A novel angiotensin I isolated from an elasmobranch fish

The whale shark genome reveals patterns of vertebrate gene family evolution

Chondrichthyes (cartilaginous fishes) are fundamental for understanding vertebrate evolution, yet their genomes are understudied. We report long-read sequencing of the whale shark genome to generate the best gapless chondrichthyan genome assembly yet with higher contig contiguity than all other cartilaginous fish genomes, and studied vertebrate genomic evolution of ancestral gene families, immunity, and gigantism. We found a major increase in gene families at the origin of gnathostomes (jawed vertebrates) independent of their genome duplication. We studied vertebrate pathogen recognition receptors (PRRs), which are key in initiating innate immune defense, and found diverse patterns of gene family evolution, demonstrating that adaptive immunity in gnathostomes did not fully displace germline-encoded PRR innovation. We also discovered a new toll-like receptor (TLR29) and three NOD1 copies in the whale shark. We found chondrichthyan and giant vertebrate genomes had decreased substitution rates compared to other vertebrates, but gene family expansion rates varied among vertebrate giants, suggesting substitution and expansion rates of gene families are decoupled in vertebrate genomes. Finally, we found gene families that shifted in expansion rate in vertebrate giants were enriched for human cancer-related genes, consistent with gigantism requiring adaptations to suppress cancer.

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.

Ontogeny of renin gene expression in the chicken, Gallus gallus

Renin or a renin-like enzyme evolved in ancestral vertebrates and is conserved along the vertebrate phylogeny. The ontogenic development of renin, however, is not well understood in nonmammalian vertebrates. We aimed to determine the expression patterns and relative abundance of renin mRNA in pre- and postnatal chickens (Gallus gallus, White Leghorn breed). Embryonic day 13 (E13) embryos show renal tubules, undifferentiated mesenchymal structures, and a small number of developing glomeruli. Maturing glomeruli are seen in post-hatch day 4 (D4) and day 30 (D30) kidneys, indicating that nephrogenic activity still exists in kidneys of 4-week-old chickens. In E13 embryos, renin mRNA measured by quantitative polymerase chain reaction in the adrenal glands is equivalent to the expression in the kidneys, whereas in post-hatch D4 and D30 maturing chicks, renal renin expressions increased 2-fold and 11-fold, respectively. In contrast, relative renin expression in the adrenals became lower than in the kidneys. Furthermore, renin expression is clearly visible by in situ hybridization in the juxtaglomerular (JG) area in D4 and D30 chicks, but not in E13 embryos. The results suggest that in chickens, renin evolved in both renal and extrarenal organs at an early stage of ontogeny and, with maturation, became localized to the JG area. Clear JG structures are not morphologically detectable in E13 embryos, but are visible in 30-day-old chicks, supporting this concept.

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.

Molecular and evolutionary perspectives of the renin-angiotensin system from lamprey

The recent advance and revision on the renin-angiotensin system in lamprey were summarized and we emphasized that presence of two types of angiotensins (Angs) in lamprey. Due to the parasitic nature on fish blood, teleost-type Angs were produced in their buccal gland and secreted into the lamphredin to evade the host immunorejection. A native lamprey angiotensinogen (AGT) was identified in genome and it retains serine-protease inhibitor activity for thrombin that regulates the blood coagulation pathway. The native lamprey angiotensin II (Lp-Ang II) is hypotensive instead of hypertensive, suggesting a functional divergence on cardiovascular regulation from the main vertebrate groups. The renin gene was absent from the lamprey genome so far, and the mutation on the renin-recognition site on lamprey AGT suggested that other proteases may have replaced the role of renin. Lp-Ang II was shown to bind to AT1 receptor and internalized, but the downstream signaling was still unknown. Molecular and phylogenetic evidence on invertebrate ACE-like proteins indicated that they were not homologous to those in vertebrates and could be acting on other native peptides. Although it was generally believed that the RAS was a well-conserved hormone system in vertebrates and invertebrates, revision by molecular data indicated that invertebrates lack homologous RAS components while lamprey possess an almost complete RAS. This suggests that the hormone cascade system was first evolved around cyclostome emergence and invertebrates could have taken up the RAS components from vertebrates through horizontal gene transfer.

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.

Immunohistochemical localization of renin-containing cells in two elasmobranch species

Renin immunoreactivity was localized at the light and electron microscopic level in two elasmobranch fish species, the Atlantic stingray, Dasyatis sabina, and river ray, Potamotrygon humerosa. At the light microscopic level, the peroxidase-anti-peroxidase method showed a positive immunoreactivity in modified smooth muscle cells in kidney afferent arterioles as well as in arterioles of several organs: rectal gland, inter-renal gland, conus arteriosus, and gill. Electron microscopic renin-positive immunogold localization was confined to the contents of membrane bound granules in the modified smooth muscle cells of these arterioles. The presence of renin-containing granules in the modified smooth muscle, "granular cells," of the renal glomerular afferent arteriole of these two stingray species adds support to earlier studies which showed the structural components of a complete juxtaglomerular apparatus and some of the biochemical and molecular components of a renin-angiotensin system (RAS) as found in teleost fish, reptiles, birds, and mammals. A notable result, however, was the renin-positive immunoreaction in the arteriolar wall of all other organs studied here. The presence of this "diffuse renin system" in the connective tissue of various organs suggests that in these two stingray species in addition to local organ-specific functions, the RAS may act as a systemic mechanism to regulate blood pressure and blood flow in the body.

Evolution of melanocortin receptors in cartilaginous fish: melanocortin receptors and the stress axis in elasmobranches

There is general agreement that the presence of five melanocortin receptor genes in tetrapods is the result of two genome duplications that occurred prior to the emergence of the gnathostomes, and at least one local gene duplication that occurred early in the radiation of the ancestral gnathostomes. Hence, it is assumed that representatives from the extant classes of gnathostomes (i.e., Chondrichthyes, Actinopterygii, Sarcopterygii) should also have five paralogous melanocortin genes. Current studies on cartilaginous fishes indicate that while there is evidence for five paralogous melanocortin receptor genes in this class, to date all five paralogs have not been detected in the genome of a single species. This mini-review will discuss the ligand selectivity properties of the melanocortin-3 receptor of the elephant shark (subclass Holocephali) and the ligand selectivity properties of the melanocortin-3 receptor, melanocortin-4 receptor, and the melanocortin-5 receptor of the dogfish (subclass Elasmobranchii). The potential relationship of these melanocortin receptors to the hypothalamus/pituitary/interrenal axis will be discussed.

Presence of functional angiotensin II receptor and angiotensin converting enzyme in the aorta of the snake Bothrops jararaca

AIM

Angiotensin II (Ang II) interacts with AT(1) and AT(2) receptors and, in some vertebrates, with an Ang II binding site showing low affinity for AT(1) and AT(2) receptor antagonists. This study was carried out to characterize the Ang II receptor, and the presence of an angiotensin-converting enzyme (ACE) in the aorta of the Bothrops jararaca snake.

MAIN METHOD

Contraction induced by Ang I or II in aortic ring from the snake was evaluated in the absence or in the presence of ACE-blocker or Ang II antagonists.

KEY FINDINGS

Ang II analogs, modified at positions 1 and 5, induced vasoconstriction with differences in their potencies. The relative rank order was: [Asp(1), Val(5)] Ang II=[Asp(1), Ile(5)] Ang II>>>[Asn(1), Val(5)] Ang II. ACE-like activity was detected, as well as an Ang II receptor with low affinity for AT(1) and AT(2) selective receptor antagonists (pK(B) values of 5.62±0.23 and 5.08±0.25). A disulfide reducing agent almost abolished the Ang II effect, while an alpha adrenoceptor antagonist, or removing the endothelium, did not modify the Ang II effect. These results indicate that the B. jararaca aorta has an Ang II receptor pharmacologically distinct from AT(1) and AT(2) receptors, and the vasoconstrictor effect observed is independent of catecholamine or endothelium modulation. ACE and the AT receptor in the aorta of B. jararaca may be part of a tissue renin-angiotensin system.

SIGNIFICANCE

The data contribute to the knowledge of the renin-angiotensin system in vertebrate species, and provide insight into the understanding of snake Ang II receptor characteristics and diversity.

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.

A teleostean angiotensinogen from Oplegnathus fasciatus responses to immune and injury challenges

Angiotensinogen (AGT) is the precursor of the renin-angiotensin system and contributes to osmoregulation, acute-phase and immune responses. A full-length cDNA of the AGT (2004 bp with a 1389 bp coding region) was isolated from rock bream (Rb), Oplegnathus fasciatus. The encoded polypeptide of 463 amino acids had a predicted molecular mass of 51.6 kDa. RbAGT possessed a deduced signal peptide of 22 residues upstream of a putative angiotensin I sequence ((23)NRVYVHPFHL(32)). RbAGT possessed a specific domain profile and a signature motif which are characteristics of the serpin family. Sequence homology and phylogenetic analysis indicated that RbAGT was evolutionarily closest to AGT of Rhabdosargus sarba. The mRNA expression profile of RbAGT was determined by quantitative RT-PCR and it demonstrated a constitutive and tissue-specific expression with the highest transcript level in the liver. Significantly up-regulated RbAGT expression was elicited by systemic injection of a lipopolysaccharide, rock bream iridovirus (RBIV) and bacteria (Edwardsiella tarda and Streptococcus iniae), revealing its pathogen inducibility. RbAGT manifested a down-regulated response to systemic injury, contemporaneously with two other serpins, protease nexin-1 (PN-1), and heparin cofactor II (HCII). In addition, a synchronized expression pattern was elicited by RbAGT and RbTNF-α in response to injury, suggesting that TNF-α might be a potential modulator of AGT transcription.

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.

Molecular classification of an elasmobranch angiotensin receptor: quantification of angiotensin receptor and natriuretic peptide receptor mRNAs in saltwater and freshwater populations of the Atlantic stingray

Among the most conserved osmoregulatory hormone systems in vertebrates are the renin-angiotensin system (RAS) and the natriuretic peptides (NPs). We examined the RAS and NP system in the euryhaline Atlantic stingray, Dasyatis sabina (Lesueur). To determine the relative sensitivity of target organs to these hormonal systems, we isolated cDNA sequences encoding the D. sabina angiotensin receptor (AT) and natriuretic peptide type-B receptor (NPR-B). We then determined the tissue-specific expression of their mRNAs in saltwater D. sabina from local Texas waters and an isolated freshwater population in Lake Monroe, Florida. AT mRNA was most abundant in interrenal tissue from both populations. NPR-B mRNA was most abundant in rectal gland tissue from both populations, and also highly abundant in the kidney of saltwater D. sabina. This study is the first to report the sequence of an elasmobranch angiotensin receptor, and phylogenetic analysis indicates that the D. sabina receptor is more similar to AT(1) vs. AT(2) proteins. This classification is further supported by molecular analysis of AT(1) and AT(2) proteins demonstrating conservation of AT(1)-specific amino acid residues and motifs in D. sabina AT. Molecular classification of the elasmobranch angiotensin receptor as an AT(1)-like protein provides fundamental insight into the evolution of the vertebrate RAS.

Adaptation of a corticosterone ELISA to demonstrate sequence-specific effects of angiotensin II peptides and C-type natriuretic peptide on 1alpha-hydroxycorticosterone synthesis and steroidogenic mRNAs in the elasmobranch interrenal gland

It is thought that a single corticosteroid, 1alpha-hydroxycorticosterone (1alpha-B), is both a glucocorticoid and mineralocorticoid in the elasmobranch fishes. We investigated the putative mineralocorticoid role of 1alpha-B by examining regulation of interrenal 1alpha-B synthesis by osmoregulatory hormones in the euryhaline stingray Dasyatis sabina. Using synthesized steroid, a commercial enzyme-linked immunoassay was validated for the quantification of 1alpha-B. In interrenal cultures, the antinatriuretic peptide angiotensin II (ANG II) was potently steroidogenic, whereas C-type natriuretic peptide had no effect on 1alpha-B titers. However, both peptides significantly decreased abundance of rate-limiting steroidogenic mRNAs (steroidogenic acute regulatory protein, StAR; cholesterol side-chain cleavage, P450scc). We also isolated cDNAs encoding ANG II from three species of elasmobranch, verifying heterogeneity among elasmobranch peptides at the first amino acid position. Potential implications of this heterogeneity were investigated by examining the effects of homologous and heterologous ANG II on interrenal steroid production and steroidogenic mRNAs. Changes at amino acid position three, but not position one, of ANG II significantly affected steroidogenic potency. Conversely, changes at position one, but not position three, significantly affected the potency of ANG II to alter levels of steroidogenic mRNAs. This study is the first to demonstrate regulation of elasmobranch steroidogenic mRNAs by osmoregulatory peptides.

Identification of angiotensinogen genes with unique and variable angiotensin sequences in chondrichthyans

The renin-angiotensin system is an enzyme-linked hormonal cascade that plays an important role in body fluid and cardiovascular regulation. The system is initiated by the action of renin on the precursor protein, angiotensinogen (AGT), whose sequence information is scarce because of its high variability among species. In the present study, we cloned AGT in chondrichthyans (elasmobranchs: Triakis scyllium, Dasyatis akajei,Leucoraja erinacea and a holocephalan: Callorhinchus milii). Homology was low among AGTs thus far identified; 25-28% between elasmobranchs and tetrapods and 33-61% even within chondrichthyans. All chondrichthyan angiotensin (ANG) II's have a unique Pro3 instead of Val3 as seen in all other species. In addition, holocephalan ANG II has an unusual His4 instead of Tyr4. In addition, and the N-terminal amino acid, which is usually Asp1 in tetrapods and Asn1 in fishes, was highly variable (Asp, Asn or Tyr) in chondrichthyans. Molecular phylogenetic analysis showed that chondrichthyan AGT precursors are clustered into a group separated from those of tetrapods and teleosts. The AGT gene was most abundantly expressed in the liver, followed by the kidney, interrenal tissue and rectal gland of Triakis where biological actions of ANG II have been demonstrated. Collectively, we identified diversified AGT genes for the first time in chondrichthyes and showed that their ANG II's have unique amino acid residues at positions 1, 3 and 4. High variability of ANG II sequences in chondrichthyans is discussed in relation to their unique regulatory mechanisms such as urea-based osmoregulation.

Characteristics of neural and humoral systems involved in the regulation of blood pressure in snakes

Cardiovascular function is affected by many mechanisms, including the autonomic system, the kallikrein-kinin system (KKS), the renin-angiotensin system (RAS) and the endothelin system. The function of these systems seems to be fairly well preserved throughout the vertebrate scale, but evolution required several adaptations. Snakes are particularly interesting for studies related to the cardiovascular function because of their elongated shape, their wide variation in size and length, and because they had to adapt to extremely different habitats and gravitational influences. To keep the normal cardiovascular control the snakes developed anatomical and functional adaptations and interesting structural peculiarities are found in their autonomic, KKS, RAS and endothelin systems. Our laboratory has characterized some biochemical, pharmacological and physiological properties of these systems in South American snakes. This review compares the components and function of these systems in snakes and other vertebrates, and focuses on differences found in snakes, related with receptor or ligand structure and/or function in autonomic system, RAS and KKS, absence of components in KKS and the intriguing identity between a venom and a plasma component in the endothelin system.

Positive feedback of hepatic angiotensinogen expression in silver sea bream (Sparus sarba)

The renin-angiotensin system (RAS) is involved in the maintenance of fluid homeostasis in vertebrates. Production of the precursor protein, angiotensinogen, is regulated by other components within the RAS. Angiotensin II (Ang II) stimulates the production and secretion of angiotensinogen in many mammalian models. However, the existence of a similar positive feedback mechanism for angiotensinogen has not been demonstrated for any non-mammalian species. In the present study, we have cloned the angiotensinogen for silver sea bream (Sparus sarba) and investigated the role of Ang II on angiotensinogen expression. The nucleotide sequence of angiotensinogen for S. sarba only exhibits a fair resemblance to other fish angiotensinogens and shows 76.6% similarity to that of Takifugu rubripes and 57.2% similarity to that of Danio rerio. Angiotensinogen transcripts have been identified in the brain, liver, kidney, and various parts of the intestine of sea bream, an observation, which probably implies the presence of a local RAS at the tissue level. The liver is probably the major source of angiotensinogen, as it exhibits the highest angiotensinogen transcript abundance among different tissues. Differential angiotensinogen expression was found among different regions of the intestine where the pyloric caeca exhibits the highest expression. Putative Ang I is identified at the N-terminal of the deduced protein with a novel sequence [Asn1, Ile5, His9]-Ang I. Hepatic angiotensinogen expression in sea bream adapted to different salinities remained constant and this is probably due to desensitization of the angiotensin receptors by angiotensin. A positive feedback mechanism of angiotensinogen by Ang II has been demonstrated as exogenous Ang II increased the amount of angiotensinogen transcript in isolated hepatocytes in vitro. Blockade of endogenous RAS by the angiotensin converting enzyme (ACE) inhibitor, captopril, significantly lowered the hepatic expression of angiotensinogen in vivo. The effect of Ang II stimulation on angiotensinogen expression is more potent in fish than that in mammals. These data suggest that the positive feedback mechanism of angiotensinogen by Ang II has already evolved in teleosts and such mechanism may be involved in the maintenance of angiotensinogen secretion under resting and hypertensive conditions.

The effects of freshwater to seawater transfer on circulating levels of angiotensin II, C-type natriuretic peptide and arginine vasotocin in the euryhaline elasmobranch, Carcharhinus leucas

This study examined the effect of transfer to increased environmental salinity on the circulating levels of angiotensin II (ANG II), C-type natriuretic peptide (CNP), and arginine vasotocin (AVT) in the euryhaline elasmobranch, Carcharhinus leucas. Plasma levels of ANG II and CNP were significantly increased in C. leucas chronically acclimated to seawater (SW) in comparison to freshwater (FW) acclimated fish. There was no difference in plasma AVT levels. Acute transfer of FW fish to 75% SW induced an increase in plasma ANG II levels within 12 h, and subsequent transfer from 75 to 100% SW further increased plasma ANG II levels at both 24 and 72 h. No change in plasma CNP was observed during acute transfer to increased salinity. However, a significant increase in plasma AVT levels was observed following 96 h in 75% SW and 24 h in 100% SW. In chronically SW acclimated C. leucas plasma osmolality, sodium, chloride, and urea were all significantly higher than FW acclimated fish but there was no difference in haematocrit. Acute transfer of C. leucas to 75% SW induced a significant increase in plasma osmolality, sodium and urea concentrations within 96 h of transfer. Subsequent transfer from 75 to 100% SW induced a further increase in these variables within 24 h in addition to a significant increase in plasma chloride above control levels. Haematocrit did not differ between the experimental and control groups throughout the acute study. Circulating levels of ANG II were significantly correlated to plasma, sodium, chloride, and urea concentrations during acclimation to SW. Conversely, circulating levels of CNP and AVT did not correlate to plasma osmolytes, however, CNP was significantly correlated to haematocrit during acclimation to seawater.

Hormonal control of salt and water balance in vertebrates

The endocrine system mediates many of the physiological responses to the homeostatic and acclimation demands of salt and water transport. Many of the hormones involved in the control of salt and water transport are common to all vertebrates, although their precise function and target tissues have changed during evolution. Arginine vasopressin (vasotocin), angiotensin II, natriuretic peptides, vasoactive intestinal peptide, urotensin II, insulin and non-genomic actions of corticosteroids are involved in acute (minutes and hours) alterations in ion and water transport. This rapid alteration in transport is primarily the result changes in behavior, blood flow to osmoregulatory organs, and membrane insertion or activation (e.g., phosphorylation) of existing transport proteins, ion and water channels, contransporters and pumps. Corticosteroids (through genomic actions), prolactin, growth hormone, and insulin-like growth factor I primarily control long-term (several hours to days) changes in transport capacity that are the result of synthesis of new transport proteins, cell proliferation, and differentiation. In addition to the important task of establishing broad evolutionary patterns in hormones involved in ion regulation, comparative endocrinology can determine species and population level differences in signaling pathways that may be critical for adaptation to extreme or rapidly changing environments.

Evolutionary aspects of intestinal bicarbonate secretion in fish

Experiments compared intestinal HCO3- secretion in the intestine of marine teleost Gulf toadfish, Opsanus beta, to representatives of early chondrostean and chondrichthyan fishes, the Siberian sturgeon, Acipenser baerii, and white-spotted bamboo shark, Chiloscyllium plagiosum, respectively. As seen in marine teleosts, luminal HCO3- concentrations were 10-fold plasma levels in all species when exposed to hyperosmotic conditions. While intestinal water absorption left Mg2+ and SO4(2-) concentrated in intestinal fluids up to four-fold ambient seawater concentrations, HCO3- was concentrated up to 50 times ambient levels as a result of intestinal HCO3- secretion. Reduced luminal Cl- concentrations in the intestine of all species suggest that HCO3- secretion also occurs via Cl-/HCO3- exchange in chondrostean and chondrichthyan fishes. Sturgeon began precipitating carbonates from the gut after only 3 days at 14 per thousand, a mechanism utilized by marine teleosts to reduce intestinal fluid osmolality and maintain calcium homeostasis. Analysis of published intestinal fluid composition in the cyclostome Lampetra fluviatilis reveals that this species likely also utilize intestinal HCO3- secretion for osmoregulation. Analysis of existing cyclostome data and our results indicate that intestinal Cl-/HCO3- exchange plays an integral role in maintaining hydromineral balance not only in teleosts, but in all fish (and perhaps other animals) with a need to drink seawater.

Effects of angiotensin II and C-type natriuretic peptide on the in situ perfused trunk preparation of the dogfish, Scyliorhinus canicula

The renal roles of physiologically relevant doses of angiotensin II (Ang II) and C-type natriuretic peptide (CNP) were investigated in the dogfish, Scyliorhinus canicula, using an in situ perfused trunk preparation. Perfusion with 10(-9) M Ang II resulted in a glomerular antidiuresis and decreases in perfusate flow rate, transport maxima for glucose and the proportion of filtering glomeruli. In addition, the renal clearances and excretion of urea, sodium, and chloride were significantly reduced, whereas the relative clearances of these parameters remained unchanged. In contrast, perfusion of 10(-9) M CNP caused a glomerular diuresis, an increase in transport maxima for glucose, but no significant change in the proportion of filtering glomeruli. In addition, the renal clearances of urea, sodium, and chloride were significantly increased but there was no effect on the relative clearances of urea, sodium, or chloride. Perfusion with 10(-10) M Ang II or CNP had no significant renal effects. Our results suggest that these hormones act at the level of the glomeruli rather than at a tubular level.

Sequence, circulating levels, and expression of C-type natriuretic peptide in a euryhaline elasmobranch, Carcharhinus leucas

The present study has examined expression and circulating levels of C-type natriuretic peptide (CNP) in the euryhaline bull shark, Carcharhinus leucas. Complementary DNA and deduced amino acid sequence for CNP in C. leucas were determined by RACE methods. Homology of CNP amino acid sequence in C. leucas was high both for proCNP and for mature CNP when compared with previously identified elasmobranch CNPs. Mature CNP sequence in C. leucas was identical to that in Triakis scyllia and Scyliorhinus canicula. Levels of expression of CNP mRNA were significantly decreased in the atrium but did not change in either the brain or ventricle following acclimation to a SW environment. However, circulating levels of CNP significantly increased from 86.0+/-7.9 fmol ml(-1) in FW to 144.9+/-19.5 fmol ml(-1) in SW. The results presented demonstrate that changes in environmental salinity influences both synthesis of CNP from the heart and also circulating levels in C. leucas. Potential stimulus for release and modes of action are discussed.

Activation of the newly discovered cyclostome renin-angiotensin system in the river lamprey Lampetra fluviatilis

This study describes the first investigations of the physiological signals involved in activating the newly discovered cyclostome renin-angiotensin system (RAS) and its role in the river lamprey Lampetra fluviatilis. Experimental manipulation showed that volume depletion (removal of 40% blood volume) rapidly activated the RAS of lampreys acclimated to water at 576 mOsm kg(-1) (21 p.p.t.), significantly increasing plasma angiotensin concentrations after 30 min and 60 min. In agreement with these results, a rapid change in environmental salinity (758 mOsm kg(-1) to freshwater (FW) and FW to 605 mOsm kg(-1)), resulted in a rapid decrease and increase in plasma [angiotensin], respectively. Intraperitoneal (i.p.) injection of FW-acclimated river lampreys with 1% body mass by volume of nominally isosmotic saline (120 mmol l(-1) NaCl; 233 mOsm kg(-1)) resulted in a significant decrease in the plasma angiotensin concentration within 15 min. In contrast, i.p. injection of hyperosmotic saline (4 mol l(-1) NaCl) at 1% body mass by volume, which significantly increased plasma osmolality, had no significant effect on plasma [angiotensin], suggesting that volume/pressure receptors and osmoreceptors interact in regulating the lamprey RAS. These results indicate an important role for volume/pressor receptors, as in teleosts, but with an additional osmoreceptor mechanism, such that circulatory [angiotensin] is determined by interaction of volume/pressure and osmoreceptors and their relative sensitivities. The volume/pressure sensitivity is in keeping with the recent evidence of a vasoconstrictor action of homologous lamprey angiotensin and provides evidence that the fundamental role of the RAS in maintaining volume and pressure is an ancient function conserved over 500 million years of vertebrate evolution.

Acute coronary syndromes and heart failure may reflect maladaptations of trauma physiology that was shaped during pre-modern evolution

We hypothesize that the pathophysiology of many cardiovascular diseases reflects a maladaptation of the triad of trauma response: adrenergia, inflammation, and coagulation. During biologic evolution, trauma has likely been a prevailing factor in natural selection. Components of the trauma triad act to limit hemorrhage, defend wounds against microorganisms, and initiate reconstruction. Response pathways that enable survival after trauma confer obvious adaptive advantages especially if the individual goes on to reproduce. Modern humans have shaped their own ecologic environment in such a way that the incidence of trauma has waned and previously unseen pathologies have emerged. Manifestations of modern diet, changing lifestyles, and extended lifespan have suddenly created new pathologic challenges to our prehistoric physiologic system. During our evolutionary heritage, endothelial injury and end-organ hypoxia were likely exclusively associated with physical trauma and the responses of the trauma triad were appropriate. Today, endothelial injury is more often precipitated by distinctly modern stressors such as hypertension, smoking, diabetes, and dyslipidemia. The once-adaptive trauma response can maladaptively initiate dangerous, self-propelling cycles of adrenergia, inflammation, and coagulation. Acute coronary syndromes perhaps best exemplify this phenomenon. Congestive heart failure, which often ensues, can similarly be seen as a maladaptation of the trauma triad. Whereas end-organ hypoxia was once commonly associated with trauma, now hypoxia is more often attributable to distinctly modern stressors such as pump failure. The fluid conservation and inflammation that results from the trauma triad was clearly adaptive in our prehistoric past, but in congestive heart failure the response is maladaptive, engendering self-propelling exacerbations of pump failure and vascular disease. Our maladaptive trauma response hypothesis portends new diagnostic and therapeutic paradigms for cardiovascular diseases and has ramifications for many other conditions such as stroke, venous thrombosis, vasculitis, aortic disease, arterial disease, pulmonary embolism, and restenosis.

Identification of angiotensin I in a cyclostome, Lampetra fluviatilis

Angiotensin I (ANG I) was isolated from incubates of plasma and kidney extracts of the river lamprey, Lampetra fluviatilis, using eel vasopressor activity as an assay during purification. Its sequence was Asn-Arg-Val-Tyr-Val-His-Pro-Phe-Thr-Leu as determined by the sequence analysis and mass spectrometry. The sequence was confirmed by identity of the elution profile with the synthetic peptide in two different reverse-phase columns of high-performance liquid chromatography. Lamprey ANG I produced dorsal-aortic pressor responses in L. fluviatilis but the rise was very small in comparison to that produced by angiotensin II. Angiotensin III produced an even bigger increase. It was not possible to demonstrate a difference in response to Asn(1) (lamprey) ANG I and Asp(1) (human) ANG I. The present study directly demonstrated the presence and biological activity of the renin-angiotensin system in the most primitive extant vertebrates, the cyclostomes. Thus the renin-angiotensin system is a phylogenetically old hormonal system that is present throughout the vertebrates.

Genomic characterization and expression analysis of the first nonmammalian renin genes from zebrafish and pufferfish

Renin is a key enzyme in the renin-angiotensin system (RAS), a pathway which plays an important physiological role in blood pressure and electrolyte homeostasis. The origin of the RAS is believed to have accompanied early evolution of vertebrates. However, renin genes have so far only been unequivocally identified in mammals. Whether or not a bona fide renin gene exists in nonmammalian vertebrates has been an intriguing question of physiological and evolutionary interest. Using a genomic analytical approach, we identified renin genes in two nonmammalian vertebrates, zebrafish (Danio rerio) and pufferfish (Takifugu rubripes). Phylogenetic analysis demonstrates that the predicted fish renins cluster together with mammalian renins to form a distinct subclass of vertebrate aspartyl proteases. RT-PCR results confirm generation of the predicted zebrafish mRNA and its expression in association with the opisthonephric kidney of adult zebrafish. Comparative in situ hybridization analysis of wild-type and developmental mutants indicates that renin expression is first detected bilaterally in cells of the interrenal primordia at 24 h postfertilization, which subsequently migrate to lie adjacent to, but distinct from, the glomerulus of the developing pronephric kidney. Our report provides the first molecular evidence for the existence of renin genes in lower vertebrates. The observation that the earliest renin-expressing cells, arising during ontogeny of this teleost vertebrate, are of adrenocortical lineage raises an interesting hypothesis as regards the origin of renin-expressing cells in the metanephric kidney of higher vertebrates.

Identification of angiotensin I in several vertebrate species: its structural and functional evolution

In order to delineate further the molecular evolution of the renin-angiotensin system in vertebrates, angiotensin I (ANG I) has been isolated after incubation of plasma and kidney extracts of emu (Dromiceus novaehollandiae), axolotl (Ambystoma mexicanum), and sea lamprey (Petromyzon marinus). The identified sequences were [Asp1, Val5, Asn9] ANG I in emu, [Asp1, Val5, His9] ANG I in axolotl, and [Asn1, Val5, Thr9] ANG I in sea lamprey. These results confirmed the previous findings that tetrapods have Asp and fishes including cyclostomes have Asn at the N-terminus, and that the amino acid residue at position 9 of ANG I was highly variable but, those at other positions were well conserved among different species. Since Asp and Asn are convertible during incubation, angiotensinogen sequences were searched in the genome and/or EST database to determine the N-terminal amino acid residue from the gene. The screening detected 12 tetrapod (10 mammalian, one avian, and one amphibian) and seven teleostean angiotensinogen sequences. Among them, all tetrapods have [Asp1] ANG except for Xenopus, and all teleosts have [Asn1] ANG, thereby confirming the above rule. Comparison of the vasopressor activity in the eel revealed that [Asn1] ANG I and II were more potent than [Asp1] peptides, which was opposite to the previous results in mammals and birds, in which [Asp1] ANG I and II were more potent. Collectively, the present results support the general rule that tetrapods have [Asp1] ANG and fishes including cyclostomes have [Asn1] ANG. However, an aquatic anuran (Xenopus) has [Asn1] ANG in its gene despite another aquatic urodele (axolotl) has [Asp1] ANG. From the functional viewpoint, homologous [Asn1] ANG was more potent in fish as is homologous [Asp1] ANG in tetrapods, suggesting that ANG II molecule has undergone co-evolution with its receptor during vertebrate phylogeny.

Urea based osmoregulation and endocrine control in elasmobranch fish with special reference to euryhalinity

Since the landmark contributions of Homer Smith and co-workers in the 1930s there has been a considerable advance in our knowledge regarding the osmoregulatory strategy of elasmobranch fish. Smith recognised that urea was retained in the body fluids as part of the 'osmoregulatory ballast' of elasmobranch fish so that body fluid osmolality is raised to a level that is iso- or slightly hyper-osmotic to that of the surrounding medium. From studies at that time he also postulated that many marine dwelling elasmobranchs were not capable of adaptation to dilute environments. However, more recent investigations have demonstrated that, at least in some species, this may not be the case. Gradual acclimation of marine dwelling elasmobranchs to varying environmental salinities under laboratory conditions has demonstrated that these fish do have the capacity to acclimate to changes in salinity through independent regulation of Na(+), Cl(-) and urea levels. This suggests that many of the presumed stenohaline marine elasmobranchs could in fact be described as partially euryhaline. The contributions of Thomas Thorson in the 1970s demonstrated the osmoregulatory strategy of a fully euryhaline elasmobranch, the bull shark, Carcharhinus leucas, and more recent investigations have examined the mechanisms behind this strategy in the euryhaline elasmobranch, Dasyatis sabina. Both partially euryhaline and fully euryhaline species utilise the same physiological processes to control urea, Na(+) and Cl(-) levels within the body fluids. The role of the gills, kidney, liver, rectal gland and drinking process is discussed in relation to the endocrine control of urea, Na(+) and Cl(-) levels as elasmobranchs acclimate to different environmental salinities.

Isolation, synthesis, and biological activity of flounder [Asn1,Ile5,Thr9] angiotensin I

A novel angiotensin I (ANG I) has been isolated from incubates of plasma and kidney extracts of the flounder, Platichthys flesus, using ion-exchange, gel-permeation, and reverse-phase high performance liquid chromatography (HPLC). Its sequence was determined as H-Asn-Arg-Val-Tyr-Ile-His-Pro-Phe-Thr-Leu-OH by sequence analysis and mass spectrometry. No vasopressor activity was detected at the elution position of [Asp(1)] ANG I in ion-exchange HPLC. The sequence was confirmed by identity of the elution position with the synthetic peptide in two different HPLC systems. When compared with ANG I isolated from other teleost fish, flounder ANG I uniquely has an isoleucine at position 5 rather than valine. Injection of angiotensin II (ANG II) into chronically cannulated flounder resulted in a dose-dependent pressor response, native [Asn(1),Ile(5)] ANG II, was found to elicit pressor responses comparable with those seen when teleost [Asn(1),Val(5)] ANG II and human [Asp(1),Ile(5)] ANG II were injected into flounder over the dose range 0.02-1.00 nmol/kg(-1). Plasma concentrations of the neurohypophysial peptide AVT were measured in chronically cannulated flounder following the injection of ANG II to examine the effect of ANG II on circulating AVT concentration. The injection of [Asn(1),Ile(5)] ANG II (1 nmolkg(-1)) or [Asp(1),Ile(5)] ANG II (2.5 nmolkg(-1)) resulted in a significant fall in the circulating levels of AVT suggesting that ANG II either directly or indirectly negatively influences AVT secretion.

Angiotensin converting enzyme-like activity in tissues from the river lamprey or lampern, Lampetra fluviatilis, acclimated to freshwater and seawater

Angiotensin converting enzyme (ACE) or kininase II is a dipeptidyl-carboxypeptidase that converts angiotensin I (Ang I) to angiotensin II (Ang II) in the renin-angiotensin system (RAS) and inactivates bradykinin in the kallikrein-kinin system (KKS). Angiotensin converting enzyme-like activity (ACELA) has been demonstrated in a wide range of vertebrates, and only in lampreys is a lack of ACELA still suggested. Though long controversial, a lamprey RAS has recently been identified by isolation and sequencing of lamprey Ang I and the measurement of circulating plasma angiotensins. We therefore re-investigated the presence of ACE in tissues from the river lamprey or lampern, Lampetra fluviatilis, using a highly sensitive fluorimetric assay. Significant detection of ACELA was found in a wide range of lamprey tissues (brain, gill, gonad, gut, heart, liver, skeletal muscle, skin, kidney, and plasma). The mammalian ACE inhibitor captopril at 10(-5)M was an effective, but variable inhibitor of the ACELA found in most lamprey tissues. The brain contained the highest ACELA, while kidney (including urinary duct), skin, gonads, and heart only contained very low ACELA. In most tissues, ACELA was similar in lampreys acclimated to freshwater (FW) and seawater (SW). However, gut ACELA was significantly higher in lampreys acclimated to SW than in FW-acclimated lampreys. Liver, skin, and gonad ACELA was significantly lower in lampreys acclimated to SW than in FW lampreys. Male and female lampreys acclimated to FW showed similar ACELA in all tissues except the kidney (including the urinary duct), where ACELA was significantly higher in male than in female lampreys. These results indicate that ACELA, a component of the RAS and KKS, is present in tissues from one of the earliest evolved groups of vertebrates.

Osmotic and volaemic effects on drinking rate in elasmobranch fish

An increase in drinking rate of two species of marine elasmobranch fish, Scyliorhinus canicula and Triakis scyllia, acclimated to 80% sea water was observed following the introduction of 100 % sea water to experimental tanks. The drinking response in both species was found to be maximal within 6 h, and a significant increase was sustained for up to 24 h in T. scyllia. Plasma osmolality was significantly increased within 6 h following introduction of 100 % sea water, and this increase was principally due to elevated plasma Na+ and Cl- concentrations. Administration of 2 mol l-1 mannitol, 75 % sucrose and vehicle(elasmobranch Ringer) did not induce a significant increase or decrease in the drinking rate of S. canicula. However, injection of 20 % NaCl was found to decrease drinking rate significantly in S. canicula 60 min after administration. Controlled haemorrhage of approximately 5.7 % of total blood volume in S. canicula induced a rapid 36-fold increase in drinking over basal levels. The present study demonstrates a physiological dipsogenesis in response to hypovolaemia in marine elasmobranch fish as part of their overall iso/hyperosmoregulatory strategy.

The dipsogenic effect of the renin-angiotensin system in elasmobranch fish

This study investigated the control of drinking in elasmobranch fish through manipulation of the homologous renin-angiotensin system (RAS). The smooth muscle relaxant papaverine was found to increase basal drinking levels in the European lesser-spotted dogfish, Scyliorhinus canicula, almost 20-fold. However, this response was significantly reduced with the coadministration of the angiotensin-converting enzyme inhibitor captopril which had no effect when administered alone. Captopril was also found to block a 7-fold increase in drinking rate following administration of homologous angiotensin I in S. canicula. Finally, administration of homologous angiotensin II produced a dose-dependent response in drinking rate in two species of elasmobranchs, S. canicula and the Japanese dogfish, Triakis scyllia. These results demonstrate a central role of the RAS in the control of drinking in elasmobranch fish.

The angiotensin system elements in invertebrates

In this review, the different components of the renin-angiotensin system (RAS) in invertebrates are discussed. This system is implicated in osmoregulation, reproduction, memory processes and immune system regulation. As the elements of this hormone-enzymatic system also exist in invertebrates, it appears that the RAS originated very early in evolution.

Circulating angiotensins in the river lamprey, Lampetra fluviatilis, acclimated to freshwater and seawater: possible involvement in the regulation of drinking

Plasma angiotensin levels were measured for the first time in a cyclostome, the river lamprey. With the demonstration that angiotensins are present in the circulation, the possibility of a physiological role in the regulation of drinking was re-examined. Angiotensin II and III concentrations and plasma osmolalities were significantly higher in lampreys acclimated to 28 ppt seawater than in those acclimated to freshwater. No changes were found in angiotensin II and III levels 4 h after transfer from freshwater to 50% seawater, although plasma osmolality had started to rise by this time. There was a suggestion that plasma angiotensin II levels might be related to osmolality in the transfer experiment. Injection of Asp(1)Val(5)- or Asn(1)Val(5)-angiotensin II (40-169 microg/kg body wt.) did not stimulate drinking in freshwater-acclimated lampreys, even when they were still capable of drinking. The angiotensin-converting enzyme inhibitor captopril and the smooth muscle relaxant papaverine both reduced drinking rate in 50% seawater-acclimated lampreys. The data do not provide direct evidence for the involvement of the renin-angiotensin system in the control of drinking behaviour in the lamprey. Indirect evidence from the captopril effect is suggestive, but could have other explanations.

Angiotensin II binding sites in the heart of Scyliorhinus canicula: an autoradiographic study

Dogfish (125)I [Asn(1), Pro(3), Ile(5)] angiotensin II ((125)I dfANG II) was used to establish the specific binding patterns of the different cardiac regions of the elasmobranch Scyliorhinus canicula by in vitro autoradiography. In the ventricular myocardium Scatchard analysis of saturation and displacement binding data revealed two classes of high- and low-affinity dfANG II binding sites (K(d) = 53 +/- 10 and 1300 +/- 900 pM). Two classes of dfANG II binding sites were also detected in the atrium (K(d) = 47 +/- 13 and 4690 +/- 930 pM) and in the outer layer of the conus arteriosus (K(d) = 16 +/- 9 and 398 +/- 83 pM). Conversely, the ventricular endocardium and the inner conal layer were characterized by a single class of dfANG II binding sites with affinity values of 48 +/- 11 and 106 +/- 3.3 pM, respectively. Competition experiments with either cold dfANG II or CV11974 or CGP42112 (specific ligands for mammalian AT(1) and AT(2) receptors, respectively) demonstrated a prevalence of CGP42112-selective dfANG II binding sites in both the inner and the outer conal layers. In the atrium, the ventricular myocardium, and the outer conal layer, dfANG II high-affinity binding sites poorly discriminated among the cold ligands. These results suggest that the dogfish heart may be a target organ of ANG II with distinct ANG II receptor subtype distributions.

Angiotensin receptors--evolutionary overview and perspectives

The structure of the angiotensin molecule has been well preserved throughout the vertebrate scale with some amino acid variations. Specific angiotensin receptors (AT receptors) that mediate important physiological functions have been noted in a variety of tissues and species. Physiological and pharmacological characterization of AT receptors and, more recently, molecular cloning studies have elucidated the presence of AT receptor subtypes. Comparative studies suggest that an AT receptor subtype homologous to the mammalian type 1 receptor subtype (AT(1)), though pharmacologically distinct, is present in amphibians and birds, whereas AT receptors cloned from teleosts show low homology to both AT(1) and AT(2) receptor subtypes. Furthermore, receptors differing from both the AT(1)-homologue receptor and AT(2) receptor exist in some non-mammalian species. This may suggest that the prototype AT receptor evolved in primitive vertebrates and diverged to more than one type of AT receptor subtype during phylogeny. Furthermore, phenotypic modulation of AT receptors appears to occur during individual development/maturation.

Angiotensin and angiotensin receptors in cartilaginous fishes

In mammals, a principal bioactive component of the renin-angiotensin system (RAS), angiotensin II (ANG II), is known to be vasopressor, dipsogenic, a stimulant of adrenocortical secretion and to control glomerular and renal tubular function. Historically, a RAS analogous to that found in mammals was thought to have first evolved in the bony fishes. Recent research has identified the unusually structured elasmobranch [Asp(1)-Pro(3)-Ile(5)] ANG II. Physiological studies have demonstrated that ANG II in elasmobranchs is vasopressor, and stimulates interrenal gland production of the elasmobranch corticosteroid 1alpha-hydroxycorticosterone. The specific binding of ANG II in elasmobranchs has been reported in gills, heart, interrenal gland, gut and rectal gland. The precise osmoregulatory role ANG II plays in cartilaginous fishes is not yet known; however, putative evidence is emerging for a role in the control of drinking rate, rectal gland secretion, and kidney function.

Comparative study of renin-containing cells. Histological approaches

The juxtaglomerular apparatus is known to be the functional unit of renin control. In the present review, the author will describe the comparative characteristics of renin-containing (RC) cells as well as extrarenal distribution, paying special attention to developmental and topographical approaches. The characteristic locality of RC cells suggests that the secretion of renin is performed at a site beside the adventitia or via the glomerular capillaries. Ontogenetical and phylogenetical investigations of RC cells have provided interesting findings on their morphogenesis. Analysis of the endocrine kidney after unilateral obstruction of the ureter provides some findings about the origin of RC cells and the processing of renin granules. Observation of developing adrenal renin suggests that there is important involvement of angiotensin II produced by renin synthesis in the morphogenesis of the adrenal gland in the fetal stage. Coagulating gland (CG) renin is characterized by testosterone-regulated and exocrine mechanisms. Recently, all or some of the components of the renin-angiotensin system (RAS) have been reported to be synthesized and secreted outside of classical organs or tissues. In the future, the real function of local RAS will be clarified by using gene targeting in mice.

Vascular angiotensin II receptor and calcium signaling in toadfish

The renin-angiotensin system evolved during the early evolution of vertebrates and regulates blood pressure/blood volume homeostasis in nonmammalian and mammalian vertebrates. Properties of vascular angiotensin (ANG) receptors and signal pathways in primitive animals are, however, not well understood. We aimed to determine whether vascular ANG II receptors in the toadfish, Opsanus tau, an aglomerular teleost, pharmacologically resemble either the ANG subtype 1 receptor (AT1) or the subtype 2 receptor (AT2) by examining (i) the effects of selective ANG receptor antagonists on ANG II-induced vasopressor action and binding and (ii) ANG II's effect on cytosolic Ca2+ signaling. [Asn1, Val5]ANG II (native teleost ANG II) dose-dependently increased the mean arterial pressure of conscious toadfish. ANG II-induced pressor responses (100-500 ng/kg) were inhibited substantially (79-83%) by [Sar1, Ile8]ANG II (5 microg x kg-1 + 5 microg x kg-1 x min-1) and moderately (34-53%) by losartan (AT1 antagonist, 10 mg/kg + 20 mg x kg-1 x h-1) and by PD 123319 (AT2 antagonist, 10 mg/kg + 20 mg x kg-1 x h-1) (36-60%). Likewise, the [Asp1, Val5, His9]ANG I-induced pressor effect was completely eliminated by an ANG I-converting enzyme inhibitor, SQ 14,225. Specific 125I-ANG II binding to vascular smooth muscle (VSM) membrane fractions was displaced completely by [Asn1, Val5]ANG II and [Sar1, Ile8]ANG II. Losartan, but not PD 123319, partly displaced ANG II binding at 10(-10)-10(-6) M. Furthermore, ANG II (10(-7) or 10(-8) M) caused a rapid, transient increase in the cytosolic Ca2+ signal (fluorescence ratio (FR) of 340/380 nm) of isolated VSM tissues measured by fura-2 and a dual wavelength fluorospectrometer, whereas extracellular K+ induced sustained, dose-dependent (P < 0.01) increases in FR. The results indicate that toadfish VSM tissues possess a rather nonselective ANG receptor; partial inhibition of ANG II binding by losartan and stimulation of cytosolic Ca2+ signaling by ANG II suggest that the receptor has some resemblance to AT1 homologous receptors.

Teleost-type angiotensin is present in Australian lungfish, Neoceratodus forsteri

Angiotensin I (ANG I) was produced from the incubation of lungfish plasma with homologous kidney extracts. The purified peptide was found to have the sequence of H-Asn-Arg-Val-Tyr-Val-His-Pro-Phe-Thr-Leu-OH, which is homologous for the first eight residues with all teleost angiotensins so far sequenced, although lungfish generally possess tetrapod-type hormones. The lungfish decapeptide (ANG I) induced dose-dependent increases in arterial pressure in the rat. The lungfish octapeptide (ANG II) released aldosterone from kidney-adrenal tissue in vitro in a dose-dependent manner and induced dose-dependent increases in arterial pressure of the lungfish. Substitution of asparagine with aspartic acid in the first position (tetrapod-type ANG II) did not alter the blood pressure response significantly, but a second substitution of the valine in the (5)-position with isoleucine (ANG II form found in human and rat) abolished the rise in arterial pressure in lungfish over the same dose range.

A radioimmunoassay for the determination of angiotensin II in elasmobranch fish

A homologous radioimmunoassay was developed to determine the concentration of angiotensin II (Asn1, Pro3, Ile5)-Ang II) in elasmobranchs. Cross-reactivity with elasmobranch angiotensin I and other heterologous angiotensins was high and therefore all potentially cross-reacting angiotensins were separated by high performance liquid chromatography after prior extraction with Sep-Pak C18 cartridges. The validity of the assay for the determination of elasmobranch Ang II was demonstrated by parallelism with a series of Ang II standards with serially diluted elasmobranch plasma extracts. Overall recovery of elasmobranch Ang II added to a plasma pool was 75.1 +/- 5.2%. Plasma Ang II concentrations measured by our RIA were similar in fish adapted to 70, 100, or 120% SW at 139 +/- 20.1, 109 +/- 15.3, and 119 +/- 16.3 fmol . ml-1, respectively.

Angiotensin, thirst, and sodium appetite

Angiotensin (ANG) II is a powerful and phylogenetically widespread stimulus to thirst and sodium appetite. When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in NaCl intake. Drinking is vigorous, highly motivated, and rapidly completed. The amounts of water taken within 15 min or so of injection can exceed what the animal would spontaneously drink in the course of its normal activities over 24 h. The increase in NaCl intake is slower in onset, more persistent, and affected by experience. Increases in circulating ANG II have similar effects on drinking, although these may be partly obscured by accompanying rises in blood pressure. The circumventricular organs, median preoptic nucleus, and tissue surrounding the anteroventral third ventricle in the lamina terminalis (AV3V region) provide the neuroanatomic focus for thirst, sodium appetite, and cardiovascular control, making extensive connections with the hypothalamus, limbic system, and brain stem. The AV3V region is well provided with angiotensinergic nerve endings and angiotensin AT1 receptors, the receptor type responsible for acute responses to ANG II, and it responds vigorously to the dipsogenic action of ANG II. The nucleus tractus solitarius and other structures in the brain stem form part of a negative-feedback system for blood volume control, responding to baroreceptor and volume receptor information from the circulation and sending ascending noradrenergic and other projections to the AV3V region. The subfornical organ, organum vasculosum of the lamina terminalis and area postrema contain ANG II-sensitive receptors that allow circulating ANG II to interact with central nervous structures involved in hypovolemic thirst and sodium appetite and blood pressure control. Angiotensin peptides generated inside the blood-brain barrier may act as conventional neurotransmitters or, in view of the many instances of anatomic separation between sites of production and receptors, they may act as paracrine agents at a distance from their point of release. An attractive speculation is that some are responsible for long-term changes in neuronal organization, especially of sodium appetite. Anatomic mismatches between sites of production and receptors are less evident in limbic and brain stem structures responsible for body fluid homeostasis and blood pressure control. Limbic structures are rich in other neuroactive peptides, some of which have powerful effects on drinking, and they and many of the classical nonpeptide neurotransmitters may interact with ANG II to augment or inhibit drinking behavior. Because ANG II immunoreactivity and binding are so widely distributed in the central nervous system, brain ANG II is unlikely to have a role as circumscribed as that of circulating ANG II. Angiotensin peptides generated from brain precursors may also be involved in functions that have little immediate effect on body fluid homeostasis and blood pressure control, such as cell differentiation, regeneration and remodeling, or learning and memory. Analysis of the mechanisms of increased drinking caused by drugs and experimental procedures that activate the renal renin-angiotensin system, and clinical conditions in which renal renin secretion is increased, have provided evidence that endogenously released renal renin can generate enough circulating ANG II to stimulate drinking. But it is also certain that other mechanisms of thirst and sodium appetite still operate when the effects of circulating ANG II are blocked or absent, although it is not known whether this is also true for angiotensin peptides formed in the brain. Whether ANG II should be regarded primarily as a hormone released in hypovolemia helping to defend the blood volume, a neurotransmitter or paracrine agent with a privileged role in the neural pathways for thirst and sodium appetite of all kinds, a neural organizer especially in sodium appetit

Tetrapod-type [Asp1] angiotensin is present in a holostean fish, Amia calva

The renin-angiotensin system has been identified in various vertebrates, from elasmobranchs to mammals. Tetrapod (amphibians to mammals) angiotensin (ANG) has Asp at the N-terminus, but Asp is replaced by Asn in elasmobranch and teleost fish. ANG I has been isolated from incubates of plasma and kidney extracts of the bowfin Amia calva, a holostean fish, using the eel vasopressor activity as an assay system; its sequence was found to be H-Asp-Arg-Val-Tyr-Val-His-Pro-Phe-Asn-Leu-OH after sequence analysis, mass spectrometry, and comparison with the synthetic peptide. This sequence is identical to bullfrog ANG I. [Asn1] ANG I was not detected. Thus the bowfin is the first fish species which contains only [Asp1] ANG I. The bowfin ANG I and II were no more vasopressor than eel peptides in the bowfin, indicating that bowfin ANG II receptors do not distinguish between [Asp1] and [Asn1] peptides. In the rat, bowfin ANG I and rat [Ile5, His9] ANG I have equipressor activities when examined in different animals, but the vasopressor activity of bowfin ANG I decreased following rat ANG I in the same animals, although the activity of rat ANG I was unaffected after bowfin ANG I. The present study directly demonstrates the presence of the renin-angiotensin system in a holostean fish and showed that its ANG II receptors have not yet fully coevolved with the homologous [Asp1] peptide.

Atypical angiotensin II receptors coupled to phosphoinositide turnover/calcium signalling in catfish hepatocytes

In catfish (Ictalurus punctatus) hepatocytes angiotensin II induced an immediate increase in cytosolic Ca2+ concentration. Other angiotensin analogues also induced this effect including: human angiotensin II, fish angiotensin II, human angiotensin III, human angiotensin I, fish angiotensin I and saralasin. CGP 42112A induced a very small effect at the highest concentration tested and angiotensin IV was without effect. Angiotensin II also increased the resynthesis of phosphatidylinositol and the production of IP3. These physiological effects were not blocked by losartan (AT1-selective antagonist) or PD 123177 (AT2-selective antagonist). [125I]Angiotensin II bound to liver plasma membranes in a saturable fashion with high affinity (K(D) 2.7 nM) and a B(max) of 185 fmol/mg of protein. Binding competition experiments showed the following order of potency: human angiotensin II = fish angiotensin II > human angiotensin III > or = human angiotensin I = fish angiotensin I. These sites were insensitive to losartan or PD 123177. The data indicate that the angiotensin II receptors expressed in catfish hepatocytes are coupled to the phosphoinositide turnover/calcium mobilization signal transduction pathway and are atypical receptors, i.e., pharmacologically distinct from mammalian AT1 and AT2 receptors.

A comparison of the leech Theromyzon tessulatum angiotensin I-like molecule with forms of vertebrate angiotensinogens: a hormonal system conserved in the course of evolution

After five steps of purification including gel permeation, anti-angiotensin I affinity column chromatography followed by reverse-phase HPLC, a peptide immunoreactive to two different antisera (anti-angiotensin II and anti-angiotensin I) was purified to homogeneity from extracts of the leech Theromyzon tessulatum. The first 14 amino acid residues of the purified peptide (DRVYIHPFHLLXWG) established by automated Edman degradation, reveal the existence in leeches of an angiotensin I-like molecule close to human angiotensin I. The sequence of the purified peptide presents 78.5% of homology with the N-terminal part of human angiotensinogen. Moreover, in its sequence, this peptide presents the cleavage sites of vertebrate angiotensin metabolic enzymes, i.e. the renin and the angiotensin-converting enzyme. This finding constitutes the first biochemical characterization of an angiotensin I in Invertebrates. It also reflects the high conservation of angiotensins in the course of evolution, suggesting a fundamental role of this family in fluid homeostasis.

Glycyl-histidyl-lysine interacts with the angiotensin II AT1 receptor

Gly-His-Lys, a tripeptide isolated from human plasma that increases the growth rate of many cells, stimulated in dose-dependent fashion the activity of phosphorylase a in isolated rat hepatocytes. Such effect was associated to increases in both IP3 production and [Ca++]i. Interestingly, these effects of Gly-His-Lys were antagonized by losartan, a nonpeptide angiotensin II receptor antagonist (AT1 selective), which suggested that these receptors were involved in its effect. Binding competition experiments using the radioligand [125I][Sar1-Ile8]angiotensin II clearly indicated that Gly-His-Lys interacts with AT1 receptors. It was also observed that other histidine-containing tripeptides were also capable of interacting with these receptors.