Cardiac hormones. I. Review on the morphology, biochemistry, and molecular biology of the endocrine heart

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.

Distribution patterns of PCNA and ANP in perinatal stages of the developing rat heart

Distribution patterns of proliferating cell nuclear antigen (PCNA) and atrial natriuretic peptide (ANP) were determined immunohistochemically and morphometrically in atrium and ventriculum of the developing rat heart in different stages of the perinatal period. During the prenatal period, PCNA and ANP were localized in opposite patterns, particularly in trabecular myocytes. A distinct reduction in the percentage of PCNA-positive nuclei was detected starting at day 19 of the prenatal period, and these cells were rarely observed on postnatal days 30 and 60. In cardiomyocytes, a distinct increase in ANP positivity was found, whereas PCNA positivity was very low. It is concluded that PCNA expression gradually decreased from prenatal day 19 onwards, whereas ANP expression increased in atria throughout the prenatal and postnatal periods, except for a decrease in ANP expression in ventricles from prenatal day 21 onwards. The opposite expression patterns of PCNA and ANP in trabecular myocytes of ventricles indicate that ANP may have antimitogenic/antiproliferative effects in trabecular myocytes.

Detection of mRNA encoding guanylate cyclase A/atrial natriuretic peptide receptor in the rat cochlea by competitive polymerase chain reaction and in situ hybridization

Expression of mRNA encoding guanylate cyclase A (GC-A)/atrial natriuretic peptide (ANP) receptor in the rat cochlea was examined by polymerase chain reaction (PCR) and in situ hybridization (ISH). After reverse-transcription, PCR amplification, subcloning, and sequencing, we found that GC-A mRNA with sequence identical to that previously cloned from the rat brain (Chinkers et al., 1989) was expressed in the rat spiral ligament as well as in the spiral ganglion. However, GC-A mRNA was not detected in the stria vascularis. Competitive PCR using internal standard DNAs indicated that the expression of GC-A in the cochlea occurred at a level approximately 16 times less than that measured in kidney cortex. ISH histochemistry using a 35S-labeled antisense riboprobe showed the highest level of expression of GC-A mRNA to be in oligodendrocytes of the cochlear nerve root. The results suggest that ANP may play a role in the cochlear nerve function.

Pituitary adenylate cyclase-activating polypeptide stimulates cardiodilatin/atrial natriuretic peptide (CDD/ANP-(99-126) secretion from cultured neonatal rat myocardiocytes

It has been reported that the highly homologous neuropeptides pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal polypeptide (VIP) exert similar cardiovascular effects in vivo. In the present study we compared the effects of these neuropeptides on myocardial cyclic AMP content and the release of immunoreactive CDD/ANP-(99-126) (atrial natriuretic peptide). In cultured neonatal rat cardiomyocytes PACAP and VIP evoke concentration-dependent increases in intracellular cyclic AMP content but responses to VIP are markedly less. PACAP stimulates the release of CDD/ ANP-(99-126) in a concentration-dependent manner with a threshold concentration of 1 nM, and up to a 6-fold increase in basal secretion at 1 microM PACAP. In contrast. VIP had no effect on the release of CDD/ANP. Pretreatment of cells with the competitive PACAP-antagonist, PACAP-6-38 (1 microM), significantly reduces the effects of PACAP on intracellular cyclic AMP and on CDD/ANP-(99-126) secretion and abolishes the effects of VIP on cyclic AMP. Pretreatment with VIP-receptor antagonist (1 microM) prevents the cyclic AMP-response to VIP while increases in cyclic AMP as well as stimulation of CDD/ANP-(99-126) release by PACAP are not affected. It is concluded that both neuropeptides directly influence cardiac myocytes through an increase in intracellular cyclic AMP. Release of CDD/ ANP-(99-126) by PACAP may be involved in the decrease in blood pressure that follows intravenous administration of this peptide. The higher potency of PACAP to induce cyclic AMP synthesis, its stimulating effect on the release of CDD/ANP-(99-126) and the finding that the VIP-receptor antagonist inhibits responses to VIP but not to PACAP suggest that PACAP activates cardiac myocytes through a PACAP-specific receptor.

Morphometric analysis of atrial natriuretic peptide-containing granules in atriocytes of rats with experimental congestive heart failure

The morphometric characteristics of atrial natriuretic peptide-containing granules were studied in atrial myoendocrine cells of rats with aorto-caval fistula, an experimental model of congestive heart failure. A total of 6680 granules of control and aorto-caval rats were analyzed by a computerized image analysis system that evaluated the number and sectioned surface area of granules and their subcellular location. Compared with control animals, rats with congestive heart failure displayed a slight increase in the number of peripheral granules, adjacent to the sarcolemma, but not centrally located in the Golgi areas. The mean sectioned surface area of granules in rats with congestive heart failure was about 50% of that in controls, both in the right and left atria. Rats with aorto-caval fistula had a higher percent of small granules and lower percent of large granules compared with controls. The data demonstrate different morphometric characteristics in atrial natriuretic peptide-containing granules in atriocytes in rats with experimental congestive heart failure; this may reflect the enhanced synthesis and release of atrial natriuretic peptide in heart failure.

Dexamethasone recruits atrial natriuretic peptide secretory cells in the rat left atrium and apex of the ventricle

The response of the atrial natriuretic peptide (ANP) secreting cells from both rat atria and the apex of the ventricles to dexamethasone (DEX) was analyzed by the plaque assay. In right atrial cardiocytes, 25% of the cells secreted ANP basally; DEX treatment did not alter this percentage. However, in the left atrial secretory population, a discordance between the basal (15%) and DEX stimulated (25%) percent plaque formation was found. ANP secreting cells from the ventricular apex responded similarly to DEX exposure (26%), with 8% of the cells basally releasing the hormone. These data suggest that in both the left atria and apex of the rat ventricles, exposure to DEX recruits ANP secretory cells from a non-secreting population. Consequently, the release of ANP from these tissues would increase after glucocorticoid stimulation.

Assessment of anthracycline cardiomyopathy by endomyocardial biopsy

Ultrastructural evaluation of endomyocardial biopsy specimens is a sensitive and effective method with which to detect and quantitate cardiotoxicity produced by anthracylines. The procedure and grading system used at the M.D. Anderson Cancer Center are described, and some of the pitfalls that may be encountered by the electron microscopist are reviewed.

Cardiac myxomas: immunohistochemical study of benign and malignant variants

Immunohistochemical investigation of 11 cardiac myxomas (CMs) including one malignant metastasizing CM showed a co-expression of epithelial (lu-5 and CAM 5.2), mesenchymal (vimentin) and neuroendocrine antigens (neuron-specific enolase) in all tumour cells. Factor VIII was found in the endothelial cells of capillaries only. In the subendocardium of fetal heart tissue close to the foramen ovale myofibroblasts reacting with the panepithelial antibody lu-5 were detected. We conclude that CMs are neoplasms that may develop from embryonic cell remnants.

Atrial natriuretic peptide-like immunoreactive cells in the guinea pig inner ear

Using specific antibodies against cardiodilatin/atrial natriuretic peptide (CDD/ANP) in a conventional immuno-histochemical method (PAP) we located ANP/CDD-like immuno-reactive cells related to the secretory area, to the sensory and to the neuronal area in the compartments of the inner ear (cochlea, utricle/ampulla, and endolymphatic sac). Immunoreactive cells were unevenly distributed in the different compartments as well as within the cochlear space. Our findings suggest that ANP/CDD may play a role in the local control of fluid and electrolyte homeostasis of the inner ear. ANP/CDD-binding sites and ANP/CDD-like immunoreactivity in the inner ear may also indicate that the peptide has an additional paracrine and/or autocrine function in the organ.

Expression of lipoprotein lipase mRNA in rat heart is localized mainly to mesenchymal cells as studied by in situ hybridization

The expression of lipoprotein lipase mRNA (LPL mRNA) was studied in rat hearts by use of a sulfur-35-labeled antisense mRNA probe. Rats were studied under three conditions: fed, fasted, and injected with cholera toxin (an irreversible agonist of adenylate cyclase) and then fasted. The highest LPL activity was found in the hearts of cholera toxin-injected, fasted rats. After injection of cholera toxin, LPL mRNA levels were 3.5-fold higher than those from fed rats. Using in situ hybridization, we studied the site of expression of LPL mRNA under the same three experimental conditions. In sections of hearts from cholera toxin-injected, fasted rats, concentrations of autoradiographic grains, representing the site of LPL mRNA, were seen over interstitial elements, which comprise capillary and perivascular cells. A more diffuse and sparse reaction was seen over cardiac myocytes and was not always distinguishable from background. A similar but much less definitive localization was seen in sections of hearts from fasted rats. The present results indicate that in the rat heart, the main site of LPL synthesis and processing, especially after stimulation with an irreversible agonist of adenylate cyclase, is localized to interstitial elements rather than to adult cardiac myocytes.

Distribution of atrial natriuretic peptide in the conduction system and ventricular muscles of the human heart

Atrial natriuretic peptide (ANP), a cardiac hormone, is known to be located in the atrial specific granules, but its presence and localization in the ventricular muscle of the human heart has not been examined fully. Using a specific antibody to human ANP, we studied the conduction system and ventricular muscle with immunohistochemical and ultrastructural methods in 30 hearts obtained at autopsy. These included 12 normal and 18 diseased hearts. In the normal hearts, ANP-positive granules, which were regularly observed in the atrial myocytes, were found in small quantities in the cells of the penetrating and branching bundles in 4 of 12, and in the cells of the ventricular free walls in 2 of the 12 hearts. In the diseased hearts, the positivity increased significantly (P less than 0.05), being found in 13 of 18 (72.2%) conduction systems and 10 of 18 (55.6%) ventricular muscles. The granules were confirmed to be immunoreactive with ANP by ultrastructural examination. Furthermore, the presence of ANP mRNA in the conduction system as well as in the ventricular myocytes was demonstrated by Northern blot hybridization for which we used the complementary DNA of human ANP. Thus, a small quantity of ANP appears to be synthesized and stored in the conduction system and ventricles of some normal hearts. However, ANP was shown to be present in a larger percentage of the diseased hearts.

Localization of atrial natriuretic peptide/cardiodilatin (ANP/CDD)-immunoreactivity in the lacrimal gland of the domestic pig

The presence of atrial natriuretic peptic/cardiodilatin-immunoreactive material was demonstrated in the lacrimal gland of the domestic pig by high performance liquid chromatography and radioimmunoassay. The immunohistochemical localization revealed a distinct population of cuboid or spindle-shaped ANP/CDD-IR cells in the epithelium of the terminal portion of the secretory tubules. In addition, a moderate number of positive cells was localized intraepithelially in the intralobular ducts as well as the connective tissue between these ducts. Our findings provide a morphological indication that ANP/CDD may play a physiological role in the regulation of sodium transport and secretion in the lacrimal gland.

Release of atrial natriuretic peptide from individual rat cardiocytes

Atrial natriuretic peptide (ANP) is released from the atria and acts to regulate blood volume and pressure. The release of ANP appears to be stimulated by atrial distension, initiated by stretch on the cardiocytes. The purpose of the present study was to develop an assay that would allow for the detection of ANP release from single, isolated cells in the absence of distension. Using the reverse hemolytic plaque assay and antibody raised against human-alpha ANP, the release of ANP was detected from trypsin dissociated rat atrial cells. The specificity of the assay was demonstrated by a 67% reduction in ANP plaque forming cells detected following preabsorption of the anti-sera with rat-alpha ANP. The assay also proved efficient in monitoring changes in ANP secreting cell populations, where an acute treatment with dexamethasone resulted in a doubling of the percentage of atrial cardiocytes detected within a 4 hour antibody incubation. Finally, the assay established that about 52% of the dispersed atrial cardiocytes release ANP. The establishment of a plaque assay for ANP release should assist in addressing questions concerning what hormones may regulate ANP secretion directly and also allow for the determination of ANP secreting cell population dynamics.

Angiotensin converting enzyme inhibitor potentiated the vasorelaxant response of atrial natriuretic peptide in toad aortic rings

1. The vasorelaxant effect of synthetic atrial natriuretic peptide (ANP) on the vascular response to angiotensin II (A II) and norepinephrine (NE) in aortic rings from Bufo arenarum toad was studied. 2. Pretreatment with ANP partially inhibited the vascular response to A II and NE. 3. Angiotensin converting enzyme inhibitor (ACEI) treatment partially inhibited the contractile response of angiotensin I (A I) and did not affect the A II response. 4. The inhibitory effect of ANP on vascular response to A II and NE were potentiated by pretreatment with ACEI. 5. Results suggest that the angiotensin converting enzyme present in the vascular wall from Bufo arenarum toad may be involved in the metabolism of ANP.

Influence of pacing mode and rate on peripheral levels of atrial natriuretic peptide (ANP)

The effect of acute modifications of pacing mode and rate on plasma ANP levels was evaluated. ANP was determined in ten resting patients with DDD pacemakers due to binodal disease or intermittent second-and third-degree AV block. At 82/minute pacing rate the ANP plasma levels (normal range 2 to 30 fmol/mL) corresponded to those under AAI (4.05 +/- 2.10 fmol/mL) and DDD (4.18 +/- 2.02 fmol/mL) pacing, but increased significantly (P 0.05) during VVI pacing (6.96 +/- 3.70 fmol/mL). Acceleration of DDD stimulation frequency from 82 to 113/minutes led to significant increase of ANP levels by the factor of three in all chosen AV delays. The lowest ANP plasma levels were measured at 175 msec AV delay under 82/minute pacing rate in DDD mode. Under 113/minutes the differences of ANP concentration after variations of AV delays were less pronounced. The influences of altered atrial pressure and tension on ANP release are discussed to account for changes in ANP plasma levels following different modes and rates of pacemaker stimulation.

Phosphorylation and dephosphorylation of the natriuretic peptide urodilatin (CDD-/ANP-95-126) and the effect on biological activity

Urodilatin (CDD-/ANP-95-126), a new peptide hormone from human urine, is comprised of the same amino acid sequence as cardiodilatin (CDD-99-126/alpha-hANP) except for N-terminal extention by four amino acid residues. The presence of the recognition sequence Arg101-Arg-Ser-Ser104 for the cyclic AMP-dependent protein kinase enables rapid phosphorylation in the Ser104-position. Phosphorylation of urodilatin is associated with decreased vasorelaxant potency, while dephosphorylation of "phospho-urodilatin" by acidic phosphatase completely restores bioactivity.

Degradation of porcine brain natriuretic peptide (pBNP-26) by endoprotease-24.11 from kidney cortical membranes

Porcine brain natriuretic peptide of 26 amino acid residues (pBNP-26) is inactivated by endoprotease-24.11 (EC 3.4.24.11) of kidney cortical membranes. In contrast to human alpha atrial natriuretic peptide/cardiodilatin (ANP/CDD) showing a single major cleavage within the disulfide-linked loop between Cys and Phe in position 7 and 8, pBNP-26 is cleaved at several sites. Although both pBNP-26 and ANP/CDD exhibit Cys-Phe peptide bonds at the corresponding positions this bond is not cleaved in BNP-26.

Demonstration of atrial natriuretic peptide/cardiodilatin (ANP/CDD)-immunoreactivity in the salt gland of the Pekin duck

A novel peptide hormone, atrial natriuretic factor/cardiodilatin (ANP/CDD), was recently isolated and characterized from mammalian heart. Its presence has been demonstrated in several organs that contribute to water and sodium homeostasis, such as salivary glands. This study demonstrates the presence of ANP/CDD immunoreactivity in the salt gland of Pekin ducks by high performance liquid chromatography, radioimmunoassay and immunocytochemistry, using a specific antibody against atriopeptide I. A small number of distinct, ovoid or cuboid shaped ANP/CDD-immunoreactive cells were localized in the connective tissue surrounding and separating the central secretory tubules, whereas no immunostaining was observed in the peripheral tubules. Salt glands of ducks that were adapted to salt water revealed a significant hypertrophy of their secretory lobules. However, no differences were found between the number or localization of immunoreactive cells in the salt gland of salt water-acclimatized ducks and nonstimulated glands of ducks that were housed with ad libitum access to fresh water. Our results indicate that ANP/CDD may play a role in the regulation of sodium secretion in the salt gland of aquatic birds.

Localization of the atrial natriuretic peptide binding sites in the inner ear tissue--possibly an additional regulating system

Specific high affinity atrial natriuretic peptide (ANP) binding sites were identified and localized by autoradiography (125J-ANP 1-28) in the inner ear of guinea pigs. ANP is a cardiac hormone known to mediate increased capillary permeability, vasodilation, and natriuresis. Since specific receptors for ANP exist in both eye and brain, possibly playing a role in regulation of fluid and pressure (14, 15), we hypothetized that ANP might be involved also in inner ear fluid dynamics via interaction with other volume/electrolyte balance regulating hormones including vasopressin, renin, and aldosteron. The distribution of ANP receptor in the inner ear tissue appears unequal, which may play a significant role in view of some functional disturbances such as Meniere's disease.

Ultrastructural aspects of atrium development: demonstration of endocardial discontinuities and immunolabeling of atrial natriuretic factor in the Syrian hamster

The endocardium ultrastructure of 13 embryonic day old hamsters was examined, especially in relationship with the atrial myocytes. The endothelial morphology was described, including the junctional attachments and their relationships with subjacent atrial myocytes. Characteristic atrial myocytes organelles were identified: myofibrils, atrial granules, lipidic inclusions, and polysomes. Immunogold labeling demonstrated that atrial natriuretic factor (ANF)-containing granules were already present in the differentiating cardiomyocytes, even before the myofibrils were completely organized. At this stage of development, while the endothelium was a narrow barrier between the blood and the cardiomyocytes, it displayed fenestrations, but also epithelial discontinuities. In addition it also contains immunoreactive-ANF products. In light of the current knowledge about ANF processing it was proposed that the endocardium lining could be an obligated passageway for transport or activating proANF into ANF before its release into the blood stream. In addition the endocardial gaps could suggest that, until about 13 to 14 days of fetal development, heart atrial tissue could be more susceptible to the effects of pathogenetic compounds than in a later state of development.

Characteristics of distension-induced release of immunoreactive atrial natriuretic peptide in isolated perfused rabbit atria

Atrial pressure- or distension-induced release of atrial natriuretic peptide (ANP) has been considered as an important regulatory mechanism of ANP release in cardiac atria. A new technique to permit graded continuous atrial distension has been developed in an isolated perfused single rabbit atrium. Graded atrial distension was induced by changing the elevation of the outflow catheter tip. Intra-atrial volume expansion resulted in an increase in immunoreactive ANP release. The graded increase in atrial distension from 43.9 +/- 10.2 to 207.7 +/- 29.1 microliter resulted in 6.2-27.1-fold increases in volume-dependent immunoreactive ANP release. A rise in immunoreactive ANP release induced by increasing atrial distension did not occur in the state of atrial distension but occurred only after return to the reduced distension. However, in the case of atrial distension with pacing, an increase in immunoreactive ANP release was observed during atrial distension with pacing and after return to the basal level. The present study shows that the new technique is applicable to the study of the 'stretch-secretion coupling' mechanism of ANP release in vitro, and that the more important factor involved in the release of immunoreactive ANP induced by atrial distension may be the atrial reduction to basal level after distension rather than the stretch itself.

Isolation and structural analysis of "urodilatin", a new peptide of the cardiodilatin-(ANP)-family, extracted from human urine

Two major forms of cardiac peptides have been established in the last few years: (a) a prohormone of 126 amino acids (CDD/ANP-1-126) in the endocrine heart and (b) the circulating CDD/ANP-99-126 (= alpha ANP) in blood plasma. The method we applied earlier to isolate the circulating form of cardiodilatin from human blood was used to detect and analyze the biologically active, predominant form of the same polypeptide family excreted by the kidneys. Each step of the isolation procedure was followed up by a bioassay using an in vitro vascular smooth muscle relaxation test and a highly specific RIA against cardiodilatin (CDD-99-126) for the initial purification steps. The polypeptides excreted in 1000 l of normal human urine were adsorbed to 2.5 kg of alginic acid, and after elution and lyophilization processed on a G-25 Sephadex column. The obtained crude polypeptide fractions were applied to ion-exchange chromatography. Thereafter four steps of HPLC were carried out to purify the polypeptide which was the suggested form of cardiodilatin (CDD) in human urine. The amino acid analysis and gas phase sequence analysis showed that the main form of urinary cardiodilatin is a 32 amino acid residue containing molecule, cardiodilatin-95-126. The molecule is N-terminally extended compared to the circulating CDD-99-126. This suggests that the analyzed urinary peptide is not the residual plasma form, filtrated and renally cleared from blood, but probably a polypeptide produced and processed in the kidney tubules and cleaved by a different postranslational process. Therefore, this vasorelaxant polypeptide is called urodilatin.

Role of atrial natriuretic factor, cyclic GMP and the renin-aldosterone system in acute volume regulation of healthy human subjects

The role of the atrial natriuretic factor (ANF), its second messenger cyclic guanosine monophosphate (cGMP), and the counteracting renin-aldosterone system in acute volume regulation was investigated in 25 healthy human subjects. Central volume stimulation by 1-h head-out water immersion (WI) into a thermoneutral water-bath increased plasma levels of ANF (mean +/- SEM) from 6.0 +/- 0.6 to 13.6 +/- 2.6 fmol ml-1. This was paralleled by a rise of plasma cGMP levels from 1.9 +/- 0.2 to 2.8 +/- 0.4 pmol ml-1, and an increase of urinary cGMP excretion from 340 +/- 64 to 692 +/- 103 pmol min-1. Water immersion reduced plasma aldosterone concentration (PAC) from 13.0 +/- 1.7 to 6.5 +/- 0.8 ng 100 ml-1 and plasma renin activity (PRA) from 5.3 +/- 0.9 to 2.4 +/- 0.3 ng AI ml-1 h-1. Volume stimulation markedly increased diuresis and natriuresis. Whereas the plasma cGMP increase correlated with plasma ANF stimulation, neither ANF nor PRA or PAC correlated with basal or stimulated renal parameters. Water immersion-induced changes in natriuresis and urinary cGMP excretion were correlated. These data suggest a role of ANF and cGMP in acute volume regulation of healthy human subjects.

Urodilatin (CDD/ANP-95-126) is not biologically inactivated by a peptidase from dog kidney cortex membranes in contrast to atrial natriuretic peptide/cardiodilatin (alpha-hANP/CDD-99-126)

Atrial natriuretic peptide (CDD/ANP-99-126) is rapidly inactivated by a membrane preparations from dog kidney cortex. Inactivation occurs by cleavage of the ring structure in the position between Cys-105 and Phe-106. A unique proteolytic product separated by HPLC on reverse-phase column appears as a single peak which elutes prior the intact peptide. In contrast, CDD/ANP-95-126 (urodilatin) which is released from the kidney is not destroyed by proteolysis using an identical membrane preparation.

Radioreceptor assay for atrial natriuretic factor

Interest in accurate measurement of atrial natriuretic factor (ANF) in biological fluids and various tissues has been stimulated by recent data indicating the possible role of ANF in the homeostasis of salt and water. The presence of high-affinity binding sites for ANF in rat glomeruli has allowed us to develop a rapid, sensitive, and simple radioreceptor assay (RRA). A saturable high-affinity binding site on the membranes of rat glomeruli has been characterized by a dissociation constant of 33 pM and binding capacity of 396 fmol/mg protein. Rat plasma extracts or atrial homogenates or standards were incubated with radioiodinated ANF and a preparation of rat glomerular membranes. The receptor-bound and free radioactivity were separated by filtration on Whatman GF/C paper after 1 h incubation at room temperature. The sensitivity of the RRA was 2.08 fmol. The effective concentration of standard ANF that displaced 50% of labeled receptor-bound ANF (EC50) was 43.3 +/- 2.6 fmol/ml (n = 7). Both intra- and interassay coefficients of variation were smaller than 11%. This RRA assay has been compared with radioimmunoassay (RIA). High correlations for 19 plasma extracts and 34 atrial homogenates (r = 0.973 and r = 0.954, respectively) tested by RRA and RIA were obtained. This good correlation between the two methods suggests that the immunoreactive material found in rat plasma and atrial homogenates also displays biological activity.

Localization of atrial natriuretic peptide mRNA and immunoreactivity in the rat heart and human atrial appendage

The localization of mRNA encoding preproatrial natriuretic peptide was investigated in tissue sections and cultures of rat heart and in sections of human right atrial appendage using the technique of in situ hybridization with 32P- and 35S-labeled RNA probes. Rat atrial natriuretic peptide (ANP) transcripts were demonstrated in numerous atrial myocytes and, to a lesser extent, in ventricular myocytes in both tissue sections and newborn rat heart cultures. These findings are consistent with those obtained by RNA blot analysis of rat heart total RNA, indicating that a single prepro-ANP transcript of approximately 900 nucleotides was present in the ventricles as well as the atria. Using a 35S-labeled RNA probe for human ANP mRNA, ANP transcripts were also localized to the majority of myocytes in the human right atrial appendage. Only background levels of autoradiographic labeling were obtained when RNA probes identical to the coding sequence of rat or human ANP mRNA were used. A close correlation was found between the distribution of ANP immunoreactivity and prepro-ANP mRNA in these preparations. These results provide unequivocal evidence for the expression of the ANP gene in the rat ventricles, as well as the atria, because myocytes in these tissues have been established as the sites of both ANP localization and precursor biosynthesis. The combined use of cardiac cultures and in situ hybridization may be of value in future studies investigating the regulation of ANP synthesis in cardiac myocytes.

Atrial natriuretic factor. Possible implications in liver disease

The discovery of the first well-defined natriuretic hormone, the Atrial Natriuretic Factor (ANF), has prompted research on its impact on volume regulation in health and disease. The natriuretic, diuretic, and smooth muscle-relaxing properties suggest an important role of this novel hormone in pathophysiological states with sodium or volume retention, such as congestive heart failure or cirrhosis of the liver. Investigations on the implications of ANF in liver disease have been performed for little more than 1 year, and results are still controversial in many respects. At present, it seems very likely that there is no absolute deficiency of plasma ANF in patients with cirrhosis. Moreover, elevated plasma levels in cirrhotics with ascites have been reported by several groups. However, as yet, a molecular characterization of this increased immunoreactivity is still lacking. There is disagreement on the reduced release of and renal response to ANF in subgroups of cirrhotics; however, stimulus-response-coupling might be impaired. Further studies are needed to elucidate the pathophysiological implications and therapeutical potential of ANF in patients with chronic liver disease.

Peptides in the mammalian cardiovascular system

Ample immunocytochemical evidence is now available demonstrating that several peptides are present in the mammalian cardiovascular system where they are localised to nerve fibres and myocardial cells. The neuropeptides (neuropeptide Y, calcitonin gene-related peptide, tachykinins and vasoactive intestinal polypeptide) are localised to large secretory vesicles in subpopulations of afferent or efferent nerves supplying the heart and vasculature of several mammals, including man. Although they often exert potent pharmacological effects on the tissues in which they occur their physiological significance has still to be established. They may act directly via specific receptors and/or indirectly by influencing the release and action of other cardiovascular transmitters. In marked contrast, atrial natriuretic peptide is produced by cardiac myocytes and considered to act as a circulating hormone.

Isolation of bovine cardiodilatin by fast protein liquid chromatography and reversed-phase high-performance liquid chromatography

Cardiodilatin (CDD), a polypeptide exhibiting vasorelaxant and diuretic natriuretic bioactivity, was isolated from bovine atria. The isolation procedure reported here is different from that originally used for the purification of porcine and bovine CDD. Instead of cation-exchange chromatography on Fractogel TSK-CM 650 S and several purification steps on different high-performance liquid chromatographic (HPLC) columns, it is now possible to obtain CDD-88 by an automated fast-protein liquid chromatography system for repeated injections and a motor valve as fraction collector and only one final step of reversed-phase HPLC on a TSK-ODS-120T column.

Organ extraction of atrial natriuretic peptide (ANP) in man. Significance of sampling site

Arterial plasma immunoreactivity of endogenous human alpha-atrial natriuretic peptide (ANP) underwent mean 54%, 28% and 40% extraction during one passage through the circulation in the kidney (n = 12), liver-intestine (n = 14) and lower limb (n = 8), respectively, in supine fasting subjects with no detectable disease or subjects with cardiovascular or hepatic disorders of minor degree undergoing a haemodynamic investigation. No extraction was identified across the lungs as evaluated by the same concentration of ANP in pulmonary and femoral arteries (n = 7). The concentration of ANP in a superficial arm vein relative to the femoral artery varied considerably and extractions from 0% up to 58% were identified (mean 18%). The results suggest a high degree of, but only to some extent selective, extraction of ANP, which may account for its proposed short plasma half-life. Due to the different concentrations of ANP in various vascular beds, sampling site should be thoroughly specified.

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.

Isolation and structural analysis of the circulating human cardiodilatin (alpha ANP)

A new method was applied to isolate a polypeptide hormone from human blood. The polypeptides from 1,000 1 of hemofiltrate with a molecular weight lower than 20 kDaltons were adsorbed to 2.5 kg alginic acid, then eluted, precipitated, and desalted on a G-25 Sephadex column, thus obtaining a crude lyophilised plasma polypeptide extract. These polypeptides were further submitted to ion-exchange chromatography. Thereafter, two steps of HPLC were carried out to purify a distinct polypeptide which was the circulating form of cardiodilatin (CDD) in this case. The amino acid analysis, C-terminal enzymatic cleavage by carboxypeptidase A, and sequence analysis showed that the only form of circulating cardiodilatin is the 28 amino acid residue containing molecule, cardiodilatin-99-126 cleaved from the C-terminus of cardiodilatin-126 and identical with alpha-ANP (alpha atrial natriuretic polypeptide). Other bioactive molecular forms of the polypeptide hormones of the cardiodilatin family were not detected in the hemofiltrate. The isolation procedure was followed up by a bioassay using in vitro vascular smooth muscle relaxation.

Immunocytochemical evidence for CDD/ANP-like peptides in strands of myoendocrine cells associated with the ventricular conduction system of the rat heart

Immunohistochemical investigations with different antisera against cardiodilatin 99-126 or alpha atrial natriuretic polypeptide revealed the presence of cardiac hormones not only in the atria of rats but also in strands of myoendocrine cells located in subendocardial regions of the ventricular septum. The localization of CDD-IR (cardiodilatin immunoreactivity) in the ventricle is associated with the location of the conduction system in the rat. The significance of the morphological relationship between cardiodilatin and the conduction system of the rat heart is discussed.

Coexistence of atrial natriuretic factor (ANF) and oxytocin in neurons of the rat hypothalamus

Sections of rat hypothalamus were examined immunocytochemically for coexistence of atrial natriuretic factor (cardiodilatin) and oxytocin. These biosynthetically unrelated neuropeptides were demonstrated using the peroxidase-antiperoxidase method followed by immunofluorescence. Neurons immunoreactive for atrial natriuretic factor were found in the supraoptic, periventricular and paraventricular nuclei. A sub-population of these neurons contained oxytocin-like immunoreactivity. They constitute only a small portion of the total number of oxytocinergic neurons.