Isolation of high-molecular-weight C-type natriuretic peptide from the heart of a cartilaginous fish (European dogfish, Scyliorhinus canicula)

The Rectal Gland of the Shark: The Road to Understanding the Mechanism and Regulation of Transepithelial Chloride Transport

Pictured, described, and speculated on, for close to 400 years, the function of the rectal gland of elasmobranchs remained unknown. In the late 1950s, Burger discovered that the rectal gland of Squalus acanthias secreted an almost pure solution of sodium chloride, isosmotic with blood, which could be stimulated by volume expansion of the fish. Twenty five years later, Stoff discovered that the secretion of the gland was mediated by adenyl cyclase. Studies since then have shown that vasoactive intestinal peptide (VIP) is the neurotransmitter responsible for activating adenyl cyclase; however, the amount of circulating VIP does not change in response to volume expansion. The humoral factor involved in activating the secretion of the gland is C-type natriuretic peptide, secreted from the heart in response to volume expansion. C-type natriuretic peptide circulates to the gland where it stimulates the release of VIP from nerves within the gland, but it also has a direct effect, independent of VIP. Sodium, potassium, and chloride are required for the gland to secrete, and the secretion of the gland is inhibited by ouabain or furosemide. The current model for the secretion of chloride was developed from this information. Basolateral NaKATPase maintains a low intracellular concentration of sodium, which establishes the large electrochemical gradient for sodium directed into the cell. Sodium moves from the blood into the cell (together with potassium and chloride) down this electrochemical gradient, through a coupled sodium, potassium, and two chloride cotransporter (NKCC1). On activation, chloride moves from the cell into the gland lumen, down its electrical gradient through apical cystic fibrosis transmembrane regulator. The fall in intracellular chloride leads to the phosphorylation and activation of NKCC1 that allows more chloride into the cell. Transepithelial sodium secretion into the lumen is driven by an electrical gradient through a paracellular pathway. The aim of this review was to examine the history of the origin of this model for the transport of chloride and suggest that it is applicable to many epithelia that transport chloride, both in resorptive and secretory directions.

Physiological and Pathophysiological Effects of C-Type Natriuretic Peptide on the Heart

Simple Summary

C-type natriuretic peptide (CNP) is the third member of the natriuretic peptide family. Unlike atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), CNP was not previously regarded as an important cardiac modulator. However, recent studies have revealed the physiological and pathophysiological importance of CNP in the heart; in concert with its cognate natriuretic peptide receptor-B (NPR-B), CNP has come to be regarded as the major heart-protective natriuretic peptide in the failed heart. In this review, I introduce the history of research on CNP in the cardiac field.

Abstract

C-type natriuretic peptide (CNP) is the third member of the natriuretic peptide family. Unlike other members, i.e., atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), which are cardiac hormones secreted from the atrium and ventricle of the heart, respectively, CNP is regarded as an autocrine/paracrine regulator with broad expression in the body. Because of its low expression levels compared to ANP and BNP, early studies failed to show its existence and role in the heart. However, recent studies have revealed the physiological and pathophysiological importance of CNP in the heart; in concert with the distribution of its specific natriuretic peptide receptor-B (NPR-B), CNP has come to be regarded as the major heart-protective natriuretic peptide in the failed heart. NPR-B generates intracellular cyclic guanosine 3′,5′-monophosphate (cGMP) upon CNP binding, followed by various molecular effects including the activation of cGMP-dependent protein kinases, which generates diverse cytoprotective actions in cardiomyocytes, as well as in cardiac fibroblasts. CNP exerts negative inotropic and positive lusitropic responses in both normal and failing heart models. Furthermore, osteocrin, the intrinsic and specific ligand for the clearance receptor for natriuretic peptides, can augment the effects of CNP and may supply a novel therapeutic strategy for cardiac protection.

B-type natriuretic peptide (BNP), not ANP, is the principal cardiac natriuretic peptide in vertebrates as revealed by comparative studies

The natriuretic peptide (NP) family consists of at least seven members; cardiac ANP, BNP and VNP and brain CNPs (CNP1-4). Phylogenetic and comparative genomic analyses showed that CNP4 is the ancestral molecule of the family, from which CNP3 and CNP1/2 were duplicated in this order, and that the three cardiac NPs were generated from CNP3 by tandem duplication. Seven members existed at the divergence of ray-finned fishes and lobe-finned fishes (tetrapods), but some of the NP genes have disappeared during the course of evolution. In ray-finned fishes, all three cardiac NPs exist in chondrostei and some migratory teleost species, but VNP is generally absent and ANP is absent in a group of teleosts (Beloniformes). In tetrapods, ANP and BNP are present in mammals and amphibians, but ANP is usually absent in reptiles and birds. Thus, BNP is a ubiquitous cardiac NP in bony fishes and tetrapods though elasmobranchs and cyclostomes have only CNP3/4 as a cardiac NP. Functional studies indicate that cardiac NPs are essential Na(+)-extruding hormones throughout vertebrates; they play critical roles in seawater (SW) adaptation in teleosts, while they are important volume-depleting hormones in mammals as water and Na(+) are regulated in parallel in terrestrial animals. In mammals, cardiac NPs become prominent in pathological conditions such as heart failure where they are used in diagnosis and treatment. Although the functional role of BNP has not yet been fully elucidated compared with ANP in non-mammalian vertebrates, it appears that BNP plays pivotal roles in the cardiovascular and body fluid regulation as shown in mammals. ANP has previously been recognized as the principal cardiac NP in mammals and teleosts, but comparative studies have revealed that BNP is the only cardiac NP that exists in all tetrapods and teleosts. This is an excellent example showing that comparative studies have created new insights into the molecular and functional evolution of a hormone family.

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.

Comparative physiology of the piscine natriuretic peptide system

The natriuretic peptide (NP) family is a seemingly ubiquitous sodium and volume reducing endocrine system of predominantly cardiac origin. Members of the NP system include ANP, BNP, CNP, VNP, their guanylate cyclase (GC)-linked receptors (NPR-A and NPR-B), and clearance receptor (NPR-C). Through the activation of their membrane-bound GC receptors, these small peptides modulate cellular functions that affect both salt and water balance. The elucidation of piscine NP sequences, structure, and functions has steadily advanced over the past 15 years spearheaded by research from Dr. Yoshio Takei's laboratory. The development of these homologous NPs has led to extensive research into both the evolutionary and physiological significance of NPs in fishes. One outcome has been the development of two seemingly disparate hypotheses of NP function; a role in salt excretion, the osmoregulatory hypothesis, versus a role in protecting the heart, the cardioprotective hypotheses. In the osmoregulatory hypothesis NPs are released in response to elevated ambient salinity and inhibit drinking and intestinal uptake of salt, thereby effectively reducing plasma sodium levels. In contrast, the cardioprotective theory depicts NPs acting to prevent debilitating cardiodilation from an excess of either venous or arterial pressure through vasodilation and a reduction of blood volume. These seemingly distinct hypotheses may be elements of a more general regulatory system and certainly require further investigation. Undoubtedly their resolution will not only give us a better perspective of the evolutionary basis of the NP system but will provide us with a greater appreciation of salt and water homeostasis in vertebrates.

Biology of the natriuretic peptides

The biology of the natriuretic peptide (NP) system is complex, yet highly phylogenetically preserved. It regulates salt and water handling, promotes vasodilatation, and exerts favorable effects on the heart in the context of processes such as heart failure. Prior assumptions about the production of B-type NP (BNP) and its amino-terminal precursor fragment (NT-proBNP) have recently been refuted. It is now recognized that rather than a 1:1 secretion of these 2 NPs, a mixture of cleaved and uncleaved NPs is released by the cardiomyocyte. It is also recognized that BNP is rapidly modified into a mixture of various fragments. Commercial assays for the detection of BNP and NT-proBNP measure a mixture of cleaved and uncleaved NPs as well as varying amounts of degraded BNP. BNP and NT-proBNP are cleared differentially: BNP is actively removed from the bloodstream and also has passive clearance mechanisms, including renal clearance; NT-proBNP is cleared more passively by organs with high rates of blood flow, including the kidney.

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.

Molecular evolution of the natriuretic peptide system as revealed by comparative genomics

The natriuretic peptide (NP) family is a group of peptides involved in cardiovascular and body fluid regulation in vertebrates. While only C-type NP (CNP) has been found in elasmobranchs, atrial NP (ANP), B-type NP (BNP) and CNP have been found in mammals, and ventricular NP (VNP) instead of BNP in teleosts. Thus, it was once hypothesized that CNP is the ancestral NP, from which ANP and BNP/VNP were generated. However, the discovery of hfNP in the hagfish, and CNP in the lamprey suggested that the ancestral NP had characteristics common to these two peptides. Genomic studies in ray-finned fish revealed multiplication processes of NP genes: The ancestral gene was duplicated into four CNP genes before the divergence of elasmobranchs, and ANP, BNP and VNP genes were generated from one of the four CNP genes by tandem duplications. From up to seven NP genes thus generated, tetrapods are supposed to have lost some of them. Concerning NP receptors, teleosts also have more subtypes (three guanylyl cyclase-coupled receptors and two clearance receptors) than mammals. It is of interest to examine how the complicated NP system in teleosts compared with tetrapods, is involved in the adaptation to a wide variety of osmotic environments.

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.

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.

C-type natriuretic peptide of rainbow trout (Oncorhynchus mykiss): primary structure and vasorelaxant activities

Natriuretic peptides (NPs) play important roles in osmoregulatory and cardiovascular systems of vertebrates. For functional studies of NPs, rainbow trout (Oncorhynchus mykiss), a euryhaline fish, is an interesting model. The information on homologous NPs of salmonid fish is, however, still incomplete with respect to C-type NP (CNP). In this study, we isolated cDNAs encoding the precursor of CNP from the brain of trout. Predicted mature CNP (CNP-22) sequence was identical to that of killifish Fundulus heteroclitus, and only one amino acid was different from that of the eel Anguilla japonica, demonstrating a greater conservation among different teleost species than is found with atrial NP (ANP) and ventricular NP (VNP). While the preprosegment of trout CNP retained 57% similarity to the eel sequence, similarities were low to those of sharks and tetrapods. The major site of expression identified by RT-PCR was the brain with minor expression in the atrium. The putative mature CNP-22 was synthesized and its biological activity was compared with other trout NPs (ANP and VNP) using trout ventral aorta, efferent branchial and celiacomesenteric arteries and anterior cardinal vein in vitro. Synthetic trout CNP-22 relaxed all pre-contracted vessels with potencies comparable to trout ANP and VNP.

Brain natriuretic peptide: role in cardiovascular and volume homeostasis

The identification of natriuretic peptides as key regulators of natriuresis and vasodilatation, and the appreciation that their secretion is under the control of cardiac hemodynamic and neurohumoral factors, has caused wide interest. The natriuretic peptides are structurally similar, but genetically distinct peptides that have diverse actions on cardiovascular, renal, and endocrine homeostasis. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are of myocardial cell origin, while cardiac natriuretic peptide (CNP) is of endothelial origin. ANP and BNP bind to the natriuretic peptide receptor (NPR-A) which, via 3' 5'-cyclic guanosine monophosphate (cGMP), mediates natriuresis, vasodialation, renin inhibition, and antimitogenic properties. CNP lacks natriuretic action but possesses vasodilating and growth inhibiting effects via the guanyl cyclase linked natriuretic peptide-B (NPR-B) receptor. All three peptides are cleared by natriuretic peptide-C receptor (NPR-C) and degraded by neutral endopeptidase, both of which are widely expressed in kidney, lung, and vascular wall. Recently, a fourth member of the natriuretic peptide, dendroaspsis natriuretic peptide (DNP) has been reported to be present in human plasma and atrial myocardium.

The natriuretic peptide system in eels: a key endocrine system for euryhalinity?

The natriuretic peptide system of a euryhaline teleost, the Japanese eel (Anguilla japonica), consists of three types of hormones [atrial natriuretic peptide (ANP), ventricular natriuretic peptide (VNP), and C-type natriuretic peptide (CNP)] and four types of receptors [natriuretic peptide receptors (NPR)-A, -B, -C, and -D]. Although ANP is recognized as a volume-regulating hormone that extrudes both Na(+) and water in mammals, ANP more specifically extrudes Na(+) in eels. Accumulating evidence shows that ANP is secreted in response to hypernatremia and acts to inhibit the uptake and to stimulate the excretion of Na(+) but not water, thereby promoting seawater (SW) adaptation. In fact, ANP is secreted immediately after transfer of eels to SW and ameliorates sudden increases in plasma Na(+) concentration through inhibition of drinking and intestinal absorption of NaCl. ANP also stimulates the secretion of cortisol, a long-acting hormone for SW adaptation, whereas ANP itself disappears quickly from the circulation. Thus ANP is a primary hormone responsible for the initial phase of SW adaptation. By contrast, CNP appears to be a hormone involved in freshwater (FW) adaptation. Recent data show that the gene expression of CNP and its specific receptor, NPR-B, is much enhanced in FW eels. In fact, CNP infusion increases (22)Na uptake from the environment in FW eels. These results show that ANP and CNP, despite high sequence identity, have opposite effects on salinity adaptation in eels. This difference apparently originates from the difference in their specific receptors, ANP for NPR-A and CNP for NPR-B. VNP may compensate the effects of ANP and CNP for adaptation to respective media, because it has high affinity to both receptors. On the basis of these data, the authors suggest that the natriuretic peptide system is a key endocrine system that allows this euryhaline fish to adapt to diverse osmotic environments, particularly in the initial phase of adaptation.

Molecular cloning of natriuretic peptide receptor A from bullfrog (Rana catesbeiana) brain and its functional expression

A comparative study of natriuretic peptide receptor (NPR) was performed by cloning the NPR-A receptor subtype from the bullfrog (Rana catesbeiana) brain and analyzing its functional expression. Like other mammalian NPR-A receptors, the bullfrog NPR-A receptor consists of an extracellular ligand binding domain, a hydrophobic transmembrane domain, a kinase-like domain and a guanylate cyclase domain. Sequence comparison among the bullfrog and mammalian receptors revealed a relatively low ( approximately 45%) similarity in the extracellular domain compared to a very high similarity ( approximately 92%) in the cytoplasmic regulatory and catalytic domains. Expression of NPR-A mRNA was detected in various bullfrog tissues including the brain, heart, lung, kidney and liver; highest levels were observed in lung. Functional expression of the receptor in COS-7 cells revealed that frog atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) elicited cyclic guanosine 3'5'-monophosphate production by stimulating the receptor in a dose-dependent manner from 10(-10) M concentrations. Rat ANP was also effective in stimulating the frog receptor whereas rat BNP and porcine BNP were less responsive to the receptor. On the other hand, frog C-type natriuretic peptide (CNP) as well as porcine CNP stimulated the receptor only at high concentrations (10(-7) M). This clearly indicates that the bullfrog receptor is a counterpart of mammalian NPR-A, and is specific for ANP or BNP but not for CNP.

Enhanced expression and release of C-type natriuretic peptide in freshwater eels

C-type natriuretic peptide (CNP) is recognized as a paracrine factor acting locally in the brain and periphery. To assess the role of CNP in teleost fish, a cDNA encoding a CNP precursor was initially cloned from the eel brain. CNP message subsequently detected by ribonuclease protection assay, using the cDNA as probe, was most abundant in the brain followed by liver, gut, gills, and heart. Expression was generally higher in freshwater (FW) than in seawater (SW) eels, but not in the brain. Plasma CNP concentration measured by a newly developed homologous radioimmunoassay for eel CNP was higher in FW than in SW eels. The CNP concentration was also higher in the heart of FW eels but not in the brain. These results show that CNP is abundantly synthesized in peripheral tissues of FW eels and secreted constitutively into the circulation. Therefore, CNP is a circulating hormone as well as a paracrine factor in eels. Together with our previous demonstration that CNP-specific receptor expression is enhanced in FW eels, it appears that CNP is a hormone important for FW adaptation. Because atrial NP (ANP) promotes SW adaptation in eels, CNP and ANP, despite high sequence identity, appear to have opposite effects on environmental adaptation of the euryhaline fish.

Does the natriuretic peptide system exist throughout the animal and plant kingdom?

Natriuretic peptides (NPs) and their receptors have been identified in vertebrate species ranging from elasmobranchs to mammals. Atrial, brain and ventricular NP (ANP, BNP and VNP) are endocrine hormones secreted from the heart, while C-type NP (CNP) is principally a paracrine factor in the brain and periphery. In elasmobranchs, only CNP is present in the heart and brain and it functions as a circulating hormone as well as a paracrine factor. Four types of NP receptors are cloned in vertebrates. NPR-A and NPR-B are guanylyl cyclase-coupled receptors, whereas NPR-C and NPR-D have only a short cytoplasmic domain. NPs are hormones important for volume regulation in mammals, while they act more specifically for Na(+) regulation in fishes. The presence of NP and its receptor has also been suggested in the most primitive vertebrate group, cyclostomes, and its molecular identification is in progress. The presence of ANP or its mRNA has been reported in the hearts and ganglia of various invertebrate species such as mollusks and arthropods using either antisera raised against mammalian ANP or rat ANP cDNA as probes. Immunoreactive ANP has also been detected in the unicellular Paramecium and in various species of plants including Metasequoia. Furthermore, the N-terminal prosegments of ANP, whose sequences are scarcely conserved even in vertebrates, have also been detected by the radioimmunoassay for human ANP prosegments in all invertebrate and plant species examined including Paramecium. Although these data are highly attractive, the current evidence is too circumstantial to be convincing that the immunoreactivity truly originates from ANP and its prosegments in such diverse organisms. The caution that has to be exercised in identification of vertebrate hormones from phylogenetically distant organisms is discussed.

Vasoactivity of the ventral aorta of the American eel (Anguilla rostrata), Atlantic hagfish ( Myxine glutinosa), and sea lamprey (Petromyzon marinus)

To determine if vascular smooth muscle from teleost and agnathan fishes expresses receptors for signaling agents that are important in vascular tension in other vertebrates, we exposed rings of aortic vascular smooth muscle from the eel (Anguilla rostrata), the hagfish (Myxine glutinosa), and the lamprey (Petromyzon marinus) to a suite of putative agonists, including: acetylcholine, endothelin, nitric oxide, natriuretic peptides, and prostanoids. Acetylcholine constricted aortic rings from the eel, but had no effect on the rings from lamprey. On the other hand, endothelin constricted rings from all three species. Use of receptor-specific ET agonists demonstrated that only ET(A) receptors are expressed in the eel and lamprey aorta. The nitric oxide donor sodium nitroprusside or nitric oxide itself dilated rings from the eel, but both agonists constricted rings from the hagfish and NO produced a biphasic response (constriction followed by dilation) in the lamprey. Two natriuretic peptides, eel ANP and porcine CNP, produced marginally significant dilation in the eel aorta, human ANP dilated the hagfish rings, and pCNP and eANP dilated the lamprey rings. The prostanoids PGE(1) and PGE(2) both dilated the eel aortic rings, and PGE(1) and carbaprostacyclin (stable PGI(2) agonist) dilated the hagfish and lamprey rings. Our results suggest that receptors for a variety of vasoactive signaling agents are expressed in the aortic smooth muscle of the earliest vertebrates (lamprey and hagfish), as well as the more advanced teleosts (eel).

Localization of salmon cardiac peptide (sCP) in the heart of salmon (Salmo salar L.)

We have previously cloned and characterized a novel cardiac hormone from the salmon (Salmo salar) which has a uniquely heart-specific distribution and a low structural similarity with any other known natriuretic peptides. Specific antibodies were raised in goat against the salmon cardiac peptide. For localization and quantification, four different methods were applied: immunohistochemistry (avidin-biotin peroxidase), transmission electron microscopy, cryoimmunoelectron microscopy (protein A-gold), and a specific radioimmunoassay. Both atrial and ventricular myocytes stained immunohistochemically. The staining was similar in all myocytes and no specific myoendocrine cells were found. Within a single myocyte, both atrial and ventricular, the staining was stronger near the nucleus. Transmission electron microscopy revealed that both the atrium and the ventricle contained small sarcoplasmic granules of similar type with a diameter of 100 to 200 nm and an electron-dense core with a clear halo. The granules were typical vesicles which can be found in secretory cells utilizing the regulatory pathway. The highest number of granules was found near the nucleus, but granules were located also near the Golgi apparatus, between myofilament bundles, and in subsarcolemmal positions. Gold particles, conjugated to antibodies raised against the salmon cardiac peptide, were deposited on similar sarcoplasmic granules found in transmission electron microscopy. Among the sarcoplasmic granules with gold particles there were granules which did not show any cardiac peptide immunoreactivity. A significantly (Student's t test, P < 0.05) higher concentration of cardiac peptide was found in the heart atrium than in the ventricle, 16.2 +/- 3.5 pmol/mg tissue (n = 8) and 4.5 +/- 1.7 pmol/mg tissue (n = 8), respectively. The findings show that the salmon cardiac peptide is localized in secretory granules in both compartments of the heart. The morphology of the granules suggests that both the atrium and the ventricle utilize the regulatory pathway to release salmon cardiac peptide.

Natriuretic peptides in fish physiology

Natriuretic peptides exist in the fishes as a family of structurally-related isohormones including atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP) and ventricular natriuretic peptide (VNP); to date, brain natriuretic peptide (or B-type natriuretic peptide, BNP) has not been definitively identified in the fishes. Based on nucleotide and amino acid sequence similarity, the natriuretic peptide family of isohormones may have evolved from a neuromodulatory, CNP-like brain peptide. The primary sites of synthesis for the circulating hormones are the heart and brain; additional extracardiac and extracranial sites, including the intestine, synthesize and release natriuretic peptides locally for paracrine regulation of various physiological functions. Membrane-bound, guanylyl cyclase-coupled natriuretic peptide receptors (A- and B-types) are generally implicated in mediating natriuretic peptide effects via the production of cyclic GMP as the intracellular messenger. C- and D-type natriuretic peptide receptors lacking the guanylyl cyclase domain may influence target cell function through G(i) protein-coupled inhibition of membrane adenylyl cyclase activity, and they likely also act as clearance receptors for circulating hormone. In the few systems examined using homologous or piscine reagents, differential receptor binding and tissue responsiveness to specific natriuretic peptide isohormones is demonstrated. Similar to their acute physiological effects in mammals, natriuretic peptides are vasorelaxant in all fishes examined. In contrast to mammals, where natriuretic peptides act through natriuresis and diuresis to bring about long-term reductions in blood volume and blood pressure, in fishes the primary action appears to be the extrusion of excess salt at the gills and rectal gland, and the limiting of drinking-coupled salt uptake by the alimentary system. In teleosts, both hypernatremia and hypervolemia are effective stimuli for cardiac secretion of natriuretic peptides; in the elasmobranchs, hypervolemia is the predominant physiological stimulus for secretion. Natriuretic peptides may be seawater-adapting hormones with appropriate target organs including the gills, rectal gland, kidney, and intestine, with each regulated via, predominantly, either A- or B-type (or C- or D-type?) natriuretic peptide receptors. Natriuretic peptides act both directly on ion-transporting cells of osmoregulatory tissues, and indirectly through increased vascular flow to osmoregulatory tissues, through inhibition of drinking, and through effects on other endocrine systems.

Mode of activation of salt secretion by C-type natriuretic peptide in the shark rectal gland

We studied the modes of activation of the salt-secreting rectal gland of the spiny dogfish, Squalus acanthias, by the native cardiac peptide CNP. The stimulatory action of CNP in isolated perfused glands is inhibited by 10 mM procaine, presumably by blocking release of vasoactive intestinal peptide (VIP) from nerves. Procaine reduces the slope of the dose-response curve of human CNP and that of shark CNP (each P < 0.0001). CNP increases short-circuit current in cultured rectal gland cells from 4.8 +/- 1.6 to 27.0 +/- 7.8 microA/cm2. It also stimulates the secretion of chloride in isolated perfused glands in the presence of 10 mM procaine from 72 +/- 31 to 652 +/- 173 microeq. h(-1). g(-1). These results suggest that CNP has a direct cellular action not mediated by the neural release of VIP. The residual stimulation of perfused glands in the presence of procaine was almost completely inhibited by staurosporine [10 nM; an inhibitor of protein kinase C (PKC)] from 652 +/- 173 to 237 +/- 61 microeq. h(-1). g(-1). Although CNP stimulates guanylyl cyclase in shark rectal gland, chloride secretion of perfused glands was not elicited by 8-bromoadenosine-cGMP (8-BrcGMP) alone nor by the activator of PKC phorbol ester. The combination of PKC activation and 8-BrcGMP infusion, however, stimulated chloride secretion in perfused glands from 94 +/- 30 to 506 +/- 61 microeq. h(-1). g(-1), a level comparable to that observed in glands blocked with procaine. Several parallel pathways appear to be synergistic in activating chloride secretion stimulated by CNP in the rectal gland.

Structural and functional evolution of the natriuretic peptide system in vertebrates

The natriuretic peptide (NP) system consists of three types of hormones [atrial NP (ANP), brain or B-type NP (BNP), and C-type NP (CNP)] and three types of receptors [NP receptor (R)-A, NPR-B, and NPR-C]. ANP and BNP are circulating hormones secreted from the heart, whereas CNP is basically a neuropeptide. NPR-A and NPR-B are membrane-bound guanylyl cyclases, whereas NPR-C is assumed to function as a clearance-type receptor. ANP, BNP, and CNP occur commonly in all tetrapods, but ventricular NP replaces BNP in teleost fish. In elasmobranchs, only CNP is found, even in the heart, suggesting that CNP is an ancestral form. A new guanylyl cyclase-uncoupled receptor named NPR-D has been identified in the eel in addition to NPR-A, -B, and -C. The NP system plays pivotal roles in cardiovascular and body fluid homeostasis. ANP is secreted in response to an increase in blood volume and acts on various organs to decrease both water and Na+, resulting in restoration of blood volume. In the eel, however, ANP is secreted in response to an increase in plasma osmolality and decreases Na+ specifically, thereby promoting seawater adaptation. Therefore, it seems that the family of NPs were originally Na(+)-extruding hormones in fishes; however, they evolved to be volume-depleting hormones promoting the excretion of both Na+ and water in tetrapods in which both are always regulated in the same direction. Vertebrates expanded their habitats from fresh water to the sea or to land during evolution. The structure and function of osmoregulatory hormones have also undergone evolution during this ecological evolution. Thus, a comparative approach to the study of the NP family affords new insights into the essential function of this osmoregulatory hormone.

Role of guanylyl cyclase receptors for CNP in salt secretion by shark rectal gland

The role of C-type natriuretic peptide (CNP) and its guanylyl cyclase-linked receptors in mediating salt secretion by the rectal gland of the spiny dogfish shark (Squalus acanthias) was investigated using HS-142-1, a competitive inhibitor of the binding of natriuretic peptides to their guanylyl cyclase receptors. CNP binds to receptors and activates guanylyl cyclase in rectal gland membranes in a way that is inhibited by HS-142-1. Guanylyl cyclase activation in rectal gland membranes is far more sensitive to CNP than to atrial natriuretic peptide, whereas the reverse is true for membranes derived from mammalian (rabbit) renal collecting duct cells. HS-142-1 inhibited the stimulatory effect of CNP on ouabain-inhibitable oxygen consumption by rectal gland tubules. In explanted rectal glands continuously perfused with blood from intact donor sharks, HS-142-1 inhibited the increase in salt secretion normally provoked by infusing isotonic saline solutions into the donor animal. These results strongly support the view that CNP released into the systemic circulation in response to volume expansion mediates the secretion of chloride by the rectal gland via receptors linked to guanylyl cyclase.

Atrial natriuretic peptide-C receptor and membrane signalling in hypertension

Atrial natriuretic peptide (ANP) regulates a variety of physiological parameters, including the blood pressure and intravascular volume, by interacting with its receptors present on the plasma membrane. ANP receptors are of three subtypes: ANP-A, -B and -C receptors. ANP-A and ANP-B receptors are guanylyl cyclase receptors, whereas ANP-C receptors are coupled to adenylyl cyclase inhibition or phospholipase C activation through inhibitory guanine nucleotide-regulating protein. Unlike other G protein-coupled receptors, ANP-C receptors have a single transmembrane domain and a short cytoplasmic domain of 37 amino acids, the cytoplasmic domain has a structural specificity like those of other single-transmembrane-domain receptors and 37 amino-acid cytoplasmic domain peptide is able to exert is inhibitory effect on adenylyl cyclase. The activation of ANP-C receptor by C-ANP(4-23) (a ring-deleted peptide of ANP) and C-type natriuretic peptide inhibits the mitogen-activated protein kinase activity stimulated by endothelin-3, platelet-derived growth factor and phorbol-12 myristate 13-acetate. C-ANP also inhibits mitogen-induced stimulation of DNA synthesis, indicating that the ANP-C receptor plays a role in cell proliferation through an inhibition of mitogen-activated protein kinase and suggesting that the ANP-C receptor might also be coupled to other signal transduction mechanism(s) or that there might be an interaction of the ANP-C receptor with some other signalling pathways. ANP receptor binding is decreased in most organs in hypertensive subjects and hypertensive animals. This decrease is consistent with there being fewer guanylyl cyclase-coupled receptors in the kidney and vasculature and selective inhibition of the ANP-C receptor in the thymus and spleen. Platelet ANP-C receptors are decreased in number in hypertensive patients and spontaneously hypertensive rats. ANP-A, -B and -C receptors are decreased in number in deoxycorticosterone acetate-salt-treated kidneys and vasculature; however, the responsiveness of adenylyl cyclase to ANP is augmented in the vasculature and heart and is attenuated completely in platelets. These alterations in ANP receptor subtypes may be related to the pathophysiology of hypertension. Several hormones such as angiotensin II, ANP and catecholamines, the levels of which are increased in hypertension, downregulate or upregulate ANP-C receptors and ANP-C receptor-mediated inhibition of adenylyl cyclase. It can be suggested that the antihypertensive action of several types of drugs such as angiotensin converting enzyme inhibitors, angiotensin type 1 receptor antagonists and beta2-adrenergic antagonists may partly be attributed to their ability to modulate the expression and function of the ANP-C receptor.

Distribution and characterization of natriuretic peptide receptors in the gills of the spiny dogfish, Squalus acanthias

The distribution and nature of natriuretic peptide receptors (NPR) in the gills of dogfish, Squalus acanthias, were examined by tissue section autoradiography, competition analysis, protein electrophoresis, guanylate cyclase (GC) assays, and molecular cloning. Specific NP binding occurred on the gill filaments, but not on the interbranchial septum or gill arch. The binding was densest on the efferent edge of the gills. Higher resolution light-microscopic examination of emulsion-coated sections showed that specific binding occurred mainly on the secondary lamellae and filament body and not on the arterial circulation. At least two types of NPR were revealed. One is linked to GC since NP binding stimulates the production of cGMP. The GC receptor may be similar to the NPR-B mammalian receptor since only pCNP stimulated cGMP production. The second receptor is not linked to GC and binds the specific ligand C-ANF [rat des(Gln18, Ser19, Gly20, Leu21, Gly22)]. The sequence of a cDNA generated using primers based on conserved regions of vertebrate NPR-C had considerable homology with mammalian and eel NPR-C and eel NPR-D. The presence of GC-linked NPR and NPR-C/ NPR-D suggests that the gills are an important target organ for NP action.

Immunocytochemical localization of atrial natriuretic factor and autoradiographic distribution of atrial natriuretic factor binding sites in the brain of the African lungfish, Protopterus annectens

The localization of atrial natriuretic factor (ANF)-immunoreactive elements was investigated in the brain of the African lungfish, Protopterus annectens, by using antisera raised against rat and human ANF(1-28). Concurrently, the distribution of ANF binding sites was studied by autoradiography using radioiodinated human ANF(1-28) as a tracer. In general, there was a good correlation between the distribution of ANF-immunoreactive structures and the location of ANF binding sites in several areas of the brain, particularly in the ventral part of the medial subpallium, the rostral preoptic region, the preoptic periventricular nucleus, the caudal hypothalamus, the neural lobe of the pituitary, and the mesencephalic tectum. In contrast, mismatching was observed in the pallium (which contained a high density of binding sites and a low concentration of ANF immunoreactive elements) as well as in the lateral subpallium and the medial region of the ventral thalamus, in which a low concentration of binding sites but a high density of ANF-immunoreactive fibers were detected. The present data provide the first localization of ANF-related peptides in the brain of dipnoans and the first anatomical distribution of ANF binding sites in the brain of fish. The results show that the ANF peptidergic systems of P. annectens exhibit similarities with those previously described in the frog Rana ridibunda, supporting the existence of relationships between dipnoans and amphibians. The location of ANF-like immunoreactivity and the distribution of ANF binding sites suggest that ANF-related peptides may act as hypothalamic neurohormones as well as neurotransmitters and/or neuromodulators in the lungfish brain.

The rectal gland of Squalus acanthias: a model for the transport of chloride

The rectal gland of the spiny dogfish shark, Squalus acanthias, secretes chloride by a furosemide sensitive process that has been termed "secondary active." Chloride enters the cell across the basolateral cell membrane via the sodium:potassium:2 chloride cotransporter. The energy for this electroneutral uptake step is provided by the electrochemical gradient for sodium directed into the cell. This is maintained by Na-K-ATPase present in the basolateral cell membrane. Present as well in the basolateral cell membrane is a potassium conductance that permits potassium to exit passively. Chloride leaves the cell across the luminal membrane via a chloride conductance closely similar to CFTR. The rectal gland is thus a model for the mechanism of secondary active chloride transport utilized by various epithelial organs throughout the vertebrate kingdom. This report reviews the humoral agents that regulate the secretion of chloride by the rectal gland and the intracellular mechanisms that mediate it. CNP, released from the heart in response to a volume stimulus, causes the release of VIP from nerves within the gland and together with VIP directly activates the rectal gland cell.

C-type natriuretic peptide

C-type natriuretic peptide is a 22-amino acid peptide that was initially identified in the central nervous system. The distribution of C-type natriuretic peptide, which has structural homology with atrial and brain natriuretic peptides, is wide and includes the endothelium, myocardium, gastrointestinal, and genitourinary tracts. The biological effects of this peptide are being elucidated in a number of sites in a number of species; however, the novel endothelial site of production of C-type natriuretic peptide and the proximal situation of its receptor in vascular smooth muscle suggest that this vascular natriuretic peptide system may play a role in concert with other local systems in the control of vascular tone.

Defective ANF-R2/ANP-C receptor-mediated signalling in hypertension

In the present studies we have shown that atrial natriuretic factor (peptide) receptor of ANF-R2/ANP-C type is coupled to adenylyl cyclase/cAMP signal transduction system through Gi-regulatory protein and is implicated in mediating some of the physiological responses of atrial natriuretic factor or peptide (ANP). ANF-R2/ANP-C receptor-mediated adenylyl cyclase inhibition was altered in hypertension. This alteration was tissue specific. In heart, aorta, brain and adrenal, the extent of inhibition of adenylyl cyclase by ANP was enhanced in SHR as compared to age-matched WKY, whereas in platelets, the ANP-mediated inhibition was completely attenuated. The enhanced inhibition of adenylyl cyclase by ANP was also observed in heart and aorta from DOCA-salt hypertensive rats. In addition, the augmented inhibition of adenylyl cyclase by ANP was observed in 2 weeks and older SHR but not in 3-5 days old SHR. Similarly, in DOCA-salt hypertensive rats, the enhanced inhibition of adenylyl cyclase by ANP was observed after 2 weeks of DOCA-salt treatment when the blood pressure was also enhanced, however one week older SHR but not in 3-5 days old SHR. Similarly, in DOCA-salt hypertensive rats, the enhanced inhibition of adenylyl cyclase by ANP was observed after 2 weeks of DOCA-salt treatment when the blood pressure and augmented ANP-mediated inhibition of adenylyl of DOCA-salt treatment did not result in an augmented blood pressure and augmented ANP-mediated inhibition of adenylyl cyclase, suggesting that blood pressure increase may be responsible for the enhanced responsiveness of ANP to adenylyl cyclase inhibition. However, in genetic model of hypertension, the increased inhibition of adenylyl cyclase by ANP at 2 weeks of age (when the blood pressure is normal) may be implicated in the pathogenesis of hypertension. The augmented inhibition of adenylyl cyclase in cardiovascular tissues from SHR and DOCA-salt hypertensive rats may be due to the upregulation of ANF-R2/ANP-C receptors or due to the amplification of post-receptor signalling mechanisms.

Quantitative analysis of messenger ribonucleic acid encoding natriuretic peptide receptors in aldosterone-producing adenoma

There exist conflicting data regarding the inhibitory effect of atrial natriuretic peptide on aldosterone production from aldosterone-producing adenoma (APA). Natriuretic peptides mediate their actions through natriuretic peptide receptors (NPRs). Whether or not NPRs are present in the tumors remains controversial. To elucidate this paradox, gene expression of NPRs was examined by Northern blot analysis and competitive polymerase chain reaction in tumorous and non-tumorous portions of APA, and in normal adrenal gland from patients with renal cell carcinoma. The results of Northern blot analysis showed the presence of messenger ribonucleic acid (mRNA) of three NPRs in all adrenal tissues, including APA. The proportional expression of NPR gene transcripts in APA was type A (0.6%), type B (18.7%), and type C (80.7%). The levels, but not the proportions, of type C and possibly type B NPR mRNAs were lower in tumorous and non-tumorous portions of APA compared to those in normal adrenal gland (type C 190.2 +/- 24.5 [means +/- SEM, normal adrenal gland] > 168.1 +/- 20.8 [non-tumorous portion] > 112.2 +/- 15.5 [tumorous portion] pg/10 micrograms total RNA, F = 3.82, P < 0.05; type B 45.2 +/- 8.5 [normal adrenal gland] > 30.0 +/- 5.2 [non-tumorous portion] > 25.1 +/- 4.1 [tumorous portion] pg/10 micrograms total RNA, F = 3.03, P = 0.065). The mRNA levels of type C, rather than type A or type B, NPR were correlated with the percentage of zona fasciculata-like cells in APA (r = 0.90, P < 0.05). In conclusion we have demonstrated the presence of mRNA encoding the three NPRs in APA.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and expression of eel natriuretic-peptide receptor B and comparison with its mammalian counterparts

A comparative study of the natriuretic-peptide receptor NPR-B was performed by cloning and expressing, in COS-1 cells, the NPR-B receptor subtype from the eel gill which exhibited a strong C-type-natriuretic-peptide (CNP)-induced guanylate cyclase activity. Like other mammalian NPR-B receptors, the eel NPR-B receptor consisted of a ligand-binding extracellular domain, a hydrophobic transmembrane domain, a kinase-like domain and a guanylate cyclase domain. Sequence comparison among the eel and mammalian receptors revealed a relatively low similarity (approximately 44%) in the extracellular domain compared to a very high similarity (approximately 84%) in the cytoplasmic regulatory and catalytic domains. This low similarity allowed identification of the amino acid residues or candidate regions important for the ligand-binding activity. RNase protection analysis of the eel NPR-B mRNA demonstrated that the message was predominantly expressed in the liver and atrium as well as in the gill with moderate-to-small amounts in the brain, ventricle, esophageal sphincter, stomach, posterior intestine and kidney. The high NPR-B mRNA levels in the liver, atrium and gill were found to decrease markedly when eels were transferred from fresh water to seawater and kept there for 2 weeks. Since similar changes are known to occur in the ligand CNP levels when eels are facing osmotic challenges, the CNP/NPR-B system appears to play an important role in their successful adaptation to salinity changes.

Cell-type-specific function of the C-type natriuretic peptide gene promoter in rat anterior pituitary-derived cultured cell lines

The promoter function of the human C-type natriuretic peptide (CNP) gene in various cultured cells was examined by transient transfection assays. The CNP promoter functioned very effectively in GH3 cells, which originated from the growth hormone-producing tumor of the rat anterior pituitary and somatomammotroph phenotype, but functioned much less effectively in GH1 cells, another type of rat pituitary-derived cell with a somatotroph phenotype, and rat primary cardiocytes. The CNP promoter did not function at all in other cells, including AtT20 cells of murine pituitary corticotroph origin. Functional analyses of the deleted promoters with various 5' deletion breakpoints revealed the existence of at least two negative and one positive regulatory regions. Within the positive regulatory region (positions -54 to -19), which conferred 90% of the promoter activity in GH3 cells, two equipotent GC-rich cis elements (positions -49 to -45 and -40 to -35) were identified. Both sites shared half of the promoter activity and binding properties to the nuclear protein in GH3 cells. Rat anterior pituitary tissue contained the binding protein of the identified cis element, which was identical or similar to that of GH3 cells. With Southwestern (DNA-protein) analysis, a 70-kDa specific binding protein distinct from known factors such as SP-1, AP-2, and Pit-1 was identified in the nuclear extract of GH3 cells.

Biochemistry and physiology of a family of eel natriuretic peptides

Three natriuretic peptides with similar structures were isolated from eels and their amino acid sequences determined; atrial natriuretic peptide (ANP) from atria, ventricular natriuretic peptide (VNP) from ventricles, and C-type natriuretic peptide (CNP) from brains. All three hormones were circulating in eel blood, and their plasma levels invariably decreased when eels were transferred from fresh water (FW) to seawater (SW). Eel ANP and VNP inhibited drinking in FW and SW eels. Eel ANP inhibited water and Na(+) absorption by the intestine of SW eels. The potency of these ANP effects was 2-3 orders greater than those of other hormones which are known to have similar effects. Eel ANP and VNP induced antidiuresis but not antinatriuresis in FW eels. Eel ANP increased plasma cortisol level in SW eels but not in FW eels. The antidiuretic effect and the stimulation of cortisol secretion in eels are opposite to the ANP effects reported in mammals. These data suggest that ANP plays a complex role in the eel osmoregulation.

Cell-type-specific function of the C-type natriuretic peptide gene promoter in rat anterior pituitary-derived cultured cell lines

The promoter function of the human C-type natriuretic peptide (CNP) gene in various cultured cells was examined by transient transfection assays. The CNP promoter functioned very effectively in GH3 cells, which originated from the growth hormone-producing tumor of the rat anterior pituitary and somatomammotroph phenotype, but functioned much less effectively in GH1 cells, another type of rat pituitary-derived cell with a somatotroph phenotype, and rat primary cardiocytes. The CNP promoter did not function at all in other cells, including AtT20 cells of murine pituitary corticotroph origin. Functional analyses of the deleted promoters with various 5' deletion breakpoints revealed the existence of at least two negative and one positive regulatory regions. Within the positive regulatory region (positions -54 to -19), which conferred 90% of the promoter activity in GH3 cells, two equipotent GC-rich cis elements (positions -49 to -45 and -40 to -35) were identified. Both sites shared half of the promoter activity and binding properties to the nuclear protein in GH3 cells. Rat anterior pituitary tissue contained the binding protein of the identified cis element, which was identical or similar to that of GH3 cells. With Southwestern (DNA-protein) analysis, a 70-kDa specific binding protein distinct from known factors such as SP-1, AP-2, and Pit-1 was identified in the nuclear extract of GH3 cells.

C-type natriuretic peptides are potent dilators of shark vascular smooth muscle

Previous studies of the effects of C-type natriuretic peptides (CNP) in intact mammals have demonstrated limited hypotensive responses, in contrast to other natriuretic peptides. Our previous studies, on isolated vascular smooth muscle (VSM) from various fish species, utilizing either mammalian or non-homologous fish atrial natriuretic peptides (ANP), have demonstrated vasodilation with a relatively high sensitivity (EC50 approximately 5 nM). The recent sequencing of a C-type natriuretic peptide from the heart of the dogfish shark, Squalus acanthias, has enabled us to compare the efficacy of this peptide on aortic VSM from that species with two other CNPs (from killifish and pig), as well as rat ANP. The EC50 of dilation for sCNP, as well as kCNP and pCNP, was 0.5 nM, over 15 times lower than the EC50 of the response to rANP. These data suggest that CNP is released from the dogfish shark heart and is a circulating hormone with potent vasodilatory effects, in sharp contrast to the apparent role of CNP predominantly as a brain neuropeptide in mammals.

Immunohistochemical localisation of natriuretic peptides in the brains and hearts of the spiny dogfish Squalus acanthias and the Atlantic hagfish Myxine glutinosa

The avidin-biotin peroxidase technique was used to determine the distribution of natriuretic peptides in the hearts and brains of the dogfish Squalus acanthias and the Atlantic hagfish Myxine glutinosa. Three antisera were used: one raised against porcine brain natriuretic peptide which cross-reacts with atrial natriuretic and C-type natriuretic peptides (termed natriuretic peptide-like immunoreactivity); the second raised against porcine brain natriuretic peptide which cross-reacts with C-type natriuretic peptide, but not with atrial natriuretic peptide (termed porcine brain natriuretic peptide-like immunoreactivity); and the third raised against rat atrial natriuretic peptide (termed rat atrial natriuretic peptide-like immunoreactivity). Only natriuretic peptide-like immunoreactivity was observed in the heart of S. acanthias which was most likely due to the antiserum cross-reacting with C-type natriuretic peptide. No immunoreactivity was found in the M. glutinosa heart. In the brain of S. acanthias, natriuretic peptide-like immunoreactive fibres were located in many areas of the telencephalon, diencephalon, mesencephalon, rhombencephalon, and spinal cord. Extensive immunoreactivity was observed in the hypothalamo-hypophyseal tract and the neurointermediate lobe of the hypophysis. Natriuretic peptide-like immunoreactive perikarya were found in ventromedial regions of the telencephalon and in the nucleus preopticus. Most perikarya had short, thick processes which extended toward the ventricle. Another group of perikarya was observed in the rhombencephalon. Porcine brain natriuretic peptide-like immunoreactive fibres were observed in the telencephalon, diencephalon, mesencephalon, and rhombencephalon, but perikarya were only present in the preoptic area. In the M. glutinosa brain, natriuretic peptide-like immunoreactive fibres were present in all regions. Immunoreactive perikarya were observed in the pallium, primordium hippocampi, pars ventralis thalami, pars dorsalis thalami, nucleus diffusus hypothalami, nucleus profundus, nucleus tuberculi posterioris, and nucleus ventralis tegmenti. Porcine brain natriuretic peptide-like immunoreactive perikarya and fibres had a similar, but less abundant distribution than natriuretic peptide-like immunoreactive structures. Although the chemical structures of natriuretic peptides in the brains of dogfish and hagfish are unknown, these observations show that a component of the natriuretic peptide complement is similar to porcine brain natriuretic peptide or porcine C-type natriuretic peptide. The presence of natriuretic peptides in the brain suggest they could be important neuromodulators and/or neurotransmitters. Furthermore, there appears to be divergence in the structural forms of natriuretic peptides in the hearts and brains of dogfish and hagfish.

C-type natriuretic peptide immunoreactivity in human breast vascular endothelial cells

The present study was undertaken to determine the presence of CNP immunoreactivity in human breast tissue (n = 9). Immunohistochemical staining of breast tissue revealed the presence of CNP immunoreactivity localized to vascular endothelial cells. This study demonstrates for the first time that CNP immunoreactivity is present in humans. Based upon the knows biological actions of CNP, these findings suggest that CNP may function as part of an endothelium-derived vasoregulatory system.

Immunohistochemical localisation of natriuretic peptides in the heart and brain of the gulf toadfish Opsanus beta

The distribution of natriuretic peptide immunoreactivity was determined in the heart and brain of the gulf toadfish Opsanus beta using the avidin-biotin peroxidase technique. Four antisera were used: the first raised against porcine brain natriuretic peptide which cross-reacts with atrial natriuretic and C-type natriuretic peptides (termed natriuretic peptide-like immunoreactivity); the second raised against porcine brain natriuretic peptide which cross-reacts with C-type natriuretic peptide but not with atrial natriuretic peptide (termed porcine brain natriuretic peptide-like immunoreactivity); the third raised against rat atrial natriuretic peptide; and the fourth raised against eel atrial natriuretic peptide. Natriuretic peptide- and porcine brain natriuretic peptide-like immunoreactivity was observed in all cardiac muscle cells of the atrium. In the ventricle, natriuretic peptide-like immunoreactivity was found in all cardiac muscle cells, however, porcine brain natriuretic peptide-like immunoreactivity was confined to muscle cells adjacent to the epicardium. There was no discernible difference in the distribution of natriuretic peptide-like immunoreactivity and porcine brain natriuretic peptide-like immunoreactivity in the brain. Immunoreactive perikarya were observed only in the preoptic region of the diencephalon, and many immunoreactive fibres were found in the telencephalon, preoptic area, and rostral hypothalamus, lateral to the thalamic region. There was no immunoreactivity in any region of the hypophysis. A pair of distinct immunoreactive fibre tracts ran caudally from the preoptic area to the thalamic region, from which fibres extended to the posterior commissure, area praetectalis, dorsolateral regions of the midbrain tegmentum, and tectum.(ABSTRACT TRUNCATED AT 250 WORDS)