Biochemical and immunological evidence for a cardiodilatin-like substance in the snail neurocardiac axis

Proliferation and Differentiation Processes in the Heart Muscle Elements in Different Phylogenetic Groups

This article reviews, discusses, and summarizes data about the generative behavior of muscle tissue cells, the mechanisms of its regulation, and the organization of the endocrine function of the heart in the main phylogenetic groups. With respect to the ratio of processes of proliferation and differentiation, cell organization, and growth mechanism, muscle tissues of propulsive organs can be divided into three types, each revealed in one of three main groups of animals, lophotrochozoans, ecdysozoans, and chordates. Ecdysterone is likely to play the key role in the regulation of proliferation and differentiation processes in the heart muscle of crustaceans, and, most probably, also of molluscs. In each of the three main phylogenetic groups the endocrine function of the heart consisting of secretion of natriuretic peptides has a peculiar organization. Vertebrate cardiomyocytes are known to combine contractile and endocrine differentiation. Such functional dualism is absent in heart muscle elements of Lophotrochozoa and Ecdysozoa; in the heart of lopfotrochozoans, secretion of natriuretic peptides is performed by endothelial cells and their derivatives. Homology of the heart muscle in the animal kingdom as well as possible mechanisms of genomic and epigenomic regulation of different types of cardiomyogenesis are discussed.

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.

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.

Natriuretic peptide immunoreactivity in nerve structures and Purkinje fibres of human, pig and sheep hearts

Atrial natriuretic peptide is a well-described peptide in cardiac Purkinje fibres and has been shown to interfere with the autonomic regulation in the heart of various species, including man. Recently, we detected immunoreactivity for the peptide in intracardial ganglionic cells and nerve fibre varicosities of bovine hearts, by the use of a modified immunostaining technique that induced an improved detection of natriuretic peptides. These findings raised the question as to whether natriuretic peptides are detectable in these tissues in man and other species. The conduction system from human, pig and sheep hearts was dissected processed with antisera against atrial natriuretic peptide and the closely related brain natriuretic peptide. Immunostaining for the brain natriuretic peptide was detected in some Purkinje fibres in all of these species. Interestingly, in pig, sheep and human hearts, some ganglionic cells and nerve fibres showed atrial natriuretic peptide immunoreactivity, particularly in the soma of human ganglionic cells. This is the first study showing immunoreactivity for the atrial natriuretic peptide in nerve structures and for the brain natriuretic peptide in Purkinje fibres of the human heart. The results give a morphological correlate for the documented effects of atrial natriuretic peptide on the heart autonomic nervous system and for the presumable effects of brain natriuretic peptide in the conduction system of man.

Presence of immunoreactive atrial natriuretic peptide in nerve fibres and conduction cells in the conduction system of the bovine heart

Previous findings of atrial (A-type) natriuretic peptide (ANP) in nervous tissue, such as the brain and the superior cervical ganglia, led us to investigate the possible occurrence of ANP in nervous tissue in the heart. The distribution of ANP in the bovine heart, particularly its conduction system, was examined by the use of immunohistochemical methods and an antiserum against alpha-hANP. ANP immunoreactivity was frequently detected in atrial myocytes and in the Purkinje fibres of the AV-bundle, and was sometimes seen in the Purkinje fibres of the bundle branches and their ramifications. On the other hand, ANP immunoreactivity was never seen in the conduction cells of SA- and AV-nodes. ANP immunoreactivity was also detected in small nerve-fibre varicosities, mainly in the AV-node and AV-bundle. Most of these varicosities were located in the proximity of the conduction cells, but some occurred close to fine blood vessels or in the walls of arterioles. These observations show for the first time that ANP immunoreactivity is present not only in atrial myocytes and conduction cells but also in nerve-fibre varicosities in the conduction system. The observations suggest that ANP may act as a neuromodulator and/or neurotransmitter in the conduction system.

Ultrastructural and immunocytochemical study of the myoendocrine cells of the fish Hypostomus cordovae

The myoendocrine cells of the heart of Hypostomus cordovae (Günther 1880), a teleost fish from South America, were investigated by electron microscopy and immunohistochemistry. By applying antibodies raised against synthetic cardiodilatin 99-126 (CDD/ANP 99-126), a specific labeling of this hormone was found in the heart of this fish, mainly in myoendocrine cells of atrial trabeculae, where specific secretory granules are stored. The distribution of secretory granules exhibited striking seasonal variations. In winter there were fewer differentiated myoendocrine cells, which were easily recognized by the presence of specific secretory granules, most of which occur clustered in perinuclear areas of the cells. By contrast, in summer the majority of the myocardic cells of the atrium are active endocrine cells. They contain abundant secretory granules widely scattered in the cytoplasm, many of them polarized toward the subendocardial aspect of the cell. The secretory granules can be easily differentiated from the Weibel-Palade granules of endothelial cells, the shape, size and content of which were typical at electron-microscopic level. In addition, these endothelial granules did not display CDD immunoreactivity. The presence of cardiodilatin in a fish such as Hypostomus cordovae further supports the view that cardiac hormones are present in many Vertebrates and may preserve analogous roles such as those reported in other species throughout the group.

Atrial natriuretic peptide in the central nervous system of the rat

1. Studies of the presence of atrial natriuretic peptide immunoreactivity and receptor binding sites in the central nervous system have revealed unusual sites of interest. 2. As a result, numerous studies have appeared that indicate that brain atrial natriuretic peptide is implicated in the regulation of blood pressure, fluid and sodium balance, cerebral blood flow, brain microcirculation, blood-brain barrier function, and cerebrospinal fluid production. 3. Alteration of the atrial natriuretic peptide system in the brain could have important implications in hypertensive disease and disorders of water balance in the central nervous system.

The amino acid sequences of frog heart atrial natriuretic-like peptide and mammalian ANF are closely related

Despite few studies conducted in non-mammalian species, there has been a number of reports pertaining to the occurrence of a natriuretic-like substance in lower organisms. Thus, an immunoreactive substance reacting with antibodies directed against mammalian atrial natriuretic factor has previously been detected both in heart atria and ventricles of a chordate, the frog. This substance was isolated and purified from frog heart atria and its amino acid sequence established. The sequence, Ala-Pro-Arg-Ser-Ser-Asp-Cys-Phe-Gly-Ser-Arg-Ile-Asp-Arg-Ile-Gly-Ala-Gln- Ser-Gly - Met-Gly-Cys-Gly-Arg-(Phe), is highly homologous to known mammalian ANF sequences. However, when aligned with the complete mammalian ANF precursor sequence at positions 121 to 151, it exhibits a single amino acid insertion at position 129 and other substitutions at positions 121, 125, 133, 135, 144, 147 and 148. Some evidence is also presented concerning the occurrence of uncleaved frog pronatriodilatin, the precursor form of ANF. This study represents the first report pertaining to the structure of a non-mammalian ANF and its precursor.

Electronmicroscopical, immunohistochemical, immunocytochemical and biological evidence for the occurrence of cardiac hormones (ANP/CDD) in chondrichthyes

As representatives of the vertebrate class of chondrichthyes the plagostomian species Squalus acanthias, Scyliorhinus canicula and Raja clavata as well as the holocephalan species Chimaera monstrosa were investigated for the presence of cardiac hormones of the atrial natriuretic polypeptide/cardiodilatin- (ANP/CDD-) family. ANP/CDD-immunoreactive cells were detected in the atria and the ventricles of all species studied. While these cells failed to react with antisera raised against the N-terminus of CDD-126 (= gamma-ANP) they reacted with all antisera directed against sequences of the C-terminus of CDD-126 (CDD 99-126) which is identical to alpha-ANP. The ANP/CDD-immunoreactive cells were found in high numbers in all regions of the atria and in moderate density also in the ventricles. In correspondence, in the electron microscope, myoendocrine cells which were characterized by dense-cored secretory granules were identified in the atrial and ventricular myocardium. With the use of the protein A-gold technique, ANP/CDD-immunoreactivity was determined within the secretory granules. Furthermore, in the bioassay, prepurified extracts of the atria and the ventricles of Scyliorhinus and Chimaera exerted dose-dependent relaxations of the pre-contracted mammalian (rabbit) aorta. In both cases the atrial extracts proved to be more potent than the ventricular extracts. The present findings indicate that myoendocrine cells occur in the atria and ventricles of chondrichthyes and that these cells contain homologous cardiac hormones of the ANP/CDD-family in their secretory granules. The results are compared with those obtained earlier for the other vertebrate classes and their phylogenetic and functional significance is discussed.

Immunocytochemistry of cardiac polypeptide hormones (cardiodilatin/atrial natriuretic polypeptide) in brain and hearts of Myxine glutinosa (Cyclostomata)

With the use of different region-specific antisera against partial sequences of porcine cardiodilatin (CDD)-126 and the peroxidase-antiperoxidase (PAP) technique, the central nervous system as well as the systemic and the portal vein heart of the cyclostomian species Myxine glutinosa were investigated for a possible existence of cardiac polypeptides. In contrast to mammals, CDD-immunoreactions were obtained only with antisera directed against the C-terminus of CDD (CDD 99-126) which is identical to human atrial natriuretic polypeptide (alpha hANP). CDD-immunoreactive myocardiocytes were found in high densities in the atrium of the systemic heart and in the portal vein heart. In the ventricle of the systemic heart, CDD-immunoreactive cells were extremely scarce. In agreement with the immunohistochemical results, myoendocrine cells analyzed by electronmicroscopy exhibited specific granules of an average diameter of 0.21 + 0.02 micron in equivalent localizations. Furthermore, with the use of the protein A-gold (PAG) technique, CDD-immunoreactivity was ultrastructurally localized within the specific granules of atrial myocardiocytes. In the central nervous system of Myxine glutinosa, CDD-immunoreactive perikarya and/or fibers were present on all levels from the telencephalon to the spinal cord. The results of the present study are compared with those obtained in mammals and their possible functional relevance and their meaning in phylogeny are discussed as well.

Cardiodilatin-immunoreactive neurons in the hypothalamus of Tupaia

Using various region specific antibodies raised against partial sequences of synthetic cardiodilatin (CDD) we detected immunoreactive neurons with their perikarya in the nucleus periventricularis of Tupaia belangeri. The fibers could be traced laterally directed towards the amygdaloid complex and some varicosities were also observed in the lateral parts of the nucleus periventricularis. It is postulated that brain CDD represents a new neuropeptide and that these CDD-IR neurons are involved in specific functions related to the modulation of the cardiovascular centers.