Physiological role of silent receptors of atrial natriuretic factor

Isolation and functional expression of the human atrial natriuretic peptide clearance receptor cDNA

The amino acid sequence of the human atrial natriuretic peptide clearance receptor (ANP C-receptor) was deduced from the nucleotide sequence of cDNA clones obtained from human placental and kidney cDNA libraries. The human sequence is highly homologous to the bovine C-receptor sequence already described, and the corresponding mRNA is expressed in human placenta, adult and fetal kidney and fetal heart. Upon transfection of this cDNA into mammalian cells, recombinant expression experiments revealed that the human ANP C-receptor has a high affinity for ANP (6 x 10(-9) M), similar to that observed for the receptor in other species. These data indicate that the human ANP C-receptor, previously characterized in other mammalian species, is highly conserved structurally and is expressed in various human tissues.

Degradation of atrial natriuretic factor in mouse blood in vitro and in vivo: role of enkephalinase (EC 3.4.24.11)

The half-life of 125I-ANF (99-126) in mouse blood in vivo, evaluated after HPLC analysis, was approximately 0.5 min. This value was nearly doubled in mice pretreated with sinorphan, an enkephalinase inhibitor. 125I-ANF is also hydrolyzed by mouse blood in vitro but its half-life is much longer and only marginally modified by an enkephalinase inhibitor. Hence, although the peptidase is present in blood plasma and cells, its rate-limiting participation in the clearance of the hormone in vivo does not occur at the blood level.

The response of atrial natriuretic factor and sodium excretion to dietary sodium challenges in patients with chronic liver disease

Despite intensive investigation, the pathogenesis of sodium retention in patients with chronic liver disease is not fully known. We have studied 19 chronic liver disease patients, 13 without (group 1) and six with (group 2) histories of clinical sodium retention (ascites or edema) by varying dietary sodium intake. The patients were placed on a 20 mmol/day constant diet for 1 wk, followed by a constant 100 mmol/day sodium diet for 1 wk under strict metabolic conditions. After 5 days of equilibration on each diet, blood and urine samples were collected for plasma atrial natriuretic factor levels and urinary sodium excretion. Group 1 patients (n = 6) achieved near sodium balance in 5 days on both a 20-mmol (urinary sodium output = 17 +/- 3 mmol/day) and a 100-mmol sodium diet (urinary sodium output = 80 +/- 5 mmol/day). Atrial natriuretic factor levels in these patients tended to be elevated, but the increase was not significantly greater than that in normal control subjects (10 +/- 4 pg/ml to 19 +/- 4 pg/ml) on the same diets. In contrast, group 2 patients (n = 5) were in significant positive sodium balance on both the 20 mmol/day sodium diet (mean urinary sodium output = 9.5 +/- 3.3 mol/day) and the 100 mmol/day sodium diet (urinary sodium output = 37 +/- 13 mmol/day). This occurred despite significantly elevated baseline atrial natriuretic factor levels and a significant increase in plasma atrial natriuretic factor levels after sodium challenge (62 +/- 9 pg/ml, p less than 0.05) on a 100 mmol/day sodium diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic factor recognizes two receptor subtypes in endothelial cells cultured from bovine pulmonary artery

In this study specific high affinity binding sites for atrial natriuretic factor (rANF(99-126] have been identified on cultured endothelial cells of bovine pulmonary artery origin (BPAEC). A time-dependent rise in cellular cGMP levels stimulated by rANF(99-126) was followed by release of the nucleotide into the incubation medium. The use of truncated, ring-deleted and linear atrial peptide analogs in competitive displacement analysis and measurement of cGMP accumulation indicated that only a minor proportion (5-11%) of the available receptor pool was of the ANF-B receptor subtype, linked to guanylate cyclase, with the remaining major proportion possibly of the ANF-C (clearance) receptor subtype. The existence of two ANF receptor subtypes in this cell culture model would suggest a significant role for the circulating peptide in modulation of pulmonary endothelial cell function, which would influence or complement its direct actions on the underlying vasculature of the pulmonary circulation.

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.

Synthesis and biological activity profiles of atrial natriuretic factor (ANF) analogs

Several analogs of the atrial natriuretic factor (ANF) were synthesized by the solid-phase method using the acetamidomethyl (Acm) group for sulfhydryl protection. The compounds were tested in a receptor binding assay using bovine adrenal zona glomerulosa cell membranes and in the rat diuresis/natriuresis assay. Substitution of tyrosine in position 116 of ANF(101-126) and of the analog [3-Mpr105]ANF(105-126)(3-Mpr = 3-mercaptopropionic acid) did not alter the biological activity profiles and, therefore, these two analogs in radioiodinated form will be useful for enzymatic degradation and clearance studies. Replacement of 3-mercaptopropionic acid with 2-mercaptopropionic acid in [3-Mpr105]ANF(105-126) resulted in an analog with very low potency in both assay systems, presumably as a consequence of the steric bulk and/or local conformational restriction produced by the methyl group attached to the alpha-carbon in position 105. The analog [3-Mpr105,Nva109]ANF(105-126)(Nva = norvaline) showed very low affinity in the receptor binding assay but displayed considerable diuretic/natriuretic activity. The obtained biological activity profiles suggest that in comparison with other ANF peptides the des-amino ANF(105-126) analogs may have a somewhat longer half-life in vivo, or alternatively, may indicate a more complex situation of ANF receptor or binding site heterogeneity.

Regulation of atrial natriuretic peptide receptors in glomeruli during chronic salt loading

The effects of chronic salt loading on atrial natriuretic peptide (ANP) receptor density and affinity were studied in isolated renal glomeruli of male Sprague-Dawley rats, which received 0.9% saline as drinking fluid (NaCl-rats) and a normal rat chow diet for 35 days (N = 12). Animals on a low sodium intake received the same diet, but deionized water and served as controls (C) (N = 12). After 35 days blood pressure was only slightly increased to 136 +/- 9 in NaCl-rats versus 120 +/- 2 mm Hg in C (NS). Glomerular filtration rate, plasma cGMP and plasma ANP remained unaltered. Determination of total ANP receptor characteristics in these rats indicated a significant down-regulation of ANP receptors in salt loaded rats. Since ANP-stimulated cGMP formation was not affected by salt loading, the roles of clearance (C) and of biologically active (B) receptors were further evaluated at 21 degrees C on freshly isolated and acid washed (pH 5) glomeruli in seven animals after 35 days of salt loading and in seven animals on a low sodium intake. B-receptors were assessed by blocking C-receptors with 4-23 cANP. C-receptor numbers were lower in NaCl-rats (97 +/- 8 vs. 184 +/- 14 fmol/mg protein in C; N = 7; P less than 0.02), while C-receptor affinity was increased (Kd: 12 +/- 3 pM in NaCl-rats vs. 22 +/- 5 pM in C; P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental pattern of cyclic guanosine monophosphate production stimulated by atrial natriuretic peptide in glomeruli microdissected from kidneys of young rats

Cyclic guanosine monophosphate (GMP) productions by alpha rat atrial natriuretic peptide 1-28 (alpha-rANP), carbamylcholine or sodium nitroprusside were assessed in isolated glomeruli microdissected from collagenase-treated kidneys of 2- to 34-day-old and adult rats. In both young and adult animals, alpha-rANP-stimulated cyclic GMP generation was proportional to the number of glomeruli and was enhanced in a dose-dependent and saturable fashion with increasing alpha-rANP concentrations. The apparent activation constant values were 6.4 nM for 5-day-old and 9.7 nM for adult rats. Maximal doses of either alpha-rANP or rANP 5-28 elicited similar responses in young and adult animals. Clear differences appeared between the developmental patterns of cyclic GMP productions stimulated by either alpha-rANP, carbamylcholine or sodium nitroprusside. The response to alpha-rANP was very large in the youngest rats tested, declined sharply during the suckling period and represented about 1.6 times the adult control level in 34-day-old rats. In contrast, the response to carbamylcholine was low after birth and rose progressively with age up to the adult level at the end of the weaning period, and the response to nitroprusside seemed to be independent of the animal's age.

Attenuated forearm vasodilative response to intra-arterial atrial natriuretic peptide in patients with heart failure

It has been shown that renal responses to atrial natriuretic peptide (ANP) are markedly attenuated in patients with heart failure. This study aimed to determine if vasodilative response to ANP is altered in patients with heart failure. In patients with heart failure (n = 7) and age-matched normal subjects (n = 7), forearm blood flow was measured using a strain-gauge plethysmograph during intra-arterial infusion of alpha-human ANP (50, 100, 200, and 400 ng/min) or nitroglycerin (100, 200, 400, and 600 ng/min). Forearm vasodilatation evoked with intra-arterial alpha-human ANP in patients with heart failure was considerably less (p less than 0.01) than that in normal subjects. In contrast, nitroglycerin produced comparable forearm vasodilatation in the two groups. Plasma ANP and cyclic guanosine monophosphate (GMP) levels at rest were higher in patients with heart failure than in normal subjects (p less than 0.05 for both), but the increases in plasma ANP and cyclic GMP in the venous effluents during intra-arterial ANP infusion did not differ between the two groups. These results indicate that the direct vasodilative effect of ANP on forearm vessels was attenuated in patients with heart failure as compared with that in normal subjects. The mechanisms responsible for this alteration are not clear but might involve mechanisms other than down-regulation of the ANP receptors because the increases in venous plasma cyclic GMP caused by intra-arterial ANP were comparable between patients with heart failure and normal subjects.

Brain natriuretic peptide stimulates particulate guanylate cyclase activity in selected areas of the rat brain

The effect of porcine brain natriuretic peptide (pBNP) on cyclic guanosine monophosphate (cGMP) production was investigated in localized rat brain areas by radioimmunoassay procedure. Porcine BNP activated particulate guanylate cyclase in the median eminence, subfornical organ, choroid plexus, olfactory bulb, paraventricular nucleus and pineal gland in a concentration-dependent fashion and its action was comparable to that of rat atrial natriuretic peptide (alpha-ANP), with ED50 values ranging from 5 to 7 x 10(-7) M for both peptides. Our results suggest that the activation of a specific receptor coupled to the guanylate cyclase system and the subsequent elevation of cGMP levels constitutes the common mechanism of the central action of BNP and ANP.

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.

Atrial natriuretic factor and central venous pressure during intermittent and continuous lower-body positive pressure in healthy humans

In eight healthy volunteers undergoing 16 experiments in a cross-over design central venous pressure (CVP) and atrial natriuretic factor (ANF) in central venous plasma were measured during a 30 min control period followed by three separated periods of 10 min lower-body positive pressure (LBPP) or 90 min continuous LBPP induced by inflation of a military anti-G suit to evaluate the effect of short repeated and of extended increases in right atrial pressure on plasma ANF levels. CVP increased significantly during each of three separate periods of intermittent LBPP, and 15 min after application of continuous LBPP (P less than 0.025 for all). Blood pressure and heart rate did not change. During intermittent LBPP plasma ANF levels increased 10 min after the first inflation of the MAGT-suit (P = 0.013), but not after the second or third inflation. During continuous LBPP plasma ANF remained unchanged until 90 min after application of LBPP where a significant rise was observed (P = 0.023). The data demonstrate that the ANF response to short-term increases in right atrial pressure, as small as 2.5 mmHg, is maximal within 10 min and that repeated pressure stimuli may decrease ANF release. Sustained increases in right atrial pressure are not associated with increases in plasma ANF until long after initiation of the pressure stimulus suggesting rapid receptor-binding of ANF and that the ANF receptors might be saturated during continuously elevated ANF levels.

Effect of prolonged high salt intake on atrial natriuretic factor's kinetics in rats

Plasma atrial natriuretic factor (ANF) paradoxically decreases after 5 weeks (but not after 3 weeks) of 8% NaCl intake in normotensive rats. As this phenomenon remains unaccounted for by changes in ANF production, we studied the disappearance of [125I]ANF(99-126) from the circulation as an alternative explanation of plasma ANF decline. Following 5 weeks (but not 3 weeks) of an 8% NaCl diet, plasma concentrations of [125I]ANF were significantly decreased and metabolic clearance rate and volume of distribution were increased compared to control rats fed a 0.8% NaCl diet. By studying [125I]ANF tissue uptake we noted significantly greater peptide uptake after 5 weeks (but not after 3 weeks) of high salt consumption in several tissues. We hypothesize that prolonged (at least 5 weeks) 8% NaCl ingestion increases the density and/or affinity of ANF binding sites. These changes may be responsible for the previously observed decline in plasma ANF concentrations after a prolonged high salt intake.

Inactivation of atrial natriuretic factor in mice in vivo: crucial role of enkephalinase (EC 3.4.24.11)

Atrial natriuretic factor (ANF) is a hormone whose potent hemodynamic and renal actions might be beneficial in several cardiovascular disorders, but whose poor oral absorption and extremely rapid inactivation in vivo have so far prevented its therapeutic use. We have developed simple tests to study the peptidases responsible for the hydrolysis of ANF in mice in vivo and to assess the effects of peptidase inhibitors. In mice injected with 125I-ANF in low amounts the radioactivity present in kidney, a major target organ for the hormone, was analysed by HPLC, precipitation with trichloracetic acid (TCA) and in a membrane binding assay. All three parameters indicated a rapid inactivation of the hormone: 20 s after injection of 125I-ANF the intact hormone represented less than 20% of the total kidney radioactivity. Oral pretreatment with acetorphan, a potent enkephalinase inhibitor resulted in a marked increase in the amount of intact 125I-ANF (6-fold), TCA-precipitated (5-fold) and membrane bound radioactivity (4-fold) in the kidney; the total kidney radioactivity was enhanced by approximately 2-fold. A similar protective effect was observed with other enkephalinase inhibitors, i.e. thiorphan and kelatorphan; the latter was effective at a 10-fold higher dosage. In contrast, a large variety of inhibitors of metallo-, cysteine, serine and aspartic proteinases had no or only marginal effects. Instead, captopril, an angiotensin-converting enzyme inhibitor, reduced the total and TCA-precipitable radioactivity in the kidneys. Aminopeptidase inhibitors, used either alone or in conjunction with acetorphan, displayed significant but limited protective effects. The crucial role of enkephalinase in ANF inactivation in vivo suggests that inhibitors of this peptidase could be used in a novel therapeutic approach to cardiovascular or renal diseases by protecting endogenous ANF.

Atrial natriuretic factor receptors in cultured renomedullary interstitial cells

To gain further insight on the cell types that may mediate the effects of atrial natriuretic factor (ANF) in the renal medulla, we determined the distribution and function of biological (B) and clearance (C) receptors of ANF in renomedullary interstitial cells (RMIC). Studies were performed in the 3rd-17th passages of RMIC obtained from a primary culture of the rat renal medulla. Electron microscopy of the cultured cells showed the typical morphological features of RMIC "in vivo," including prominent lipid droplets. RMIC have a very high density of high-affinity specific binding sites of ANF-(1-28) [23,000 sites/cell; dissociation constant (Kd) = 50 pM]. There was only minimal binding of C-ANF-(4-23) (less than 2,500 sites/cell), a specific ligand of C-ANF receptors. ANF-(1-28) markedly increased guanosine 3',5'-cyclic monophosphate (cGMP) from 1.3 +/- 0.3 to 106 +/- 22 pmol cGMP/10(6) cells [50% effective dosage (ED50) = 1.2 nM]. The effect of ANF-(1-28) on cGMP was nearly additive to that of sodium nitroprusside and was not potentiated or antagonized by C-ANF-(4-23). The density of guanylate cyclase-coupled B-ANF receptors and the ANF-induced increase in cGMP in RMIC are higher than those reported to date in other target cells. This suggests that RMIC may mediate some of the known effects of ANF in the renal medulla.

Kinetic analysis of the downregulation of epidermal growth factor receptors in rats in vivo

We previously clarified the specific binding sites for epidermal growth factor (EGF) in several organs in rats based on in vivo kinetic analysis (D. C. Kim, Y. Sugiyama, H. Sato, T. Fuwa, T. Iga, and M. Hanano. J. Pharm. Sci. 77: 200-207, 1988). In the present study, we have determined the extent of the receptor downregulation and the recovery rate of the available receptors for EGF in several organs in vivo. At the specified times (30 min-24 h) after intravenous administration of excess unlabeled EGF (300 micrograms/kg), the early-phase (less than 3 min) uptake clearances (k1) of the tracer amount of 125I-EGF, which are proportional to the cell-surface available receptor densities, were determined in the liver, kidney, duodenum, jejunum, ileum, stomach, and spleen. As the result, the k1 value in each organ at 30 min after intravenous administration of unlabeled EGF was lowered close to the receptor-independent clearance value, indicating that the cell-surface receptors were almost completely downregulated, and thereafter, the k1 value showed gradual recovery to the control level. Furthermore, the recovery half-lives showed interorgan differences, namely the half-life (20 min) in the liver was much shorter than those (2-4.5 h) in other organs. These results were considered to reflect the processes of the recycling of internalized EGF receptors to the cell-surface or recruitment of new receptors. It was concluded that the recovery rate of the downregulated receptors in the liver, which is most responsible for the plasma clearance of EGF, is much faster than those in other organs.

Characterisation of atrial natriuretic peptide receptors in bovine ventricular sarcolemma

Analysis of [125I]-ANP binding data in an isolated bovine ventricular sarcolemmal membrane fraction revealed a single high affinity binding site (Kd approximately 5 x 10(-11) M). The ring deleted ANP analogue des [QSGLG]-ANP (4-23)-NH2 bound with a 1000-fold lower affinity indicating the absence of C-type receptors in this preparation. ANP stimulated guanylate cyclase activity by up to 2-fold with half-maximal activation at approximately 10(-9) M. Crosslinking [125I]-ANP to its receptor with disuccinimidyl suberate (DSS) revealed two radiolabelled bands of 120 kDa and 65 kDa on non-denaturing SDS-PAGE. Radioactive signals from both bands were lost by reducing the sample with beta-mercaptoethanol prior to electrophoresis, in which case a radioactive fragment of less than 5 kDa migrated with the dye front. These results suggest that the binding of ANP to both high and low molecular weight "receptor" proteins may be associated with the hydrolysis of the peptide.

Binding of atrial and brain natriuretic peptides in brains of hypertensive rats

Displacement of bound [125I]alpha-atrial natriuretic peptide (alpha-ANP) by brain natriuretic peptide (BNP) was used to map receptors common to both peptides in rat brain by in vitro autoradiography. Both spontaneously hypertensive rats (SHR) and the normotensive Wistar-Kyoto control strain (WKY) were studied. In both strains, [125I]alpha-ANP bound densely to subfornical organ, choroid plexus and arachnoid mater. Binding at these sites in either strain was displaced similarly by 1 microM unlabelled alpha-ANP or BNP. However, no [125I]alpha-ANP was displaced by peptides unrelated to alpha-ANP or BNP. In WKY, both alpha-ANP and BNP competed with similarly high affinities for binding sites occupied by [125I]alpha-ANP. This was also true for SHR. However, SHR showed a substantial reduction in the maximum number of binding sites in the subfornical organ and choroid plexus which were competed for by the peptides. Therefore, BNP may be a significant high affinity ligand for brain receptors previously thought specific for atrial natriuretic peptides, including receptors which vary between WKY and SHR.

Partial characterization of a metalloendopeptidase activity produced by cultured endothelial cells that removes the COOH-terminal tripeptide from 125I-atrial natriuretic factor

The presence of the COOH-terminal region of human atrial natriuretic factor-(99-126) (hANF) is necessary for the full expression of its biological activity. Here, we report on the partial characterization of a proteolytic activity in the conditioned medium from cultured bovine aortic endothelial cells that cleaves the Ser123-Phe124 bond of 125I-hANF generating the COOH-terminal tripeptide. The concentrated conditioned medium was fractionated by gel filtration high performance liquid chromatography and fractions were assayed for the ability to generate the COOH-terminal tripeptide from 125I-hANF. This analysis indicated that the protein responsible for this activity had an approximate molecular weight of 200,000 daltons. Of 16 protease inhibitors tested, only 1,10 phenanthroline, EDTA, EGTA and N-ethylmaleimide significantly inhibited the endopeptidase activity. Thus, we conclude that cultured bovine aortic endothelial cells produce a potentially novel phosphoramidon-insensitive metalloendopeptidase that removes the COOH-terminal tripeptide from 125I-hANF.

Specific [125I]brain natriuretic peptide-26 binding sites in rat and pig kidneys

Specific binding sites for porcine brain natriuretic peptide-26 (BNP-26), a member of the atrial natriuretic peptide family (ANPs), were investigated in the kidney by using receptor autoradiographic and membrane binding techniques with [125I]BNP-26. The binding sites were discretely localized in rat and porcine kidney areas corresponding anatomically to the glomeruli and inner medulla. There were no differences between the localization of [125I]BNP-26 and [125I]alpha-rat ANP binding sites in the kidney. [125I]BNP-26 binding to solubilized membranes from isolated glomeruli of the rat kidney was saturable, and a single class of high-affinity sites was labeled with a KD of 372 pM. The radioligand bound to two sites in solubilized inner medullary membranes of the rat, a low-affinity site with a KD of 30 nM, and a high-affinity site with a KD of 33 pM. The rank order of potency to inhibit binding was BNP-26 = alpha-rat ANP-(1-28) greater than atriopeptin III (ANP-(103-126)) much greater than atriopeptin I (ANP-(103-123)) greater than des-Cys105,Cys121- ANP-(104-126). Thus, [125I]BNP-26 presumably recognizes ANP receptors in the kidney. The possibility that BNP-26 regulates, as a circulating hormone, kidney functions by binding to ANP receptors would have to be considered.

Effects of atrial and brain natriuretic peptides upon cyclic GMP levels, potassium transport, and receptor binding in rat astrocytes

The ability of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) to alter cyclic GMP levels and NaKCl cotransport in rat neocortical astrocytes was determined. At concentrations of 10(-9)-10(-6) M, rat ANP99-126 (rANF), rat ANP102-126 (auriculin B), and rat ANP103-126 (atriopeptin III) stimulated 6- to 100-fold increases in cyclic GMP levels. Porcine BNP (pBNP) and rat BNP (rBNP) were 20%-90% as effective as rANF over most of this concentration range, although 10(-6) M pBNP produced a greater effect than rANF. NaKCl cotransport as measured by bumetanide-sensitive 86Rb+ influx was not altered by exposure of astrocytes to 10(-6)M rANF, pBNP, or rBNP. Both pBNP and rBNP, as well as rat ANP103-123 (atriopeptin I) and des[gl18, ser19, gly20, leu21, gly22] ANF4-23-NH2 (C-ANF4-23) strongly competed for specific 125I-rANF binding sites in astrocyte membranes with affinities ranging from 0.03 to 0.4 nM, suggesting that virtually all binding sites measured at subnanomolar concentrations of 125I-rANF were of the ANP-C (ANF-R2) receptor subtype. These receptors are thought to serve a clearance function (Maack et al.: Science 238:675-678, 1987) and may be linked to a guanylate cyclase activity that is chemically and pharmacologically distinct from that coupled to ANP-A (ANF-R1) receptors (Féthiere et al.: Mol Pharmacol 35:584-592, 1989). ANP receptors on astrocytes may function in limiting the access of ANP and BNP to neurons involved in body fluid and cardiovascular regulation.

Developmental patterns of renal atrial natriuretic peptide receptors: [125I]alpha-rat atrial natriuretic peptide binding in glomeruli and inner medullary collecting tubules microdissected from kidneys of young rats

The ontogenic developmental patterns of atrial natriuretic peptide (ANP) receptors of glomeruli and inner medullary collecting tubules (IMCT) were studied by measuring the specific binding of [125I]alpha-rat ANP 1-28 ([125I]alpha-RANP) to isolated glomeruli and IMCT microdissected from collagenase-treated kidneys of young rats aged from 2 to 35 days post-partum. For glomeruli and IMCT from young and adult animals, total and non-specific binding increased linearly with glomerulus number or tubular length. ANP receptors detected in glomeruli and IMCT from young rats showed the same stereospecifities as those from adult rats for recognition of ANP analogues (alpha-RANP 1-28, ANP 3-28, atriopeptin III and atriopeptin II). The numbers of ANP receptors in glomeruli and IMCT (expressed in terms of 10(-18) mol labelled ANP bound per glomerulus or per mm IMCT length, respectively) exhibited marked variations during postnatal ontogenesis; they were low after birth and rose progressively with age up to the corresponding adult levels (20 +/- 2 X 10(-18) mol.glom-1 and 4.4 +/- 0.8 X 10(-18) mol.mm-1) at the end of the 5th week of postnatal life.

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.

Vascular receptors for atrial natriuretic peptide in hypertension

Atrial natriuretic peptide (ANP) is secreted by the heart in response mainly to atrial distension and circulates in plasma in picomolar concentrations. It binds to receptors in blood vessels which it relaxes, renal glomeruli where it induces increased glomerular filtration rate, renal papilla to produce natriuresis, adrenal glomerulosa cells to inhibit aldosterone secretion, and median eminence and pituitary where it may inhibit vasopressin secretion. In experimental models of hypertension plasma levels of ANP are uniformly elevated, except in spontaneously hypertensive rats, in which plasma ANP may only rise transiently. The action of ANP on smooth muscle cells of the blood vessel wall results in production of cyclic GMP, which appears to be the second messenger producing relaxation of pre-contracted blood vessels. Mechanisms other than cGMP generation have been proposed but remain unproven as mediators of ANP action. Receptors for ANP in blood vessels are of two subtypes: B-receptors (or R1-receptors), which contain guanylate cyclase in their structure, and C-receptors (or R2-receptors), which have not been shown to the present to be biologically active. Our studies on vascular ANP receptors are reviewed. In several experimental models of hypertension such as saralasin-insensitive 2-kidney, 1-clip and 1-kidney, 1-clip Goldblatt hypertensive rats and in DOCA-salt hypertensive rats, we have found elevated plasma ANP, as well as decreased binding and ANP-induced vascular relaxation and blood pressure-lowering effects of ANP. Both the B and C ANP receptors appear decreased in density, even after acid washing of membranes to remove any retained circulating ANP.(ABSTRACT TRUNCATED AT 250 WORDS)

Guanylate cyclase receptor family

The plasma membrane forms of guanylate cyclase contain a highly conserved catalytic domain, which is also conserved in the soluble form of the enzyme and in mammalian adenylate cyclase. A protein kinase-like domain lies to the amino-terminal side of the catalytic domain and appears to be required for signaling via cGMP; it might also signal, itself, through phosphotransferase activity. This domain is present in the growth factor receptors, but appears not to be a component of other guanylate cyclases or adenylate cyclases. A single transmembrane domain then separates the cyclase catalytic and protein kinase-like domains from the putative ligand-binding domain. At least two plasma membrane forms of gunaylate cyclase (i.e., GC-A and GC-B) have now been identified, and their ligand specificities appear to be distinctly different. The tissue/cellular distribution of this family of receptors is now of potential importance, since specific agonists might differentially regulate physiological processes via the secondary messenger, cGMP, dependent on cellular distribution of the receptors.

Mechanisms of atrial natriuretic factor-induced vasodilation

ANF can potentially elicit vasorelaxation in vitro which is typically associated with an elevation in tissue levels of cGMP. Hypotension with vasodilation can be observed upon injection of ANF in vivo, however, infusion of the peptide often results in a decreased blood pressure due to a fall in cardiac output, This apparent discrepancy may reflect some of the distinguishing characteristics of ANF-induced vasorelaxation which include activation of particulate guanylate cyclase, a marked regional vascular selectivity, species differences in the relaxation profile and a variable sensitivity depending on the type and degree of contractile preload.

Degradation and clearance of atrial natriuretic factors (ANF)

Atrial natriuretic factor, the first well defined natriuretic hormone is synthesized in the human heart as 151 aminoacid (AA) preprohormone and stored as 126 AA prohormone in atrial granules. Upon appropriate stimulation, the prohormone is cleaved into a 98 AA N-terminal fragment and a 28 AA C-terminal fragment, the biological active ANF(99-126), both circulating in plasma. Circulating ANF(99-126) is cleared by various organs, such as lung, liver and intestine, kidney and upper and lower limbs. Reported arterial-venous extraction ratios vary greatly, but are not much different between organs, the average extraction ratio being about 35%. Due to marked differences of organ blood flow, the contribution of various organs to total body ANF clearance differs considerably. Major mechanisms for ANF clearance are uptake by clearance receptors and degradation by an endoprotease (EC 3.4.24.11.). Clearance receptors, distinct from the receptors mediating the biological actions of ANF, have been demonstrated in various organs. Characterization of the ANF degrading enzyme activity has been performed in kidney tissue. Whether and how pathophysiological states affect ANF clearance is still poorly understood. Inhibition of clearance by ANF analogues binding to clearance receptors and by inhibitors of degrading peptidase can increase the biological action of circulating ANF. This may prove to be a therapeutic approach in diseases with smooth muscle contraction or volume overload.

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.

Enkephalinase (EC 3.4.24.11) inhibitors: protection of endogenous ANF against inactivation and potential therapeutic applications

Atrial natriuretic factor (ANF) is a cardiac hormone exerting potent cardiovascular and renal effects but its poor intestinal absorption and rapid inactivation have prevented so far its therapeutic utilisation. However inhibition of endogenous ANF metabolism progressively emerges as a novel therapeutic approach in cardiovascular and renal disorders. The critical role played by enkephalinase (membrane metalloendopeptidase, EC 3.4.24.11) in ANF inactivation was deduced from the effects of inhibitors. These compounds not only protect partially exogenous ANF from hydrolysis by some tissue preparations in vitro but also, in vivo, they increase the half-life of the exogenous hormone in plasma and, even more markedly, its recovery in intact form in kidney, a major target organ. In addition, enkephalinase inhibitors increase by two- to three-fold the circulating level of endogenous ANF, even when the latter is already markedly elevated, such as in patients with chronic heart failure. Finally, enkephalinase inhibitors induce a series of ANF-like responses such as natriuresis, diuresis or increase in cGMP excretion which are attributable to the hormone. These pharmacological observations, as well as preliminary clinical trials, suggest that enkephalinase inhibitors may represent a novel class of therapeutic agents with potential applications in congestive heart failure, essential hypertension and various sodium-retaining states.

Binding and intracellular degradation of atrial natriuretic factor by cultured vascular smooth muscle cells

Binding studies were performed on vascular smooth muscle cells (VSMC) from the rat aorta, using 125I-atrial natriuretic factor (Ser99-Tyr126) (ANF (Ser99-Tyr126] as the ligand. Kinetic studies at 37 degrees C indicated a rapid onset of binding with a maximum total binding of 25% being reached by 60 min. Crosslinking experiments demonstrated that ANF bound to a 120 kDa and a 60 kDa protein with the former dissociating into the 60 kDa species in presence of beta-mercaptoethanol. Of the total radioactivity bound, 15% represented internalized material. Analysis of the medium after different incubation periods revealed a 42% degradation of 125I-ANF by 120 min. At 4 degrees C, no internalization of 125I-ANF was observed. However, surface binding occurred, albeit at a much slower rate, and not reaching a maximum even at the end of 3 h. No degraded material was detected in the extracellular medium even after a 2-h incubation. Chloroquine (100 microM) and monensin (10 microM) significantly increased the cell-associated radioactivity, causing a 2- to 3-fold elevation of internalized material and a 1.5- to 2-fold rise in the surface-bound ligand. Both lysosomotropic agents also inhibited ANF degradation by 70-80%. Kinetic of the intracellular labeled material was analyzed: within 5-10 min it reaches a maximum level and it decreases rapidly. In presence of monensin the intracellular signal was amplified and the decay was minimized. The intracellular material was found to be mostly bound to a 60 kDa protein. These studies suggest an intracellular degradation of ANF, probably in the lysosomal compartment, following receptor-mediated endocytosis.

Atrial natriuretic factor modulates proximal glomerulotubular balance in anesthetized rats

The extent to which the natriuretic effect of a prolonged low dose infusion of atrial natriuretic factor (30 ng/kg/min) is dependent on interference with the prevailing intrarenal actions of angiotensin II was examined before and after blockade of angiotensin production with the converting enzyme inhibitor enalaprilat (5 mg/kg). Lithium clearance was used to assess proximal tubular sodium and water reabsorption. Atrial natriuretic factor and enalaprilat caused similar increases in sodium excretion (10-fold and sevenfold, respectively) and glomerular filtration rate (each 34%) and similar decreases in fractional proximal reabsorption of sodium (17% and 13%, respectively) and blood pressure. Each also caused a major disruption in the effectiveness of proximal glomerulotubular balance (30% and 50% of perfect balance). Infusion of atrial natriuretic factor during converting enzyme inhibition increased glomerular filtration rate further by 23%, reaching 63% above control without change in renal blood flow but with a rise in filtration fraction to 0.48. Sodium excretion increased further but fractional proximal sodium reabsorption remained constant and proximal glomerulotubular balance appeared to improve. Atrial natriuretic factor therefore possesses a glomerular action that persists during converting enzyme inhibition and is indeed additive to the removal of angiotensin II when the proximal effect of atrial natriuretic factor is no longer apparent. It is concluded that failure of atrial natriuretic factor to further suppress fractional proximal sodium reabsorption during converting enzyme inhibition is caused by either prior removal of the stimulatory action of angiotensin II on proximal tubular transport or extreme changes in peritubular physical factors consequent on the high filtration fraction.

Does the olfactory system mediate water- and mineral-regulating mechanisms? Evidence of immunoreactive atrial natriuretic factor within olfactory mucosa

The immunoreactivity of atrial natriuretic factor (ANF) was studied in the rat olfactory mucosa (OM). Endogenous immunoreactive ANF (IR-ANF) was purified from OM using Vycor glass beads for extraction and reverse-phase HPLC: two of three IR-ANF peaks, identified by retention time, were identical to both the circulating form of ANF (Ser99-Tyr126) and the ANF pro-hormone (Asn1-Tyr126). A radioreceptor assay, employing rat renal glomerular membranes, revealed that endogenous IR-ANF competed with radiolabelled ANF. IR-ANF was localized by immunocytochemistry in secretory cells of Bowman's gland and in some cells of the epithelial layer. The relatively low concentration of IR-ANF in the OM (2.5 ng/mg protein) suggests a local role of ANF in this tissue. This hypothesis is supported by the presence in OM of ANF-binding sites, characterized by a KD of 95 pM and a Bmax of 130 fmol/mg protein. We propose that ANF could be released from the OM and act throughout in a paracrine (if not autocrine) manner on some yet-unidentified targets containing ANF-binding sites.

Linear alpha-human atrial natriuretic peptide analogs display receptor binding activity and inhibit alpha-hANP-induced cGMP accumulation

We have synthesized a series of [Cys(R)7,23]alpha-hANP analogs, in which the two Cys residues were modified with various alkyl groups(R); i.e., R=Acm, Pe, Qe, Cam, Me, Ae, Bzl, Cm, Ocam and sulfo. The Acm-, Cam-, and Me-analogs exhibited binding activity as potent as alpha-hANP in rat vascular smooth muscle cells (VSMC). Binding activity of the analogs decreased progressively as the bulkiness of the R group increased. None of the analogs caused accumulation of cGMP in VSMC and vasorelaxant activity in rat aorta. Acm-, Cam- and Me-analogs substantially antagonized alpha-hANP-induced cGMP accumulation, but did not antagonize vasorelaxation induced by alpha-hANP in vitro.

Purification and properties of active atrial-natriuretic-peptide receptor (type C) from bovine lung

Atrial-natriuretic-peptide (ANP) receptor, previously identified as a 140 kDa protein with a disulphide-linked homodimeric structure, was purified from bovine lung by (NH4)2SO4 fractionation and affinity chromatography on ANP-Affi-Gel 10. The purified receptor had a binding capacity of 4.2 nmol of ANP/mg of protein and an affinity constant of 6.5 pM. The isoelectric point of the receptor was 5.8, consistent with the acidic nature of the protein (amino acid analysis revealed a predominance of glutamic acid and aspartic acid residues). Treatment with endoglycosidase H and glycopeptidase F revealed that the receptor has three complex types of oligosaccharide chains per 70 kDa subunit. Deglycosylation of the receptor did not affect its binding activity. Reduction with dithiothreitol and reoxidation by dialysis revealed a strong tendency of the receptor subunits to dimerize via disulphide cross-linking; however, carboxymethylation of the reduced receptor indicated that the intersubunit disulphide bond is not necessary for the ligand-binding activity.

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.

HeLa cells contain the atrial natriuretic peptide receptor with guanylate cyclase activity

Receptors for atrial natriuretic peptide (ANP) are heterogeneous: an approximately 140-kDa receptor exhibits ANP-stimulated guanylate cyclase activity whereas an approximately 65-kDa receptor is thought to act only as a clearance-storage protein. We have used photoaffinity labeling techniques to show that the human cell line, HeLa, contains predominantly the approximately 140-kDa ANP receptor. In contrast, several other cell lines contain primarily the approximately 65-kDa receptor. In HeLa cells, ANP bound specifically to high affinity binding sites (Kd approximately 2 nM) and stimulated a rapid, dose-dependent accumulation of cGMP. These cell lines can thus provide useful models to study the multiple mechanisms of ANP action.

Competitive peptide antagonists of ANF-induced cyclic guanosine monophosphate production

Atrial natriuretic factor (isoleucine ANF 101-126), cleaved ANF (isoleucine ANF 101-105/106-126) and des (Gln 18, Ser 19, Gly 20, Leu 21, Gly 22) ANF 4-23-NH2 (C-ANF 4-23) stimulated cyclic guanosine monophosphate production (cGMP) by rat aortic vascular smooth muscle cells (VSMC) in culture. Cleaved ANF and ANF C4-23 also antagonised or diminished the response to ANF 101-126. Agonist and antagonist actions of both peptides were dose-related. In contrast, prepro ANF (104-123), an ANF precursor fragment, exhibited no agonist or antagonist effect on cGMP production.

Differential activation by atrial and brain natriuretic peptides of two different receptor guanylate cyclases

Alpha atrial natriuretic peptide (alpha-ANP) and brain natriuretic peptide are homologous polypeptide hormones involved in the regulation of fluid and electrolyte homeostasis. These two natriuretic peptides apparently share common receptors and stimulate the intracellular production of cyclic GMP as a second messenger. Molecular cloning has defined two types of natriuretic peptide receptors: the ANP-C receptor of relative molecular mass (Mr) 60-70,000 (60-70 K), which is not coupled to cGMP production and may function in the clearance of ANP and the ANP-A receptor of Mr 120-140 K, which is a membrane form of guanylate cyclase in which ligand binding to the extracellular domain activates the cytoplasmic domain of the enzyme. Here we report the cloning and expression of a second human natriuretic peptide-receptor guanylate cyclase, the ANP-B receptor. The ANP-B receptor is preferentially activated by porcine brain natriuretic peptide rather than human alpha-ANP, whereas the ANP-A receptor responds similarly to both natriuretic peptides. These observations may have important implications for our understanding of the central and peripheral control of cardiovascular homeostasis.

cGMP-Producing, Atrial Natriuretic Factor-Responding Cells in the Rat Brain

Using an in vitro incubation method, we stimulated cGMP production in rat brain slices by rat ANF-(103 - 126). The localization of the cells responding to this ANF stimulation with an increase in cGMP production was studied by cGMP immunocytochemistry. ANF-responding cells were found in specific loci throughout the central nervous system of the rat. Regions containing the highest number of these cells were: the olfactory bulb, the lateral septum, the bed nucleus of the accessory olfactory tract, the mediobasal amygdala, the central grey area, the medial vestibular nucleus, and the nucleus of the solitary tract. Scattered ANF-responding, cGMP-immunoreactive cells were found in the hippocampus, the cingulate cortex, the ventral pallidum, the medial preoptic area, and the endopeduncular nucleus. ANF-responding cells in these areas had the same morphology, that is, multipolar with numerous processes. The nature of these ANF-responding cells was studied by sequential staining with an antiserum against glial fibrillary acidic protein (GFAP). In the hippocampus it was demonstrated that all ANF-responding cells are astroglial cells. However, not all astroglial cells in this area showed a cGMP response, demonstrating a regional heterogeneity. ANF-responding cells, having the appearance of neuronal cell bodies, could be found in the subfornical organ, and the hypothalamic paraventricular nucleus. Fibres producing cGMP immunoreactivity in response to ANF were found in the median preoptic nucleus, the medial preoptic area, and the dorsal hypothalamus.

Harnessing the therapeutic potential of atrial natriuretic peptide

The biological actions of Atrial Natriuretic Peptide (ANP) make it potentially useful in the treatment of hypertension and heart failure. We review here the physiology of ANP, the effects of infusion in heart failure and hypertension and preliminary data suggesting that inhibition of endopeptidase 24.11, the enzyme degrading ANP, is an effective mechanism of raising circulating levels of endogenous ANP. Due to the rate of progress in this field we have restricted ourselves to recent work much of which is still available only in abstract form. For more complete accounts the reader is referred to recent reviews [9, 11, 14].

Enkephalinase inhibition increases plasma atrial natriuretic peptide levels, glomerular filtration rate, and urinary sodium excretion in rats with reduced renal mass

To investigate the in vivo effects of inhibition of endopeptidase 24.11, an enkephalinase enzyme shown to be involved in atrial natriuretic peptide (ANP) breakdown in vitro, we infused phosphoramidon, a specific inhibitor of endopeptidase 24.11, into rats with reduced renal mass (and chronic extracellular volume expansion) and into normal rats. Relative to baseline values in rats with remnant kidneys, phosphoramidon led to elevations of plasma ANP levels and concomitant increases in urinary sodium excretion, fractional excretion of sodium, glomerular filtration rate, filtration fraction, and urinary cyclic GMP excretion. Similar changes in renal function and urinary cyclic GMP excretion were obtained with thiorphan, another endopeptidase 24.11 inhibitor. These enhanced ANP levels and renal actions were not observed with phosphoramidon in normal rats. These results show that plasma ANP levels can be modulated in rats with reduced renal mass by inhibition of endopeptidase 24.11.