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

Coping with salinity extremes: Gill transcriptome profiling in the black-chinned tilapia (Sarotherodon melanotheron)

Steeper and sometimes extreme salinity gradients increasingly affect aquatic organisms because of climate change. Hypersalinity habitats demand powerful physiological adaptive strategies. Few teleost species have the capacity to spend their whole life cycle in salinities way over seawater levels. Focusing on the multifunctional gill, we unraveled the tilapia S. melanotheron key strategies to cope with different environmental conditions, ranging from freshwater up to hypersaline habitats. De novo transcriptome assembly based on RNAseq allowed for the analysis of 40,967 annotated transcripts among samples collected in three wild populations at 0, 40 and 80 ‰. A trend analysis of the expression patterns revealed responses across the salinity gradient with different gene pathways involved. Genes linked to ion transport, pH regulation and cell surface receptor signaling were mainly upregulated in the high salinity habitat. We identified tight junction proteins that were critical in high salinity habitats and that were different from the well-known tightening junctional proteins identified and expressed in fresh water. Expression profiles also suggest a change in the vascular tone that could be linked to an osmorespiratory compromise not only in fresh water, but also in high salinity environments. A striking downregulation of genes linked to the immune system and to the heat shock response was observed suggesting an energetic trade-off between immunity and acclimation/adaptation in the hypersaline habitat. The high expression of transcripts coding for immune and heat shock response in the freshwater habitat suggests the establishment of powerful mechanisms to protect gills from environmental threats and to maintain protein integrity. Non-directional expression trends were also detected with an upregulation of genes only in the hypersaline habitat (80 ‰) or only in the marine habitat (40 ‰). Unravel physiological strategies in S. melanotheron populations will help to better understand the molecular basis of fish euryhalinity in salinity-contrasted environments.

The origins of gas exchange and ion regulation in fish gills: evidence from structure and function

Gill function in gas exchange and ion regulation has played key roles in the evolution of fishes. In this review, we summarize data from the fields of palaeontology, developmental biology and comparative physiology for when and how the gills first acquired these functions. Data from across disciplines strongly supports a stem vertebrate origin for gas exchange structures and function at the gills with the emergence of larger, more active fishes. However, the recent discovery of putative ionocytes in extant cephalochordates and hemichordates suggests that ion regulation at gills might have originated much earlier than gas exchange, perhaps in the ciliated pharyngeal arches in the last common ancestor of deuterostomes. We hypothesize that the ancestral form of ion regulation served a filter-feeding function in the ciliated pharyngeal arches, and was later coopted in vertebrates to regulate extracellular ion and acid-base balance. We propose that future research should explore ionocyte homology and function across extant deuterostomes to test this hypothesis and others in order to determine the ancestral origins of ion regulation in fish gills.

Effect of hypoxia and air-breathing restricted on respiratory physiology of air-breathing loach (Paramisgurnus dabryanus)

This aim of this study was to determine the respiratory physiology response in the gill and gut of Paramisgurnus dabryanus under different breathing treatment patterns. The experimental design included the following three conditions: a control group without any stress treatments, an inhibited group with intestinal respiration inhibited, and an air-exposed group with gill respiration inhibited. The results indicated that the total static metabolic rate in the air-exposed group (188.92 ± 13.67 mg h^-1 kg^-1) was much higher than that of the other group after 7 days, decreased significantly after the first day of recovery (81.64 ± 7.85 mg h^-1 kg^-1). The air metabolic rate in the air-exposed group increased significantly after 7 days (P < 0.05). There was no significant difference among the groups. Histological observation on the gill and hindgut of P. dabryanus showed that the gill filament area of inhibited group became larger, while the gill structure of air exposed group showed some damage. The number of capillariesin the hindgut mucosal epithelial in air-exposed group showed a rapidly increase (P < 0.05). Likewise, the gas diffusion distance (1.24 ± 0.36 μm) became significantly shorter (P < 0.05). Lactate dehydrogenase activity of gill in the air-exposed group (846.68 ± 88.78 U mg^-1 protein) significantly increased after 7 days whereas succinate dehydrogenase (1.02 ± 0.21 U mg^-1 protein) and Na⁺/K⁺ ATPase (0.57 ± 0.20 U mg^-1 protein) activity decreased significantly (P < 0.05). However, there was no significant change in the hindgut. After recovery, there was no significant difference in lactate dehydrogenase, succinate dehydrogenase, and Na⁺/K⁺ ATPase activity in the gill or hindgut in groups. P. dabryanus had a high viability in air-exposed condition. When recovery occurred under normoxic conditions, the physical levels of respiration returned back to the normal level quickly.

Effects of temperature, salinity and body size on routine metabolism of coastal largemouth bass Micropterus salmoides

Routine metabolism (i.e. standard metabolism plus a low level of activity) of coastal largemouth bass Micropterus salmoides from Mobile-Tensaw Delta, AL, U.S.A. was examined as a function of temperature (15, 20, 25 and 30° C), salinity (0, 4, 8 and 12) and body mass (range 24-886 g) using flow-through respirometry. Functionally, a cubic relationship best described the effect of salinity on respiration; the magnitude of these effects increased with temperature and body mass. The best model predicted that specific respiration (mg O(2) g(-1) h(-1)) at temperatures >20° C was lowest at salinities of 0·0 and 9·7, and elevated at 3·2 and 12·0; salinity had little to no effect at temperatures ≤20° C. Respiration increased exponentially with temperature, but when compared with previously published respiration rates for M. salmoides from northern latitudes, predicted respiration was higher at cool temperatures and lower at high temperatures. The reduced energetic cost near the isosmotic level (i.e. c. 9) may be an adaptive mechanism to tolerate periods of moderate salinity levels and may help explain why M. salmoides do not flee an area in response to increased salinity. Further, these results suggest that salinity has high energetic costs for coastal populations of M. salmoides and may contribute to the observed slow growth and small maximum size within coastal systems relative to inland freshwater populations.

Nervous control of the gills

The fish gill is a highly complex organ that performs a wide variety of physiological processes and receives extensive nervous innervation from both afferent (sensory) and efferent (motor) fibres. Innervation from the latter source includes autonomic nerve fibres of spinal (sympathetic) and cranial (parasympathetic) origin whose primary role is to induce vasomotor changes within the respiratory or nonrespiratory pathways of the gill vasculature. Autonomic control of the gill occurs by nerve fibres identified as adrenergic, cholinergic, and more recent evidence indicates that nonadrenergic-noncholinergic (NANC) nerve fibres, such as those that express amines, peptides, or nitric oxide, may also play an important role. The distribution and physiological function of NANC nerve fibres, however, is less clear. This review primarily discusses histochemical studies that have characterized the nervous innervation and autonomic control of the gill vasculature. In addition, supporting evidence from recent studies for the efferent control, or modulation, of other homeostatic processes in the gill is examined.

New developments on gill innervation: insights from a model vertebrate

The fish gill is a highly specialized and complex organ that performs a variety of important physiological functions. In this article, we briefly review the innervation of important structures of the branchial region, such as the gill filaments, respiratory lamellae and pseudobranch, and discuss the physiological significance of this innervation within the context of homeostatic functions of the gill, such as oxygen sensing and ion regulation. Studies in zebrafish utilizing techniques of confocal microscopy and immunolabelling, with specific antibodies against neuronal markers, have recently led to the characterization of innervation patterns in the gills not attained with traditional techniques of histochemistry and electron microscopy. We will discuss the association of putative sensory nerve fibres with O2-chemoreceptive neuroepithelial cells and the implications of dual sensory pathways for cardiorespiratory and vascular control. In addition, the idea of the neural control of ion regulation in the gill based on the apparent innervation of mitochondria-rich cells, and the role of innervation in the pseudobranch, will be presented.

Endothelin and endothelin converting enzyme-1 in the fish gill:evolutionary and physiological perspectives

In euryhaline fishes like the killifish (Fundulus heteroclitus)that experience daily fluctuations in environmental salinity, endothelin 1(EDN1) may be an important regulator molecule necessary to maintain ion homeostasis. The purpose of this study was to determine if EDN1 and the endothelin converting enzyme (ECE1; the enzyme necessary for cleaving the precursor proendothelin-1 to EDN1) are present in the killifish, to determine if environmental salinity regulates their expression, and to examine the phylogenetic relationships among the EDNs and among the ECEs. We sequenced killifish gill cDNA for two EDN1 orthologues, EDN1A and EDN1B, and also sequenced a portion of ECE1 cDNA. EDN1A and ECE1 mRNA are expressed ubiquitously in the killifish while EDN1B mRNA has little expression in the killifish opercular epithelium or gill. Using in situ hybridization and immunohistochemistry, EDN1 was localized to large round cells adjacent to the mitochondrion-rich cells of the killifish gill, and to lamellar pillar cells. In the gill, EDN1A and EDN1B mRNA levels did not differ with acute (&lt;24 h) or chronic (30 days) acclimation to seawater (SW); however, EDN1B levels increased threefold post SW to freshwater (FW) transfer,and ECE1 mRNA levels significantly increased twofold over this period. ECE1 mRNA levels also increased sixfold over 24 h post FW to SW transfer. Chronic exposure to SW or FW had little effect on ECE1mRNA levels. Based upon our cellular localization studies, we modeled EDN1 expression in the fish gill and conclude that it is positioned to act as a paracrine regulator of gill functions in euryhaline fishes. It also may function as an autocrine on pillar cells, where it is hypothesized to regulate local blood flow in the lamellae. From our phylogenetic analyses, ECE is predicted to have an ancient origin and may be a generalist endoprotease in non-vertebrate organisms, while EDNs are vertebrate-specific peptides and may be key characters in vertebrate evolution.

Characterization of the column and autocellular junctions that define the vasculature of gill lamellae

Novel adhesion junctions have been characterized that are formed at the interface between pillar cells and collagen columns, both of which are essential constituents of the gill lamellae in fish. We termed these junctions the "column junction" and "autocellular junction" and determined their molecular compositions by immunofluorescence microscopy using pufferfish. We visualized collagen columns by concanavalin A staining and found that the components of integrin-mediated cell-matrix adhesion, such as talin, vinculin, paxillin, and fibronectin, were concentrated on plasma membranes surrounding collagen columns (column membranes). This connection is analogous to the focal adhesion of cultured mammalian cells, dense plaque of smooth muscle cells, and myotendinous junction of skeletal muscle cells. We named this connection the "column junction." In the cytoplasm near the column, actin fibers, actinin, and a phosphorylated myosin light chain of 20 kDa are densely located, suggesting the contractile nature of pillar cells. The membrane infoldings surrounding the collagen columns were found to be connected by the autocellular junction, whose components are highly tyrosine-phosphorylated and contain the tight junction protein ZO-1. This study represents the first molecular characterization and fluorescence visualization of the column and autocellular junctions involved in both maintaining structural integrity and the hemodynamics of the branchial lamellae.

Local control of pulmonary blood flow and lung structure in reptiles: Implications for ventilation perfusion matching

Lung structure of reptiles is very diverse ranging from single chambered lungs with a simple structure to more complex and multi-chambered lungs. Increased structural complexity resulted from the evolution of smaller gas exchange units and larger surface area, which increases the pulmonary diffusive capacity for O(2). However, increased structural complexity probably also increases the possibilities for ventilation-perfusion (V /Q ) heterogeneity, which exerts significant constraints on gas exchange. In most reptiles, the ventricle is anatomically and functionally undivided so blood pressures are equal in the systemic and pulmonary circulations. In these species, blood flow distribution between pulmonary and systemic circulations are primarily determined by pulmonary and systemic vascular resistances. Thus, increased pulmonary resistance lowers pulmonary blood flow through increasing cardiac right-to-left shunt decreasing systemic oxygen levels. It has been proposed that local mechanisms regulating pulmonary blood flow are more pronounced in reptiles with complex lungs as they are more prone to V /Q heterogeneity. However, local control of pulmonary blood flow has also been suggested to primarily exist when hearts are functionally divided because altered pulmonary vascular resistance does not affect cardiac shunt patterns. Data suggest that, while there seems to be a general trend of increased local regulation of pulmonary blood flow in species with structurally complex lungs and divided hearts, it is also possible that other factors, such as breathing pattern, have been important for the evolutionary development of local regulatory mechanisms in the lungs.

Fluorescence Visualization of Branchial Collagen Columns Embraced by Pillar Cells

A collagen column is a structure of the extracellular matrix that helps to maintain the flatness and width of gill lamella. Collagen columns are unique in that they are enfolded by plasma membrane of pillar cells that form two-dimensional vascular networks between parallel sheets of respiratory epithelia. Despite their unique structure and fundamental importance in the physiology of aquatic animals, little is known about their properties and molecular components, owing to the lack of detection methods. In this study we demonstrated that collagen columns can be visualized by staining with fluorescencelabeled concanavalin A (ConA), a lectin that specifically recognizes the trimannoside core of N-glycosylated proteins and histidine-tagged green fluorescent protein (His6-Xpress-GFP), a fluorescent substrate for transglutaminase. We constructed a three-dimensional image of a pillar cell and visualized the spatial relationship between collagen columns and contractile apparatuses within the pillar cell body. This manuscript contains online supplemental material at http://www.jhc.org . Please visit this article online to view these materials.

Endothelin-1 causes systemic vasodilatation in anaesthetised turtles(Trachemys scripta) through activation of ETB-receptors

The effects of endothelin-1 (ET-1) on systemic and pulmonary circulation were investigated in anaesthetised freshwater turtles (Trachemys scripta) instrumented with arterial catheters and blood flow probes. Bolus intra-arterial injections of ET-1 (0.4–400 pmol kg-1)caused a dose-dependent systemic vasodilatation that was associated with a decrease in systemic pressure (Psys) and a rise in systemic blood flow (Q̇sys),causing systemic conductance (Gsys) to increase. ET-1 had no significant effects on the pulmonary vasculature, heart rate(fh) or total stroke volume(Vstot). This response differs markedly from mammals, where ET-1 causes an initial vasodilatation that is followed by a pronounced pressor response. In mammals, the initial dilatation is caused by stimulation of ETB-receptors, while the subsequent constriction is mediated by ETA-receptors. In the turtles, infusion of the ETB-receptor agonist BQ-3020 (150 pmol kg-1) elicited haemodynamic changes that were similar to those of ET-1, and the effects of ET-1 were not affected by the ETA-antagonist BQ-610 (0.15 μmol kg-1). Conversely, all effects of ET-1 were virtually abolished after specific ETB-receptor blockade with the ETB-antagonist BQ-788 (0.15 μmol kg-1). The subsequent treatment with the general ET-receptor antagonist tezosentan (15.4μmol kg-1) did not produce effects that differed from the treatment with ETB-antagonist, and the blockade of ET-1 responses persisted. This present study indicates, therefore, that ETB-receptors are responsible for the majority of the cardiovascular responses to ET-1 in Trachemys.

Hypoxia induces adaptive and reversible gross morphological changes in crucian carp gills

We show that crucian carp (Carassius carassius) living in normoxic (aerated) water have gills that lack protruding lamellae, the primary site of O(2) uptake in fish. Such an unusual trait leads to a very small respiratory surface area. Histological examination showed that the lamellae (secondary lamellae) of these fish were embedded in a cell mass (denoted embedded lamellae). When the fish were kept in hypoxic water, a large reduction in this cell mass occurred, making the lamellae protrude and increasing the respiratory surface area by approximately 7.5-fold. This morphological change was found to be reversible and was caused by increased apoptosis combined with reduced cell proliferation. Carp with protruding lamellae had a higher capacity for oxygen uptake at low oxygen levels than fish with embedded lamellae, but water and ion fluxes appeared to be increased, which indicates increased osmoregulatory costs. This is, to our knowledge, the first demonstration of an adaptive and reversible gross morphological change in the respiratory organ of an adult vertebrate in response to changes in the availability of oxygen.

Neuroepithelial cells and associated innervation of the zebrafish gill: a confocal immunofluorescence study

Peripheral chemoreceptors responsive to hypoxia have been well characterized in air-breathing vertebrates, but poorly in water-breathers. The present study examined the distribution of five populations of neuroepithelial cells (NECs), putative O(2) chemoreceptors, and innervation patterns in the zebrafish gill using whole-mounts and confocal immunofluorescence. Nerve bundles and fibers of the gill were labeled with zn-12 (a zebrafish-specific neuronal marker) and SV2 antisera and NECs were characterized by serotonin (5-HT) immunoreactivity (IR), SV2-IR and the purinoceptor P2X(3)-IR. A zn-12-IR nerve bundle extended the length of the gill filament and gave rise to a nerve plexus surrounding the efferent filament artery (eFA) and a rich network of fibers that innervated both serotonergic and nonserotonergic NECs of the filament and lamellar epithelium. Three populations of serotonergic, SV2-IR neurons intrinsic to the gill filaments are described, one of which provided innervation to NECs of the filament epithelium. Degeneration of nerve fibers in gill arches maintained in explant culture for 2 days revealed the extrinsic origin of nerve fibers of the plexus and lamellae and the innervation of filament NECs by both intrinsic and extrinsic fibers. Intrinsic innervation surrounding the eFA survived in explant cultures, suggesting a mechanism of local vascular control within the gill. In addition, NECs survived in explants after degeneration of extrinsic nerve fibers. Thus, NECs of the zebrafish gill are organized in a manner reminiscent of O(2) chemoreceptors of mammalian vertebrates, suggesting a role in respiratory regulation.

Gill circulation: regulation of perfusion distribution and metabolism of regulatory molecules

The fish gill is the primary regulatory interface between internal and external milieu and a variety of neurocrine, endocrine, paracrine, and autocrine signals coordinate and control gill functions. Many of these messengers also affect gill vascular resistance, and they, in turn, may be inactivated (or activated) by branchial vessels. Few studies have critically addressed how flow is distributed within the gill filament, the physiological consequences thereof, or the impact of gill hormone metabolism on gill and systemic homeostasis. In most fish, the entire cardiac output perfuses the arterioarterial pathway, and this network probably accounts for the majority of passive- and stimulus-induced changes in vascular resistance. The in-series arrangement of the extensive gill microcirculation with systemic vessels is also indicative of a high capacity for metabolism of plasma-borne messengers as well as xenobiotics. Adenosine, arginine vasotocin (AVT), and endothelin (ET) are the most potent gill constrictors identified to date, and all decrease lamellar perfusion. Perhaps not surprising, they are also inactivated by gill vessels. Acetylcholine favors perfusion of the alamellar filamental vasculature, although the physiological relevance of acetylcholine-mediated responses remains unclear. Angiotensin, bradykinin, urotensin, natriuretic peptides, prostaglandins, and nitric oxide are vasoactive to varying degrees, but their effects on intrafilamental blood flow are unknown. If form befits function, then the complex vascular anatomy of the gill suggests a level of regulatory sophistication unparalleled in other vertebrate organs. Resolution of these issues will be technically challenging but unquestionably rewarding.

Endothelin induced cerebral vasoconstriction in rainbow trout, detected in a novel in vitro preparation

We have here developed an in vitro method for examining cerebral vasoconstriction/vasodilation in fish brain tissue, relying on microscopic observation of the diameter of an artery (denoted tectal artery) on the ventral side of the isolated optic tectum of rainbow trout (Oncorhynchus mykiss). Using this technique, we show that endothelin-1 (ET-1) is a powerful vasoconstrictor in rainbow trout brain. When surperfused over the optic tectum, 1.0-2.5 nM of ET-1 caused 23 and 46% reductions, respectively, in the tectal artery diameter. This vasoconstriction could be completely blocked by the endothelin receptor antagonist bosentan. ET-1 is the first substance shown to constrict cerebral arteries in fish.

The CO2/pH ventilatory drive in fish

That ventilation in fish is driven by O2 has long been accepted. The O2 ventilatory drive reflects the much lower capacitance of water for O2 than for CO2, and is mediated by O2 receptors that are distributed throughout the gill arches and that monitor both internal and external O2 levels. In recent years, however, evidence has amassed in support of the existence of a ventilatory drive in fish that is keyed to CO2 and/or pH. While ventilatory responses to CO2/pH may be mediated in part by the O2 drive through CO2/pH-induced changes in blood O2 status, CO2/pH also appear to stimulate ventilation directly. The receptors involved in this pathway are as yet unknown, but the experimental evidence available to date supports the involvement of branchial CO2-sensitive chemoreceptors with an external orientation. Internally-oriented CO2-sensitive chemoreceptors may also be involved, although evidence on this point remains equivocal. In the present paper, the evidence for a CO2/pH-keyed ventilatory drive in fish will be reviewed.

Evidence of a guanylyl cyclase natriuretic peptide receptor in the gills of the new zealand hagfish Eptatretus cirrhatus (Class Agnatha)

Natriuretic peptide binding sites were examined in the gills of the hagfish Eptatretus cirrhatus (Class Agnatha, subfamily Eptatretinae) using radio-ligand binding techniques, molecular cloning and guanylyl cyclase assays. Iodinated rat atrial natriuretic peptide ((125)I-rANP) and iodinated porcine C-type natriuretic peptide ((125)I-pCNP) bound specifically to the lamellar folds and cavernous tissue of E. cirrhatus gills, and 0.3 nmol l(−1) rat ANP competed for 50 % of specific (125)I-rANP binding sites. Affinity cross-linking of (125)I-rANP to gill membranes followed by sodium dodecylsulphate-polyacrylamide gel electrophoresis revealed a single binding site of 150 kDa. In the presence of Mn(2+), 0.1 nmol l(−1) rANP inhibited cGMP production, whereas 1 micromol l(−1) rANP stimulated cGMP production rates. At 1 micromol l(−1), pCNP also stimulated cGMP production. The production of cGMP was also measured in the presence and absence of ATP with either Mn(2+) or Mg(2+). Reverse transcriptase polymerase chain reaction (RT-PCR) of hagfish gill RNA, followed by cloning and sequencing of PCR products, produced a partial cDNA sequence of a natriuretic peptide guanylyl cyclase receptor. The deduced amino acid sequence indicated 87–91 % homology with other natriuretic peptide guanylyl cyclase receptors. This study indicates the presence of a natriuretic peptide guanylyl cyclase receptor in the gills of E. cirrhatus that is similar to the natriuretic peptide guanylyl cyclase receptors in higher vertebrates. These observations demonstrate that the coupling of natriuretic peptide receptors with guanylyl cyclase has a long evolutionary history.

Digital motion analysis as a tool for analysing the shape and performance of the circulatory system in transparent animals

The analysis of perfusion parameters using the frame-to-frame technique and the observation of small blood vessels in transparent animals using video microscopy can be tedious and very difficult because of the poor contrast of the images. Injection of a fluorescent probe (fluorescein isothiocynate, FITC) bound to a high-molecular-mass dextran improved the visibility of blood vessels, but the gray-scale histogram showed blurring at the edges of the vessels. Furthermore, injection of the fluorescent probe into the ventricle of small zebrafish (Danio rerio) embryos (body mass approximately 1 mg) often resulted in reduced cardiac activity. Digital motion analysis, however, proved to be a very effective tool for analysing the shape and performance of the circulatory system in transparent animals and tissues. By subtracting the two fields of a video frame (the odd and the even frame), any movement that occurred within the 20 ms necessary for the acquisition of one field could be visualised. The length of the shifting vector generated by this subtraction, represented a direct measure of the velocity of a moving particle, i.e. an erythrocyte in the vascular system. By accumulating shifting vectors generated from several consecutive video frames, a complete trace of the routes over which erythrocytes moved could be obtained. Thus, a cast of the vascular system, except for those tiny vessels that are not entered by erythrocytes, could be obtained. Because the gray-scale value of any given pixel or any given group of pixels increased with the number of erythrocytes passing it, digital motion analysis could also be used to visualise the distribution of blood cells in transparent tissues. This method was used to describe the development of the peripheral vascular system in zebrafish larvae up to 8 days post-fertilisation. At this stage, food intake resulted in a clear redistribution of blood between muscle tissue and the gut, and alpha-adrenergic control of peripheral blood flow was established.

Characterization of an endothelin ET(B) receptor in the gill of the dogfish shark Squalus acanthias

Endothelins (ETs) are potent vasoconstrictive peptides that are secreted by the vascular endothelium and other tissues in vertebrates. Previous studies have demonstrated that ETs are expressed in a variety of fish tissues and contract various blood vessels. In order to determine if receptors for ET are expressed in fish gill tissue, we examined the binding kinetics of (125)I-labeled, human ET-1 to membrane fragments isolated from the gill of the dogfish shark, Squalus acanthias. (125)I-ET-1 bound at a single site, with a dissociation constant (K(d)) and binding site number (B(max)) very similar to those described in a variety of mammalian blood vessels. ET-1 and ET-3 competed equally with (125)I-ET-1, suggesting that the receptor was ET(B), which has been shown in mammalian systems to bind to both ligands equally. The ET(B)-specific agonists sarafotoxin S6c, IRL-1620, and BQ-3020 also competed against (125)I-ET-1 at a single site, supporting this hypothesis. We conclude that the shark gill expresses an ET(B) receptor with substantial homology to the mammalian receptor and that ET may play an important role in modulating such vital gill functions as gas exchange, ion regulation, acid-base balance, and excretion of nitrogen.