A new natriuretic peptide isolated from cardiac atria of trout, Oncorhynchus mykiss

Responses of the trout cardiac natriuretic peptide system to manipulation of salt and water balance

Natriuretic peptides (NPs) are evolutionarily conserved hormones that affect blood pressure and fluid volume through membrane-bound guanylate cyclase (GC)-linked natriuretic peptide receptors-A and -B (NPR-A and NPR-B, respectively) in a variety of vascular, renal, and other tissues. The principal physiological stimulus for cardiac NPs in fish is somewhat debated between two prominent theories: regulation of salt balance (osmoregulatory hypothesis) or prevention of volume expansion (cardioprotective hypothesis). In the present study, we examined atrial and ventricular expression of trout NPs, atrial (ANP), brain (BNP), and ventricular (VNP) using Northern (mRNA), Western (NP pro-hormone), and qPCR (GC-NPR-A and -B mRNA) blot analysis following independent manipulation of plasma salt and volume levels after chronic exposure to freshwater (FW; volume loaded, salt depleted), saltwater (SW; volume depleted, salt loaded), or freshwater trout fed a high-salt diet (FW-HSD; volume and salt loaded). We also measured NP transcriptional response to acute (2 h) volume expansion with dialyzed plasma (VE; 80% blood vol) or volume depletion by hemorrhage (VD, 20% blood volume every 30 min for 2 h) with real-time PCR. In essentially all instances, increased expression of the NP system was associated with FW-HSD or plasma expansion. There were no differences in NP expression between chronically adapted FW and SW fish, and hemorrhage decreased atrial ANP and VNP mRNA. These results indicate that rainbow trout cardiac NPs and cardiovascular GC-NPRs respond principally to volume, not salt overload, and this suggests that the primary function of trout cardiac NP system is to protect the heart.

Identification and expression of natriuretic peptide receptor type-A and -B mRNA in freshwater and seawater rainbow trout

Natriuretic peptide receptors mediate the physiological response of natriuretic peptide hormones. One of the natriuretic peptide receptor types is the particulate guanylyl cyclase receptors, of which there are two identified: NPR-A and NPR-B. In fishes, these have been sequenced and characterized in eels, medaka, and dogfish shark (NPR-B only). The euryhaline rainbow trout provides an opportunity to further pursue examination of the system in teleosts. In this study, partial rainbow trout NPR-A-like and NPR-B-like mRNA sequences were identified via PCR and cloning. The sequence information was used in real-time PCR to examine mRNA expression in a variety of tissues of freshwater rainbow trout and rainbow trout acclimated to 35 parts per thousand seawater for a period of 10 days. In the excretory kidney and posterior intestine, real-time PCR analysis showed greater expression of NPR-B in freshwater fish than in those adapted to seawater; otherwise, there was no difference in the expression of the individual receptors in fresh water or seawater. In general, the expression of the NPR-A and NPR-B type receptors was quite low. These findings indicate that NPR-A and NPR-B mRNA expression is minimally altered under the experimental regime used in this study.

Haemoconcentration via diuresis in short-term hypoxia: a possible role for cardiac natriuretic peptide in rainbow trout

Rainbow trout, exposed to acute hypoxia (decrease of oxygen level from full to 30% air saturation for 1 h, stable 30% air saturation for 2 h), showed more than twofold increase in urine flow rate. Hypoxic diuresis was associated with a sustained increase in dorsal aortic cardiac peptide (sCP) level, and the diuresis could be completely inhibited by a bolus injection of sCP antiserum. These results suggest that hypoxic haemoconcentration, which is partially achieved via increased urine flow rate in vertebrates, is caused by cardiac peptides. The results further suggest that cardiac peptide receptors in hypoxic fish gills modulate the postbranchial systemic level of sCP.

Comparative aspects of natriuretic peptide physiology in non-mammalian vertebrates: a review

The natriuretic peptide system is a complex family of peptides and receptors that is primarily linked to the maintenance of osmotic and cardiovascular homeostasis. A natriuretic peptide system is present in each vertebrate class but there are varying degrees of complexity in the system. In agnathans and chondrichthyians, only one natriuretic peptide has been identified, while new data has revealed that multiple types of natriuretic peptides are present in bony fish. However, it seems in tetrapods that there has been a reduction in the number of natriuretic peptide genes, such that only three natriuretic peptides are present in mammals. The peptides act via a family of guanylyl cyclase receptors to generate the second messenger cGMP, which mediates a range of physiological effects at key targets such as the gills, kidney and the cardiovascular system. This review summarises the current knowledge of the natriuretic peptide system in non-mammalian vertebrates and discusses the physiological actions of the peptides.

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.

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.

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.

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.

Similarity of vasorelaxant effects of natriuretic peptides in isolated blood vessels of salmonids

Natriuretic peptides (NPs) have been implicated in cardiovascular regulation in rainbow trout (Oncorhyncus mykiss), and it has been observed that the vasorelaxant activity of distinct trout and human NPs is similar in isolated trout arteries. This study characterizes the response of a variety of vessels from rainbow trout and other salmonids to different NPs. The effects of heterologous (rat atrial and human atrial) and homologous (rainbow trout atrial and rainbow trout ventricular) NPs were examined in precontracted efferent branchial arteries from rainbow trout (O. mykiss, Kamloops strain), lake whitefish (Coregonus clupeaformis), and in rainbow trout celiacomesenteric arteries and anterior cardinal veins. The response to mammalian NPs was also examined in efferent branchial arteries from the steelhead (O. mykiss, Skamania strain), coho salmon (Oncorhyncus kisutch), brook trout (Salvelinus fontinalis), and brown trout (Salmo trutta). In general, there were relatively few differences that were species, peptide, or vessel specific. There was no difference in the sensitivity (concentration producing a half-maximal response EC(50)) or efficacy (percent relaxation, i.e., E(max)) of trout or whitefish efferent branchial arteries to any NP, except human NP, which was significantly less effective (greater EC(50) and lower E(max)) in whitefish arteries. There were no differences in E(max) of mammalian NPs in efferent branchial arteries from any species, and only coho and brook trout had significantly different EC(50)'s (coho, 1.0+/-0.2 nM; brook trout, 4. 2+/-0.6 nM; and other species, from 1.9 to 3.5 nM). Rainbow and coho anterior cardinal veins were less sensitive than arteries to mammalian NPs (EC(50)'s; 8.8+/-2.0, 2.0+/-0.1 vs. 3.0+/-0.9, 1.0+/-0. 2, respectively), whereas brown trout veins were more sensitive (1. 0+/-0.2, 3.5+/-1.3, respectively). Sodium nitroprusside (SNP), which activates soluble guanylate cyclase, was vasodilatory, albeit significantly less potent than all NPs, in efferent branchial arteries of all species. SNP was significantly more potent in trout than whitefish efferent branchial arteries, whereas it was equally efficacious in these vessels. These results demonstrate that multiple vessels from various salmonids are similarly responsive to the vasorelaxant effects of a variety of NPs and that the salmonid NP receptor has relatively little ability to discriminate between homologous and heterologous peptides. We conclude that the vascular NP receptor complex is highly conserved among salmonids. Further, salmonids utilize cyclic guanosine monophosphate (cGMP) elevations for reductions of vascular tonus by both particulate and soluble guanylate cyclase pathways.

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.

Cardiac natriuretic peptides: a physiological lineage of cardioprotective hormones?

Vertebrate hearts from fish to mammals secrete peptide hormones with profound natriuretic, diuretic, and vasodilatory activity; however, the specific role of these cardiac natriuretic peptides (NPs) in homeostasis is unclear. NPs have been suggested to be involved in salt excretion in saltwater teleosts, whereas they are proposed to be more important in volume regulation in mammals. In this review, we consider an alternative (or perhaps complementary) function of NPs to protect the heart. This hypothesis is based on a number of observations. First, evidence for NPs, or NP-like activity has been found in all vertebrate hearts thus far examined, from osmoconforming saltwater hagfish to euryhaline freshwater and saltwater teleosts to terrestrial mammals. Thus the presence of cardiac NPs appears to be independent of environmental conditions that may variously affect salt and water balance. Second, cardiac stretch is a universal, and one of the most powerful, NP secretagogues. Furthermore, stretch-induced NP release in euryhaline teleosts appears relatively independent of ambient salinity. Third, excessive cardiac stretch that increases end-diastolic volume (EDV) can compromise the mechanical ability of the heart by decreasing actin-myosin interaction (length-tension) or through Laplace effects whereby as EDV increases, the wall tension necessary to maintain a constant pressure must also increase. Excessive cardiac stretch can be produced by factors that decrease cardiac emptying (i.e., increased arterial pressure), or by factors that increase cardiac filling (i.e., increased blood volume, increased venous tone, or decreased venous compliance). Fourth, the major physiological actions of cardiac NPs enhance cardiac emptying and decrease cardiac filling. In fish, NPs promote cardiac emptying by decreasing gill vascular resistance, thereby lowering ventral aortic pressure. In mammals a similar effect is achieved through pulmonary vasodilation. NPs also decrease cardiac filling by decreasing blood volume and increasing venous compliance, the latter producing a rapid fall in central venous pressure. Fifth, the presence of NP clearance receptors in the gill and lung (between the heart and systemic circulation) suggest that these tissues may be exposed to considerably higher NP titers than are systemic tissues. Thus, a decrease in outflow resistance immediately downstream from the heart may be the first response to increased cardiac distension. Because the physiology of cardiac NPs is basically the same in fish and mammals, we propose that the cardioprotective effects of NPs have been well preserved throughout the course of vertebrate evolution. It is also likely that the cardioprotective role of NPs was one of the most primordial homeostatic activities of these peptides in the earliest vertebrates.

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.

Cloning, sequence analysis, tissue-specific expression, and prohormone isolation of Eel atrial natriuretic peptide

A complementary DNA (cDNA) encoding eel atrial natriuretic peptide (ANP) precursor was specifically amplified from eel atrial mRNAs by rapid-amplification polymerase chain reaction. The sequence analysis of the cDNA using multiple clones revealed that the preproANP consists of 140 amino acid residues carrying a signal sequence at its N-terminus and a mature ANP at its C-terminus. An additional glycine residue was attached to the C-terminus of previously isolated eel ANP. The glycine residue may be used for amidation of the C-terminus or removed after processing. The cleavage site of a signal peptide with 22 amino acid residues was confirmed by isolation of proANP protein from eel atria. The proANP sequence deduced from the cDNA was also confirmed for 71% of the isolated protein. Sequence comparison with other natriuretic peptides revealed that eel ANP is more similar to mammalian ANP than to B-type natriuretic peptide (BNP) at both amino acid and nucleotide sequence levels. The eel ANP gene was a single copy gene as shown by Southern blot analysis. Northern blot analysis showed that eel ANP mRNA is approximately 0.8 kb in size and exclusively detected in the atrium. Thus, eel ANP is a true atrial hormone judging from both the sequence and the site of production. However, reverse transcription-polymerase chain reaction detected ANP message in the brain, gill, cardiac ventricle, red body of swim bladder (rete mirabilis), intestine, head kidney (including interrenal and chromaffin tissues) and kidney. Most of these tissues are involved in ion and/or gas exchange in fishes.