Identification of "B" receptor for natriuretic peptide in human kidney

Innocent heart murmurs and enuresis: Examining a possible link

OBJECTIVE

The aim of this study was to evaluate the prevalence of innocent heart murmurs in children affected by nocturnal enuresis (NE).

RESULTS

The prevalence of innocent heart murmurs in G2 was 6.34%. This condition was significantly more frequent in children who suffered from NE. Indeed, in G1, the prevalence of innocent heart murmurs was 21.45%, although there were few differences between the children with monosymptomatic NE and non-monosymptomatic NE; moreover, this prevalence was higher in males.

MATERIAL AND METHODS

We enrolled a total of 401 children (G1), 300 males and 101 females, aged 5-15 years, affected by NE and referred to the Service of Pediatrics, "Campus Bio-Medico" University Hospital of Rome, from September 2013 to September 2018, into the study. The control group was composed of 394 children without NE (G2). The study was carried out in compliance with the Helsinki Declaration.

CONCLUSION

These findings made us aware of the presence of possible underlying mechanisms, which explain the association between a higher prevalence of innocent heart murmurs and enuresis, and further studies are required to explore this issue.

Regulation of Dopamine Uptake by Vasoactive Peptides in the Kidney

Considering the key role of renal dopamine in tubular sodium handling, we hypothesized that c-type natriuretic peptide (CNP) and Ang-(1-7) may regulate renal dopamine availability in tubular cells, contributing to Na(+), K(+)-ATPase inhibition. Present results show that CNP did not affect either (3)H-dopamine uptake in renal tissue or Na(+), K(+)-ATPase activity; meanwhile, Ang-(1-7) was able to increase (3)H-dopamine uptake and decreased Na(+), K(+)-ATPase activity in renal cortex. Ang-(1-7) and dopamine together decreased further Na(+), K(+)-ATPase activity showing an additive effect on the sodium pump. In addition, hydrocortisone reversed Ang-(1-7)-dopamine overinhibition on the enzyme, suggesting that this inhibition is closely related to Ang-(1-7) stimulation on renal dopamine uptake. Both anantin and cANP (4-23-amide) did not modify CNP effects on (3)H-dopamine uptake by tubular cells. The Mas receptor antagonist, A-779, blocked the increase elicited by Ang-(1-7) on (3)H-dopamine uptake. The stimulatory uptake induced by Ang-(1-7) was even more pronounced in the presence of losartan, suggesting an inhibitory effect of Ang-(1-7) on AT1 receptors on (3)H-dopamine uptake. By increasing dopamine bioavailability in tubular cells, Ang-(1-7) enhances Na(+), K(+)-ATPase activity inhibition, contributing to its natriuretic and diuretic effects.

Molecular Physiology of Membrane Guanylyl Cyclase Receptors

cGMP controls many cellular functions ranging from growth, viability, and differentiation to contractility, secretion, and ion transport. The mammalian genome encodes seven transmembrane guanylyl cyclases (GCs), GC-A to GC-G, which mainly modulate submembrane cGMP microdomains. These GCs share a unique topology comprising an extracellular domain, a short transmembrane region, and an intracellular COOH-terminal catalytic (cGMP synthesizing) region. GC-A mediates the endocrine effects of atrial and B-type natriuretic peptides regulating arterial blood pressure/volume and energy balance. GC-B is activated by C-type natriuretic peptide, stimulating endochondral ossification in autocrine way. GC-C mediates the paracrine effects of guanylins on intestinal ion transport and epithelial turnover. GC-E and GC-F are expressed in photoreceptor cells of the retina, and their activation by intracellular Ca(2+)-regulated proteins is essential for vision. Finally, in the rodent system two olfactorial GCs, GC-D and GC-G, are activated by low concentrations of CO2and by peptidergic (guanylins) and nonpeptidergic odorants as well as by coolness, which has implications for social behaviors. In the past years advances in human and mouse genetics as well as the development of sensitive biosensors monitoring the spatiotemporal dynamics of cGMP in living cells have provided novel relevant information about this receptor family. This increased our understanding of the mechanisms of signal transduction, regulation, and (dys)function of the membrane GCs, clarified their relevance for genetic and acquired diseases and, importantly, has revealed novel targets for therapies. The present review aims to illustrate these different features of membrane GCs and the main open questions in this field.

Protective effects of C-type natriuretic peptide on cisplatin-induced nephrotoxicity in mice

PURPOSE

Cisplatin is a highly effective chemotherapeutic agent used to treat various malignancies, but its utility is compromised by its nephrotoxicity. C-type natriuretic peptide (CNP), a member of the natriuretic peptide family, exhibits anti-inflammatory effects by activating its specific receptor, guanylyl cyclase (GC)-B. CNP and GC-B receptor are known to be expressed in both the vascular endothelium and the kidney. The objective of this study was to investigate the renoprotective effects of CNP in a mouse model of cisplatin-induced nephrotoxicity.

METHODS

C57BL/6 mice were divided into three groups: normal control mice; cisplatin (20 mg/kg, intraperitoneal) mice treated with vehicle; and cisplatin mice treated with CNP (2.5 µg/kg/min, subcutaneous). At 72 h after cisplatin injection, urine, blood and kidney samples were collected. Urine and blood samples were examined biochemically. Histological findings and gene expression in kidney tissue were evaluated.

RESULTS

CNP reduced histological renal tubular damage and apoptosis induced by cisplatin and suppressed plasma blood urea nitrogen and creatinine levels, which were elevated by cisplatin administration. CNP treatment decreased the expression of kidney injury molecule-1 and monocyte chemoattractant protein-1, which were elevated in the kidney by cisplatin administration. CNP treatment attenuated the decrease in GC-B expression in cisplatin-induced kidney injury.

CONCLUSIONS

The present study is the first to show that CNP inhibits nephrotoxicity and kidney cell damage induced by cisplatin. The mechanism of action may involve down-regulation of inflammatory cytokine expression in cisplatin-induced kidney injury and attenuation of apoptosis in renal tubular cells.

Urinary C-type natriuretic peptide excretion: a potential novel biomarker for renal fibrosis during aging

Renal aging is characterized by structural changes in the kidney including fibrosis, which contributes to the increased risk of kidney and cardiac failure in the elderly. Studies involving healthy kidney donors demonstrated subclinical age-related nephropathy on renal biopsy that was not detected by standard diagnostic tests. Thus there is a high-priority need for novel noninvasive biomarkers to detect the presence of preclinical age-associated renal structural and functional changes. C-type natriuretic peptide (CNP) possesses renoprotective properties and is present in the kidney; however, its modulation during aging remains undefined. We assessed circulating and urinary CNP in a Fischer rat model of experimental aging and also determined renal structural and functional adaptations to the aging process. Histological and electron microscopic analysis demonstrated significant renal fibrosis, glomerular basement membrane thickening, and mesangial matrix expansion with aging. While plasma CNP levels progressively declined with aging, urinary CNP excretion increased, along with the ratio of urinary to plasma CNP, which preceded significant elevations in proteinuria and blood pressure. Also, CNP immunoreactivity was increased in the distal and proximal tubules in both the aging rat and aging human kidneys. Our findings provide evidence that urinary CNP and its ratio to plasma CNP may represent a novel biomarker for early age-mediated renal structural alterations, particularly fibrosis. Thus urinary CNP could potentially aid in identifying subjects with preclinical structural changes before the onset of symptoms and disease, allowing for the initiation of strategies designed to prevent the progression of chronic kidney disease particularly in the aging population.

C-type natriuretic-derived peptides as biomarkers in human disease

The natriuretic peptide system comprises three structurally related peptides: atrial natriuretic peptide, B-type natriuretic peptide and C-type natriuretic peptide. In circulation, they play an important endocrine role in the regulation of cardiovascular homeostasis by maintaining blood pressure and extracellular fluid volume. Atrial natriuretic peptide and B-type natriuretic peptide have gained considerable diagnostic interest as biomarkers in cardiovascular disease. By contrast, C-type natriuretic peptide has not yet been ascribed a role in human diagnostics. This perspective aims at recapitulating the present biochemical and clinical issues concerning C-type natriuretic peptide measurement in plasma as a potential biomarker.

CNP production in the kidney and effects of protein intake restriction in nephrotic syndrome

C-type natriuretic peptide (CNP) possesses well-established cardiovascular properties. Although present in the mammalian kidney, CNP production in human kidney and its modulation in human renal disease remain less defined. We investigated the presence of CNP in normal human kidney and in patients with nephrotic syndrome (NS). We also addressed whether or not a low-protein diet (LPD) alters plasma CNP and urinary CNP excretion in NS. In situ hybridization studies demonstrated CNP mRNA expression in tubular cells and glomeruli of normal human kidneys. CNP immunoreactivity was positive in proximal, distal, and medullary collecting duct tubular cells in both controls and patients with NS. The ratios of plasma CNP and urinary CNP to creatinine were significantly higher in patients with NS compared with controls. Urinary CNP, but not plasma CNP, was significantly lowered in patients with NS after an LPD. Similarly, the ratios of urinary protein to creatinine and urinary albumin to creatinine, but not urinary guanosine 3',5'-cyclic monophosphate to creatinine, decreased significantly with an LPD. These data confirm and extend previous reports and demonstrate for the first time the presence of CNP in human kidney with NS. We also report increased plasma CNP concentration and urinary CNP excretion in NS patients and a significant reduction of CNP excretion with an LPD. Our findings demonstrate that CNP metabolism is altered in patients with NS and support the hypothesis that activation of renal CNP can be partially offset by an LPD. These results underscore that the beneficial effect of an LPD on protein excretion is paralleled by a substantial reduction in intrarenal CNP release.

G protein-dependent activation of smooth muscle eNOS via natriuretic peptide clearance receptor

In gastrointestinal smooth muscle, the neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) induce relaxation by interacting with VIP2/PACAP3 receptors coupled via Gs to adenylyl cyclase and with distinct receptors coupled via Gi1 and/or Gi2 to a smooth muscle endothelial nitric oxide synthase (eNOS). The present study identifies the receptor as the single-transmembrane natriuretic peptide clearance receptor (NPR-C). RT-PCR and Northern analysis demonstrated expression of the natriuretic peptide receptors NPR-C and NPR-B but not NPR-A in rabbit gastric muscle cells. In binding studies using 125I-labeled atrial natriuretic peptide (125I-ANP) and 125I-VIP as radioligands, VIP, ANP, and the selective NPR-C ligand cANP(4-23) bound with high affinity to NPR-C. ANP, cANP-(4-23), and VIP initiated identical signaling cascades consisting of Ca2+ influx, activation of eNOS via Gi1 and Gi2, stimulation of cGMP formation, and muscle relaxation. NOS activity and cGMP formation were abolished (93 +/- 3 to 96 +/- 2% inhibition) by nifedipine, pertussis toxin, the NOS inhibitor, NG-nitro-L-arginine, and the antagonists ANP-(1-11) and VIP-(10-28). NOS activity stimulated by all three ligands in muscle membranes was additively inhibited by Gi1 and Gi2 antibodies (82 +/- 2 to 84 +/- 1%). In reconstitution studies, VIP, cANP-(4-23), and guanosine 5'-O-(3-thiotriphosphate) stimulated NOS activity in membranes of COS-1 cells cotransfected with NPR-C and eNOS. The results establish a unique mechanism for G protein-dependent activation of a constitutive NOS expressed in gastrointestinal smooth muscle involving interaction of the relaxant neuropeptides VIP and PACAP with a single-transmembrane natriuretic peptide receptor, NPR-C.

Low-dose C-type natriuretic peptide does not affect cardiac and renal function in humans

In experimental animals, C-type natriuretic peptide (CNP) has vasodilating, hypotensive, and natriuretic activities. The role of circulating CNP in the overall regulation of cardiac and renal function in humans is less defined, in both health and disease. We measured cardiac volumes, diastolic and systolic functions, systemic (Doppler echocardiography) and renal hemodynamics, intrarenal sodium handling (lithium clearance method), plasma and urinary cGMP, plasma renin concentration, and plasma aldosterone level in six healthy volunteers (mean age, 33+/-3 years) receiving CNP (2 and 4 pmol/kg per minute for 1 hour each) in a single-blind, placebo-controlled, random-order, crossover study. During CNP infusion, plasma CNP increased from 1.17+/-0.23 to 41.52+/-4.61 pmol/L (ie, 4- to 10-fold higher levels than those observed in disease states) without affecting plasma and urinary cGMP, cardiac volumes, dynamics of left and right heart filling, cardiac output, arterial pressure, renal hemodynamics, intrarenal sodium handling, sodium excretion, or plasma levels of renin and aldosterone. The finding that increments in plasma CNP within the pathophysiological range have no effects on systemic hemodynamics, renal function, or the renin-angiotensin system do not support the hypothesis that CNP may act as a circulating hormone in humans.

Effect of BNP on renal hemodynamics, tubular function and vasoactive hormones in humans

The effect of a continuous infusion of human brain natriuretic peptide (BNP) was studied in 48 healthy men. The study was randomized, placebo controlled, and single blind. BNP was given in doses of 1, 2, or 4 pmol.kg-1.min-1 for 60 min, and peak values of BNP in plasma were 38, 85, and 199 pmol/l, giving increments in plasma as seen in heart or renal failure. BNP infusion increased the urinary flow rate and the excretion of sodium in a dose-dependent way. The maximal effects were +65 and +156%, respectively. GFR increased and RPF decreased, the latter in a dose-dependent manner. Blood pressure, heart rate, angiotensin II, and aldosterone were all unaffected by infusion of BNP, whereas a direct inhibition of renin secretion was seen. With the use of the lithium clearance technique, we concluded that the tubular site of action is in both the proximal and distal segments, and the major effect on sodium handling is in the distal parts of the nephron.

Renal and vascular effects of C-type and atrial natriuretic peptides in humans

C-type natriuretic peptide (CNP) may affect renal and vascular functions differently from atrial natriuretic peptide (ANP). The objective of this study was to compare the renal and vascular actions of CNP to those of ANP in normal men. CNP or ANP (0.005, 0.01, and 0.05 microg x kg(-1) x min(-1)) were given by infusion to eight healthy volunteers. CNP caused dose-dependent increases in natriuresis (U(Na)) and in the fractional excretion of sodium (FE(Na)) with no effect on diuresis (UV), renal plasma flow, and glomerular filtration rate (GFR). Fraction of filtration (FF) increased only with the 0.05 microg x kg(-1) x min(-1) CNP dose. ANP caused larger increases in U(Na), FE(Na), and FF than CNP and also increased UV at 0.01 and 0.05 microg x kg(-1) x min(-1) and GFR at 0.05 microg x kg(-1) x min(-1). Although the ANP and CNP infusions produced similar elevation in the respective peptides plasma levels, urinary and nephrogenous guanosine 3',5'-cyclic monophosphate increased less in response to CNP than to ANP. Blood pressure, forearm blood flow, plasma renin activity, and aldosterone remained unaffected during the peptides infusion. Plasma ANP increased slightly during CNP infusion. Our data indicate a higher threshold of renal response to CNP than to ANP. In contrast to ANP, CNP probably may not act as an endocrine factor in humans.

Neutral endopeptidase regulates C-type natriuretic peptide metabolism but does not potentiate its bioactivity in vivo

C-type natriuretic peptide (CNP) is a newly described 22-amino acid peptide of endothelial and renal cell origin with selective cardiovascular actions. Recent in vitro studies have reported that CNP is the most susceptible of all natriuretic peptides to enzymatic degradation by neutral endopeptidase 24.11 (NEP). The present study was undertaken to define the role of NEP in total and regional CNP metabolism and the modulatory actions of NEP inhibition on the biological actions of CNP. CNP (10 ng x kg(-1) x min(-1)) followed by candoxatrilat (240 microg x kg(-1) bolus and 8 microg x kg(-1) x min(-1)), a potent and selective NEP inhibitor, was administered intravenously to a group of anesthetized mongrel dogs (group 1) to permit calculation of total metabolic clearance rate (MCR); results were compared with those in a group receiving vehicle infusion followed by candoxatrilat (group 2; both groups, n=7). NEP inhibition increased circulating CNP achieved by exogenous infusion and reduced total MCR in group 1. The regional CNP MCRs increased after CNP administration. While the pulmonary MCR did not change during concomitant candoxatrilat infusion, renal MCR was suppressed. Hemodynamic changes were not different between groups. A mild natriuretic and diuretic effect in association with an increase in circulating and urinary ANP levels was not different between groups. Urinary CNP excretion did not change with CNP infusion but markedly increased after NEP inhibition. We conclude that (1) circulating CNP achieved by exogenous CNP infusion is regulated by NEP in vivo, (2) regional MCRs are heterogeneous with NEP inhibition, (3) NEP inhibition does not potentiate acute cardiovascular actions of CNP, and (4) a mild natriuretic and diuretic effect observed with CNP and NEP inhibition may be ANP dependent.

Receptors for natriuretic peptides in a human cortical collecting duct cell line

The aim of the present study was to analyze the expression of natriuretic peptide receptors in human collecting duct, by using a newly established SV40 cell line (HCD). ANP and C-type natriuretic peptide (CNP) induced a concentration-dependent increase in cGMP suggesting the presence of type-A (NPR-A) and type-B (NPR-B) receptors, respectively. Threshold concentrations were 1 pM and 1 nM, respectively, and stimulated over basal cGMP ratios were 500 and 160 at 0.1 microM ANP and CNP. The urodilatin concentration-response curve was similar to that of ANP. [125I]-ANP bound specifically to HCD cells in a time-dependent fashion, reaching a plateau-phase between one and two hours at 4 degrees C. Equilibrium saturation binding curves suggested a single group of receptor sites (Kd = 421 +/- 55 pM, Bmax = 49.2 +/- 8.8 fmol/mg protein, Hill coefficient = 1.44 +/- 0.1, N = 6). Binding of [125I]-ANP was not displaced by CNP or by C-ANP (4-23), a specific ligand of clearance receptors (NPR-C), and thus occurred mainly via NPR-A. Neither Northern blot analysis nor RT-PCR could detect NPR-C mRNA, although the latter was clearly identified in control human glomerular visceral epithelial cells. In contrast, PCR products with the expected lengths were obtained for NPR-A and NPR-B. In conclusion, HCD cells express both NPR-A and NPR-B, as demonstrated by mRNA and cGMP production studies, but fail to produce NPR-C. This suggests that the human cortical collecting duct is a target for ANP, CNP and urodilatin.

Expression of natriuretic peptide receptors in human adipose and other tissues

To characterize natriuretic peptide receptor (NPr) gene expression in human tissues, we cloned portions of the cDNAs codifying for NPr with guanylyl cyclase activity (NPr-A and NPr-B) and without guanylyl cyclase activity (NPr-C). Total RNA was extracted from samples taken at surgery from normal human tissues. NPr-A and NPr-B cDNAs obtained from lung as well as NPr-C cDNA obtained from renal cortex were cloned, characterized, and used for comparative Northern analysis. NPr-A mRNA (approximately 4 kb) was most abundant in adipose tissue (8 patients) independently on the site of sampling, whereas it was approximately 2.5-fold and 5-fold less abundant, respectively, in kidney (either renal cortex or papilla from 3 patients) and adrenal (4 patients), known target tissues of natriuretic peptides. NPr-C mRNAs (approximately 7.7 and 6.8 kb) had a similar tissue distribution but the highest levels were found in renal tissue and only very low expression levels were found in adrenals (approximately 20-fold lower than renal cortex). The ratio of NPrA versus NPr-C mRNA levels were highest in adrenal and lowest in renal tissue. NPr-B mRNA (approximately 4 kb), which encodes the receptor for the C-type natriuretic peptide, had a different and wide tissue distribution, including expression in ileum and liver, with the highest levels in venous and prostatic tissue. These results indicate that, in humans, different patterns of NPr expression with different NPr-A/NPr-C mRNA level ratios, are present in known target tissues of natriuretic peptides. "Non-classic" target tissues, such as the adipose one, maximally expressed NPr-A and also NPr-C, suggesting that natriuretic peptides may have wider functional activities than those previously demonstrated.

Natriuretic peptide receptors on human trabecular meshwork cells

The effects of natriuretic peptides on cGMP formation and [125I]ANP binding in human trabecular meshwork cells were investigated. CNP at 1 microM stimulated cGMP formation approximately 18-25 fold, with a half maximal effective concentration approximately 20-30nM. BNP at 1 microM stimulated approximately 7 fold, while ANP stimulated cGMP formation 2-fold at 1 microM but had little or no effect at concentrations below 1 microM. Displacement binding of [125I]ANP to intact TM cells in the presence of unlabeled ANP indicated a single binding site with a dissociation constant approximately 0.15nM.c-ANP, which binds specifically to natriuretic peptide C receptors, displaced > 95% [125I]ANP binding to surface receptor sites with a half-maximal effective concentration comparable to that of ANP or BNP. c-ANP had no inhibitory effect on CNP stimulation of cGMP formation. The data suggest that human TM cells possess natriuretic peptide B receptors as the primary guanylyl cyclase-containing subtype and C receptors as the numerically predominant subtype of natriuretic peptide receptors.

C-type natriuretic peptide

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

Renal atrial natriuretic factor receptors in hamster cardiomyopathy

Hamsters with cardiomyopathy (CMO), an experimental model of congestive heart failure, display stimulated renin-angiotensin-aldosterone and enhanced sympathetic nervous activity, all factors that lead to sodium retention, volume expansion and subsequent elevation of plasma atrial natriuretic factor (ANF) by the cardiac atria. However, sodium and water retention persist in CMO, indicating hyporesponsiveness to endogenous ANF. These studies were undertaken to fully characterize renal ANF receptor subtypes in normal hamsters and to evaluate whether alterations in renal ANF receptors may contribute to renal resistance to ANF in cardiomyopathy. Transcripts of the guanylyl cyclase-A (GC-A) and guanylyl cyclase B (GC-B) receptors were detected by quantitative polymerase chain reaction (PCR) in renal cortex, and outer and inner medullas. Compared to normal controls, the cardiomyopathic hamster's GC-A mRNA was similar in cortex but significantly increased in outer and inner medulla. Levels of GC-B mRNA were not altered by the disease. On the other hand, competitive binding studies, autoradiography, and affinity cross-linking demonstrated the absence of functional GC-B receptors in the kidney glomeruli and inner medulla. Also, C-type natriuretic peptide (CNP), the natural ligand for the GC-B receptors, failed to stimulate glomerular production of its second messenger cGMP. In CMO, sodium and water excretion were significantly reduced despite elevated plasma ANF (50.5 +/- 11.1 vs. 309.4 +/- 32.6 pg/ml, P < 0.001). Competitive binding studies of renal glomerular ANF receptors revealed no change in total receptor density, Bmax (369.6 +/- 27.4 vs. 282.8 +/- 26.2 fmol/mg protein), nor in dissociation constant, Kd (647.4 +/- 79.4 vs. 648.5 +/- 22.9 pM). Also, ANF-C receptor density (254.3 +/- 24.8 vs. 233.8 +/- 23.5 fmol/mg protein), nor affinity were affected by heart failure. Inner medullary receptors were exclusively of the GC-A subtype with Bmax (153.2 +/- 26.4 vs. 134.5 +/- 21.2 fmol/mg protein) and Kd (395.7 +/- 148.0 vs. 285.8 +/- 45.0 pM) not altered by cardiomyopathy. The increase in ANF-stimulated glomerular cGMP production was similar in normal and CMO hamsters (94- vs. 75-fold). These results demonstrate that renal ANF receptors do not contribute to the attenuated renal responses to ANF in hamster cardiomyopathy.

Blunted natriuresis and abnormal systemic hemodynamic responses to C-type and brain natriuretic peptides in rats with cirrhosis

BACKGROUND/AIMS

The effects of C-type and brain natriuretic peptides (CNP and BNP, respectively) on renal excretion of sodium and hemodynamics have not yet been studied in cirrhosis.

METHODS

This study aimed to examine the effects of saline, CNP (300 ng.kg-1.min-1 i.v. for 30 min) and BNP (600 ng.kg-1.min-1 i.v. for 30 min) on natriuresis and diuresis in normal and rats with cirrhosis. Moreover, regional and systemic hemodynamics were measured prior to and following CNP and BNP administration in normal rats and rats with cirrhosis with ascites.

RESULTS

In rats with cirrhosis, the effects of CNP or BNP or natriuresis and diuresis did not significantly differ from the effects of saline. CNP significantly decreased portal pressure and systemic vascular resistance and significantly increased the cardiac index. BNP significantly decreased portal tributary blood flow, portal pressure and cardiac index. In normal rats, natriuresis and diuresis were significantly higher with CNP and BNP than with saline. Systemic hemodynamics were not changed by CNP. A decrease in arterial pressure was the sole BNP-induced hemodynamic change.

CONCLUSIONS

In conclusion, this study shows that the natriuretic response to pharmacological doses of CNP and BNP is blunted in rats with cirrhosis. This blunting may be related to an activation of the endogenous antinatriuretic systems secondary to systemic vasodilation (by CNP) or to a decreased cardiac index (by BNP). Finally, this study shows that CNP and BNP have a portal hypotensive action.

Natriuretic peptides in the human kidney

We studied the presence of three natriuretic peptides--atrial natriuretic peptide (ANP), human brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP)--in the human kidney by radioimmunoassay and immunocytochemistry. Immunoreactive ANP, immunoreactive human BNP, and immunoreactive CNP concentrations in six kidneys were 0.12 +/- 0.07 (mean +/- SD), 0.23 +/- 0.08, and 0.37 +/- 0.07 pmol/g wet wt, respectively. Sephadex G-50 superfine column chromatography and reversed-phase high-performance liquid chromatography of kidney extracts revealed a broad peak of immunoreactive ANP comigrating with ANP-28 and urodilatin. Renal immunoreactive human BNP consisted of three components; the major component comigrated with human BNP-32. Renal immunoreactive CNP consisted of at least two components; the major component comigrated with CNP-22, and the minor component eluted in a position similar to that of authentic human CNP-53. Immunocytochemistry showed that immunoreactive human BNP was colocalized with immunoreactive ANP in the segments of distal tubules, whereas immunoreactive CNP was found predominantly in the proximal tubules. These findings indicate that these three natriuretic peptides are present in the human kidney and raise the possibility that they form a renal natriuretic peptide system that participates in the local regulation of sodium and water transport and renal circulation in the human kidney.

Identification of renal natriuretic peptide receptor subpopulations by use of the non-peptide antagonist, HS-142-1

1. The renal actions of natriuretic peptides are dictated by the distribution of guanylyl cyclase-linked (NPRA and NPRB) and non-guanylyl cyclase-linked (NPRC) receptors. Natriuretic peptide receptors have previously been distinguished on the basis of their differential affinity for peptide fragments and analogues; however, most of the available ligands are not fully selective. We have used the specific guanylyl cyclase-linked receptor antagonist, HS-142-1, to investigate the differential distribution of natriuretic peptide receptor subtypes in the human, bovine and rat kidney. 2. Specific, high affinity 3-([125I]-iodotyrosyl)-rat-ANP-(1-28)([125I]-rANP1-28) binding sites were identified in all three species, localized to glomeruli, inner medulla, intrarenal arteries and regions in the outer medulla corresponding to vasa recta bundles. Binding sites were also identified in the smooth muscle lining of the hilar region in the bovine and rat kidney. 3. In the rat, [125I]-rANP1-28 binding was inhibited by unlabelled peptide sequences with a rank order of potency (rANP1-28 > pCNP1-22 > C-ANP4-23). The glomeruli exhibited a heterogeneous population of binding sites, C-ANP4-23 and pCNP1-22 producing a significantly better fit to a two component inhibition curve compared to the single component curve for rANP1-28. 4. Competitive inhibition experiments with the receptor selective ligands, C-ANP4-23 and HS-142-1, suggested that, like the rat, human and bovine glomeruli possessed a heterogeneous population of binding sites, whilst those in the inner medulla and intrarenal arteries of all three species represented a homogeneous population. Rat glomeruli exhibited a high proportion (>80%) of the NPRc receptor subtype whereas in human and bovine glomeruli this receptor represented less than 20% of the total population, the majority of binding sites being HS-142-1-sensitive.5. C-ANP4-23 exhibited a significantly higher inhibitory potency for binding sites in rat glomeruli compared to those in human and bovine kidney whilst HS-142-1 was significantly more potent in the rat and bovine kidney compared to man. No evidence was found to suggest the presence of a renal NPRBreceptor subtype.6. The relative density, affinity and proportion of natriuretic receptor subtypes in the kidney exhibit significant species differences. HS-142-1 may be a valuable tool in further elucidating the localization and function of these receptors, but heterogeneity between species should be considered when selecting experimental models.

Presence of C-type natriuretic peptide in human kidney and urine

The current study was undertaken to investigate the presence of CNP immunoreactivity in both human kidney and urine. Immunohistochemical staining with an indirect immunoperoxidase method utilizing an antibody which is 100% cross-reactive to both CNP-53 and CNP-22 was performed on five human kidney specimens (three biopsies of normal cadaveric donor kidneys and two of normal autopsy specimens). CNP immunoreactivity was positive in proximal, distal and medullary collecting duct tubular cells in a cytoplasmic and granular staining pattern. CNP immunoreactivity was also determined in the urine of five healthy volunteers utilizing a sensitive and specific double-antibody radioimmunoassay with a mean concentration of 10.8 +/- 1.0 pg/ml. With the utilization of high pressure liquid chromatography, this immunoreactivity proved to be consistent with both the low molecular weight form, CNP-22, as well as the high molecular weight form, CNP-53. Urinary excretion of CNP was also measured in normal subjects (N = 5) and in patients with congestive heart failure (CHF, N = 6). CHF patients excreted over three times more CNP than normals (27.2 +/- 2.8 vs. 8.7 +/- 0.81 pg/min, P < 0.004) despite no difference between the two groups in plasma CNP concentrations (6.97 +/- 0.28 vs. 8.08 +/- 1.52 pg/ml, P = NS). This study demonstrates for the first time the presence of CNP immunoreactivity in human kidney and suggests that renal tubular cells may be an additional non-vascular site of synthesis for this cardiorenal acting peptide. This study also demonstrates an increase in urinary CNP excretion in congestive heart failure.

Characterization of C-type natriuretic peptide receptors in human mesangial cells

Our aim was to examine whether the human glomerulus was a target for C-type natriuretic peptide (CNP) and how A, B and C receptors of natriuretic peptides (ANPR-A, ANPR-B, ANPR-C) were distributed in glomerular mesangial and epithelial cells. CNP stimulated cyclic GMP production in cultured human mesangial and epithelial cells with similar threshold concentrations (1 nM) and maximum effects (basal value x 30 at 1 microM). In contrast, atrial natriuretic peptide (ANP) was only stimulatory in epithelial cells. [125I] CNP bound specifically to mesangial cells with a Kd of 0.47 nM and Bmax of 42 fmol/mg. Equilibrium of binding was obtained after four to five hours at +4 degrees C and nonspecific binding represented 10 to 20% of total binding. HS142-1 (100 micrograms/ml), a specific inhibitor of ANPR-A and ANPR-B, suppressed 90% of CNP-dependent cyclic GMP production whereas it had little effect on [125I]-CNP binding, suggesting that C receptors were largely predominant in mesangial cells. No biological effect of CNP on mesangial cells, including change in basal or angiotensin II-induced contractility and inhibition of basal or serum-dependent proliferation, could be demonstrated. Similar results were obtained with 8-bromo-cyclic GMP and sodium nitroprusside. Intraglomerular localization of ANPR-A, ANPR-B and ANPR-C mRNA was studied using reverse transcriptase-polymerase chain reaction with amplification of their corresponding cDNA by different primers. Amplification products were identified on gel electrophoresis by their predicted sizes and sequencing. ANPR-A, ANPR-B and ANPR-C mRNA were present in epithelial cells whereas only ANPR-B and ANPR-C mRNA were detected in mesangial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Widespread co-localization of mRNAs encoding the guanylate cyclase-coupled natriuretic peptide receptors in rat tissues

Natriuretic peptides modulate vasorelaxation, diuresis, and natriuresis through the stimulation of cGMP production by the guanylate cyclase-coupled natriuretic peptide receptors, GC-A and GC-B. We used reverse transcription-polymerase chain reaction to determine the distribution of mRNA encoding both receptors in rat tissues. GC-A and GC-B transcripts were detected in all peripheral and neural tissues examined. Since the atrial natriuretic peptide gene is expressed in all these tissues, our widespread detection of GC-A and GC-B mRNAs now suggests that natriuretic peptides may act as endocrine and paracrine hormones as well as neurotransmitters via both GC-A and GC-B receptors.

The natriuretic peptides and their receptors

Atrial natriuretic factor (ANF) is released from the cardiac atrium in response to stretch and acts through receptors to cause an increase in urinary flow and sodium excretion, vasodilatation, and a reduction in blood volume. Recently, two new natriuretic peptides, brain natriuretic peptide (BNP) and C-type natriuretic peptide (C-typeNP), have been isolated, and three different natriuretic peptide receptors have been identified. Two of the receptors, ANP-RGC(A) and ANP-RGC(B), mediate biologic actions. The natural ligand of ANP-RGC(A) is ANF, whereas that of ANP-RGC(B) is C-typeNP. In view of clear differences in ligand specificity and tissue distribution of these receptors, it has been proposed that ANF and its receptor, ANP-RGC(A), and C-typeNP and its receptor, ANP-RGC(B), represent two distinct natriuretic peptide regulatory systems. Whether a separate system exists that incorporates BNP awaits clarification of its natural receptor that mediates a biologic action. The third receptor, ANP-Rc, binds all three natriuretic peptides. Its messenger RNA lacks the guanylyl cyclase sequence present in the mRNA of the other natriuretic peptide receptors, suggesting that the principal function of ANP-Rc is to remove natriuretic peptides from the circulation, that is, to regulate plasma levels of the natriuretic peptides. However, ANP-Rc may also mediate a biologic effect. These findings raise several intriguing questions about the functional role of this family of natriuretic peptides.