Localization of angiotensin-converting enzyme in the trout gill

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.

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.

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

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

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.

Response by the corpuscles of Stannius to hypotensive stimuli in three divergent ray-finned fishes (Amia calva, Anguilla rostrata, and Catastomus commersoni): cardiovascular and morphological changes

In accordance with their vital role in cardiovascular physiology () corpuscles of Stannius (CS) from two teleosts and an holostean species showed marked and consistent degranulation and exocytotic responses to hypotensive stimuli. In eels (Anguilla rostrata LeSueur) acute blood withdrawal (hypovolemic hypotension) was followed by a prompt decrease in cardiac output (CO) and dorsal aortic pressure (P(DA)), a compensatory tachycardic response and an increase in systemic vascular resistance (R(SYS)). Isovolemic hypotension induced by papaverine i.v., led to a similar, but more prolonged, decrease in P(DA) but the heart rate (HR) continued to accelerate, thereby counterbalancing the severe and persistent decrease in R(SYS). Both hypovolemic and isovolemic hypotension were followed by a significant depletion of cytoplasmic granules from eel CS even though plasma concentrations of Ca, Mg, Na(+), and K(+) were normal. In an ancient holostean fish, the bowfin, Amia calva and a generalized teleost fish, Catastomus commersoni, the number of cytoplasmic granules decreased by 39% and 54%, respectively, 120 min after the acute withdrawal of 8 ml kg bw(-1) of blood. These findings suggest that a primary role of the CS is to release cytoplasmic granules containing renin or isorenin into the blood circulation, in response to hypotension and/or hypovolemia.

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.

Corpuscles of Stannius secrete renin or an isorenin that regulates cardiovascular function in freshwater North American eels, Anguilla rostrata LeSueur

Dorsal aortic blood flow (DABF) and caudal venous blood flow (CVBF) were measured in free-swimming conscious freshwater (FW) North American eels (Anguilla rostrata) with Doppler-flow probes. DABF and CVBF increased in a dose-dependent manner following iv doses of [Asn(1), Val(5), Gly(9)]-angiotensin I (ANG I), [Asn(1), Val(5)]-angiotensin II (ANG II), and [Val(4)]-angiotensin III (ANG III) ranging from 5 to 50 ng x kg bw(-1). A minimum effective dose for ANG I and ANG II was 5 ng x kg bw(-1); that for ANG III was 10 ng x kg bw(-1). DABF and CVBF rates increased during the first 2 min and remained elevated for 20-50 min. Flow responses similar to those of ANG II in form and magnitude followed iv injections of extracts of corpuscles of Stannius (CS-EXT). Increases in DABF and CVBF following injections of ANG I, human renin substrate (hRS), and CS-EXT were all blocked by the angiotensin-converting enzyme inhibitor Captopril. Increases in DABF and CVBF which followed injections of hRS and CS-EXT were blocked completely by pepstatin A. [Sar(1), Val(5)]-ANG II (Sarile) blocked completely the DABF and CVBF responses to ANG II and CS-EXT, but the mammalian receptor antagonists losartan (AT1) and PD123319 (AT2) only partially blocked them. These findings support strongly the hypothesis that the corpuscles of Stannius secrete renin or isorenin and that the renin-angiotensin system regulates cardiovascular function in freshwater eels and other bony fishes that possess them.

Hormone metabolism by the fish gill

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

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

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

Distribution and characterization of angiotensin-converting enzyme-like activity in tissues of the blue crab Callinectes sapidus

Angiotensin-converting enzyme-like activity (ACELA) was determined in tissue homogenates of the blue crab, Callinectes sapidus, using the synthetic substrate hippuryl-histidyl-leucine (Hip-His-Leu) and the specific inhibitor, captopril. ACELA was highest in gill homogenates followed by the hepatopancreas and hemolymph with specific activities of 1.69, 0.37 and 0.10 nmol/min/mg protein, respectively. Gill enzyme activity was membrane-associated and no difference in activity was noted between anterior and posterior gills. The enzyme preparation from gill membranes was activated by chloride and had a Km of 4.1 +/- 0.4 mmol and a Vmax of 39.5 +/- 2.0 nmol/min/mg protein. The enzyme was strongly inhibited by captopril and lisinopril with IC50 s of 3.8 x 10(-8) and 2.6 x 10(-8) M. The enzyme was less strongly inhibited by angiotensin II and SQ-20881 with IC50 s of 5.1 x 10(-5) and 2.5 x 10(-6) M, respectively.

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

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

Vasculature of the fish gill: anatomical correlates of physiological functions

The fish gill is a multifunctional organ responsible for respiration, osmoregulation, acid base balance, nitrogen excretion, and metabolism of circulating hormones. Two or more microcirculatory systems subserve these activities and form one of the most complex vascular networks found in any vertebrate. In this article the vascular anatomy of the teleost gill and the role of gill vessels in mediating physiological function are examined. Vascular corrosion replication techniques have been instrumental in resolving the spatial organization of gill microcirculation. Variations in the replication procedures provide information on the interrelationships between the vascular pathways, factors that govern flow distribution, and the physical characteristics of the vessels themselves. Anatomically, gill vessels are as diverse as their physiological functions. Pillar cells, unique to the fish gill, form the lining of the respiratory vasculature and may have substantial metabolic effects on circulating hormones. The non-respiratory pathways appear to be lined with both typical and unusual endothelial cells, although the fine structure and function(s) of these vessels are largely unknown. To date most of the information on gill vessels has been derived from descriptive morphological studies. Further evaluation of the anatomical and physiological correlations of these tissues is predicated upon the application of histocytochemical, morphometric, and other quantitative methodologies as well as an examination of gills from fish with various evolutionary and environmental backgrounds.