Vasculature of the fish gill: anatomical correlates of physiological functions

Amoeba species colonizing the gills of rainbow trout (Oncorhynchus mykiss) in Swiss aquaculture

Nodular gill disease (NGD) is an infectious condition characterized by proliferative gill lesions leading to respiratory problems, oxygen deficiency and mortality in fish. Globally, NGD primarily impacts freshwater salmonids in intensive aquaculture systems. In recent years, numerous outbreaks of severe gill disease have affected more than half of the larger rainbow trout (Oncorhynchus mykiss) farms in Switzerland, mainly during spring and early summer. Mortality has reached up to 50% in cases where no treatment was administered. Freshwater amoeba are the presumed aetiologic agent of NGD. The gross gill score (GS) categorising severity of gill pathology is a valuable first-line diagnostic tool aiding fish farmers in identifying and quantifying amoebic gill disease (AGD) in farmed marine salmonids. In this study, the GS was adapted to the NGD outbreak in farmed trout in Switzerland. In addition to scoring disease severity, gill swabs from NGD-affected rainbow trout were sampled and amoeba were cultured from these swabs. Morphologic and molecular methods identified six amoeba strains: Cochliopodium sp., Naegleria sp., Vannella sp., Ripella sp., Saccamoeba sp. and Mycamoeba sp. However, the importance of the different amoeba species for the onset and progression of NGD still has to be evaluated. This paper presents the first description of NGD with associated amoeba infection in farmed rainbow trout in Switzerland.

Toxic Effects of Cadmium on Fish

Large amounts of enriched cadmium (Cd) in the environment seriously threatens the healthy and sustainable development of the aquaculture industry and greatly restricts the development of the food processing industry. Studying the distribution and toxic effects of Cd in fish, as well as the possible toxic effects of Cd on the human body, is very significant. A large number of studies have shown that the accumulation and distribution of Cd in fish are biologically specific, cause tissue differences, and seriously damage the integrity of tissue structure and function, the antioxidant defense system, the reproductive regulation system, and the immune system. The physiological, biochemical, enzyme, molecular, and gene expression levels change with different concentrations and times of Cd exposure, and these changes are closely related to the target sites of Cd action and tissues in fish. Therefore, the toxic effects of Cd on fish occur with multiple tissues, systems, and levels.

Comparative Transcriptome Analysis During the Seven Developmental Stages of Channel Catfish (Ictalurus punctatus) and Tra Catfish (Pangasianodon hypophthalmus) Provides Novel Insights for Terrestrial Adaptation

Tra catfish (Pangasianodon hypophthalmus), also known as striped catfish, is a facultative air-breather that uses its swim bladder as an air-breathing organ (ABO). A related species in the same order (Siluriformes), channel catfish (Ictalurus punctatus), does not possess an ABO and thus cannot breathe in the air. Tra and channel catfish serve as great comparative models for investigating possible genetic underpinnings of aquatic to land transitions, as well as for understanding genes that are crucial for the development of the swim bladder and the function of air-breathing in tra catfish. In this study, hypoxia challenge and microtomy experiments collectively revealed critical time points for the development of the air-breathing function and swim bladder in tra catfish. Seven developmental stages in tra catfish were selected for RNA-seq analysis based on their transition to a stage that could live at 0 ppm oxygen. More than 587 million sequencing clean reads were generated, and a total of 21,448 unique genes were detected. A comparative genomic analysis between channel catfish and tra catfish revealed 76 genes that were present in tra catfish, but absent from channel catfish. In order to further narrow down the list of these candidate genes, gene expression analysis was performed for these tra catfish-specific genes. Fourteen genes were inferred to be important for air-breathing. Of these, HRG, GRP, and CX3CL1 were identified to be the most likely genes related to air-breathing ability in tra catfish. This study provides a foundational data resource for functional genomic studies in air-breathing function in tra catfish and sheds light on the adaptation of aquatic organisms to the terrestrial environment.

The osmorespiratory compromise in the fish gill

August Krogh made fundamental discoveries about both respiratory gas exchange and osmo/iono-regulation in fish gills. Dave Randall and co-workers identified a tradeoff between these two functions such that high functional surface area and low diffusion distance would favour O₂ uptake (e.g. exercise, hypoxia), whereas low functional surface area and high diffusion distance would favour osmo/iono-regulation (rest, normoxia). Today we call this concept the "osmorespiratory compromise" and realize that it is much more complex than originally envisaged. There are at least 6 mechanisms by which fish can change functional branchial area and diffusion distance. Three involve reorganizing blood flow pathways: (i) flow redistribution within the secondary (respiratory) lamellae; (ii) flow shunting between "respiratory" and "ionoregulatory" pathways in the filament; (iii) opening up more distal lamellae on the filament and closing non-respiratory pathways. Three more involve "reversible gill remodeling": (iv) proliferation of the interlamellar gill cell mass (ILCM); (v) proliferation of ionocytes up the sides of the lamellae; (vi) covering over the apical exposure of ionocytes by extension of pavement cells. In ways that remain incompletely understood, these mechanisms allow dynamic regulation of the osmorespiratory compromise, such that ion and water fluxes can be decoupled from O₂ uptake during continuous exercise. Furthermore, hypoxia-tolerant species can reduce branchial ion and water fluxes below normoxic levels despite hyperventilating during hypoxia. In marine fish, the osmorespiratory conflict is intensified by the greater ionic and osmotic gradients from seawater to blood, but underlying mechanisms remain poorly understood.

Comparison of bacterial diversity and distribution on the gills of Atlantic salmon (Salmo salar L.): an evaluation of sampling techniques

AIMS

Assess bacterial diversity and richness in mucus samples from the gills of Atlantic salmon in comparison to preserved or fixed gill filament tissues. Ascertain whether bacterial diversity and richness are homogeneous upon different arches of the gill basket.

METHODS AND RESULTS

Bacterial communities contained within gill mucus were profiled using 16S rRNA gene sequencing. No significant difference in taxa richness, alpha (P > 0·05) or beta diversity indices (P > 0·05) were found between the bacterial communities of RNAlater preserved gill tissues and swab-bound mucus. A trend of lower richness and diversity indices were observed in bacterial communities from posterior hemibranchs.

CONCLUSIONS

Non-lethal swab sampling of gill mucus provides a robust representation of bacterial communities externally upon the gills. Bacterial communities from the fourth arch appeared to be the least representative overall.

SIGNIFICANCE AND IMPACT OF THE STUDY

The external mucosal barriers of teleost fish (e.g. gill surface) play a vital role as a primary defence line against infection. While research effort on the role of microbial communities on health and immunity of aquaculture species continues, the collection and sampling processes to obtain these data require evaluation so methodologies are consistently applied across future studies that aim to evaluate the composition of branchial microbiomes.

Enzymatic and Histological Biomarkers in Ucides cordatus (Crustacea, Decapoda) in an Industrial Port on the North Coast of Brazil

The aim of this study was to evaluate enzymatic (glutathione-S-transferase and catalase) and histological (branchial lesions) biomarkers in Ucides cordatus (Crustacea, Decapoda) from an industrial port region on the north coast of Brazil. The crabs were collected in two distinct locations of the Brazilian coast: A1 = region under influence of port activities; and A2 = low-impacted area. We performed histological examination in the gills and glutathione-S-transferase (GST) and catalase activity in the hepatopancreas. The most frequent and severe histological lesions were found in A1, especially rupture of pilaster cells and lamellar collapse. Catalase activity did not show a pattern capable of differentiating the two analyzed areas. On the other hand, GST activity presented a more pronounced response in the crabs of the port area (p < 0.05), coinciding with the most frequent branchial lesions in these same organisms. These results suggest that the species is susceptible to environmental stress, once alterations at different organizational levels were verified.

Fish response to hypoxia stress: growth, physiological, and immunological biomarkers

Water quality encompasses the water physical, biological, and chemical parameters. It generally affects the fish growth and welfare. Thus, the success of a commercial aquaculture project depends on supplying the optimum water quality for prompt fish growth at the minimum cost of resources. Although the aquaculture environment is a complicated system, depending on various water quality variables, only less of them have a critical role. One of these vital parameters is dissolved oxygen (DO) level, which requires continuous oversight in aquaculture systems. In addition, the processes of natural stream refinement require suitable DO levels in order to extend for aerobic life forms. The depletion of DO concentration (called hypoxia) in pond water causes great stress on fish where DO levels that remain below 1-2 mg/L for a few hours can adversely affect fish growth resulting in fish death. Furthermore, hypoxia has substantial effects on fish physiological and immune responses, making them more susceptible to diseases. Therefore, to avoid disease outbreak in modern aquaculture production systems where fish are intensified and more crowded, increasing attention should be taken into account on DO levels.

Cytotoxic and genotoxic affects of acid mine drainage on fish Channa punctata (Bloch)

The investigation deals with the effects of Acid Mine Drainage (AMD) of coal mine on fish Channa punctata (Bloch) by examining the incidence of haematological, morphological, histological changes and DNA fragmentation in tissues of C. punctata in laboratory condition. For this study fishes were exposed to 10% of AMD for a period of 30 days. The fusion of the primary and secondary gill lamellae, distortion, loss of alignment, deposition of worn out tissues and mucous on the surface of the lamella in the gills; degeneration of morphological architecture, loss of alignment of tubules, mucous deposition in the kidney; cellular damage, cellular necrosis, extraneous deposition on the surface, pore formation in the liver are some important changes detected by scanning electron microscopy. Fishes of AMD treated group showed gradual significant decrease in TEC, Hb and, increase in TLC and DLC as compared to that of the control. DNA fragmentation observed in kidney of fishes from treated group indicates an intricate pollutant present in the AMD. The high incidence of morphological and histological alterations, haematological changes along with DNA breakage in C. punctata is an evidence of the cytotoxic and genotoxic potential of AMD of coal mines.

Arteriosclerosis in the ventral aorta and epicarditis in the bulbus arteriosus of Atlantic salmon (Salmo salar L)

Spontaneous mortality of seemingly healthy, farmed Atlantic salmon (Salmo salar L) is an increasing problem in Norwegian aquaculture. In this study, we present a morphological study of the previously undescribed syndrome of arteriosclerosis of the ventral aorta and epicarditis of the adjacent bulbus arteriosus found in farmed Atlantic salmon, with wild-captured fish as a control group. Both the ventral aorta and epicardium are vital for correct arterial compliance and vascular resistance in the respiratory capillaries of the gills. We discuss the possible implications of ventral aorta arteriosclerosis and epicarditis for blood vascular health and in particular for the increasing frequency of spontaneous gill bleeding in farmed salmon. As both these conditions primarily occur in farmed salmon, we suggest that they should be considered pathological.

Comparative physiology of the pulmonary circulation

Two selective pressures have shaped the evolution of the pulmonary circulation. First, as animals evolved from heterothermic ectotherms to homeothermic endoderms with their corresponding increase in the ability to sustain high oxygen consumptions, the blood-gas barrier had to become successively thinner, and also provide an increasingly large area for diffusive gas exchange. Second, the barrier had to find a way to maintain its mechanical integrity in the face of extreme thinness, and this was assisted by the increasing separation of the pulmonary from the systemic circulation. A remarkable feature throughout the evolution of air-breathing vertebrates has been the tight conservation of the tripartite structure of the blood-gas barrier with its three layers: capillary endothelium, extracellular matrix, and alveolar epithelium. The strength of the barrier can be ascribed to the very thin layer of type IV collagen in the extracellular matrix. In the phylogenic progression from amphibia and reptiles to mammals and birds, the blood-gas barrier became successively thinner. Also, the area increased greatly reflecting the greater oxygen demands of the organism. The gradual separation of the pulmonary from the systemic circulation continued from amphibia through reptiles to mammals and birds. Only in the last two classes are the circulations completely separate with the result that the pulmonary capillary pressures can be maintained low enough to avoid stress failure of the blood-gas barrier. Remarkably, the barrier is generally much thinner in birds than mammals, and it is also much more uniform in thickness. These advantages for gas exchange can be explained by the support of avian pulmonary capillaries by the surrounding air capillaries. This arrangement was made possible by the adoption of the flow-through system of ventilation in birds as opposed to the reciprocating pattern in mammals.

Surtos de infecção por Piscinoodinium pillulare e Henneguya spp. em pacus (Piaractus mesopotamicus) criados intensivamente no Sudoeste de Goiás

Três surtos de infecção por Piscinoodinium pillulare e Henneguya spp. em Piaractus mesopotamicus criados intensivamente no Sudoeste de Goiás são descritos. Dois surtos ocorreram no inverno e um no verão. As taxas de morbidade (58,57%-90%) e mortalidade (100%) foram determinadas. Episódios de estresse ocorreram previamente em todos os casos. Sinais clínicos incluíam mudanças comportamentais e movimentos erráticos com perda de equilíbrio. Os principais achados macroscópicos foram tumefação das brânquias com exsudato mucoso esbranquiçado. Microscopicamente, as brânquias apresentaram inflamação linfoplasmocítica e necrosante, multifocal, leve a moderada, com hipertrofia e fusão de lamelas secundárias. Adicionalmente, numerosos trofontes intralesionais de P. pillulare e cistos de Henneguya spp. foram notados. Para o conhecimento dos autores, essa parece ser a primeira descrição de infecção por P. pillulare e Henneguya spp. em peixes criados no Sudoeste de Goiás.

A role for inositol monophosphatase 1 (IMPA1) in salinity adaptation in the euryhaline eel (Anguilla anguilla)

This study investigated the expression and tissue distribution of inositol monophosphatase (IMPA1) and characterized its role in salinity adaptation in the eel. The coding sequence of eel IMPA1 was determined and confirmed to be orthologous to the mammalian gene/enzyme by phylogenetic analysis and structural modeling. Quantitative real-time PCR and Western blot techniques indicated up to 17-fold increases in mRNA expression and 2-fold increases in protein abundance in major osmoregulatory tissues following transfer of fish to seawater (SW). This was accompanied by up to 5-fold increases in enzyme activity, and 1.8- and 3-fold increases in inositol contents within the gill and kidney, respectively. Immunohistological studies revealed that IMPA1 protein expression predominated in SW-acclimated fish within basal epithelial/epidermal layers of the gill, esophagus, intestine, skin, and fins. SW transfer also induced a 10-fold increase in inositol content in the fin. IMPA1 immunoreactivity was also identified in chondrocytes within the cartilagenous matrix of the gills and fins, as well as in clusters of interstitial cells surrounding the kidney tubules. The observed increases in expression of IMPA1 highlight a protective role for inositol within various eel tissues following SW acclimation. This constitutes an adaptive mechanism in teleost fish naturally exposed to hypertonic environments.

Adenosinergic and cholinergic control mechanisms during hypoxia in the epaulette shark (Hemiscyllium ocellatum), with emphasis on branchial circulation

Coral reef platforms may become hypoxic at night during low tide. One animal in that habitat, the epaulette shark (Hemiscyllium ocellatum), survives hours of severe hypoxia and at least one hour of anoxia. Here, we examine the branchial effects of severe hypoxia (<0.3 mg oxygen l(-1) for 20 min in anaesthetized epaulette shark), by measuring ventral and dorsal aortic blood pressure (P(VA) and P(DA)), heart rate (fh), and observing gill microcirculation using epi-illumination microscopy. Hypoxia induced a flow of blood in two parallel blood vessels, termed longitudinal vessels, in the outer borders of the free tip of the gill filament. Hypoxia also induced significant falls in fh, P(VA) and P(DA), and a biphasic change in ventilation frequency (increase followed by decrease). Adenosine injection (1 micromol kg(-1)) also initiated blood flow in the longitudinal vessels, in addition to significant drops in P(VA), P(DA) and fh, and a biphasic response in ventilation frequency (decrease followed by increase) indicating that adenosine influences ventilation. Aminophylline (10 mg kg(-1)), an A(1) and A(2) adenosine receptor antagonist, blocked the effects of adenosine injection, and also significantly reduced blood flow in the longitudinal vessels during hypoxia. In the second part of the study, we examined the cholinergic influence on the cardiovascular circulation during severe hypoxia (<0.3 mg l(-1)) using antagonists against muscarinic (atropine 2 mg kg(-1)) and nicotinic (tubocurarine 5 mg kg(-1)) receptors. Injection of acetylcholine (ACh; 1 micromol kg(-1)) into the ventral aorta caused a marked fall in fh, a large increase in P(VA), but small changes in P(DA) (suggesting increased R(gill)). Atropine was able to inhibit the branchial vascular responses to ACh but not the hypoxic bradycardia, suggesting the presence of muscarinic receptors on the heart and gill vasculature, and that the hypoxia induced bradycardia is of non-cholinergic origin. The results suggest that adenosine mediates increases in the arterio-venous circulation in the gill during hypoxia. This may serve to increase blood supply to heart and gill tissue.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Ultrastructural study of the late stages of Loma salmonae development in the gills of experimentally infected rainbow trout

The main objective of this investigation was to examine the ultrastructural features of gills from rainbow trout experimentally infected with Loma salmonae to determine the morphological events that occur during the late stages of development of this parasite. Peripheral distribution of the mature parasites inside round xenomas was observed at weeks 5 and 6 postexposure (PE), but eventually the parasite occupied the entire xenoma. Degenerative changes were observed only in immature parasites at week 7 PE, and eventually an inflammatory reaction with a cellular infiltration was directed against mature spores. Round, flattened, and irregular shaped xenomas were observed at week 8 PE. The round xenomas showed a severe inflammatory response with disintegration of the xenoma membrane. This event was accompanied by eversion of polar tubes within the attacked xenoma and by the simultaneous presence of 2 tubular appendages, the type I and II tubules. Flattened xenomas were observed below the endothelium of gill lamella arteries. The irregular xenomas were located in the connective tissue of the gill filament and showed multiple projections occupied by spores. Both flattened and irregular xenomas showed no evidence of inflammatory reaction. An earlier proposed hypothesis is expanded to explain how L. salmonae is implanted beneath lamellar endothelium and within filament connective tissue.

Vascular anatomy of the gills in a high energy demand teleost, the skipjack tuna (Katsuwonus pelamis)

Tunas (family: Scombridae, Tribe: Thunnini) exhibit anatomical, physiological, and biochemical adaptations that dramatically increase the ability of their cardiorespiratory systems to transfer oxygen from the water to the tissues. In the present study the vascular anatomy of the skipjack tuna, Katsuwonus pelamis, gill was examined by light and scanning electron microscopic analysis of methyl methacrylate vascular corrosion replicas prepared under physiological pressure. The gill filament contains three distinct blood pathways, respiratory, interlamellar, and nutrient. The respiratory, or arterio-arterial (AA) pathway, is the site of gas exchange and consists of the afferent and efferent filamental arteries (AFA and EFA) and arterioles (ALA and ELA) and the lamellae. Each ALA in the basal filament supplies ten or more lamellae and they anastomose with their neighbor to form a continuous vascular arcade. Four modifications in the lamellar circulation appear to enhance gas exchange efficiency. 1) The ALA deliver blood directly to the outer margin of the lamellae where unstirred boundary layer effects are predicted to be minimal and water PO2 highest. 2) Pillar cells are closely aligned along the outer boundary of the inlet side and the inner boundary of the outlet side of the lamellae to form multiple distributing and receiving blood channels. 3) Elsewhere in the lamella, pillar cells are aligned to form diagonal channels that direct blood from the outer to the inner lamellar margins, thereby reducing vascular resistance. 4) The lamellar sinusoid is especially widened near the efferent end to augment oxygen saturation of blood flowing through the inner margin. These adaptations, plus the presence of a bow-shaped interlamellar septum, and a thinned filament core appear to decrease gill vascular resistance and maximize gas-exchange efficiency. The interlamellar (IL) and nutrient systems originate from post-lamellar vessels and are arterio-venous (AV) pathways. IL vessels form an extensive ladder-like lattice on both sides of the filamental cartilage and are supplied in part by narrow-bore vessels from the medial wall of the EFA. Their function is unknown. Nutrient vessels are formed from the confluence of a myriad of tortuous, narrow-bore vessels arising from the basal region of the EFA and from efferent branchial arteries. They re-enter the filament and eventually drain into the IL system or filamental veins. As these AV pathways are retained despite considerable reduction in filamental tissue, it is evident that they are integral components of other non-respiratory homeostatic activities of the gill.

Vascular anatomy of the fish gill

The fish gill is the most physiologically diversified vertebrate organ, and its vasculature the most intricate. Application of vascular corrosion techniques that couple high-fidelity resins, such as methyl methacrylate, with scanning electron microscopy yields three-dimensional replicas of the microcirculation that have fostered a better appreciate gill perfusion pathways. This is the focus of the present review. Three vascular networks can be identified within the gill filament. The arterioarterial (respiratory) pathway consists of the lamellae and afferent and efferent segments of the branchial and filamental arteries and lamellar arterioles. The body of the filament contains two post-lamellar pathways: the interlamellar and nutrient. The interlamellar system is an extensive ladder-like network of thin-walled, highly distensible vessels that traverses the filament between, and parallel to, the lamellae and continues around the afferent and efferent borders of the filament. Interlamellar vessels are supplied by short, narrow-bore feeder vessels from the medial wall of the efferent filamental artery. A myriad of narrow-bore, tortuous arterioles arise from the basal efferent filamental artery and efferent branchial artery and anastomose to form the nutrient circulation of the arch and filament. In the filament body, nutrient capillaries and interlamellar vessels are often closely associated, and the former may ultimately drain into the latter. Many of the anatomical characteristics of interlamellar vessels are strikingly similar to those of mammalian lymphatic capillaries, with the exception that interlamellar vessels are directly fed by arteriovenous-like anastomoses. It is likely that gill interlamellar and mammalian lymphatics are physiologically, if not embryologically, equivalent.

Ultrastructural study of the early development and localization of Loma salmonae in the gills of experimentally infected rainbow trout

The early ultrastructural stages of Loma salmonae were studied in the gills of experimentally infected rainbow trout. No parasitic stages were identified during the first 2 wk of the infection. By week 3 postexposure (PE), uninucleate and binucleate meronts were recognized within host cells (no xenomas) associated with the capillary channels of secondary lamellae and lamellar arteries. An inflammatory reaction was absent. In secondary lamellae, infected cells were isolated from the capillary lumen, and some were recognized as pillar cells. In lamellar arteries, infected cells were localized beneath the endothelium and not in the lumen. Inflammatory reaction and destruction of parasites inside blood cells in the lumen of secondary lamellae were observed by week 4 PE. Three hypotheses, i.e., isolation, internalization, and evasion, are proposed to explain the localization of the infected cells in the gills. It is concluded that meronts are the earliest parasitic stage observed by week 3 PE, pillar cells are secondarily infected by phagocytosis of infected cells in the blood, endothelial cells of gills are not infected, and inflammatory response to the parasite starts by week 4 PE.

Effect of cyanide concentrations on the secondary structures of protein in the crude homogenates of the fish gill tissue

The effect of cyanide concentrations on the secondary conformation of protein in the fish gill homogenate was determined using an attenuated total reflectance (ATR)/Fourier transform infrared (FT-IR) microspectroscopy. Gills from male Tilapia zillii were isolated and homogenized in pH 8.0 Tris buffer solution and subjected to FT-IR study. The results indicate that the amide I and III bands of protein in fish gill homogenate deformed markedly with the increase of cyanide concentration. The fish gill homogenate shows a maximum peak at 1650 cm(-1) in amide I band, suggesting the predominant proportion of alpha-helical conformation. Once the KCN was added into the gill homogenate, the maximum peak shifted gradually from 1650 to 1643 cm(-1) due to the random coil structure, with the increase of cyanide concentration used. Two additional shoulders at 1657 (alpha-helix) and 1627 (beta-sheet) cm(-1) also appeared gradually, implying that the cyanide can in part induce changes in protein conformation of fish gill homogenate from alpha-helix to random coil and beta-sheet conformations.

Control of vascular tone in notothenioid fishes is determined by phylogeny, not environmental temperature

We examined potential vasomotor control mechanisms in an Antarctic fish (Trematomus bernacchii; usual core temperature approximately -1 degrees C), comparing sensitivity to agonists by means of the cumulative dose response and potency with reference to depolarization by 50 mM KCl. In efferent branchial arteries, norepinephrine (NE) produced approximately 20% of the maximal KCl tension and ~40% in the presence of 10(-3)M sotalol, suggesting a modest contribution of alpha- and beta-adrenergic tonus [half-maximal response (pEC(50)) = 6.29 +/- 0.37 M]. Carbachol (CBC) and serotonin (5-HT) had different sensitivities (pEC(50) = 4.50 +/- 0.40 and 6.82 +/- 0.08 M, respectively) but similar potencies (21.6 +/- 11.1 and 31.1 +/- 5.3% of KCl). A related species from warmer waters around New Zealand, Paranotothenia angustata, had similar vascular reactivity for NE (pEC(50) = 5.48 +/- 0.31 M), CBC (pEC(50) = 4.94 +/- 0.22 M), and methysergide-sensitive vasoconstriction with 5-HT (pEC(50) = 6.22 +/- 0.40 M). Agonist potencies were 9, 65, and 45% that of KCl, respectively. Bovichtus variegatus, a member of the phylogenetic sister group to the notothenioids, also gave broadly similar responses. In contrast, Dissostichus mawsoni, a pelagic Antarctic notothenioid, showed a dominance of vasodilatation over vasoconstriction, with sensitive isoprenaline (pEC(50) = 6.66 +/- 0.05 M) but weak serotonergic (5.2 +/- 1.5% KCl) responses. The unusual dominance of serotonergic control appears to be primarily a consequence of evolutionary lineage rather than low environmental temperature, but the pattern may be modified according to functional demand.

Biological functions of trout pavement-like gill cells in primary culture on solid support: pH(i) regulation, cell volume regulation and xenobiotic biotransformation

This review presents results obtained on rainbow trout gill cells in primary culture on solid support. Ultrastructural analysis showed that cultured gill cells displayed features of pavement cells in situ. Several biological functions have been investigated on these cultured cells. First, it was shown that their intracellular pH at rest and after acidosis is regulated by a Na+/H+ exchanger. Second, gill cells in primary culture can regulate their volume after a cell swelling. Intracellular calcium appears to be involved in this regulation. The effects of different xenobiotics on the capacity of gill cells to regulate their volume are presented. Third, cultured pavement cells contain biotransformation enzymes to metabolize xenobiotics. All these results demonstrate that gill cells in primary culture on solid support represent a promising in vitro model for the study of pavement cells physiology. In conclusion, applications of this culture are discussed and compared with the permeable filter method, together with the limitations and prospects of this in vitro model on solid support.

Is the sheet-flow design a 'frozen core' (a Bauplan) of the gas exchangers? Comparative functional morphology of the respiratory microvascular systems: illustration of the geometry and rationalization of the fractal properties

The sheet-flow design is ubiquitous in the respiratory microvascular systems of the modern gas exchangers. The blood percolates through a maze of narrow microvascular channels spreading out into a thin film, a "sheet". The design has been convergently conceived through remarkably different evolutionary strategies. Endothelial cells, e.g. connect parallel epithelial cells in the fish gills and reptilian lungs; epithelial cells divide the gill filaments in the crustacean gills, the amphibian lungs, and vascular channels on the lung of pneumonate gastropods; connective tissue elements weave between the blood capillaries of the mammalian lungs; and in birds, the blood capillaries attach directly and in some areas connect by short extensions of the epithelial cells. In the gills, skin, and most lungs, the blood in the capillary meshwork geometrically lies parallel to the respiratory surface. In the avian lung, where the blood capillaries anastomose intensely and interdigitate closely with the air capillaries, the blood occasions a 'volume' rather than a 'sheet.' The sheet-flow design and the intrinsic fractal properties of the respiratory microvascular systems have produced a highly tractable low-pressure low-resistance region that facilitates optimal perfusion. In complex animals, the sheet-flow design is a prescriptive evolutionary construction for efficient gas exchange by diffusion. The design facilitates the internal and external respiratory media to be exposed to each other over an extensive surface area across a thin tissue barrier. This comprehensive design is a classic paradigm of evolutionary convergence motivated by common enterprise to develop corresponding functionally efficient structures. With appropriate corrections for any relevant intertaxa differences, use of similar morphofunctional models in determining the diffusing capacities of various gas exchangers is warranted.

Cardiovascular and gill microcirculatory effects of endothelin-1 in atlantic cod: evidence for pillar cell contraction

Endothelin-1 (ET-1) has been shown to cause a considerable increase in the vascular resistance of fish gills. In trout, recent evidence suggest that this is the result of pillar cell contraction in the gill lamellae. Using epi-illumination microscopy to observe the gill lamellae of anaesthetised Atlantic cod (Gadus morhua), we show that ET-1 (100 ng kg-1, injected into the ventral aorta) causes an increase in pillar cell diameter, consistent with pillar cell contraction, and a shift of intralamellar blood flow from the lamellar sheet to the outer marginal channels. Simultaneously, there was an increase in ventral aortic blood pressure, a reduction in cardiac output, an increase in gill vascular resistance and a reduction in the oxygen partial pressure of venous blood. All these effects were blocked by the ETA/ETB receptor antagonist bosentan (5 mg kg-1). Pillar cell contraction is likely to be a mechanism for matching the functional respiratory surface area with the instantaneous respiratory needs of the fish.

Hormone metabolism by the fish gill

The fish gill, like the mammalian lung, is ideally situated to process circulating biomolecules because: 1) the gill is the only organ perfused by the entire cardiac output, 2) the in-series positioning of branchial and systemic circulations permits "conditioning" of blood immediately before systemic perfusion and 3) gill microcirculation is extensive, providing substantial endothelial/pillar cell surface in contact with plasma. In addition, two or three distinct circulatory pathways within the gill may differentially affect plasma substrates, raising the possibility of vasoactive control of hormone titers. Hormones may be activated or inactivated by the gill, the latter involving extraction (uptake) from the plasma, metabolism by enzymes on the endothelial surface without uptake or uptake plus intracellular metabolism. Over 60% of angiotensin I (ANG I) is activated to angiotensin II (ANG II) in a single transit through the gill lamellae by pillar cell angiotensin-converting enzyme, whereas both ANG I and II are inactivated by the non-lamellar filamental vasculature. Gills may accumulate and store (uptake 1) or degrade (uptake 2) catecholamines via intracellular monoamine oxidase and catechol-O-methyl transferase enzymes, and they show substrate preference for norepinephrine over epinephrine. Similar processes may exist for serotonin. Atrial natriuretic peptides are efficiently (60-90%) extracted from plasma in vivo by C-type clearance receptors. Fifty percent of an endothelin-1 bolus is removed in a single transit through the gill circulation, arginine vasotocin extraction is modest and bradykinin is virtually unaffected. Arachidonic acid is completely extracted by the gill, whereas extraction of prostaglandins I2 and E2 is only 13 and 5%, respectively. Intense cytochrome P450 immunofluorescence in the pillar cells suggests that the gill vasculature may be an important site of detoxification and production of biologically active epoxides. Thus, gills appear to be potent and selective effectors of hormonal signals.

Gill blood flow control

The arrangement of the fish gill vasculature is quite complex, and varies between the different fish groups. The use of vascular casting techniques has greatly enhanced our knowledge of the anatomy of the branchial microcirculation, not least through the contributions of Pierre Laurent and co-workers at Strasbourg. At different physiological situations, the contact surface between water and blood (functional surface area) varies to balance oxygen uptake against osmotic water flow ("respiratory-osmoregulatory compromise"). This is controlled by nerves and by blood-borne or locally released substances that affect blood flow patterns in the gill. Histochemical techniques have been used to demonstrate neurotransmitter substances in the branchial innervation. In combination with physioly-osmoregulatory compromise" at different physiological situations.

Effects of sodium nitroprusside on blood circulation and acid–base and ionic balance in rainbow trout: indications for nitric oxide induced vasodilation

The effects of the nitric oxide (NO) releasing antihypertensive agent sodium nitroprusside (SNP) on rainbow trout (Oncorhynchus mykiss) were investigated using either waterborne exposures of up to 1.3 mM SNP over 60 and 90 min or injections via the dorsal aorta, and cardiovascular responses were compared. Doses of up to 40 μg∙kg−1into the blood stream resulted within 5 min in significantly reduced blood pressure and pulse pressure with tachycardia. Waterborne SNP below 0.53 mM had no effect, but concentrations above this level (to 1.3 mM) after 30–60 min produced significant tachycardia and vasodilation, resulting in dose- and time-dependent reductions in dorsal aorta pulse pressure, with maximum decreases of 50–57%. At and above 0.4 mM SNP waterborne exposure, blood [Formula: see text] was significantly increased, plasma Na+, K+, and Cl−values were unchanged, and there was a mild alkalosis. The cardiovascular effects of waterborne isosorbide dinitrate at 40 μM were similar to those of SNP. Exposure of fish to waterborne potassium ferricyanide (0.67 mM), which is structurally similar to SNP but does not release NO, produced only minor cardiovascular effects compared with those of SNP. Pretreatment of fish with propranolol (1.9 mg∙kg−1via the dorsal aorta) followed by exposure to 1 mM waterborne SNP showed that adrenergic responses were unlikely to be involved in the vasodilation. The results of pretreatment with methylene blue (1.5 mg∙kg−1via the dorsal aorta) followed by 1 mM waterborne SNP suggested that the vasodilation was induced by NO activation of soluble guanylyl cyclase. This study demonstrates that NO-releasing compounds cause vasodilation in rainbow trout in vivo and provides a novel way of studying the effects of altered vascular resistance in fish.

Drinking rate in juvenile Atlantic salmon,Salmo salar L fry in response to a nitric oxide donor, sodium nitroprusside and an inhibitor of angiotensin converting enzyme, enalapril

Drinking in freshwater juvenile salmon was investigated in response to vasodilation by sodium nitroprusside (SNP), a nitric oxide donor, which significantly increased blood vessel diameter in Atlantic salmon alevins. Atlantic salmon fry (1-3 g), as previously shown, drank at a significant rate in fresh water which doubled to about 1.2 ml kg(-1) h(-1) following injection of SNP (100 μmol kg(-1)), through dilation of body vasculature and activation of a vasoconstrictive mechanism, the endogenous renin angiotensin system (RAS). This response was 50% inhibited by injection of about 100 mg kg(-1) enalapril. Fry increased drinking in response to SNP administered in the water, though the concentration required for maximal response, 1.6 mmol l(-1), was much greater than for injected SNP; this response was also inhibited by enalapril injection. Possible involvement of the gill vasculature and branchial osmoreceptors or baroreceptors in control of the drinking response is discussed.

Microcirculation of gills and accessory respiratory organs of the walking catfish Clarias batrachus

BACKGROUND

An ability to extract oxygen directly from the atmosphere enables air-breathing fish to survive otherwise debilitating hypoxic environments. Addition of accessory respiratory organs (ARO) necessitates changes in both the general circulatory system and the microcirculation of the respiratory epithelia. Understanding these modifications provides information on the efficiency of gas exchange organs as well as an indication of the evolutionary processes associated with adaptation to terrestrial habitats.

METHODS

Vascular organization and structure of gills and ARO of the facultative air-breathing walking catfish Clarias batrachus were examined by scanning electron microscopy of vascular replicas and fixed tissue.

RESULTS

Well-developed filaments are present on all four pairs of gill arches and they possess three vascular pathways: respiratory (arterioarterial), nutrient (arteriovenous), and interlamellar (arteriovenous), typical of teleosts. ARO, consisting of gill fans, dendritic organs on the second and fourth gill arch, and the suprabranchial epithelium are derived from gill tissue and retain structural features and arterioarterial vessels similar to gill filaments. Gill and ARO vessels are in parallel with each other, and together they are in series with the systemic circulation. Nutrients and interlamellar vessels are reduced in ARO.

CONCLUSIONS

Other than the presence of multiple ventral aortas, and an additional vessel connecting the suprabranchial epithelium to the dorsal aorta, there are no vascular shunts or anatomical modifications that indicate spatial separation of flow through the heart or between gills and ARO. However, a mechanism is proposed that would prevent unsaturation of dorsal aortic blood by local myogenic vasoconstriction of gill vessels when the fish is in hypoxic water. Despite considerable differences in the gross features of ARO in Clarias and Heteropneustes fossilis (Olson et al. 1990 J. Morphol., 203:165), there are striking similarities in vascular organization and respiratory islet structure that suggest these ARO evolved in a common silurid ancestor and were later modified into an everted arborescent organ or inverted air sac, respectively.

Stress failure of pulmonary capillaries as a limiting factor for maximal exercise

The pulmonary blood-gas barrier has a basic physiological dilemma. On the one hand it needs to be extremely thin for efficient gas exchange. On the other hand it also needs to be immensely strong because the stresses on the pulmonary capillary wall become extremely high when the capillary pressure rises on exercise. Maximal hydrostatic pressures in human pulmonary capillaries during exercise are not accurately known but must exceed 30 mmHg. In some animals, for example thoroughbred horses, the capillary pressure rises to about 100 mmHg. These pressures cause stresses in the capillary wall of 5-10 x 10(4) N.m-2 (50-100 kPa) which approach the breaking strength of collagen. The strength of the capillary wall on the thin side of the blood-gas barrier can be attributed to the type IV collagen of the extracellular matrix. Raising the capillary pressure to similar levels in experimental preparations causes ultrastructural changes in the wall including disruption of the capillary endothelium, alveolar epithelium, and basement membrane in the interstitium. Essentially all thoroughbred racehorses bleed into their lungs during exercise because they break their capillaries, and some elite human athletes apparently do the same. Avoiding stress failure of pulmonary capillaries poses a challenging problem for some species. Stress failure is a hitherto overlooked factor limiting maximal exercise.

Microcirculation of gills and accessory respiratory organs from the air-breathing snakehead fish, Channa punctata, C. gachua, and C. marulius

Snakehead fish of the genus Channa have well-developed air-breathing organs (ABO) yet retain their gill arches for respiratory and non-respiratory functions. Alterations in the macrocirculation accompany inclusion of the ABO and appear to enhance gas exchange efficiency (Munshi et al., 1994. Anat. Rec. 238:77-91). In the present study, the microcirculatory anatomy of gill and ABO from two facultative air-breathing Channa, C. punctata and C. gachua, and one obligate air-breather, C. marulius, were examined in detail using scanning electron microscopy (SEM) of vascular corrosion replicas and fixed whole-sectioned tissue. The results show that the circulation in the filaments from the first, second, and third gill arches is similar to that found in water-breathing teleosts. Fourth gill arch microcirculation of C. punctata is not different from the other three, whereas in C. marulius, it has been greatly modified into a network of low-resistance vascular shunts, although remnants of an intralamellar filamental microcirculation remain. The vascular shunts are formed from extensions of afferent and efferent lamellar arterioles and the complete, or nearly complete, loss of a lamellar sinus. The vasculature of the ABO has been highly modified in all species into a coiled-spiral capillary network with a constricted aperture guarding a dilated capillary dome at the epithelial surface. Microvilli are found congregated on the aperture endothelium of C. punctata but they are virtually absent from C. marulius endothelium. Less than 15% of the ABO capillary surface appears to face the epithelium and thereby contributes directly to gas exchange. These findings suggest that the microvascular modifications observed in Channa entail more than a simple increase in the contact surface between ABO vessels and air and they may serve other unknown physiological functions.