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

Tetra-ARMS PCR analysis of angiotensinogen AGT T174M (rs4762) genetic polymorphism in diabetic patients: a comprehensive study

Background and purpose

Hypertension (HTN) is a multifactorial chronic disease that poses a significant global health burden and is associated with increased mortality rates. It often coexists with other conditions, such as cardiovascular, liver, and renal diseases, and has a strong association with diabetes mellitus. Insulin resistance and endothelial dysfunction commonly occur in individuals with both HTN and type 2 diabetes mellitus (T2DM). Genetic factors, along with environmental and pathological factors, play a role in the development of HTN. Recent studies have revealed the influence of single nucleotide polymorphisms (SNPs) in various genes on HTN. In this study, we aimed to investigate the genetic polymorphism of angiotensinogen (AGT) T174M (rs4762) and its association with HTN in diabetic patients.

Methods

A total of 300 participants were enrolled in this study and divided into three groups: control, hypertensive, and hypertensive diabetic. Blood samples were collected, and predetermined biochemical parameters were assessed. Genotyping of the AGT T174M (rs4762) gene was conducted using Tetra ARMS PCR with specific primers.

Results

The study findings revealed a significant association between AGT T174M (rs4762) genotype and HTN in diabetic patients within the Pakistani population. The C/T genotype of AGT T174M (rs4762) was found to be significant in both the hypertensive and hypertensive diabetic participants compared to the control group. This genotype was identified as a risk factor for developing HTN in both the hypertensive and hypertensive diabetic participants.

Conclusion

This study demonstrates a significant association between AGT T174M (rs4762) genetic polymorphism and HTN in diabetic patients. The C/T genotype of AGT T174M (rs4762) may serve as a potential marker for identifying individuals at risk of developing HTN, specifically in the hypertensive and hypertensive diabetic populations. Further research is warranted to elucidate the underlying mechanisms and validate these findings in larger cohorts.

Genetic Polymorphism in Angiotensinogen and Its Association with Cardiometabolic Diseases

Angiotensinogen (AGT) is one of the most significant enzymes of the renin-angiotensin-aldosterone system (RAAS) which is involved in the regulation and maintenance of blood pressure. AGT is involved in the production of angiotensin I which is then converted into angiotensin II that leads to renal homeostasis. However, various genetic polymorphisms in AGT have been discovered in recent times which have shown an association with various diseases. Genetic polymorphism increases the level of circulating AGT in blood which exaggerates the effects produced by AGT. The associated diseases occur due to various effects produced by increased AGT levels. Several cardiovascular diseases including myocardial infarction, coronary heart disease, heart failure, hypertrophy, etc. are associated with AGT polymorphism. Other diseases such as depression, obesity, diabetic nephropathy, pre-eclampsia, and liver injury are also associated with some variants of AGT gene. The most common variants of AGT polymorphism are M235T and T174M. The two variants are associated with many diseases. Some other variants such as G-217A, A-6G, A-20C and G-152A, are also present but they are not as significant as that of M235T and T174M variants. These variants increase the level of circulating AGT and are associated with prevalence of different diseases. These diseases occur through various pathological pathways, but the initial reason remains the same, i.e., increased level of AGT in the blood. In this article, we have majorly focused on how genetic polymorphism of different variants of AGT gene is associated with the prevalence of different diseases.

The vasopressor action of angiotensin II (ANG II) in ball pythons (Python regius)

Angiotensin II (ANG II) is part of the renin-angiotensin system (RAS) in vertebrates and exert vasoconstriction in all species studied. The present study examines the vasopressor effect of ANG II in the ball python (Python regius), and examines whether ANG II exert its effect through direct angiotensin receptors or through an activation of α-adrenergic receptors. The studies were conducted in snakes with chronic arterial catheters that had recovered from anesthesia. In addition to demonstrating a clear and pronounced dose-dependent rise in arterial blood pressure upon repeated injections of boluses with ANG II (0.001-1 μg/kg), we demonstrate that the pressor response persisted following α-adrenergic blockade using the α-adrenergic antagonist phentolamine (2.5 mg/kg). Unfortunately, it proved impossible to block the ANG receptors using losartan (1, 3 or even 10 mg/kg). The pressor response to ANG II was associated with a significant rise in heart rate at the higher dosages, pointing to a resetting of the barostatic mechanism for heart rate regulation. The responses were similar in fasting and digesting pythons despite the expected rise in baseline values for blood pressure and heart rate of the digesting snakes.

The Cardiopulmonary Effects of the Calcitonin Gene-related Peptide Family

Cardiopulmonary diseases are very common among the population. They are high-cost diseases and there are still no definitive treatments. The roles of members of the calcitonin-gene related-peptide (CGRP) family in treating cardiopulmonary diseases have been studied for many years and promising results obtained. Especially in recent years, two important members of the family, adrenomedullin and adrenomedullin2/intermedin, have been considered new treatment targets in cardiopulmonary diseases. In this review, the roles of CGRP family members in cardiopulmonary diseases are investigated based on the studies performed to date.

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.

Association of angiotensinogen gene SNPs and haplotypes with risk of hypertension in eastern Indian population

BACKGROUND

Angiotensinogen (AGT) enzyme comprises a vital module of RAAS system that effectively controls the blood pressure and related cardiovascular functions. Ample association studies have reported the importance of AGT variants in cardiovascular and non-cardiovascular adversities. But lately, owing to the complexity of the many anomalies, the haplotype based examination of genetic variation that facilitates the identification of polymorphic sites which are located in the vicinity of the causative polymorphic site, gets greater appreciation.

METHODS

In the present study, we have done genotype and haplotype analysis of AGT gene in reference to hypertension to confirm the association of the two in an Indian population. To accomplish this, we performed candidate SNPs analysis and construct possible haplotypes across the AGT promoter and gene region in 414 subjects (256 Hypertensive cases and 158 controls).

RESULTS

We found four SNPs (rs11568020: A-152G and rs5050: A-20C in promoter; rs4762 and rs699 in exon2) and 3 haplotypes (H4, H7 and H8) that showed a stronger positive association with hypertension. The haplotype H2 was showing protective association with hypertension.

CONCLUSION

The results of the present study confirmed and reestablished the role of AGT gene variants and their haplotypes in the causation of hypertension in Indian population and showed that haplotypes can provide stronger evidence of association.

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.

Classical Renin-Angiotensin system in kidney physiology

The renin-angiotensin system has powerful effects in control of the blood pressure and sodium homeostasis. These actions are coordinated through integrated actions in the kidney, cardiovascular system and the central nervous system. Along with its impact on blood pressure, the renin-angiotensin system also influences a range of processes from inflammation and immune responses to longevity. Here, we review the actions of the "classical" renin-angiotensin system, whereby the substrate protein angiotensinogen is processed in a two-step reaction by renin and angiotensin converting enzyme, resulting in the sequential generation of angiotensin I and angiotensin II, the major biologically active renin-angiotensin system peptide, which exerts its actions via type 1 and type 2 angiotensin receptors. In recent years, several new enzymes, peptides, and receptors related to the renin-angiotensin system have been identified, manifesting a complexity that was previously unappreciated. While the functions of these alternative pathways will be reviewed elsewhere in this journal, our focus here is on the physiological role of components of the "classical" renin-angiotensin system, with an emphasis on new developments and modern concepts.

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.

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.

Molecular and Pathophysiological Features of Angiotensinogen: A Mini Review

The renin-angiotensin system is an essential regulatory system for blood pressure and fluid homeostasis. Angiotensinogen is the only known precursor of all the peptides generated in this system. While many of the basic understandings of angiotensinogen have come from research efforts to define its role in blood pressure regulation, novel pathophysiological functions of angiotensinogen have been discovered in the last two decades including kidney developmental abnormalities, atherosclerosis, and obesity. Despite the impressive advance in the understanding of angiotensinogen gene structure and protein functions, some fundamental questions remain unanswered. In this short review, we provide contemporary insights into the molecular characteristics of angiotensinogen and its pathophysiological features. In light of the recent progress, we emphasize some newly recognized functional features of angiotensinogen other than its regulation on blood pressure.

Antidiuretic action of angiotensin II in the river lamprey Lampetra fluviatilis: evidence for endocrine control of kidney function in cyclostomes

Intravenous infusion of angiotensin II ([Asn¹ Val⁵]-Ang II) at 10⁻⁹ mol min⁻¹ kg⁻¹ body mass produced a significant antidiuresis in river lamprey Lampetra fluviatilis, captured during upstream migration and maintained in fresh water. Although the renin-angiotensin hormonal system (RAS) is now recognized in jawless fishes, until this study, the role of homologous Ang II in L. fluviatilis kidney function had not been examined. This study provides the first evidence for an antidiuretic action of Ang II in cyclostomes and, in evolutionary terms, suggests a renal function for the RAS in early vertebrates.

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.

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.

Potent cardiovascular actions of homologous adrenomedullins in eels

Adrenomedullin (AM), known as a multifunctional hormone in mammals, forms a unique family of five paralogous peptides in teleost fish. To examine their cardiovascular effects using homologous AMs in eels, we isolated cDNAs encoding four eel AMs, and named AM1 (ortholog of mammalian AM), AM2, AM3 (paralog of AM2 generated only in teleost lineage), and AM5 according to the known teleost AM sequences. Unlike pufferfish, not only AM1 but AM2/3 and AM5 were expressed ubiquitously in various eel tissues. Synthetic mature AM1, AM2, and AM5 exhibited vasodepressor effects after intra-arterial injections, and the effects were more potent at dorsal aorta than at ventral aorta. This indicates that AMs preferentially act on peripheral resistance vessels rather than on branchial arterioles. The potency was in the order of AM2 = AM5 ≫ AM1 in both freshwater (FW) and seawater (SW) eels, which is different from the result of mammals in which AM1 is as potent as, or more potent than, AM2 when injected peripherally. The minimum effective dose of AM2 and AM5 in eels was 1/10 that of AM1 in mammals. The hypotension reached 50% at 1.0 nmol/kg of AM2 and AM5, which is much greater than atrial natriuretic peptide (20%), another potent vasodepressor hormone. Even with such hypotension, AMs did not change heart rate in eels. In addition, AM1 increased blood pressure at ventral aorta and dorsal aorta immediately after an initial hypotension at 5.0 nmol/kg, but not with AM2 and AM5. These data strongly suggest that specific receptors for AM2 and AM5 exist in eels, which differ from the AM1 receptors identified in mammals.

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.

The His-Pro-Phe motif of angiotensinogen is a crucial determinant of the substrate specificity of renin

The amino acid sequence His-Pro-Phe as N-terminal residues 6–8 of the natural renin substrate, angiotensinogen, is conserved among species. We investigated whether this His-Pro-Phe motif functions as the determinant of the substrate specificity of renin. Mutant angiotensinogens in which the Ile-His-Pro-Phe-His-Leu sequence at positions 5–10 of wild-type angiotensinogen was replaced by either His-Pro-Phe-His-Leu-Leu or Ala-Ile-His-Pro-Phe-His were cleaved by renin at the C-terminal side of residues 9 and 11, respectively, while wild-type angiotensinogen was cleaved at residue 10. A triple Ala substitution for the His-Pro-Phe motif of angiotensinogen prevented its cleavage by renin. In contrast, triple Ala substitution for residues 9–11, including the natural site of cleavage by renin, allowed cleavage between the two Ala residues at positions 10 and 11. Furthermore, the 33-residue C-terminal peptide of human megsin, which carries a naturally occurring His-Pro-Phe sequence, was cleaved by renin at the C-terminal side of the His-Pro-Phe-Leu-Phe sequence. These results indicate that the His-Pro-Phe motif of angiotensinogen is a crucial determinant of the substrate specificity of renin. By binding to a corresponding pocket on renin, the His-Pro-Phe motif may act as a molecular anchor to recruit the scissile peptide bond to a favorable site for catalysis.

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.

Differential status of the renin-angiotensin system of silver sea bream (Sparus sarba) in different salinities

Silver sea bream (Sparus sarba) is extremely euryhaline and can survive in a wide range of salinities (0-70 per thousand). The status of the renin-angiotensin system (RAS) in sea bream adapted to different salinities was studied. As indicated by plasma Ang II levels, a suppressed status of the RAS was found to occur under brackish water conditions; while under hypersaline conditions, an activated RAS prevailed, especially in fish adapted to double strength seawater (70 per thousand). Captopril successfully blocked the conversion of Ang I to Ang II, causing a dramatic drop in plasma Ang II levels, and such decrease was accompanied by lowered plasma cortisol levels. The pattern of changes in branchial Na-K-ATPase activity in different salinities was similar to those of plasma Ang II and cortisol, suggesting a causal regulatory role of Ang II on branchial Na-K-ATPase activity. Intraperitoneal injection of Ang II elicited a dose-dependent increase in branchial Na-K-ATPase activity in both 33- and 6 per thousand-adapted sea bream, but a relatively more intense stimulation of enzyme activity occurred in hyposmotic-adapted fish. Abrupt hyposmotic transfer rapidly lowered plasma Ang II level but elevated branchial Na-K-ATPase and transiently elevated plasma cortisol, indicating that these parameters are not solely controlled by Ang II but are also influenced by other hormonal factors that change during salinity transfer. Blood volumes of both 33- and 6 per thousand-adapted sea bream exhibited high stability during short-term salinity transfers and after long-term salinity adaptation. Captopril significantly reduced resting blood pressure in both 33- and 6 per thousand-adapted sea bream, indicating that the RAS was involved in maintenance of resting blood pressure in both hyperosmotic and hyposmotic environments. Blood pressure was highly stable during abrupt salinity transfer and captopril blockade did not alter such stability. The vasopressive effect of angiotensins was more potent in 6 per thousand-adapted sea bream. These results showed that the RAS is involved in the maintenance of fluid and pressure homeostasis in sea bream and hyposmotic-adapted sea bream has an abated RAS status.

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.

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.