Angiotensin signaling and receptor types in teleost fish

The role of the renin-angiotensin system (RAS) in salinity adaptation in Pacific white shrimp (Litopenaeus vannamei)

The renin-angiotensin system (RAS) is a hormonal system that plays an important role in the regulation of blood pressure and cardiovascular homeostasis in mammals. In fishes, the RAS pathway participates in osmoregulation and salinity adaptation. However, the role of the RAS pathway in invertebrates, particularly in crustaceans, remains unknown. In this study, four key genes of the RAS pathway (LV-ACE, LV-APN, LV-AT1R, and LV-RR) were cloned, characterized, and their expression levels were detected in the eyestalk, hepatopancreas, and muscle of Litopenaeus vannamei during long-term and short-term low salinity stress. The results showed that LV-ACE, LV-APN, LV-AT1R, and LV-RR encode 666, 936, 175, and 323 amino acids, respectively. Low salinity stress downregulated the expression levels of LV-ACE, LV-APN, LV-AT1R, and LV-RR in L. vannamei, indicating that the RAS pathway was suppressed under low salinity. Moreover, these genes play important roles in the regulation of drinking rate, controlling urine output, blood glucose, and blood pressure, indicating that their downregulation probably affected the homeostasis of shrimps. These findings provide novel insights into the mechanism of salinity adaptation in L. vannamei.

Transcriptomic profiles of Florida pompano (Trachinotus carolinus) gill following infection by the ectoparasite Amyloodiniumocellatum

The dinoflagellate Amyloodinium ocellatum is an important pathogenic parasite infecting cultured marine and brackish water fishes worldwide. This includes cultured Florida pompano (Trachinotus carolinus), which is one of the most desirable marine food fish with high economic value in the USA. A. ocellatum infects fish gills and causes tissue damage, increased respiratory rate, reduced appetite, and mortality, especially in closed aquaculture systems. This study mimicked the natural infection of A. ocellatum in cultured pompano and conducted a transcriptomic comparison of gene expression in the gills of control and A. ocellatum infected fish to explore the molecular mechanisms of infection. RNA-seq data revealed 604 differentially expressed genes in the infected fish gills. The immunoglobulin genes (including IgM/T) augmentation and IL1 inflammation suppression were detected after infection. Genes involved in reactive oxygen species mediating parasite killing were also highly induced. However, excessive oxidants have been linked to oxidative tissue damage and apoptosis. Correspondingly, widespread down-regulation of collagen genes and growth factor deprivation indicated impaired tissue repair, and meanwhile the key executor of apoptosis, caspase-3 was highly expressed (25.02-fold) in infected fish. The infection also influenced the respiratory gas sensing and transport genes and established hypoxic conditions in the gill tissue. Additionally, food intake and lipid metabolism were also affected. Our work provides the transcriptome sequencing of Florida pompano and provides key insights into the acute pathogenesis of A. ocellatum. This information can be utilized for designing optimal disease surveillance strategies, future selection for host resistance, and development of novel therapeutic measures.

An ACE2-Alamandine Axis Modulates the Cardiac Performance of the Goldfish Carassius auratus via the NOS/NO System

Alamandine is a peptide of the Renin Angiotensin System (RAS), either generated from Angiotensin A via the Angiotensin Converting Enzyme 2 (ACE2), or directly from Ang-(1-7). In mammals, it elicits cardioprotection via Mas-related G-protein-coupled receptor D (MrgD), and the NOS/NO system. In teleost fish, RAS is known to modulate heart performance. However, no information is available on the presence of a cardioactive ACE2/Alamandine axis. To fill this gap, we used the cyprinid teleost Carassius auratus (goldfish) for in silico and in vitro analyses. Via the NCBI Blast P suite we found that in cyprinids ace2 is phylogenetically detectable in a subcluster of proteins including ace2-like isoforms, and is correlated with a hypoxia-dependent pathway. By real-time PCR, Western Blotting, and HPLC, ACE2 and Alamandine were identified in goldfish heart and plasma, respectively. Both increased after chronic exposure to low O₂ (2.6 mg O₂ L^-1). By using an ex-vivo working goldfish-heart preparation, we observed that in vitro administration of exogenous Alamandine dose-dependently stimulates myocardial contractility starting from 10^-11 M. The effect that involved Mas-related receptors and PKA occurred via the NOS/NO system. This was shown by exposing the perfused heart to the NOS inhibitor L-NMMA (10^-5 M) that abolished the cardiac effect of Alamandine and was supported by the increased expression of the phosphorylated NOS enzyme in the extract from goldfish heart exposed to 10^-10 M Alamandine. Our data are the first to show that an ACE2/Alamandine axis is present in the goldfish C. auratus and, to elicit cardiac modulation, requires the obligatory involvement of the NOS/NO system.

Zone of Interaction Between the Parasite and the Host: Protein Profile of the Body Cavity Fluid of Gasterosteus aculeatus L. Infected with the Cestode Schistocephalus solidus (Muller, 1776)

PURPOSE

During infection, the host and the parasite "communicate" with each other through various molecules, including proteins. The aim of this study was to describe the excretory-secretory proteins from the helminth Schistocephalus solidus and its intermediate host, the three-spined stickleback Gasterosteus aculeatus L., which are likely to be involved in interactions between them.

METHODS

Combined samples of washes from the G. aculeatus sticklebacks cavity infected with the S. solidus, and washes from the parasite surface were used as experimental samples, while washes from the uninfected fish body cavity were used as control. The obtained samples were analyzed using mass-spectrometry nLC-MS/MS.

RESULTS

As a result of mass-spectrometry analysis 215 proteins were identified. Comparative quantitative analysis revealed significant differences in LFQ intensity between experimental and control samples for 20 stickleback proteins. In the experimental samples, we found an increase in the content of serpins, plasminogen, angiotensin 1-10, complement component C9, and a decrease in the content of triosephosphate isomerase, creatine kinase, fructose-biphosphate aldolase, superoxide dismutase, peroxidoxin-1, homocysteine-binding and fatty acid-binding proteins, compared to uninfected fish samples. In the experimental group washes, 30 S. solidus proteins were found, including malate dehydrogenase, annexin family proteins, serpins, peptidyl-prolyl cis-trans isomerase and fatty acid-binding protein.

CONCLUSIONS

Thus, the protein composition of washes from the helminth S. solidus surface and the body cavity of infected and uninfected stickleback G. aculeatus were studied. As a result, it was shown that various components of the immune defense system predominated in the washes of infected fish and helminths.

The SARS-CoV-2 receptor and other key components of the Renin-Angiotensin-Aldosterone System related to COVID-19 are expressed in enterocytes in larval zebrafish

People with underlying conditions, including hypertension, obesity, and diabetes, are especially susceptible to negative outcomes after infection with coronavirus SARS-CoV-2, which causes COVID-19. Hypertension and respiratory inflammation are exacerbated by the Renin-Angiotensin-Aldosterone System (RAAS), which normally protects from rapidly dropping blood pressure via Angiotensin II (Ang II) produced by the enzyme Ace. The Ace paralog Ace2 degrades Ang II, counteracting its chronic effects, and serves as the SARS-CoV-2 receptor. Ace, the coronavirus, and COVID-19 comorbidities all regulate Ace2, but we do not yet understand how. To exploit zebrafish (Danio rerio) to help understand the relationship of the RAAS to COVID-19, we must identify zebrafish orthologs and co-orthologs of human RAAS genes and understand their expression patterns. To achieve these goals, we conducted genomic and phylogenetic analyses and investigated single cell transcriptomes. Results showed that most human RAAS genes have one or more zebrafish orthologs or co-orthologs. Results identified a specific type of enterocyte as the specific site of expression of zebrafish orthologs of key RAAS components, including Ace, Ace2, Slc6a19 (SARS-CoV-2 co-receptor), and the Angiotensin-related peptide cleaving enzymes Anpep (receptor for the common cold coronavirus HCoV-229E), and Dpp4 (receptor for the Middle East Respiratory Syndrome virus, MERS-CoV). Results identified specific vascular cell subtypes expressing Ang II receptors, apelin, and apelin receptor genes. These results identify genes and cell types to exploit zebrafish as a disease model for understanding mechanisms of COVID-19.

Morpho-functional remodelling of the adult zebrafish (Danio rerio) heart in response to waterborne angiotensin II exposure

Angiotensin II (AngII), the principal effector of the Renin-Angiotensin System, is a pluripotent humoral agent whose biological actions include short-term modulations and long-term adaptations. In fish, short-term cardio-tropic effects of AngII are documented, but information on the role of AngII in long-term cardiac remodelling is not fully understood. Here, we describe a direct approach to disclose long-term morpho-functional effects of AngII on the zebrafish heart. Adult fish exposed to waterborne teleost analogue AngII for 8 weeks showed enhanced heart weight and cardio-somatic index, coupled to myocardial structural changes (i.e. augmented compacta thickness and fibrosis), and increased heart rate. These findings were paralleled by an up-regulation of type-1 and type-2 AngII receptors expression, and by changes in the expression of GATA binding protein 4, nuclear factor of kappa light polypeptide gene enhancer in B-cells 1 and superoxide dismutase 1 soluble mRNAs, as well as of cytochrome b-245 beta polypeptide protein, indicative of cardiac remodelling. Our results suggest that waterborne AngII can sustain and robustly affect the cardiac morpho-functional remodelling of adult zebrafish.

An intestinal cell type in zebrafish is the nexus for the SARS-CoV-2 receptor and the Renin-Angiotensin-Aldosterone System that contributes to COVID-19 comorbidities

People with underlying conditions, including hypertension, obesity, and diabetes, are especially susceptible to negative outcomes after infection with the coronavirus SARS-CoV-2. These COVID-19 comorbidities are exacerbated by the Renin-Angiotensin-Aldosterone System (RAAS), which normally protects from rapidly dropping blood pressure or dehydration via the peptide Angiotensin II (Ang II) produced by the enzyme Ace. The Ace paralog Ace2 degrades Ang II, thus counteracting its chronic effects. Ace2 is also the SARS-CoV-2 receptor. Ace , the coronavirus, and COVID-19 comorbidities all regulate Ace2 , but we don't yet understand how. To exploit zebrafish ( Danio rerio ) as a disease model to understand mechanisms regulating the RAAS and its relationship to COVID-19 comorbidities, we must first identify zebrafish orthologs and co-orthologs of human RAAS genes, and second, understand where and when these genes are expressed in specific cells in zebrafish development. To achieve these goals, we conducted genomic analyses and investigated single cell transcriptomes. Results showed that most human RAAS genes have an ortholog in zebrafish and some have two or more co-orthologs. Results further identified a specific intestinal cell type in zebrafish larvae as the site of expression for key RAAS components, including Ace, Ace2, the coronavirus co-receptor Slc6a19, and the Angiotensin-related peptide cleaving enzymes Anpep and Enpep. Results also identified specific vascular cell subtypes as expressing Ang II receptors, apelin , and apelin receptor genes. These results identify specific genes and cell types to exploit zebrafish as a disease model for understanding the mechanisms leading to COVID-19 comorbidities.

SUMMARY STATEMENT

Genomic analyses identify zebrafish orthologs of the Renin-Angiotensin-Aldosterone System that contribute to COVID-19 comorbidities and single-cell transcriptomics show that they act in a specialized intestinal cell type.

Exploring cardiac plasticity in teleost: the role of humoral modulation

The fish heart represents an established natural model for evaluating basic mechanisms of the coordinated physiological reactions which maintain cardiac steady-state. This is due to its relatively simple design, but also to its multilevel morpho-functional flexibility which allows adequate responses to a variety of intrinsic (body size and shape, swimming performance, etc.), and extrinsic (temperature, salinity, oxygen level, water chemistry, etc.) factors related to the animal life style. Nowadays, although many gaps are still present, a huge literature is available about the mechanisms that fine-tune fish cardiac performance, particularly in relation to the influence exerted by substances possessing cardio-modulatory properties. Based on these premises, this review will provide an overview of the existing current knowledge regarding the humoral control of cardiac performance in fish. The role of both classic (i.e. catecholamines, angiotensin II and natriuretic peptides), and emerging cardioactive substances (i.e. the chromogranin-A-derived peptides vasostatins, catestatin and serpinin) will be illustrated and discussed. Moreover, an example of cardiomodulation elicited by peptides (e.g., nesfatin-1) associated to the regulation of feeding and metabolism will be provided. The picture will hopefully emphasize the complex circuits that sustain fish cardiac performance, also highliting the power of the teleost heart as an experimental model to deciphering mechanisms that could be difficult to explore in more elaborated cardiac morpho-functional designs.

Testing the Translational Power of the Zebrafish: An Interspecies Analysis of Responses to Cardiovascular Drugs

The zebrafish is rapidly emerging as a promising alternative in vivo model for the detection of drug-induced cardiovascular effects. Despite its increasing popularity, the ability of this model to inform the drug development process is often limited by the uncertainties around the quantitative relevance of zebrafish responses compared with nonclinical mammalian species and ultimately humans. In this test of concept study, we provide a comparative quantitative analysis of the in vivo cardiovascular responses of zebrafish, rat, dog, and human to three model compounds (propranolol, losartan, and captopril), which act as modulators of two key systems (beta-adrenergic and renin–angiotensin systems) involved in the regulation of cardiovascular functions. We used in vivo imaging techniques to generate novel experimental data of drug-mediated cardiovascular effects in zebrafish larvae. These data were combined with a database of interspecies mammalian responses (i.e., heart rate, blood flow, vessel diameter, and stroke volume) extracted from the literature to perform a meta-analysis of effect size and direction across multiple species. In spite of the high heterogeneity of study design parameters, our analysis highlighted that zebrafish and human responses were largely comparable in >80% of drug/endpoint combinations. However, it also revealed a high intraspecies variability, which, in some cases, prevented a conclusive interpretation of the drug-induced effect. Despite the shortcomings of our study, the meta-analysis approach, combined with a suitable data visualization strategy, enabled us to observe patterns of response that would likely remain undetected with more traditional methods of qualitative comparative analysis. We propose that expanding this approach to larger datasets encompassing multiple drugs and modes of action would enable a rigorous and systematic assessment of the applicability domain of the zebrafish from both a mechanistic and phenotypic standpoint. This will increase the confidence in its application for the early detection of adverse drug reactions in any major organ system.

The Amphibious Mudskipper: A Unique Model Bridging the Gap of Central Actions of Osmoregulatory Hormones Between Terrestrial and Aquatic Vertebrates

Body fluid regulation, or osmoregulation, continues to be a major topic in comparative physiology, and teleost fishes have been the subject of intensive research. Great progress has been made in understanding the osmoregulatory mechanisms including drinking behavior in teleosts and mammals. Mudskipper gobies can bridge the gap from aquatic to terrestrial habitats by their amphibious behavior, but the studies are yet emerging. In this review, we introduce this unique teleost as a model to study osmoregulatory behaviors, particularly amphibious behaviors regulated by the central action of hormones. Regarding drinking behavior of mammals, a thirst sensation is aroused by angiotensin II (Ang II) through direct actions on the forebrain circumventricular structures, which predominantly motivates them to search for water and take it into the mouth for drinking. By contrast, aquatic teleosts can drink water that is constantly present in their mouth only by reflex swallowing, and Ang II induces swallowing by acting on the hindbrain circumventricular organ without inducing thirst. In mudskippers, however, through the loss of buccal water by swallowing, which appears to induce buccal drying on land, Ang II motivates these fishes to move to water for drinking. Thus, mudskippers revealed a unique thirst regulation by sensory detection in the buccal cavity. In addition, the neurohypophysial hormones, isotocin (IT) and vasotocin (VT), promote migration to water via IT receptors in mudskippers. VT is also dipsogenic and the neurons in the forebrain may mediate their thirst. VT regulates social behaviors as well as osmoregulation. The VT-induced migration appears to be a submissive response of subordinate mudskippers to escape from competitive and dehydrating land. Together with implications of VT in aggression, mudskippers may bridge the multiple functions of neurohypophysial hormones. Interestingly, cortisol, an important hormone for seawater adaptation and stress response in teleosts, also stimulates the migration toward water, mediated possibly via the mineralocorticoid receptor. The corticosteroid system that is responsive to external stressors can accelerate emergence of migration to alternative habitats. In this review, we suggest this unique teleost as an important model to deepen insights into the behavioral roles of these hormones in relation to osmoregulation.

Renin-angiotensin system in vertebrates: phylogenetic view of structure and function

Renin substrate, biological renin activity, and/or renin-secreting cells in kidneys evolved at an early stage of vertebrate phylogeny. Angiotensin (Ang) I and II molecules have been identified biochemically in representative species of all vertebrate classes, although variation occurs in amino acids at positions 1, 5, and 9 of Ang I. Variations have also evolved in amino acid positions 3 and 4 in some cartilaginous fish. Angiotensin receptors, AT₁ and AT₂ homologues, have been identified molecularly or characterized pharmacologically in nonmammalian vertebrates. Also, various forms of angiotensins that bypass the traditional renin-angiotensin system (RAS) cascades or those from large peptide substrates, particularly in tissues, are present. Nonetheless, the phylogenetically important functions of RAS are to maintain blood pressure/blood volume homeostasis and ion-fluid balance via the kidney and central mechanisms. Stimulation of cell growth and vascularization, possibly via paracrine action of angiotensins, and the molecular biology of RAS and its receptors have been intensive research foci. This review provides an overview of: (1) the phylogenetic appearance, structure, and biochemistry of the RAS cascade; (2) the properties of angiotensin receptors from comparative viewpoints; and (3) the functions and regulation of the RAS in nonmammalian vertebrates. Discussions focus on the most fundamental functions of the RAS that have been conserved throughout phylogenetic advancement, as well as on their physiological implications and significance. Examining the biological history of RAS will help us analyze the complex RAS systems of mammals. Furthermore, suitable models for answering specific questions are often found in more primitive animals.

Vasotrophic regulation of age-dependent hypoxic cerebrovascular remodeling

Hypoxia can induce functional and structural vascular remodeling by changing the expression of trophic factors to promote homeostasis. While most experimental approaches have been focused on functional remodeling, structural remodeling can reflect changes in the abundance and organization of vascular proteins that determine functional remodeling. Better understanding of age-dependent hypoxic macrovascular remodeling processes of the cerebral vasculature and its clinical implications require knowledge of the vasotrophic factors that influence arterial structure and function. Hypoxia can affect the expression of transcription factors, classical receptor tyrosine kinase factors, non-classical G-protein coupled factors, catecholamines, and purines. Hypoxia's remodeling effects can be mediated by Hypoxia Inducible Factor (HIF) upregulation in most vascular beds, but alterations in the expression of growth factors can also be independent of HIF. PPARγ is another transcription factor involved in hypoxic remodeling. Expression of classical receptor tyrosine kinase ligands, including vascular endothelial growth factor, platelet derived growth factor, fibroblast growth factor and angiopoietins, can be altered by hypoxia which can act simultaneously to affect remodeling. Tyrosine kinase-independent factors, such as transforming growth factor, nitric oxide, endothelin, angiotensin II, catecholamines, and purines also participate in the remodeling process. This adaptation to hypoxic stress can fundamentally change with age, resulting in different responses between fetuses and adults. Overall, these mechanisms integrate to assure that blood flow and metabolic demand are closely matched in all vascular beds and emphasize the view that the vascular wall is a highly dynamic and heterogeneous tissue with multiple cell types undergoing regular phenotypic transformation.

Angiotensin-II promotes Na+ uptake in larval zebrafish, Danio rerio, in acidic and ion-poor water

The contribution of the renin-angiotensin system (RAS) to Na(+) uptake was investigated in larval zebrafish (Danio rerio). At 4 days post fertilization (dpf), the level of whole-body angiotensin-II (ANG-II) was significantly increased after 1- or 3-h exposure to acidic (pH=4.0) or ion-poor water (20-fold dilution of Ottawa tapwater), suggesting rapid activation of the RAS. Long-term (24 h) treatment of 3 dpf larvae with ANG-I or ANG-II significantly increased Na(+) uptake which was accompanied by an increase in mRNA expression of the Na(+)-Cl(-) cotransporter (zslc12a10.2). Induction of Na(+) uptake by exposure to ANG-I was blocked by simultaneously treating larvae with lisinopril (an angiotensin-converting enzyme inhibitor). Acute (2 h) exposure to acidic water or ion-poor water led to significant increase in Na(+) uptake which was partially blocked by the ANG-II receptor antagonist, telmisartan. Consistent with these data, translational knockdown of renin prevented the stimulation of Na(+) uptake following exposure to acidic or ion-poor water. The lack of any effects of pharmacological inhibition (using RU486), or knockdown of glucocorticoid receptors on the stimulation of Na(+) uptake during acute exposure to acidic or ion-poor environments, indicates that the acute effects of RAS occur independently of cortisol signaling. The results of this study demonstrate that the RAS is involved in Na(+) homeostasis in larval zebrafish.

Humoral control of cardiac remodeling in fish: role of Angiotensin II

Angiotensin II (AngII), the principal effector of the Renin-Angiotensin-System (RAS), is a multipotent hormone whose biological actions include short-term modulation as well as long-term adjustments. In the eel heart, AngII elicits short-term inotropic and chronotropic effects. However, information regarding the influence of AngII on cardiac remodeling, expressed as morphological and hemodynamic changes, is lacking. To clarify the putative actions of AngII on eel cardiac remodeling, we used freshwater eels (Anguilla anguilla) intraperitoneally injected for 4 weeks with saline or AngII (0.4 or 1.2 nmol g BW(-1)) or AngII (1.2 nmol g BW(-1)) plus the AT₂ receptor antagonist CGP42112. Using an in vitro working heart preparation, the cardiac response (stroke volume changes) to preload and afterload increases has been evaluated. Hearts of all groups showed similar Frank-Starling responses. However, in response to afterload increases, stroke volume rapidly decreased in control hearts, while it was better maintained in AngII-treated counterparts. These effects were abolished by an antagonist of the AT₂ receptor, whose cardiac expression was revealed by western blotting analysis. We also found by immunolocalization and immunoblotting that AngII influences both expression and localization of molecules which regulate cell growth [such as c-kit, heat shock protein 90 (Hsp-90), endothelial Nitric Oxide Synthase "(eNOS)-like" isoform] and apoptosis [i.e. apoptosis repressor with CARD domain (ARC)], thus playing a role in cardiac long-term adjustments. These results point to a role of AngII in eel heart remodeling, providing new insights regarding the modulation of cardiac plasticity in fish.

The eel heart: multilevel insights into functional organ plasticity

The remarkable functional homogeneity of the heart as an organ requires a well-coordinated myocardial heterogeneity. An example is represented by the selective sensitivity of the different cardiac cells to physical (i.e. shear stress and/or stretch) or chemical stimuli (e.g. catecholamines, angiotensin II, natriuretic peptides, etc.), and the cell-specific synthesis and release of these substances. The biological significance of the cardiac heterogeneity has recently received great attention in attempts to dissect the complexity of the mechanisms that control the cardiac form and function. A useful approach in this regard is to identify natural models of cardiac plasticity. Among fishes, eels (genus Anguilla), for their adaptive and acclimatory abilities, represent a group of animals so far largely used to explore the structural and ultrastructural myoarchitecture organization, as well as the complex molecular networks involved in the modulation of the heart function, such as those converting environmental signals into physiological responses. However, an overview on the existing current knowledge of eel cardiac form and function is not yet available. In this context, this review will illustrate major features of eel cardiac organization and pumping performance. Aspects of autocrine–paracrine modulation and the influence of factors such as body growth, exercise, hypoxia and temperature will highlight the power of the eel heart as an experimental model useful to decipher how the cardiac morpho-functional heterogeneities may support the uniformity of the whole-organ mechanics.

Angiotensin AT2 receptor activates the cyclic-AMP signaling pathway in eel

A unique angiotensin type 2 receptor (AT2) that induces a cAMP signaling pathway was cloned and characterized for the first time in fish, Anguilla japonica. Phylogeny and synteny results showed that the AT2s among fishes and tetrapods share the same origin despite a sub-cluster formation among eel, salmon, and zebrafish. The eel AT2 was expressed abundantly in the spleen and localized at straight arterioles and ellipsoid regions prior to the sinusoid, suggesting a role in the regulation of microcirculation and/or immune response. Various angiotensin (Ang) peptides, including Ang II, Ang III, and Ang IV, were detected in the spleen by a radioimmunoassay coupled with HPLC separation, and these endogenous peptides stimulated a cAMP signaling, which has no crosstalk with cGMP pathway. The common and contrasting features of AT2 between fishes and mammals imply some ancestral characters of AT2, which are important information for receptor binding and evolutionary studies.

Changes in plasma angiotensin subtypes in Japanese eel acclimated to various salinities from deionized water to double-strength seawater

Our knowledge of complexity of the renin-angiotensin system (RAS) has grown in recent years and various angiotensin peptides including Ang II, Ang III, Ang IV, and Ang (1-7) were found to have specific functions. Using a combination of HPLC and radioimmunoassay (RIA), we established a high resolution method to quantify various angiotensin subtypes in the plasma of eel acclimated to deionized water (dW), freshwater (FW), seawater (SW), and double-strength seawater (DSW). [Asn(1), Val(5)]-Ang II, [Asp(1), Val(5)]-Ang II, [Val(4)]-Ang III, and [Val(3)]-Ang IV are all present in the circulation and both Ang II subtypes were significantly higher in DSW eel. When the eel was transferred from FW to SW, plasma immunoreactive (ir) Ang II concentration increased and its levels were highly correlated to plasma osmolality, suggesting that the elevated plasma osmolality is the major stimulus for activating the RAS during high salinity transfer. To examine the conversion of [Asn(1)] to [Asp(1)] residue in vivo and in vitro, synthetic [Asn(1), Val(5)]-Ang II was injected into the circulation or incubated with plasma, but the production of [Asp(1), Val(5)]-Ang II was insignificant, which implies that the conversion may occur at the angiotensinogen level. An asparaginase assay was further developed for measuring asparaginase activity and the highest activity was in liver in both FW and SW eel. This new method of analysis can be extended to study the endogenous angiotensin ligands in the local RAS. The potential significance of [Asn(1)] to [Asp(1)] conversion on Ang II metabolism and function is discussed.

The Combinatorial Nature of Osmosensing in Fishes

Organisms exposed to altered salinity must be able to perceive osmolality change because metabolism has evolved to function optimally at specific intracellular ionic strength and composition. Such osmosensing comprises a complex physiological process involving many elements at organismal and cellular levels of organization. Input from numerous osmosensors is integrated to encode magnitude, direction, and ionic basis of osmolality change. This combinatorial nature of osmosensing is discussed with emphasis on fishes.

Central ventilatory and cardiovascular actions of angiotensin peptides in trout

In the brains of teleosts, angiotensin II (ANG II), one of the main effector peptides of the renin-angiotensin system, is implicated in various physiological functions notably body fluid and electrolyte homeostasis and cardiovascular regulation, but nothing is known regarding the potential action of ANG II and other angiotensin derivatives on ventilation. Consequently, the goal of the present study was to determine possible ventilatory and cardiovascular effects of intracerebroventricular injection of picomole doses (5-100 pmol) of trout [Asn(1)]-ANG II, [Asp(1)]-ANG II, ANG III, ANG IV, and ANG 1-7 into the third ventricle of unanesthetized trout. The central actions of these peptides were also compared with their ventilatory and cardiovascular actions when injected peripherally. Finally, we examined the presence of [Asn(1)]-ANG II, [Asp(1)]-ANG II, ANG III, and ANG IV in the brain and plasma using radioimmunoassay coupled with high-performance liquid chromatography. After intracerebroventricular injection, [Asn(1)]-ANG II and [Asp(1)]-ANG II two ANG IIs, elevated the total ventilation through a selective stimulatory action on the ventilation amplitude. However, the hyperventilatory effect of [Asn(1)]-ANG II was threefold higher than the effect of [Asp(1)]-ANG II at the 50-pmol dose. ANG III, ANG IV, and ANG 1-7 were without effect. In addition, ANG IIs and ANG III increased dorsal aortic blood pressure (P(DA)) and heart rate (HR). After intra-arterial injections, none of the ANG II peptides affected the ventilation but [Asn(1)]-ANG II, [Asp(1)]-ANG II, and ANG III elevated P(DA) (50 pmol: +80%, +58% and +48%, respectively) without significant decrease in HR. In brain tissue, comparable amounts of [Asn(1)]-ANG II and [Asp(1)]-ANG II were detected (ca. 40 fmol/mg brain tissue), but ANG III was not detected, and the amount of ANG IV was about eightfold lower than the content of the ANG IIs. In plasma, ANG IIs were also the major angiotensins (ca. 110 fmol/ml plasma), while significant but lower amounts of ANG III and ANG IV were present in plasma. In conclusion, our study suggests that the two ANG II isoforms produced within the brain may act as a neurotransmitter and/or neuromodulator to regulate the cardioventilatory functions in trout. In the periphery, two ANG IIs and their COOH-terminal peptides may act as a circulating hormone preferentially involved in cardiovascular regulations.

Brain neuropeptides in central ventilatory and cardiovascular regulation in trout

Many neuropeptides and their G-protein coupled receptors (GPCRs) are present within the brain area involved in ventilatory and cardiovascular regulation but only a few mammalian studies have focused on the integrative physiological actions of neuropeptides on these vital cardio-respiratory regulations. Because both the central neuroanatomical substrates that govern motor ventilatory and cardiovascular output and the primary sequence of regulatory peptides and their receptors have been mostly conserved through evolution, we have developed a trout model to study the central action of native neuropeptides on cardio-ventilatory regulation. In the present review, we summarize the most recent results obtained using this non-mammalian model with a focus on PACAP, VIP, tachykinins, CRF, urotensin-1, CGRP, angiotensin-related peptides, urotensin-II, NPY, and PYY. We propose hypotheses regarding the physiological relevance of the results obtained.

Amplitude-encoded calcium oscillations in fish cells

The reaction of intracellular Ca(2+) to different agonist stimuli in primary hepatocytes from rainbow trout (Oncorhynchus mykiss) as well as the permanent fish cell line RTL-W1 was investigated systematically. In addition to "classical" agonists such as phenylephrine and ATP, model environmental toxicants like 4-nitrophenol and 3,4-dichloroaniline were used to elucidate possible interactions between toxic effects and Ca(2+) signaling. We report Ca(2+) oscillations in response to several stimuli in RTL-W1 cells and to a lesser extent in primary hepatocytes. Moreover, these Ca(2+) oscillations are amplitude-encoded in contrast to their mammalian counterpart. Bioinformatics and computational analysis were employed to identify key players of Ca(2+) signaling in fish and to determine likely causes for the experimentally observed differences between the Ca(2+) dynamics in fish cells compared to those in mammalian liver cells.

Effects of chronic dietary salt loading on the renin angiotensin and adrenergic systems of rainbow trout (Oncorhynchus mykiss)

Previous studies have demonstrated that chronic dietary salt loading causes hypertension and a decreased sensitivity of the systemic vasculature to α-adrenergic stimulation and other hypertensive stimuli (e.g. hypercapnia) in rainbow trout (Oncorhynchus mykiss). This reduced sensitivity to hypertensive stimuli is consistent with a possible blunting of homeostatic responses normally aimed at raising blood pressure. To test this idea, we examined the consequences of long-term salt feeding and the associated hypertension on the interactive capacities of the renin angiotensin system (RAS) and adrenergic systems to elevate blood pressure in trout. Secretion of catecholamines in response to a range of doses of homologous ANG II in vivo and in situ (using a perfused posterior cardinal vein preparation) was reduced in the salt-fed fish. The reduced sensitivity to ANG II could not be explained by alterations in stored catecholamine (adrenaline or noradrenaline) levels or the general responsiveness of the chromaffin cells to depolarizing stimuli (60 mmol/l KCl). Despite the decreased responsiveness of the chromaffin cells to ANG II, plasma catecholamines were increased to a greater extent in the salt-fed fish during acute hypoxia (a condition that activates the RAS). Interestingly, the pressor effects of ANG II in vivo were actually heightened in the salt-fed fish. The increased pressor response to exogenous ANG II was likely attributable to its direct interaction with vascular ANG II receptors because the effect persisted even after blockade of α-adrenergic receptors. Treating fish with the vascular smooth muscle relaxant papaverine caused similar reductions in blood pressure and increases in plasma ANG II levels regardless of diet. Similarly, inhibition of angiotensin converting enzyme with lisinopril reduced blood pressure equally in control and salt-fed fish. These results indicate that, while long-term dietary salt loading blunts the response of trout chromaffin cells to ANG II, the RAS itself appears to be unaffected. Indeed, the capacity of ANG II to elevate blood pressure is not compromised nor do fish exhibit a reduced capacity to mount an acute humoral adrenergic stress response during acute hypoxia.

Evolutionary conservation of human drug targets in organisms used for environmental risk assessments

Pharmaceuticals are typically found in very low concentrations in the aquatic environment. Accordingly, environmental effects clearly assigned to residual drugs are consistent with high affinity interactions with conserved targets in affected wildlife species rather than with a general toxic effect. Thus, evolutionarily well-conserved targets in a given species are associated with an increased risk. In this study orthologs for 1318 human drug targets were predicted in 16 species of which several are relevant for ecotoxicity testing. The conservation of different functional categories of targets was also analyzed. Zebrafish had orthologs to 86% of the drug targets while only 61% were conserved in Daphnia and 35% in green alga. The predicted presence and absence of orthologs agrees well with published experimental data on the potential for specific drug target interaction in various species. Based on the conservation of targets we propose that aquatic environmental risk assessments for human drugs should always include comprehensive studies on aquatic vertebrates. Furthermore, individual targets, especially enzymes, are well conserved suggesting that tests on evolutionarily distant organisms would be highly relevant for certain drugs. We propose that the results can guide environmental risk assessments by improving the possibilities to identify species sensitive to certain types of pharmaceuticals or to other contaminants that act through well defined mechanisms of action. Moreover, we suggest that the results can be used to interpret the relevance of existing ecotoxicity data.

Central cardiovascular actions of angiotensin II in trout

In mammals, a large body of evidence supports the existence of a brain renin-angiotensin system (RAS) acting independently or synergistically with the endocrine RAS to maintain diverse physiological functions, notably cardiovascular homeostasis. The RAS is of ancient origin and although most components of the RAS are present within the brain of teleost fishes, little is known regarding the central physiological actions of the RAS in these vertebrates. The present review encompasses the most relevant functional data for a role of the brain RAS in cardiovascular regulations in our experimental animal model, the unanesthetized trout Oncorhynchus mykiss. This paper mainly focuses on the central effect of angiotensin II (ANG II) on heart rate, blood pressure, heart rate variability and cardiac baroreflex, after intracerebroventricular injection or local microinjection of the peptide within the dorsal vagal motor nucleus. The probable implications of the parasympathetic nervous system in ANG II-evoked changes in the cardiac responses are also discussed.

Osmotic stress sensing and signaling in fishes

In their aqueous habitats, fish are exposed to a wide range of osmotic conditions and differ in their abilities to respond adaptively to these variations in salinity. Fish species that inhabit environments characterized by significant salinity fluctuation (intertidal zone, estuaries, salt lakes, etc.) are euryhaline and able to adapt to osmotic stress. Adaptive and acclimatory responses of fish to salinity stress are based on efficient mechanisms of osmosensing and osmotic stress signaling. Multiple osmosensors, including calcium sensing receptor likely act in concert to convey information about osmolality changes to downstream signaling and effector mechanisms. The osmosensory signal transduction network in fishes is complex and includes calcium, mitogen-activated protein kinase, 14-3-3 and macromolecular damage activated signaling pathways. This network controls, among other targets, osmosensitive transcription factors such as tonicity response element binding protein and osmotic stress transcription factor 1, which, in turn, regulate the expression of genes involved in osmotic stress acclimation. In addition to intracellular signaling mechanisms, the systemic response to osmotic stress in euryhaline fish is coordinated via hormone- and paracrine factor-mediated extracellular signaling. Overall, current insight into osmosensing and osmotic stress-induced signal transduction in fishes is limited. However, euryhaline fish species represent excellent models for answering critical emerging questions in this field and for elucidating the underlying molecular mechanisms of osmosensory signal transduction.

The venous circulation: a piscine perspective

Vascular capacitance describes the pressure-volume relationship of the circulatory system. The venous vasculature, which is the main capacitive region in the circulation, is actively controlled by various neurohumoral systems. In terrestrial animals, vascular capacitance control is crucial to prevent orthostatic blood pooling in dependent limbs, while in aquatic animals like fish, the effects of gravity are cancelled out by hydrostatic forces making orthostatic blood pooling an unlikely concern for these animals. Nevertheless, changes in venous capacitance have important implications on cardiovascular homeostasis in fish since it affects venous return and cardiac filling pressure (i.e. central venous blood pressure), which in turn may affect cardiac output. The mean circulatory filling pressure is used to estimate vascular capacitance. In unanaesthetized animals, it is measured as the central venous plateau pressure during a transient stoppage of cardiac output. So far, most studies of venous function in fish have addressed the situation in teleosts (notably the rainbow trout, Oncorhynchus mykiss), while any information on elasmobranchs, cyclostomes and air-breathing fishes is more limited. This review describes venous haemodynamic concepts and neurohumoral control systems in fish. Particular emphasis is placed on venous responses to natural cardiovascular challenges such as exercise, environmental hypoxia and temperature changes.

Venous responses during exercise in rainbow trout, Oncorhynchus mykiss: α-adrenergic control and the antihypotensive function of the renin–angiotensin system

The role of the alpha-adrenergic system in the control of cardiac preload (central venous blood pressure; P(ven)) and venous capacitance during exercise was investigated in rainbow trout (Oncorhynchus mykiss). In addition, the antihypotensive effect of the renin-angiotesin system (RAS) was investigated during exercise after alpha-adrenoceptor blockade. Fish were subjected to a 20-min exercise challenge at 0.66 body lengths s(-1) (BL s(-1)) while P(ven), dorsal aortic blood pressure (P(da)) and relative cardiac output (Q) was recorded continuously. Heart rate (f(H)), cardiac stroke volume (SV) and total systemic resistance (R(sys)) were derived from these variables. The mean circulatory filling pressure (MCFP) was measured at rest and at the end of the exercise challenge, to investigate potential exercise-mediated changes in venous capacitance. The protocol was repeated after alpha-adrenoceptor blockade with prazosin (1 mg kg(-1)M(b)) and again after additional blockade of angiotensin converting enzyme (ACE) with enalapril (1 mg kg(-1)M(b)). In untreated fish, exercise was associated with a rapid (within approx. 1-2 min) and sustained increase in Q and P(ven) associated with a significant increase in MCFP (0.17+/-0.02 kPa at rest to 0.27+/-0.02 kPa at the end of exercise). Prazosin treatment did not block the exercise-mediated increase in MCFP (0.25+/-0.04 kPa to 0.33+/-0.04 kPa at the end of exercise), but delayed the other cardiovascular responses to swimming such that Q and P(ven) did not increase significantly until around 10-13 min of exercise, suggesting that an endogenous humoral control mechanism had been activated. Subsequent enalapril treatment revealed that these delayed responses were in fact due to activation of the RAS, because resting P(da) and R(sys) were decreased further and essentially all cardiovascular changes during exercise were abolished. This study shows that the alpha-adrenergic system normally plays an important role in the control of venous function during exercise in rainbow trout. It is also the first study to suggest that the RAS may be an important modulator of venous pressure and capacitance in fish.

Distribution of peptidase activity in teleost and rat tissues

Peptides play important roles in cell regulation and signaling in many tissues. The actions of peptides are regulated by peptidases. Although the activity of these enzymes has been thoroughly characterized in mammals, little is known about their presence or function in fish. In the present study, we compared the activity of several peptidases in selected tissues (pituitary gland, different brain areas, kidney and gills) of the gilthead sea bream and rainbow trout with that found in similar rat tissues (lungs studied in place of gills). Soluble puromycin-sensitive aminopeptidase showed the highest values in the pituitary gland of the sea bream, whereas the membrane-bound form was found to be more active in the trout kidney. Very high levels of activity of aminopeptidase N were detected in trout and sea bream plasma. In contrast, the highest levels of activity of aminopeptidase B were found in rat tissues, with the exception of the gills of the trout. Aminopeptidase N levels tended to be higher in sea bream tissues with respect to those of trout. In contrast, the level of activity of aminopeptidase B was found to be consistently much higher in trout tissues than in those of the sea bream. Prolyl endopeptidase activity was principally detected in the pituitary gland and in the brain areas of teleosts. These differences between species could be related to different mechanisms of osmoregulation in saltwater- and in freshwater-adapted fish.

Ontogeny of osmoregulation in postembryonic fish: a review

Salinity and its variations are among the key factors that affect survival, metabolism and distribution during the fish development. The successful establishment of a fish species in a given habitat depends on the ability of each developmental stage to cope with salinity through osmoregulation. It is well established that adult teleosts maintain their blood osmolality close to 300 mosM kg(-1) due to ion and water regulation effected at several sites: tegument, gut, branchial chambers, urinary organs. But fewer data are available in developing fish. We propose a review on the ontogeny of osmoregulation based on studies conducted in different species. Most teleost prelarvae are able to osmoregulate at hatch, and their ability increases in later stages. Before the occurrence of gills, the prelarval tegument where a high density of ionocytes (displaying high contents of Na+/K+-ATPase) is located appears temporarily as the main osmoregulatory site. Gills develop gradually during the prelarval stage along with the numerous ionocytes they support. The tegument and gill Na+/K+-ATPase activity varies ontogenetically. During the larval phase, the osmoregulatory function shifts from the skin to the gills, which become the main osmoregulatory site. The drinking rate normalized to body weight tends to decrease throughout development. The kidney and urinary bladder develop progressively during ontogeny and the capacity to produce hypotonic urine at low salinity increases accordingly. The development of the osmoregulatory functions is hormonally controlled. These events are inter-related and are correlated with changes in salinity tolerance, which often increases markedly at the metamorphic transition from larva to juvenile. In summary, the ability of ontogenetical stages of fish to tolerate salinity through osmoregulation relies on integumental ionocytes, then digestive tract development and drinking rate, developing branchial chambers and urinary organs. The physiological changes leading to variations in salinity tolerance are one of the main basis of the ontogenetical migrations or movements between habitats of different salinity regimes.

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.

Molecular biology of major components of chloride cells

Current understanding of chloride cells (CCs) is briefly reviewed with emphasis on molecular aspects of their channels, transporters and regulators. Seawater-type and freshwater-type CCs have been identified based on their shape, location and response to different ionic conditions. Among the freshwater-type CCs, subpopulations are emerging that are implicated in the uptake of Na(+), Cl(-) and Ca(2+), respectively, and can be distinguished by their shape of apical crypt and affinity for lectins. The major function of the seawater CC is transcellular secretion of Cl(-), which is accomplished by four major channels and transporters: (1). CFTR Cl(-) channel, (2). Na(+),K(+)-ATPase, (3). Na(+)/K(+)/2Cl(-) cotransporter and (4). a K(+) channel. The first three components have been cloned and characterized, but concerning the K(+) channel that is essential for the continued generation of the driving force by Na(+),K(+)-ATPase, only one candidate is identified. Although controversial, freshwater CCs seem to perform the uptake of Na(+), Cl(-) and Ca(2+) in a manner analogous to but slightly different from that seen in the absorptive epithelia of mammalian kidney and intestine since freshwater CCs face larger concentration gradients than ordinary epithelial cells. The components involved in these processes are beginning to be cloned, but their CC localization remains to be established definitively. The most important yet controversial issue is the mechanism of Na(+) uptake. Two models have been postulated: (i). the original one involves amiloride-sensitive electroneutral Na(+)/H(+) exchanger (NHE) with the driving force generated by Na(+),K(+)-ATPase and carbonic anhydrase (CA) and (ii). the current model suggests that Na(+) uptake occurs through an amiloride-sensitive epithelial sodium channel (ENaC) electrogenically coupled to H(+)-ATPase. While fish ENaC remains to be identified by molecular cloning and database mining, fish NHE has been cloned and shown to be highly expressed on the apical membrane of CCs, reviving the original model. The CC is also involved in acid-base regulation. Analysis using Osorezan dace (Tribolodon hakonensis) living in a pH 3.5 lake demonstrated marked inductions of Na(+),K(+)-ATPase, CA-II, NHE3, Na(+)/HCO(3)(-) cotransporter-1 and aquaporin-3 in the CCs on acidification, leading to a working hypothesis for the mechanism of Na(+) retention and acid-base regulation.

Captopril blocks the cardiac actions of centrally administered angiotensin I in the trout Oncorhynchus mykiss

The present study was performed in order to gain new insights into the existence of a brain renin-angiotensin system (RAS) in teleost fish. For this purpose, we investigated the effects of centrally administered angiotensin (ANG) I ([Asn(1),Val(5),Asn(9)]ANG I) and ANG II ([Asn(1),Val(5)]ANG II) on heart rate (HR) and heart rate variability (HRV) in the unanesthetized trout. The animals were studied before and after treatment with captopril, an angiotensin-converting enzyme (ACE) inhibitor. Trout were equipped with two subcutaneous electrocardiographic electrodes and with an intracerebroventricular (i.c.v.) cannula inserted within the third ventricle of the brain. The i.c.v. injection of vehicle had no effect on the recorded parameters. The i.c.v. injections of ANG I and ANG II at doses of 5 and 50 pmol had a marked effect on HR and HRV. At a dose of 50 pmol, ANG I and ANG II produced a progressive and significant increase in HR (+36% and+45%, respectively) but elicited a profound decrease in HRV (-88% and-92%, respectively). I.c.v. injection of captopril (10 microg) had no effect on HR or HRV. However, this ACE inhibitor prevented the tachycardia and abolished the decrease in HRV mediated by 50 pmol of ANG I. In contrast, captopril had no effect upon the cardiac actions of 50 pmol of ANG II. These results give the first support for the existence of functional important ACE-like activity in the brain of a teleost fish and suggest that the brain RAS in this class of vertebrate may be involved in the control of cardiac chronotropic activity.

Dynamic synchronization analysis of venous pressure-driven cardiac output in rainbow trout

Measurement of venous function in vivo is inherently difficult. In this study, we used the Hilbert transform to examine the dynamic relationships between venous pressure and cardiac output (CO) in rainbow trout whose blood volume was continuously increased and decreased by ramp infusion and withdrawal (I/W). The dorsal aorta and ductus Cuvier were cannulated percutaneously and connected to pressure transducers; a flow probe was placed around the ventral aorta. Whole blood from a donor was then I/W via the dorsal aortic cannula at a rate of 10% of the estimated blood volume per minute, and the duration of I/W was varied from 40, 60, 80, 90, 120, 230, 240, 260, 300, and 340 s. Compliance [change in (delta) blood vol/deltavenous pressure] was 2.8 +/- 0.2 ml x mmHg-1x g-1 (N = 25 measurements; 6 fish with closed pericardium) and 2.8 +/- 0.3 ml. mmHg-1x kg-1 (N = 19 measurements, 4 fish with open pericardium). Compliance was positively correlated with the duration of I/W, indicative of cardiovascular reflex responses at longer I/W durations. In trout with closed pericardium, CO followed venous pressure oscillations with an average time lag of 4.2 +/- 1.0 s (N = 9); heart rate (HR) was inversely correlated with CO. These studies show that CO is entrained by modulation of venous pressure, not by HR. Thus, although trout have a rigid pericardium, venous pressure (vis-a-tergo), not cardiac suction (vis-a-fronte), appears to be the primary determinant of CO. Estimation of venous compliance by ramp-modulation of venous pressure is faster and less traumatic than classical capacitance measurements and appears applicable to a variety of vertebrate species, as does the Hilbert transform, which permits analysis of signals with disparate frequencies.

Maturation-dependent changes of angiotensin receptor expression in fowl

An angiotensin (ANG) receptor homologous to the type 1 receptor (AT1) has been cloned in chickens (cAT1). We investigated whether cAT1 expression in various tissues shows maturation/age-dependent changes. cAT1 mRNA levels detected in renal glomeruli [in situ hybridization (ISH)] and kidney extract (RT-PCR) are significantly (P < 0.01) higher in 19-day embryos (EB) than in chicks (CH, 2-3 wk) and pullets/cockerels (PL/CK, 14-16 wk). The levels in adrenal glands (concentrated in subcapsular regions) are high in EB and further increased in CH and PL/CK. cAT1 mRNA is also detectable in smooth muscle (SM)/adventitia of EB and CH aorta and in the adventitia, but not SM, from PL/CK aortas. The endothelia from small arteries and arterioles, but not from aorta, express cAT1 mRNA (ISH). In all age groups, ANG II induces profound endothelium-dependent relaxation of abdominal aorta, partly (37-47%) inhibitable (P < 0.01) by Nomega-nitro-l-arginine methyl ester (l-NAME, 10(-4) M), suggesting the presence of ANG receptor in endothelium. l-NAME-resistant ANG II relaxation, examined in a limited number of EB or CH aortas, was reduced by 125 mM K+ or apamin plus charybdotoxin. The results suggest that 1) cAT1 is present in kidney, adrenal gland, and vascular endothelium (heterogeneity exists among arteries) of EB, CH, and PL/CK, and in aortic SM/adventitia of EB/CH but only in adventitia of PL/CK; 2) levels of cAT1 gene expression change during maturation in a tissue-specific manner; and 3) ANG II-induced relaxation may be partly attributable to nitric oxide and potassium channel activation.

Transvascular and intravascular fluid transport in the rainbow trout: revisiting Starling's forces, the secondary circulation and interstitial compliance

The kinetics of transvascular fluid transport across fish capillaries and redistribution of fluids between intravascular compartments in intact fish are unknown. Cannulae were placed in the dorsal aorta (DA) and caudal vein (CV) of rainbow trout Oncorhynchus mykiss (mass 0.45-0.85 kg) and the fish spleenectomized. The following day a peristaltic pump was fitted to complete the extracorporeal arterio-venous circulation. Hematocrit (Hct) was monitored in unanesthetized fish either manually, by collecting blood from the extracorporeal loop at 5 min intervals for a period of 1 h (groups 1 and 2), or continuously (instantaneously) with an impedance flow-cell inserted in the aortic cannula (group 3). Fish in group 1 were volume expanded by injecting a volume of saline (0.9 g% NaCl; SI) or trout plasma (PI) equivalent to 40% of the plasma volume. In group 2, 20% or 35% of the blood volume was removed, and in group 3, 35% of the blood volume was removed. Plasma volume (Vp) was calculated from an assumed blood volume of 35 ml kg(-1) and the Hct. Vp declined mono-exponentially after SI with a half-time of 6.8 min and Vp reached a new steady state at 28.1 ml kg(-1); 30% of the injected volume remained in the vasculature. Volume recovery after PI was also mono-exponential, but slower (half-time=15.4 min) than SI, whereas the steady-state Vp (27.3 ml kg(-1)) was similar and 30% of the injected volume remained in the vasculature. Thus the presence of plasma proteins delayed fluid efflux from the vasculature, but did not affect the volume lost. Transvascular fluid filtration coefficients calculated from this data were 5.5 (SI) and 4.5 ml mmHg(-1) kg(-1) min(-1) (PI), and interstitial compliance was 11.8 (SI) and 9.7 ml mmHg(-1) kg(-1) (PI). The rate of volume recovery after 20% or 35% hemorrhage was independent of the hemorrhage volume (half-time=13.3 and 15.1 min, respectively) and similar to the half-time of PI, indicating that protein-rich interstitial fluid is returned to the vasculature. There is a nearly instantaneous change in Hct that occurs during the hemorrhage period; it is dependent on hemorrhage duration and volume and not associated with the subsequent mono-exponential recovery. This initial response is best explained by a rapid fluid shift from a large-volume (approximately 40% of total blood volume), low-hematocrit (less than half of systemic Hct) microcirculation into the higher-hematocrit macrocirculation. These studies are consistent with transcapillary fluid flux across a barrier that is highly permeable to protein, and cannot be explained by fluid shift between primary and secondary circulations, or by transcapilllary flux across a capillary bed that is impermeable to plasma proteins. The results support the hypothesis that whole-body reflection coefficients in trout are very low and that plasma oncotic pressure is not a determinant of transcapillary fluid balance. They also show that both transvascular and intravascular fluid movements are important effectors of central volume homeostasis.

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.

Effects of hypoxia on isolated vessels and perfused gills of rainbow trout

Local hypoxia dilates systemic and constricts pulmonary blood vessels in mammals without neural or humoral involvement. The direct effects of hypoxia on isolated vessels from bony fish have not been examined. In the present study, isolated vessels (efferent branchial artery, EBA; coeliacomesenteric artery, CMA; ventral aorta, VA; and anterior cardinal vein, ACV) from rainbow or steelhead trout (Oncorhynchus mykiss) were subjected to either passive load (resting tension) or contracted with a ligand or 50 mM KCl and then subjected to 60 min of hypoxia by N(2) administration and an additional 30 min of normoxia. All vessels were usually refractory to hypoxia under conditions of resting tension. EBAs, CMAs and VAs pre-contracted with a receptor-mediated ligand were all significantly relaxed by hypoxia and only VAs recovered significantly upon subsequent restoration of normoxia. In contrast, tension in all arteries pre-contracted with 50 mM KCl was elevated further in response to hypoxia. Conversely, ligand-contracted ACVs responded to hypoxia with a further increase in tension, whereas KCl-contracted ACVs relaxed. During apparently random 2-3-week periods EBA and CMA from steelhead and EBA from rainbow trout were hyper-reactive to hypoxia. Steelhead vessels responded to hypoxia with a rapid contraction that increased in magnitude over 3 days. These contractions were independent of pre-stimulation and they were dose-dependent upon PO(2). In isolated gills, hypoxic perfusate produced an immediate but transient elevation of resistance (R(GILL)) in all four gill arches. R(GILL) increased by as much as 30% of initial values and this response was unaltered upon a second hypoxic exposure. These studies demonstrate that isolated vascular segments of rainbow trout are indeed responsive to hypoxia and that these differential responses are vessel and tone dependent and the overall response may be altered by as yet unknown seasonal or environmental factors. Hypoxia-induced arterial relaxation is blocked by elevated external [K(+)], implicating alteration of transmembrane K(+) conductance and/or membrane potential in this depressor response. K(+)-channel closure or voltage-gated Ca(2+) influx cannot account for arterial vasoconstriction due to hypoxia during KCl contractions. Vascular responses to hypoxia could have a profound impact on local flow in vivo and could mediate ventilation-perfusion matching in the branchial circulation of fish.