M.), and to Prof. G. M. Hughes in whose department the work was carried out.     

Absorption of intact proteins by the intestinal epithelium of trout,Salmo gairdneri

In the rainbow trout, a carnivorous species with a stomach, orally administered horseradish peroxidase (HRP) crossed the intestinal epithelium into the circulation. The protein first appeared in the blood 7 to 8 h after intubation, and could be assayed up to 24 h after administration. The method used, which combines ELISA (enzyme-linked immunoabsorbant assay) and chemiluminescence, enabled the transfer to be measured quantitatively. There was a direct correlation between the quantity ingested and the quantity transferred to the plasma within the experimental limits chosen. The clearance was monophasic and exponential (clearance rate: 3% per minute). Up to 6% of the ingested HRP was transferred to the blood. By cytochemistry it was possible to demonstrate that the protein crossed the intestinal cells at the level of the posterior segment, escaping the particularly intense intracellular lysosomal digestion. After entering the intercellular space, HRP was transferred to the interstitial space of the subepithelial lamina propria. During this transfer the HRP was in close contact with infiltrated macrophages and leukocytes resembling lymphoid cells. Thus, the passage of these protein particles could be the first indispensable step in the possible triggering of a local and/or a systemic immune response.     

The fine structure of the gastric epithelium of the rainbow trout, Salmo gairdneri, Richardson

Fine structural characteristics of the columnar mucus cells lining the surface and pits of the gastric mucosa, oxyntic cells lining the glands and the gastric endocrine cells were studied. The surface mucus cells, in addition to their primary involvement in production of mucus, showed structural adaptations. Release of the mucus vesicles was achieved by exocytosis. Transition from pit gland cells was abrupt since no mucus neck cells were observed. The oxyntic cells possessed apical and basal microvillous processes, a well developed tubulovesicular system, zymogen granules and extensive RER associated with many large mitochondria. When stimulated by distension of the stomach, the apical cytoplasm was converted into a labyrinth of cytoplasmic processes, while annular lamellae, each of which showed a short peripheral linear density appeared in the basal cytoplasm. The endocrine cells showed apical modifications as microvilli, cilia and reduced glycocalyx covering. Three types were distinguished on the basis of their granular morphology.     

Arteriovenous anastomoses in the afferent region of trout gill filaments (Salmo gairdneri richardson,teleostei)

Summary Arteriovenous anastomoses (AVAs) in the afferent region of trout gill filaments originate from two small filament arteries (Fromm's arteries), which parallel the main afferent filament vessel on either side. As in the efferent filament arteries the origin of AVAs is bordered by specialized endothelial cells. Fromm's arteries originate from efferent filament or branchial arteries. A few extremely narrow connections between the afferent filament artery and Fromm's arteries (= afferent shunts) do exist in some gill filaments. Nevertheless, the AVAs in the afferent filament region carry mainly arterialized blood, or blood plasma, to the central venous sinus of the filament.Supported by the Deutsche Forschungsgemeinschaft (Vo 229/1)     

Effects of cortisol on gill chloride cell morphology and ionic uptake in the freshwater trout,Salmo gairdneri

Summary Daily intramuscular injection of cortisol (4 mg kg^–1 body weight) in rainbow trout,Salmo gairdneri, for 10 days caused significant increases in the number and individual apical surface area of gill chloride cells per mm² of filament epithelium. Concomitantly, whole body influxes of sodium (Na⁺) and chloride (Cl^–) increased. Acute (3 h) intra-arterial infusion of cortisol did not affect whole body Na⁺ or Cl^– influx. A significant correlation was observed between both Na⁺ and Cl^– influxes and the fractional apical surface area of filament chloride cells in control, sham (saline-injected) and experimental (cortisol-injected) fish. The chloride cells displayed similar ultrastructural modifications in trout undergoing cortisol treatment as in trout transferred to ion-deficient water. These findings suggest the existence of structure/function relationships in which branchial chloride cell morphology is an important determinant of Na⁺ and Cl^– transport capacity. We conclude that chronic cortisol treatment enhances whole body Na⁺ and Cl^– influxes by promoting proliferation of branchial chloride cells. The results of correlation analysis indicate that the chloride cell is an important site of NaCl uptake in freshwater rainbow trout.     

Avoidance response of two-year-old rainbow trout, Salmo gairdneri Richardson, to air-supersaturated water: hydrostatic compensation

Vertical swimming responses of 2-year-old rainbow trout were tested with air saturated and air supersaturated water in tank experiments. The level of supersaturation varied between 115 and 125% total gas pressure. Control groups offish were kept in tanks with saturated water. No consistent vertical avoidance response was observed in the fish tested, but the mortality of fish restricted to the upper 30 cm of the tanks was significantly greater than fish free to sound to the whole depth of the tanks. It is concluded that the Norwegian stock of rainbow trout do not avoid air-supersaturation at levels from 115 to 125% TGP by active hydrostatic pressure compensation. An incidental type of hydrostatic pressure compensation seemed to be taking place during these experiments, as indicated by different levels of mortality in tanks with different depths. This incidental type of hydrostatic compensation may explain the observed difference in tolerance to supersaturation between wild fish and fish kept in experimental tanks.     

The vascular and epithelial serotonergic innervation of the actinopterygian gill filament with special reference to the trout,Salmo gairdneri

Summary Antibodies against serotonin and 5-methoxytryptamine reveal indolaminergic neurons innervating the proximal part of the efferent arterial vasculature, the filament epithelia, the central venous sinus, and certain other serotonergic cells of the teleost gill filament. In the same area, acetylcholinesterase-positive and indoleaminergic neurons have already been described. We propose that these populations of neurons belong to a single neuronal type but express different agents. Our current results support this idea; in particular, they point to the presence of a single type of serotonin-containing nerve terminal, impinging on vascular smooth muscle. These results are in agreement with physiological data showing (i) the existence of non-cholinergic (atropine-resistant) vasoconstriction of the gill vasculature after nerve stimulation, and (ii) a potent vasoconstrictory action of infused serotonin. In addition, the above-mentioned serotonergic neurons have synaptic contacts with catecholaminergic nerve fibers, suggesting the existence of a modulatory relationship between the sympathetic and the cranial autonomic nerves supplying the teleost gill. Finally, these neurons show morphological relationships with a previously undescribed type of branchialserotonergic cell. The role of the parasympathetic nerve plexus of the teleost gill filament in the control of respiration and ionoregulation is discussed.     

Rapid changes in the phospholipid composition of gill membranes during thermal acclimation of the rainbow trout,Salmo gairdneri

Summary The phospholipid composition of gill tissue was determined in rainbow trout (Salmo gairdneri) undergoing thermal acclimation between 5°C and 20°C for a period of up to 28 days. Proportions of phosphatidylethanolamine (PE) and cardiolipin (CL) increased during cold acclimation and decreased during warm acclimation; proportions of phosphatidylcholine (PC) changed in the opposite direction (i.e., decreased during cold acclimation). In contrast, levels of phosphatidylserine,-inositol, and sphingomyelin did not vary significantly. Thermal modulation of headgroup composition occurred rapidly as reflected by changes in the ratio of PC-to-PE, which rose significantly from 2.40±0.09 to 2.92±0.09 within 72 h of transfer from 5 to 20°C; adaptation to 5°C was equally rapid. Proportions of PE changed more rapidly than those of PC during cold adaptation, whereas the opposite was true during warm acclimation. Both the time course and the direction of the observed changes in phospholipid composition suggest that such adjustments may contribute to the homeoviscous regulation of membrane properties, particularly during the initial stages of thermal adaptation.