Specific membrane receptors for atrial natriuretic factor in renal and vascular tissues

Functional Screening Identifies MicroRNA Regulators of Corin Activity and Atrial Natriuretic Peptide Biogenesis

Atrial natriuretic peptide (ANP) represents an attractive therapeutic target in hypertension and heart failure. The biologically active form of ANP is produced by the cardiac serine protease corin, and modulation of its activity might therefore represent a novel approach for ANP augmentation. MicroRNAs (miRNAs) are pervasive regulators of gene expression, but their potential role in regulating corin activity has not been elucidated. Our aim was to systematically identify and characterize miRNA regulators of corin activity in human cardiomyocytes. An assay for measuring serine protease activity in human induced pluripotent stem cell (iPS)-derived cardiomyocytes was used to perform a comprehensive screening of miRNA family inhibitors (n = 42). miRNA 1-3p (miR-1-3p) was identified as a potent inhibitor of corin activity. The interaction between miR-1-3p and a specific target site in the CORIN 3' untranslated region (3' UTR) was confirmed through argonaute 2 (AGO2)-RNA immunoprecipitation and reporter assays. Inhibition of miR-1-3p resulted in upregulation of CORIN gene and protein expression, as well as a concomitant increase in extracellular ANP. Additionally, miR-1-3p was found to interact with and inhibit the expression of several transcriptional activators of ANP gene expression. In conclusion, we have identified a novel regulator of corin activity and ANP biogenesis in human cardiomyocytes that might be of potential future therapeutic utility.

Atrial natriuretic peptide and regulation of vascular function in hypertension and heart failure: implications for novel therapeutic strategies

Atrial natriuretic peptide (ANP) plays a pivotal role in modulation of vascular function and it is also involved in the pathophysiology of several cardiovascular diseases. We provide an updated overview of the current appraisal of ANP vascular effects in both animal models and humans. We describe the physiological implications of ANP vasomodulatory properties as well as the involvement of ANP, through its control of vascular function, in hypertension and heart failure. The principal molecular mechanisms underlying regulation of vascular tone, that is natriuretic peptide receptor type A/cyclic guanylate monophosphate, natriuretic peptide receptor type C, nitric oxide system, are discussed. We review the literature on therapeutic implications of ANP in hypertension and heart failure, examining the potential use of ANP analogues, neutral endopeptidase (NEP) inhibitors, ACE/NEP inhibitors, angiotensin receptor blocker (ARB)/NEP inhibitors, the new dual endothelin-converting enzyme (ECE)/NEP inhibitors and ANP-based gene therapy. The data discussed support the role of ANP in different pathological conditions through its vasomodulatory properties. They also indicate that ANP may represent an optimal therapeutic agent in cardiovascular diseases.

Atrial natriuretic peptide inhibits the spontaneous contractions of rabbit isolated ileum

The present study investigates the effects of atriopeptin II on spontaneous phasic contractions of rabbit isolated ileum. Atriopeptin II caused a significant and concentration-dependent decrease in ileum motor activity. This effect was mimicked by 8-Br-cGMP and it was not affected by pretreatment with tetrodotoxin. Verapamil significantly decreased ileum contractions; however, in the presence of this calcium blocker, atriopeptin II further reduced ileal motility. These findings demonstrate that atriopeptin II depresses the motility of rabbit ileum through a cGMP-dependent mechanism and suggest that neither ileal neural networks nor extracellular calcium are involved in this effect.

Peptide receptors as molecular targets for cancer diagnosis and therapy

During the past decade, proof of the principle that peptide receptors can be used successfully for in vivo targeting of human cancers has been provided. The molecular basis for targeting rests on the in vitro observation that peptide receptors can be expressed in large quantities in certain tumors. The clinical impact is at the diagnostic level: in vivo receptor scintigraphy uses radiolabeled peptides for the localization of tumors and their metastases. It is also at the therapeutic level: peptide receptor radiotherapy of tumors emerges as a serious treatment option. Peptides linked to cytotoxic agents are also considered for therapeutic applications. The use of nonradiolabeled, noncytotoxic peptide analogs for long-term antiproliferative treatment of tumors appears promising for only a few tumor types, whereas the symptomatic treatment of neuroendocrine tumors by somatostatin analogs is clearly successful. The present review summarizes and critically evaluates the in vitro data on peptide and peptide receptor expression in human cancers. These data are considered to be the molecular basis for peptide receptor targeting of tumors. The paradigmatic peptide somatostatin and its receptors are extensively reviewed in the light of in vivo targeting of neuroendocrine tumors. The role of the more recently described targeting peptides vasoactive intestinal peptide, gastrin-releasing peptide, and cholecystokinin/gastrin is discussed. Other emerging and promising peptides and their respective receptors, including neurotensin, substance P, and neuropeptide Y, are introduced. This information relates to established and potential clinical applications in oncology.

Novel approaches to the treatment of acute renal failure

Acute renal failure (ARF) occurs frequently in hospitalised patients and is associated with significant morbidity and mortality. Many therapeutic strategies have been undertaken both to prevent acute renal injury and, once ARF occurs, to improve renal function and reduce mortality. Among the available pharmacological options, no specific therapy has been shown to alter the course of ARF. This article reviews the efficacy of several strategies in experimental renal disease and raises the possibility that similar interventions might be available to the clinician in the near future for the prevention and management of ARF. The prospect of these novel strategies together with the ever-increasing understanding of the complex pathophysiology of ARF, offers the promise of effective and more physiological therapeutic interventions in this new millennium.

Inhibition of kidney neutral endopeptidase after administration of the neutral endopeptidase inhibitor candoxatril: quantitation by autoradiography

Inhibition of neutral endopeptidase (NEP) in the kidney was studied ex vivo after oral administration of candoxatril (UK79300), an NEP inhibitor, to rats to study the time course and dose response by quantitative in vitro autoradiography by using the NEP inhibitor 125I-SCH47896 as a radioligand. In control rats, high NEP binding was demonstrated in the deep proximal tubule. After oral administration of candoxatril (10 mg/kg), kidney NEP binding was rapidly decreased and recovered gradually over a period of 24 h. The inhibition was maximal at 1 h (13.3 +/- 2.5% of control). Increasing doses of candoxatril administered to rats produced progressive inhibition of NEP binding in the kidney. A dose of 100 mg/kg inhibited kidney NEP binding to 2.6 +/- 0.2% of the control value at 1 h after administration. Candoxatrilat (UK73967), an active metabolite of candoxatril, given intravenously inhibited kidney NEP binding also in a time- and dose-dependent manner. This inhibition of NEP activity at the tissue level may be important in the actions of NEP inhibitors.

Atriopeptin lowers aqueous humor formation and intraocular pressure and elevates ciliary cyclic GMP but lacks uveal vascular effects in the bovine perfused eye

The effects of atriopeptin (AP) on intraocular pressure (IOP), aqueous humor formation (AHF) and ciliary cyclic GMP in bovine eyes perfused in vitro through a ciliary artery were investigated. AHF was monitored fluorophotometrically by perfusing the anterior chamber with Bárány's mock aqueous humor containing fluorescein. To study the effect of AP on cyclic GMP synthesis, eyes were injected with an intra-arterial bolus of AP prior to dissection of the ciliary processes. For comparison, individual ciliary processes, or isolated ciliary epithelial cells were incubated with AP. Cyclic GMP was extracted by homogenization, aqueous partition and column chromatography, and measured by radioimmunoassay (RIA). Decreases in IOP or AHF were seen approximately 15 min after the injection of AP and persisted throughout a 60-80 min perfusion. Arterial perfusion pressure was not significantly altered by AP, even when vascular tone was raised by adding noradrenaline to the perfusate. In whole eyes, ciliary cyclic GMP increased in response to AP. Cyclic GMP in isolated ciliary processes, or cultured ciliary epithelial cells incubated with AP also increased. We conclude that the decrease in IOP in the bovine eye by AP is due to a reduction in AHF, which is independent of any vascular effect. Ciliary cyclic GMP may be the intracellular mediator involved.

Expression of mRNA for atrial natriuretic peptide receptor-A in human liver: detection using RT-PCR

cDNA preparations obtained from human liver poly A+ RNA by reverse transcription were subjected to polymerase chain reaction. Primers used in the reaction were designed from the reported cDNA sequence for human placental atrial natriuretic peptide receptor-A (ANPRA). Sequence analysis of the amplified product (approximately 700 bp) showed 100% identity to the 2476-3189 bp region of the reported cDNA sequence of human placental ANPRA. Northern blot analysis of human liver poly A+ RNA fractions showed a hybridization signal at approximately 4.4 kb, identical to the signal obtained from the poly A+ RNA fractions of human placenta. These results indicate the possible mRNA expression for atrial natriuretic peptide receptor-A in human liver.

Renal effects of human atrial natriuretic peptide in patients after major vascular surgery

The effects were studied postoperatively of an infusion of atrial natriuretic peptide (ANP) 7.5 pMol.kg-1.min-1 on renal function and haemodynamics in seven patients who had been operated with insertion of an abdominal aortic graft. Urine flow, glomerular filtration rate (GFR), renal plasma flow (RPF) and excretion of electrolytes and osmoles were measured for three periods of 20 minutes during infusion of ANP, in the morning of the day after surgery. Haemodynamic studies were conducted, and serum levels of ANP, catecholamines and plasma renin activity were measured. ANP levels increased from 52 to approximately 250 pMol.L-1 during ANP infusion and decreased after infusion to a level equal to baseline. GFR increased from 92 mL.min-1 by 58, 20 and 21%, respectively. RPF was unchanged. Urine flow rate increased from 1.99 mL.min-1 by 81, 151 and 173%, respectively. Fractional clearances of sodium, chloride and osmoles were increased during the second and third ANP periods whereas fractional potassium clearance did not change during the study. There were no changes in catecholamine levels or plasma renin activity during the study. Heart rate, mean arterial pressure and calculated systemic and pulmonary vascular resistance did not change whereas reductions occurred in cardiac index, mean pulmonary artery pressure, pulmonary artery wedge pressure and mean right atrial pressure. We conclude that infusion of ANP also in the postoperative situation increases GFR, diuresis and sodium excretion.

Increased cyclic guanosine monophosphate production and overexpression of atrial natriuretic peptide A-receptor mRNA in spontaneously hypertensive rats

Atrial natriuretic peptide (ANP) specifically stimulates particulate guanylate cyclase, and cyclic guanosine monophosphate (cGMP) has been recognized as its second messenger. Spontaneously hypertensive rats (SHR) have elevated plasma ANP levels, but manifest an exaggerated natriuretic and diuretic response to exogenous ANP when compared to normotensive strains. In isolated glomeruli, the maximal cGMP response to ANP corresponds to a 12- to 14-fold increase over basal levels in normotensive strains (Wistar 13 +/- 2; Wistar-Kyoto 12 +/- 2; Sprague-Dawley 14 +/- 2) while a maximal 33 +/- 3-fold elevation occurs in SHR (P < 0.001). This hyperresponsiveness of cGMP is reproducible in intact glomeruli from SHR from various commercial sources. Furthermore, this abnormality develops early in life, even before hypertension is clearly established, and persists despite pharmacological modulation of blood pressure, indicating that it is a primary event in hypertension. In vitro studies have revealed a higher particulate guanylate cyclase activity in membranes from glomeruli and other tissues from SHR. This increase is not accounted for by different patterns of ANP binding to its receptor subtypes between normotensive and hypertensive strains, as assessed by competitive displacement with C-ANP102-121, an analog which selectively binds to one ANP receptor subtype. The hyperactivity of particulate guanylate cyclase in SHR and its behavior under basal, ligand (ANP), and detergent-enhanced conditions could be attributed either to increased expression or augmented sensitivity of the enzyme. Radiation-inactivation analysis does not evoke a disturbance in the size of regulatory elements normally repressing enzymatic activity, while the expression of particulate guanylate cyclase gene using mutated standard of A- and B-receptors partial cDNAs, quantified by polymerase chain reaction (PCR) transcript titration assay, manifests a selective increase of one guanylate cyclase subtype. Our data suggest that in hypertension, genetic overexpression of the ANP A-receptor subtype is related to the exaggerated biological response to ANP in this disease.

A quantitative autoradiographic study of 125I atrial natriuretic factor in the heart of a teleost fish (Conger conger)

Quantitative receptor autoradiographic study of 125I-atrial natriuretic peptide factor (ANF) in the heart of a teleost fish Conger conger has shown that a heterogenous distribution of 125I-ANF binding exists in the different cardiac regions. Elevated ANF binding densities (3,790 fmol/mg protein) were encountered in the innermost layer (tunica intima) of the bulbus arteriosus while lower binding levels (293-403 fmol/mg protein) were revealed in atrium and ventricle. In order to determine 125I-ANF binding characteristics (KD, Bmax) in the above cardiac sites, saturation binding assays were carried out. The results show that low 125I-ANF KD values (28.8-52.6 pM) were found in the atrium and in the bulbus arteriosus with respect to the higher KD values (373 pM) of the ventricle. The number of binding sites were respectively 632 and 1,279 fmol/mg protein for the atrium and the ventricle, while a substantially elevated Bmax of 7,235 fmol/mg protein was found for the bulbus arteriosus. These results may furnish some insights concerning ANF receptor binding activity and its putative regulatory role of different cardiac functions.

Plasma alpha-human atrial natriuretic peptide concentration in patients with acute lung injury

To elucidate the pathophysiologic role of alpha-human atrial natriuretic peptide (alpha-hANP) in acute lung injury, plasma alpha-hANP concentrations were measured in 15 patients with severe lung injury, and the relationships of plasma alpha-hANP levels to the severity of lung injury, diuresis/natriuresis, and fluid balance were examined. The mean concentrations of plasma alpha-hANP (188.0 +/- 94.6 pg/ml) in patients with severe lung injury at the entry into the study were significantly (p less than 0.001) higher than those in normal subjects (31.7 +/- 12.0 pg/ml). Plasma alpha-hANP levels decreased in parallel with the improvement of lung injury in nine of 15 patients, whereas they changed little, if any, in the patients who did not recover. Plasma alpha-hANP concentrations correlated positively with urine volume, urinary sodium excretion, and excreted fraction of filtered sodium, but they correlated negatively with fluid balance at the onset of the disease as well as during the clinical course. It is suggested that elevation of circulatory alpha-hANP may reflect an adaptative mechanism to remove excessive fluid retention and reduce pulmonary hypertension for acute lung injury.

Mechanisms and amelioration of acute renal allograft failure in the cyclosporine era

The fairly wide-ranging spectrum of tactics under investigation for ameliorating acute renal allograft dysfunction caused by harvest/preservation-related ischemia, acute CsA nephrotoxicity, and acute immunologic crises reflect the fact that no single approach has emerged as universally useful for mitigating the vasomotor nephropathy produced by the combined effects of each of these vectors of vasomotor renal allograft injury. Given the clinical heterogeneity of patients and allografts, it is the author's bias that, in addition to careful donor and recipient hemodynamic management, induction immunosuppressive regimens should be individualized on the basis of allograft function in the immediate postreperfusion period (judged by rate of diuresis, intraoperative parenchymal tone, renal scan profiles, and rate of decline of serum creatinine concentration) as well as patient-specific immunologic and general medical risk factors. Promising laboratory and clinical investigations of such agents as calcium channel blockers, substances promoting intrarenal vasodilator vs. vasoconstrictor prostaglandin formation, and atriopeptins have the potential to provide clinically helpful options with regard to adjunctive therapy for ameliorating acute renal allograft dysfunction associated with INF and ACR.

Renal tubular sodium handling and plasma atrial natriuretic peptide, renin activity and aldosterone in untreated men under normal living conditions

The associations between renal tubular sodium handling and plasma levels of atrial natriuretic peptide, renin activity and aldosterone were studied in 295 untreated men under normal living conditions. The renal clearance of ingested lithium was used as a marker of proximal tubular sodium handling. Plasma atrial natriuretic peptide was inversely related to creatinine clearance (r = -0.148, P less than 0.01) and directly and significantly related to the overall fractional excretion of sodium (r = 0.213, P less than 0.001) and to distal (r = 0.151, P less than 0.01) fractional sodium excretion. Plasma renin activity was inversely related to sodium excretion at both proximal (r = -0.145, P less than 0.05) and distal (r = -0.236, P less than 0.001) tubular site, whereas plasma aldosterone was significantly and inversely related to distal sodium excretion only (r = -0.305, P less than 0.001). The association between plasma atrial natriuretic peptide and distal sodium excretion in a large sample of men under normal living conditions supports the view of a possible tubular effect of the hormone of the overall control of sodium excretion in man.

The biological activity of atrial natriuretic factor cleaved by endoprotease 3.4.24.11

Ring cleavage of atrial natriuretic peptide (ANF) between Cys7 and Phe8 by endoprotease 3.4.24.11 yields X-ANF. Since endoprotease 3.4.24.11 may contribute to ANF metabolism in vivo, the present study determined if X-ANF exhibits reduced biological activity in comparison to the parent molecule.

Atrial natriuretic factor has a weak insulinotropic action in the isolated perfused rat pancreas

The effect of atrial natriuretic factor (ANF; 1, 10, 100, 1000 pmol/l) on insulin release from the isolated perfused rat pancreas was studied. ANF weakly augmented the glucose (10 mmol/l)-stimulated insulin release during the second (controls: 100%; 1 pmol/l: 99%; 10 pmol/l: 149%, P less than 0.05; 100 pmol/l: 111%; 1000 pmol/l 135%), but not the first phase of the secretory response. In contrast, the first, but not the second phase of arginine (10 mmol/l)-stimulated insulin release was significantly enhanced by ANF (1000 pmol/l; controls: 100%; 1000 pmol/l: 235%, P less than 0.05). The hormone did not influence basal insulin secretion. Our data indicate an insulinotropic effect of ANF on the rat pancreas, which is dependent on the utilized background secretagogues.

Increased secretion of erythropoietin in human renal carcinoma cells in response to atrial natriuretic factor

The present studies were undertaken to assess the effects of atrial natriuretic factor (ANF) on erythropoietin (Ep) secretion in Ep-producing renal carcinoma (RC) cells using a sensitive radioimmunoassay for Ep. Human ANF produced a significant dose-related increase in Ep secretion at concentrations of 10(-7) and 10(-6) M when compared with vehicle controls. ANF (greater than or equal to 10(-9) M) also significantly increased the intracellular guanosine 3',5'-cyclic monophosphate (cGMP) concentration after 5-min incubation with the RC cells. Scatchard analysis of the human 125I-labeled ANF binding data indicated that the RC cells contain a single class of binding sites with a dissociation constant (Kd) of 93 +/- 1 pM and a binding capacity of 2,190 +/- 750 sites/cell. Incubation of the RC cells with 8-bromo-cGMP in concentrations of 10(-7)-10(-5) M also produced a significant dose-related enhancement of Ep secretion. These findings suggest that the increase in Ep secretion in response to ANF can be attributed, at least in part, to activation of guanylate cyclase, which is coupled to specific ANF receptors on the RC cell.

Autoradiographic localization of specific atrial natriuretic peptide binding sites on immunocytochemically identified cells in cultures from rat and guinea-pig hearts

Dissociated cell culture preparations from rat and guinea-pig atria and interatrial septum, and from rat ventricles were studied using a combined autoradiographic and immunocytochemical approach. Alpha-atrial natriuretic peptide (125I-ANP1-128) bindings sites were confined to subpopulations of identified non-neuronal cells in each type of culture preparation, and had distinct patterns of labelling. The density of ANP1-28 binding sites was substantially greater in guinea-pig cultures than in rat cultures and was least in rat ventricular cultures. ANP1-28-labelled subpopulations of S-100-like immunoreactive glial cells were only seen in guinea-pig cultures. Von Willebrand factor (vWF)-like immunoreactive endothelial cells and vWF-negative endothelioid cells expressed ANP1-28 binding sites in both the guinea-pig and rat atrial cultures, but were unlabelled in rat ventricular cultures. In contrast, labelled subpopulations of fibronectin-like immunoreactive fibroblasts were present in all of the three types of culture preparation studied. ANP-like immunoreactive myocytes were present in both atrial and ventricular cultures. These cells did not, however, express ANP1-28 binding sites.

Neutral endopeptidase inhibition potentiates the renal actions of atrial natriuretic factor

Atrial natriuretic factor (ANF) is degraded by neutral endopeptidase. We hypothesized that neutral endopeptidase inhibition (NEP-I) increases sodium excretion and that this effect would be potentiated in the presence of an isolated increase in intrarenal ANF. In seven anesthetized dogs, ANF was infused into one renal artery to produce pathophysiologic concentrations in the supplemented kidney while the control kidney received physiologic circulating concentrations of ANF. In the control kidney, NEP-I (SQ 28,603) produced significant increases in urine flow, absolute sodium excretion and fractional sodium excretion while glomerular filtration rate (GFR) remained constant. These renal actions of NEP-I were associated with marked increases in urinary excretion of ANF and cyclic GMP consistent with decreased renal degradation and increased biologic activity of ANF. All of these effects were significantly greater in the supplemented kidney. The present study suggests that NEP-I produces natriuresis which appears to be independent of changes in GFR. In addition, while NEP-I mimics the renal action of pathophysiologic levels of ANF, NEP-I also potentiates the natriuretic effects of pathophysiologic concentrations of ANF as observed in congestive heart failure or hypertension.

Rapid receptor-mediated catabolism of 125I-atrial natriuretic factor by vascular endothelial cells

The binding, internalization and degradation of 200 pM monoiodinated human atrial natriuretic factor-(99-126) (125I-hANF) by cultured bovine aortic endothelial cells (BAECs) were studied at 37 degrees C. 125I-hANF was rapidly cleared from the extracellular medium (t1/2 approximately 10 min), whereas preincubation of the cells in the presence of 20 mM-NH4Cl or 0.2 mM-chloroquine resulted in a significant inhibition of this process. The BAECs rapidly produce three major degradation products of 125I-hANF, namely [125I]iodotyrosine 126 (125I-Y), Arg125-[125I]iodotyrosine126 (125I-RY) and Phe124-Arg125-[125I]iodotyrosine126(125I-FRY), which were detected in the extracellular medium. NH4Cl and chloroquine acted to inhibit the generation of 125I-Y and 125I-RY, but not that of 125I-FRY. Furthermore, excess unlabelled hANF (300 nM) completely blocked the rapid production of 125I-Y and 125I-RY in the first 5 min, but only partially (49%) inhibited the generation of 125I-FRY. Thus, in contrast with our previous findings with cultured smooth-muscle cells [Johnson, Arik & Foster (1989) J. Biol. Chem. 264, 11637-11642], BAECs bind, internalize and rapidly degrade 125I-hANF, resulting in the release of 125I-Y and 125I-RY into the extracellular medium. Similarly to smooth-muscle cells, the BAECs generate 125I-FRY from 125I-hANF via an extracellular proteolytic event. The rapidity of the receptor-mediated process and its sensitivity to NH4Cl and chloroquine suggest that the 125I-hANF is proteolytically processed in the endosomes of BAECs and that its receptors cycle between the cell surface and intracellular stores.

Photoaffinity labelling of atrial natriuretic factor (ANF)-R1 receptor by underivatized 125I-ANF. Involvement of lipid peroxidation

In bovine adrenal zona glomerulosa, atrial natriuretic factor (ANF) exerts its physiological effect through high-affinity binding to specific membrane receptors. On studying further the molecular properties of the ANF receptor binding domain, we have observed that incubation of intact or solubilized bovine adrenal zona glomerulosa membranes with 125I-ANF-(99-126) followed by u.v. irradiation results in the irreversible labelling of a 130 kDa protein corresponding to the ANF-RI receptor. This process is time-, protein- and 125I-ANF-dependent. The apparently covalent nature of this complex is documented by its resistance to heat, guanidine hydrochloride, urea and trichloroacetic acid denaturation. Photolabelling with underivatized 125I-ANF is much more efficient with the ANF-R1 than with the ANF-R2 receptor. After photolysis, the covalently linked 125I-ANF is still sensitive to digestion by carboxypeptidase A, suggesting that ANF is linked by its N-terminal end to the receptor upon u.v. irradiation and that its C-terminal end is still freely accessible. Aerobic conditions and lipids are required for the photolabelling, suggesting a role in this process for malondialdehyde, a highly reactive secondary product associated with u.v.-induced lipid peroxidation. This simple method should provide a powerful tool in the accurate characterization of the hormone-binding domain of the ANF receptor.

Visualisation of specific binding sites for atrial natriuretic peptide on non-neuronal cells of cultured rat sympathetic ganglia

The distribution of atrial natriuretic peptide binding sites on cells in dissociated culture preparations of neonatal rat superior cervical ganglia and in explant cultures of rat thoracic sympathetic chain ganglia has been studied. The autoradiographic visualisation of atrial natriuretic peptide binding sites has been combined with the use of specific immunocytochemical markers for glial cells (antiserum to S-100 protein), fibroblasts (antiserum to fibronectin) and neurones (antiserum to protein gene product 9.5) in order to achieve unambiguous identification of the cell types in culture. Specific binding sites for rat 125I-atrial natriuretic peptide(1-28) were observed over subpopulations of fibronectin-like-immunoreactive fibroblasts and S-100-like-immunoreactive glia in the dissociated superior cervical ganglion cultures. However, only a subpopulation of fibronectin-like-immunoreactive fibroblasts possessed atrial natriuretic peptide binding sites in the explant culture preparations. No atrial natriuretic peptide-like-immunoreactive cells were present in either culture. The distribution of autoradiographic grains over individual cell surfaces in culture was uniform, but there were distinct differences in the density of labelling of single cells of the same type. This apparent variation in the number of binding sites on glial cells and fibroblasts in culture did not seem to be related to the morphology of the cells or the surrounding cell types. No sympathetic neurones were labelled with autoradiographic grains in either the dissociated or explant culture preparations. However, the presence of atrial natriuretic peptide binding sites on non-neuronal cells of sympathetic ganglia in culture may be linked to the relationship between atrial natriuretic peptide and the sympathetic nervous system.

Atrial natriuretic factor plays a significant role in body fluid homeostasis

Atrial natriuretic factor (ANF) is a hormone with the physiological characteristics of a regulator of body fluid volume. It is potent, has a short duration of action, and responds to a physiologically relevant stimulus in a negative feedback-controlled system. It can act directly or indirectly (via inhibition of aldosterone biosynthesis) on the kidney to alter sodium transport and may regulate fluid distribution within the extracellular space. The peptide circulates at low (nanomolar) levels, and recent studies with renal inner medullary cells document relevant receptor binding and second messenger activation in this concentration range. In vivo data support a direct action on the kidney to enhance natriuresis, and blockade of a primary catabolic pathway for ANF within the kidney results in augmented natriuresis at concurrent endogenous peptide concentrations. Long-term, low dose infusion directly into the renal artery of conscious dogs supports a physiological action of ANF to promote urinary sodium excretion. Nevertheless, under certain circumstances, natriuresis does not occur even at high circulating levels of ANF. Apparently other factors such as renal perfusion pressure, volume status, and renal nerve activity are important in determining the natriuretic response to a given level of peptide. We hypothesize that the role played by ANF in volume regulation is highly complex, and the kidney responds with increased sodium excretion only when a constellation of variables is appropriately arrayed. That is, ANF is a necessary, but not sufficient, condition to induce natriuresis.

Atrial natriuretic peptide has no direct effect on proximal tubule sodium and water reabsorption

Infusion of ANP has been shown to increase the urinary excretion of sodium and water. However it is still controversial in which tubular segment sodium reabsorption is inhibited. To clarify this problem we have performed in vivo and in vitro studies to examine the direct effect of ANP on rat proximal tubules. The in vivo effect of ANP has been tested by using the micropuncture technique and in particular the shrinking droplet method that allows each investigated tubule to serve as its own control. Addition of either low (10(-9) M) or high (2 x 10(-6) M) concentrations of ANP to the luminal perfusate resulted in no significant change in isotonic fluid reabsorption (Jv). The same holds when the proximal tubules were perfused on both the tubular and peritubular side, with modified Ringer solution containing 10(-9) M ANP. To examine possible in vitro effects of ANP we prepared highly purified proximal tubule suspension derived from rat renal cortex and monitored oxygen consumption (QO2) that is tightly coupled to sodium transport in this segment. Synthetic ANP, either at low (10(-9) M) or at high (10(-6) M) concentrations, did not affect basal rate of tubular respiration. Moreover the peptide hormone (10(-9) M) did not inhibit nystatin stimulated and ouabain sensitive QO2. These results indicate that the enhancement of renal sodium excretion induced by ANP is not related to a direct inhibition of sodium transport in the proximal tubule.

Glomerular atrial natriuretic factor receptors in primary glomerulopathies: studies on human renal biopsies

Human renal biopsies are currently used to provide information about morphologic changes, chronicity of disease, patterns of inflammation, and immunoglobulin deposition. This practice has provided only limited insight into functional aberrations and has failed to provided information necessary for disease classification based on pathophysiology. To expand the potential of the renal biopsy in this regard and to determine whether differences in glomerular atrial natriuretic factor (ANF) binding exist in different forms of primary renal disease, quantitative autoradiography and 125I-human ANF (1-28) were used to determine the location and pharmacological characteristics of ANF binding sites in the normal human kidney. Specific ANF binding was highest in the glomeruli, but lower levels of specific binding were localized to the inner medulla and the interlobular arteries. ANF binding sites in the human kidney were found to be highly stable and similar in both location and pharmacology to those observed in experimental animals. As determined by saturation experiments, the equilibrium dissociation constants for glomeruli, inner medulla, and interlobular arteries were almost identical at 4.0 x 10(-11) mol/L. Competitive binding inhibition studies with unlabeled human ANF (1-28) demonstrated highly specific binding shared by the glomerulus, inner medulla, and interlobular artery, with apparent half-maximal inhibition concentrations of 9.2 x 10(-10) mol/L, 8.0 x -10 mol/L, and 8.2 x 10(-10) mol/L, respectively. Quantitation of specific binding of ANF to glomeruli in needle biopsy specimens of three primary glomerulopathies, ie, minimal-change disease, membranous nephropathy, and focal glomerulosclerosis, showed no differences among the groups. This study demonstrates the feasibility of studying receptor physiology on biopsy specimens of the human kidney and should allow renal diseases, particularly of glomerular origin, to be characterized according to differences in hormone binding and hormone responsiveness. The absence of significant differences in glomerular ANF binding in the primary glomerulopathies studied is consistent with other studies that have failed to delineate important pathophysiological differences in renal function and volume homeostasis in these disease states.

Atrial natriuretic peptide stimulates submandibular gland synthesis and secretion of cGMP

Binding of atrial natriuretic peptide (ANP) to rat submandibular gland and its effect on guanosine 3',5'-cyclic monophosphate (cGMP) formation and salivary secretion were investigated. Membranes rapidly and specifically bound 125I-ANP. Binding was inhibited by unlabeled ANP (IC50 approximately 1.6 nM), but not by atriopeptin I, other COOH- and NH2-terminal deleted ANP fragments, or agents such as pilocarpine or substance P. Scatchard analysis revealed a single class of high-affinity sites (dissociation constant 0.74 +/- 0.25 nM; maximal binding capacity 20.5 +/- 6.3 pmol/mg protein). Intravenous infusion of ANP with pilocarpine caused a significant dose-dependent increase in the levels of cGMP detected in plasma and saliva. Because salivary cGMP may have originated in plasma, the effect of ANP on cGMP formation was evaluated in dispersed cells. ANP evoked a concentration-dependent increase in both cGMP synthesis and secretion (EC50 approximately 1.7 x 10(-8) M). The atrial peptide did affect basal or l-isoproterenol-stimulated adenosine 3',5'-cyclic monophosphate synthesis in dispersed cells. When infused by itself and/or with pilocarpine, ANP did not alter the rate of spontaneous or pilocarpine-induced salivary flow, secretion of chloride, or protein release. The data demonstrate the presence of guanylate cyclase-coupled ANP receptors in submandibular gland; the atrial peptide, however, does not exert an effect of the secretory function of the gland.

Effect of exogenous ANP on initial renal function following 24-hour cold preservation

The impact of synthetic atrial natriuretic peptide (sANP) on renal function following cold ischemic injury was studied in a canine autotransplant model. Following a prenephrectomy inulin clearance determination (CIn), the left kidney was excised, flushed with Eurocollins solution, and cold-stored for 24 hours. Immediately following reperfusion and a 10 minute equilibration period, baseline CIn was measured over a 20-minute time interval (Collection Period I). Experimental animals (N = 11) then received 1 mcg/kg sANP by intravenous bolus followed by a continuous infusion at 0.3 mcg/kg/min for 30 minutes. CIn was measured throughout the infusion (Collection Period II). Normal saline was substituted for sANP in control animals (N = 11). CIn was also measured 24 hours following reimplantation in seven control and seven sANP-treated animals. Autograft inulin clearance increased from 0.32 +/- 0.11 ml/min during Period I to 2.5 +/- 0.6 ml/min during sANP infusion (P less than 0.01). This increase in CIn associated with ANP infusion was accompanied by increases in urine flow rate (V) (0.15 +/- 0.05 ml/min to 0.98 +/- 0.21 ml/min, P less than 0.01) and renal blood flow (RBF) measured by electromagnetic flow probe (85 +/- 17 ml/min to 171 +/- 13 ml/min, P less than 0.05). No significant changes in CIn, V, or RBF occurred in control animals between periods I and II. Although systemic blood pressure declined during sANP infusion, it did not decrease to an extent that compromised peripheral perfusion. CIn determined 24 hours after autograft reimplantation in the ANP-treated animals approximated or exceeded values determined during ANP infusion (Period II).(ABSTRACT TRUNCATED AT 250 WORDS)

Protection of atrial natriuretic factor against degradation: diuretic and natriuretic responses after in vivo inhibition of enkephalinase (EC 3.4.24.11) by acetorphan

Atrial natriuretic factor (ANF) might be beneficial in several cardiovascular disorders, but its poor oral absorption and rapid inactivation in vivo have so far prevented its use in therapeutics. We have assessed the role of enkephalinase (membrane metallo-endopeptidase, EC 3.4.24.11) in the in vivo inactivation of ANF in mice and healthy human volunteers by evaluating the effects of acetorphan, a potent inhibitor. In mice, the degradation of 125I-labeled ANF was markedly delayed, as shown by the levels of the intact peptide in the plasma and the kidney, a major target organ. The effect of acetorphan was due to the inhibition of enkephalinase activity, since it occurred at an ED50 very close to this drug's ID50 for the inhibition of the specific binding of radioactive material to the kidney or lung peptidase that was measured after administration of [3H]acetorphan. The effects of acetorphan were also studied in eight healthy human volunteers by using a randomized double-blind, placebo-controlled design. Oral administration of acetorphan elicited a lasting elevation of plasma ANF-like immunoreactivity, with a time course parallel to that of the inhibition of plasma enkephalinase activity. These effects were accompanied by significant increases in urinary volume and sodium excretion, two well-established renal responses to ANF peptides. These results indicate that enkephalinase plays a critical role in ANF degradation in vivo and that its inhibition enhances the levels of circulating endogenous ANF, which, in turn, results in diuresis and natriuresis. Enkephalinase inhibition may constitute another therapeutic approach to the treatment of cardiovascular diseases, such as congestive heart failure or essential hypertension, on which ANF is postulated to have a beneficial effect.

Characterization and expression of the human rhoH12 gene product

The rho genes constitute an evolutionarily conserved family having significant homology to the ras oncogene family. These genes have been found in Saccharomyces cerevisiae, Drosophila melanogaster, rat, and human; their 21,000-dalton products show strong conservation of structure. In humans, three classes of rho cDNA clones have been identified which differ by virtue of the presence of variable C-terminal domains: rhoH12, rhoH6, and rhoH9. The predicted 193 amino acids of human rhoH12 protein show 88% similarity with those of the human rhoH6 clone, 96.8% similarity with those of the Aplysia rho product, and 81.8% similarity with those of the yeast RHO1 protein. Rat-1 and NIH 3T3 mouse fibroblasts were transfected with clones containing the normal human rhoH12 allele as well as the variants encoding valine in place of the glycine and leucine in place of the glutamine normally found at residues 14 and 64, respectively. These replacements mirror the changes responsible for oncogenic activation of the related ras-encoded p21 proteins. These mutant rhoH12 clone alleles did not cause focus formation in monolayers or growth in soft agar. However, amplification of normal rhoH12 via cotransfection with a dihydrofolate reductase gene resulted in colonies that displayed reduced dependence on serum for growth, grew to higher saturation densities, and were tumorigenic when inoculated into nude mice. Normal p21rho protein was detected in the transfected cell lines as well as in normal cell lines by Western immunoblot and immunoprecipitation analysis with rabbit antibodies raised against the peptide corresponding to amino acids 122 to 135.

Developmental change of kidney receptor for atrial natriuretic factor in spontaneously hypertensive rat

Properties of human atrial natriuretic factor (ANF) binding to the crude membrane fraction of rat kidney were studied using the ANF-radiolabeled receptor assay; the developmental change of renal ANF receptors in three age groups of spontaneously hypertensive rats (SHR) was also investigated with the methods of radiolabeled receptor assay and the quantitative approach of in vitro macro-autoradiography. Temperature and incubation time greatly influenced ANF binding capacities because of the degradation of radiolabeled ligand. Addition of 5 mM MgCl2 to assay buffer was useful for the stabilization of ANF specific binding. Scatchard analysis suggested that the crude membrane fraction of rat's kidney had a single binding site with the apparent dissociation constant of 0.55 nM. In the study of the developmental change of renal ANF receptor in SHR, systolic blood pressure of the SHR at the age of 5 weeks and 12 weeks was significantly higher than that of age-matched Wistar-Kyoto (WKY) rats, but there was no significant difference in blood pressure between SHR and WKY rats at the age of 3 weeks. Concerning the radiolabeled receptor assay of ANF, the apparent dissociation constant and maximum binding capacity in SHR were low in all age groups when compared with those of WKY rats. In the in vitro macro-autoradiographic observation, the specific binding of ANF was localized mainly in the renal cortex, and these binding patterns of SHR and WKY rats were the same in all age groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease of atrial natriuretic peptide content in rat superior cervical sympathetic ganglion after denervation and axotomy

We found atrial natriuretic peptide (ANP), known as a humoral factor in regulating body fluid volume and blood pressure, in considerable quantities in rat superior cervical sympathetic ganglion (SCG) by radioimmunoassay after separation with reverse-phase HPLC. Although the ANP content of the immature rat 1 week after birth was low, it doubled at 2 weeks and then increased gradually, until it reached the adult level. Denervation caused a rapid decrease in the ANP content to half of the intact SCG level after 3 h, which then fell to 10% of the control value on day 2 after operation. The time course of ANP content reduction after denervation was similar but rather faster than that of activity of the acetylcholine-synthesizing enzyme, choline acetyltransferase, an observation suggesting that ANP may partly contribute to cholinergic synaptic transmission. On the other hand, axotomy produced a rather slower decrease in the ANP content than did denervation. Enucleation and sialoadenectomy also caused a considerable reduction of the ANP content. Thus, part of the ANP found in the ganglion is apparently transported from sympathetically innervated extraganglionic organs via retrograde axoplasmic flow.

Characterization and expression of the human rhoH12 gene product

The rho genes constitute an evolutionarily conserved family having significant homology to the ras oncogene family. These genes have been found in Saccharomyces cerevisiae, Drosophila melanogaster, rat, and human; their 21,000-dalton products show strong conservation of structure. In humans, three classes of rho cDNA clones have been identified which differ by virtue of the presence of variable C-terminal domains: rhoH12, rhoH6, and rhoH9. The predicted 193 amino acids of human rhoH12 protein show 88% similarity with those of the human rhoH6 clone, 96.8% similarity with those of the Aplysia rho product, and 81.8% similarity with those of the yeast RHO1 protein. Rat-1 and NIH 3T3 mouse fibroblasts were transfected with clones containing the normal human rhoH12 allele as well as the variants encoding valine in place of the glycine and leucine in place of the glutamine normally found at residues 14 and 64, respectively. These replacements mirror the changes responsible for oncogenic activation of the related ras-encoded p21 proteins. These mutant rhoH12 clone alleles did not cause focus formation in monolayers or growth in soft agar. However, amplification of normal rhoH12 via cotransfection with a dihydrofolate reductase gene resulted in colonies that displayed reduced dependence on serum for growth, grew to higher saturation densities, and were tumorigenic when inoculated into nude mice. Normal p21rho protein was detected in the transfected cell lines as well as in normal cell lines by Western immunoblot and immunoprecipitation analysis with rabbit antibodies raised against the peptide corresponding to amino acids 122 to 135.

Cardiorenal effects of atrial natriuretic factor administration in congestive heart failure: natriuresis and diuresis without hemodynamic alterations

The effects of low bolus dose (70 +/- 6 micrograms [mean +/- SEM]) atrial natriuretic factor (ANF) administration was assessed in 16 patients with chronic congestive heart failure. Measurements were made for at least 60 minutes before and after the dose of ANF. There was a significant increase in urine flow rate (0.81 +/- 0.06 to 1.81 +/- 0.23 ml/min, p less than 0.01), sodium excretion rate (56 +/- 14 to 80 +/- 23 microEq/min, p less than 0.01), fractional excretion of sodium (1.23 +/- 0.49 to 1.63 +/- 0.60 percent, p less than 0.01) and potassium excretion rate (35 +/- 7 to 42 +/- 6 microEq/min, p less than 0.02). However, no significant alterations in renal plasma flow or glomerular filtration rate were observed. Furthermore, there was no significant correlation between the change in urine flow rate or sodium excretion rate and the change in renal plasma flow or glomerular filtration rate, respectively. In addition, there was no significant effect on cardiac index, mean aortic or left ventricular filling pressures, or systemic vascular resistance. There also was no discernible relationship between the response to ANF and the baseline concentrations of plasma ANF, aldosterone, or plasma renin activity. Thus, in patients with congestive heart failure, low dose ANF boluses may produce an increase in urine flow rate and sodium excretion rate that is independent of renal plasma flow or glomerular filtration rate. This suggests a meaningful direct renal tubular effect of exogenous ANF in this setting.

Localization of atrial natriuretic factor (ANF) binding sites in the central nervous system of the frog

The distribution of atrial natriuretic factor (ANF) binding sites was investigated in the central nervous system of the frog Rana ridibunda using the technique of in vitro receptor autoradiography by means of [125I]-labeled ANF-28. The anatomic distribution of ANF recognition sites was determined on Kodak ARX films apposed onto tissue sections, and their distribution was examined in greater detail by analysis of autoradiograms generated by using emulsion-coated sections. The highest levels of ANF binding sites were found in the olfactory bulb, the dorsal pallium, the septum, the habenular nucleus, the dorsal infundibular nucleus, the interpeduncular nucleus, and in the tectum. Moderate levels of ANF binding sites were observed in the thalamus and throughout the mesencephalon, whereas low levels were detected in the lateral and medial pallium, the medial forebrain bundle, and the nucleus rotondus. In the pituitary gland, the neural and distal lobes were densely loaded with ANF binding sites, whereas no autoradiographic labeling was observed in the pars intermedia. In general, there was a good correlation between the location of ANF receptors and the distribution of ANF-containing neurons, as previously determined by immunocytochemistry. Together these results support the view that ANF may act as a neurotransmitter or a neuromodulator in various regions of the frog brain.

Receptor sites for atrial natriuretic factors in brain and associated structures: an overview

1. Recent data have clearly shown the existence of specific receptor binding sites for atrial natriuretic factors (ANF) or polypeptides in mammalian brain tissues. 2. Ligand selectivity pattern and coupling to cGMP production suggest that brain ANF sites are similar to high-affinity/low-capacity sites found in various peripheral tissues (kidney, adrenal gland, blood vessels). These brain ANF sites possibly are of the B-ANP subtype. 3. High densities of ANF binding sites are found especially in areas of the central nervous system associated with the control of various cardiovascular parameters (such as the subfornical organ and area postrema). However, high densities of sites are also present in other regions such as the hippocampus, cerebellum, and thalamus in the brain of certain mammalian species, suggesting that brain ANF could act as a neuromodulator of noncardiovascular functions. 4. The density of brain ANF binding sites is modified in certain animal models of cardiovascular disorders and during postnatal ontogeny, demonstrating the plasticity of these sites in the central nervous system (CNS). 5. Specific ANF binding sites are also found in various other CNS-associated tissues such as the eye, pituitary gland, and adrenal medulla. In these tissues ANF appears to act as a modulator of fluid production and hormone release. 6. Thus, ANF-like peptides and ANF receptor sites are present in brain and various peripheral tissues, demonstrating the existence of a family of brain/heart peptides.

Atrial natriuretic peptide (ANP) acts via specific binding sites on cGMP system of rat pancreatic islets without affecting insulin release

Atrial natriuretic peptide (ANP) has been shown to increase plasma insulin levels in vivo and to act on various target cells as a potent stimulator of the cGMP system. It has, therefore, been investigated whether ANP has a direct insulinotropic effect mediated by specific binding sites and by affecting the cGMP system in isolated rat pancreatic islets. Unlabelled ANP inhibited 125I-ANP binding in a concentration-related manner (Kd1 and Kd2 = 0.02 and 11.2 nM, Bmax1 and Bmax2 = 0.0147 and 0.0328 pmoles per 1 mg protein). ANP was able to augment cGMP levels in islets, but was not able to enhance insulin secretion at various glucose concentrations. Since the role of cGMP for the glucose-mediated insulin release is controversial, in addition to ANP M&B 22,948 (a cGMP phosphodiesterase inhibitor) was investigated to evaluate the possible role of cGMP for insulin release more precisely. Like ANP M&B 22,948 increased cGMP levels but did not affect insulin release. The data indicate no direct insulinotropic effect of ANP, although ANP binding sites are present on rat pancreatic islets and question the claimed role of cGMP for insulin secretion in general. Therefore, the recently observed in vivo elevation of plasma insulin levels in response to ANP is rather an indirect than a direct effect.