Autoradiographic discrimination of brain and atrial natriuretic peptide-binding sites in the rat kidney

Osmoregulation of natriuretic peptide receptors in bromoethylamine-treated rat kidney

Extracellular osmolarity is known as an important factor for the regulation of natriuretic peptide receptors (NPRs). We investigated the intra-renal osmoregulation of NPRs using renal medullectomized rats with bromoethylamine hydrobromide (BEA, 200mg/kg). The administration of BEA caused the decreased food intake and body weight. Water intake was decreased on the first day and then increased from the second day. Urine volume was persistently increased from the first day and free water clearance was also increased from the second day. Urinary excretions of sodium and potassium were decreased on the second day and then recovered to control level. Plasma levels of atrial natriuretic peptide (ANP) and Dendroaspis natriuretic peptide (DNP) in BEA-treated rats were not different from control rats. The inactive renin was increased. The maximum binding capacities of (125)I-ANP as well as (125)I-DNP decreased in glomeruli and medulla of BEA-treated rat kidneys but the binding affinity was not changed. In renal cortex, the gene expressions of ANP, NPR-A, and NPR-B were not changed but that of NPR-C decreased. In renal medulla, the gene expressions of NPR-A, -B, and -C decreased without change in ANP mRNA. Both renal medullary osmolarity and sodium concentration by BEA treatment were lower than those in control kidney. The cGMP concentrations in renal medulla and urine in BEA-treated rats were higher than those in control rats. These results suggest that the increased cGMP production may be partly involved in the decrease in NPRs mRNA expression and their binding capacities by BEA-induced medullectomy.

Urinary immunoreactive brain natriuretic peptide in patients with renal disease

Urinary immunoreactive brain natriuretic peptide (BNP) was studied by radioimmunoassay in patients with renal disease. Urinary immunoreactive human BNP excretion measured in 11 normal subjects was 3.82 +/- 0.62 pmol/day (mean +/- SEM). Significantly increased 24-h urinary secretion of immunoreactive human BNP was noted in patients with chronic renal failure (11.07 +/- 1.73 pmol/day, n = 9, P < 0.05 to normal subjects). A significant correlation was noted between 24-h urinary excretion of immunoreactive human BNP and creatinine clearance in patients with various renal diseases (r = -0.43, P < 0.01, n = 45). Gel chromatography of the urine extracts obtained from normal subjects and patients with chronic renal failure showed multiple immunoreactive peaks; two eluting earlier, one in the position of human BNP-32 and others eluting later. Reverse-phase high-performance liquid chromatography of the urine extracts showed a peak in the position of human BNP-32 and a peak eluting earlier. These findings indicate that: (1) immunoreactive human BNP is present in human urine; (2) urinary immunoreactive human BNP consists of multiple components, i.e., human BNP-32 itself or a substance very similar to it, smaller molecular forms which are probably metabolic products of human BNP-32, and larger molecular forms; and (3) 24-h urinary excretion of immunoreactive human BNP is increased in patients with renal dysfunction.

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.

Characterization of binding sites in rat for A, B and C-type natriuretic peptides

Binding studies, affinity cross-linking and guanylate cyclase assays allowed a comparison of receptors with which the rat forms of atrial/A-type natriuretic peptide (rANP), brain/B-type natriuretic peptide (rBNP) and C-type natriuretic peptide (rCNP) interact in rat kidney cortex and lung. This work represents the first study in which the rat form of BNP (= rBNP-45/iso-rANP(1-45)) has been used as a radiolabelled tracer to further characterize its receptors in these tissues. In addition, these studies stress the use of the same species of natriuretic peptide and assay system, an important experimental des ign given that BNPs show species-specific differences in structure. rBNP-45 bound with lower affinity to rANP (99-126) receptors, namely guanylate cyclase-linked receptor(s) and C-receptor. No receptor which interacted with only rBNP-45 was detectable in lung and kidney cortex. Since rBNP-45 interacted preferentially with the C-receptor and was less potent than rANP(99-126) in stimulating glomerular guanylate cyclase, rBNP-45 may signal through another second messenger in addition to cyclic GMP. Work with truncated analogues of this hormone pinpointed regions of this peptide which may contribute to receptor binding affinity and guanylate cyclase activation. CNP-22 bound to only a subset of ANP receptors and was least effective in stimulating glomerular guanylate cyclase, suggesting a differential mode of action from ANP.

Quantitative determination of natriuretic peptides in human biological samples with a bioassay using cultured cells

Up to now, members of the natriuretic peptide family have usually been determined by radioimmunoassays using antibodies more or less specific for the distinct peptides so far identified. However, natriuretic peptides differing significantly in their amino acid sequence from the one against which the antibody has been raised cannot be determined by this means, and still unknown natriuretic peptides cannot be detected. We therefore developed a new bioassay system sensitive to all members of the natriuretic peptide family by taking advantage of the biological activity of these hormones, the activation of the guanylate cyclase/cyclic GMP system. In this assay, cultured cells are incubated with the natriuretic peptides, and the amount of cyclic GMP produced by the cells is determined by radioimmunoassay. From the relative stimulation of the cellular cyclic GMP production, the concentration of natriuretic peptides in the sample is determined after calibration with synthetic atrial natriuretic peptide. For a qualitative identification of the various peptides, the bioassay is combined with a reversed-phase HPLC step. Using cultured bovine aortic endothelial or bovine kidney epithelial cells for the bioassay, we achieved detection limits of 1 fmol or 50 fmol, respectively, for human atrial as well as brain natriuretic peptide. Intra-assay coefficients of variation of 4.3% (aortic endothelial cells, at 0.65 nmol/l peptide) and 5.8% (kidney epithelial cells, at 6.5 nmol/l peptide) were obtained. The total content of natriuretic peptides as well as the amounts of the individual natriuretic peptides following HPLC separation were determined in extracts of human atria obtained at aortocoronary bypass operations.(ABSTRACT TRUNCATED AT 250 WORDS)

Response to atrial natriuretic peptide, endopeptidase 24.11 inhibitor and C-ANP receptor ligand in the rat

1. The present studies compared the renal and hypotensive response to (a) exogenous atrial natriuretic peptide (ANP) (99-126), (b) an endopeptidase-24.11 inhibitor (candoxatrilat) and (c) an antagonist of ANP clearance receptors (SC 46542) in conscious rats. 2. Infusion of low-dose-ANP (100 ng kg-1 min-1) produced a gradual increase in urinary sodium and guanosine 3':5'-cyclic monophosphate (cyclic GMP) excretion without significant change in glomerular filtration rate (GFR) or fractional lithium clearance (FeLi). There was a significant fall in blood pressure. 3. Infusion of high-dose ANP (300 ng kg-1 min-1) produced a brisk, 3 fold increase in urinary sodium and cyclic GMP excretion along with a rise in GFR, but had no significant effect on FeLi compared to the control group. The renal response was accompanied by a pronounced fall in blood pressure. 4. Candoxatrilat or SC 46542, alone, had no significant effect on sodium excretion compared to control animals. Both compounds enhanced the natriuretic and cyclic GMP responses to a low-dose ANP infusion (100 ng kg-1 min-1) to levels similar to, or greater than, those observed with the high-dose ANP (300 ng kg-1 min-1). However, unlike high-dose ANP, these renal effects were not accompanied by a significant change in GFR and neither compound potentiated the hypotensive effect of the low-dose ANP infusion. Only candoxatrilat when given with ANP produced a marked rise in FeLi.5. Similarly, combined administration of candoxatrilat and SC 46542 (without exogenous ANP) induced an increase in sodium and cyclic GMP excretion comparable to high-dose ANP but did so without a significant increase in GFR and with a significantly smaller fall in blood pressure. Interestingly, there was no increase in FeLi with the combination of the two compounds, suggesting that the major contribution to sodium excretion came from SC 46542.6. Both candoxatrilat and SC 46542 increased sodium and cyclic GMP excretion in the rat A-V fistula model of heart failure, a model hyporesponsive to infusions of ANP, without significant change in blood pressure.7. These data show that candoxatrilat and SC 46542 do not simply reproduce the effects of an ANP infusion but preferentially enhance the natriuretic response to ANP. Inhibition of E-24.11 may potentiate a tubule action of ANP while the renal mechanism of action of the C-ANP receptor ligand needs further study. Both manipulations are of potential value in the management of heart failure.

Plasma levels of brain natriuretic peptide in patients with cirrhosis

Plasma levels of brain natriuretic peptide, a recently identified cardiac hormone with natriuretic activity, were measured in 11 healthy subjects, 13 cirrhotic patients without ascites, 18 nonazotemic cirrhotic patients with ascites and 6 patients with cirrhosis, ascites and functional kidney failure. Plasma levels of brain natriuretic peptide were similar in healthy subjects and cirrhotic patients without ascites (5.56 +/- 0.65 and 7.66 +/- 0.68 fmol/ml, respectively). In contrast, cirrhotic patients with ascites, with and without functional kidney failure, had significantly higher plasma concentrations of brain natriuretic peptide (19.56 +/- 1.37 and 16.00 +/- 1.91 fmol/ml, respectively) than did healthy subjects and patients without ascites (p less than 0.01); no significant difference was found between the two groups of cirrhotic patients with ascites with respect to this parameter. In the whole group of cirrhotic patients included in the study, brain natriuretic peptide level was directly correlated with the degree of impairment of liver and kidney function, plasma renin activity and plasma levels of aldosterone and atrial natriuretic peptide. The results of this study indicate that brain natriuretic peptide is increased in cirrhotic patients with ascites and suggest that sodium retention in cirrhosis is not due to deficiency of this novel cardiac hormone.

Construction and use of two alpha-human atrial natriuretic peptide-fragment affinity chromatography columns in the isolation of C- and N-terminal epitope-specific antibodies for use in a prototype alpha-hANP biosensor

Two alpha-human atrial natriuretic peptide (alpha-hANP) based affinity chromatography columns were produced by covalently immobilizing the C- and N-terminal epitopes of alpha-hANP. The stationary phase was made from a controlled-pore-glass bead solid support, which was silanized and treated with sulphosuccinimidyl 4-(maleimidomethyl)cyclohexyl carboxylate before the individual fragments were immobilized by substitution at their thiol groups. These columns were used to isolate alpha-hANP-specific antibodies from a goat anti-alpha-hANP serum, which were then further sorted according to their epitope specifity. These C- and N-terminal epitope-specific antibodies were in turn used as components in the construction of an alpha-hANP biosensor based on an enzyme-linked immunosorbent assay (ELISA) sandwich principle. Initial in vitro testing of the sensor using a physiological alpha-hANP solution showed a reproducible response to the peptide. There is to date no other equally fast, sensitive and precise method available to detect this peptide. This alpha-hANP sensor may prove to be an invaluable aid in human medicine as a monitor of patient status during transplant surgery, for example, an area inaccessible to radioimmunoassay and normal ELISA techniques.

Endocardial localization and characterization of natriuretic peptide binding sites in human fetal and adult heart

Specific, high affinity binding sites for 125I-human-alpha-atrial natriuretic peptide-(1-28)) (125I-hANP-(1-28)) were identified in human fetal and adult heart and the binding characterized using quantitative in vitro autoradiography. Binding sites were localized to atrial and ventricular endocardium, aorta, pulmonary arteries and epicardial mesothelium. Kinetic studies indicated a Kd value of 32 pM for ventricular endocardial 125I-hANP-(1-28) binding. The binding was completely inhibited by an excess (1 microM) of unlabelled hANP-(1-28), human brain natriuretic peptide-(1-32) (hBNP-(1-32)) and by the 'clearance receptor' specific ring-deleted analogue, C-ANP-(4-23). Competitive inhibition studies indicated a relative inhibitory potency for hBNP-(1-32) and C-ANP-(4-23) of 6% and 3% respectively. The data suggest that a distinct natriuretic peptide receptor subtype is expressed in the endocardium and in addition to a possible clearance function, may represent a site for feedback regulation and peptide interaction.