Identification of a receptor for atrial natriuretic factor in rabbit aorta membranes by affinity cross-linking

Binding sites in rabbit aorta membranes for atrial natriuretic factor (ANF) have been specifically and covalently labeled by two methods. In the first, the photoreactive analog of ANF, 125I-azidobenzoyl-ANF, was synthesized and used to photoaffinity label ANF receptors. In the second, 125I-ANF was covalently attached to its binding site by treatment of the 125I-ANF-receptor complex with bifunctional cross-linking agents. Analysis of the labeled proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed that by both methods the same three protein bands were labeled. These bands had apparent molecular masses of 60,000, 70,000, and 120,000 daltons. With the photoaffinity label, half-maximal inhibition of labeling of each of these bands was achieved when approximately 200 pM of unlabeled ANF was included in the binding assay. These results suggest that these three different polypeptides are specific components of ANF receptors in rabbit aorta membranes.     

Regulation of ANF receptor internalization: involvement of extracellular calcium.

Receptor mediated internalization of 125I-ANF (99-126) and the underlying mechanism was studied in PC12 cells. Phosphorylation of PC12 cell plasma membrane proteins at 0 degrees C or 37 degrees C was not altered in presence of ANF (99-126) or c-ANF (4-23). Exposure of cells to phorbol 12-myristate 13-acetate (PMA, 100 ng/ml) did not alter the endocytic rate or extent of 125I-ANF (99-126) internalization. When cells were treated with a combination of PMA and the calcium ionophore A23187, internalization was not stimulated. Incubation with A23187 (10 microM) alone decreased 125I-ANF (99-126) internalization by 22% in Ca2+ containing medium. Cell surface binding increased 10% in the presence of Ca2+ compared to Ca2+ free medium, irrespective of the presence of A23187. Ca2+ appears to play an important role in the binding of ANF to the receptor and initiation of ligand-receptor complex internalization. Activation of protein kinase C or receptor phosphorylation is not an essential step in initiating ANF receptor internalization.     

Identification of the receptor for atrial natriuretic factor on cultured vascular cells

Binding experiments with 125I-atrial natriuretic factor (ANF) followed by covalent attachment with disuccimidyl suberate show that the peptide binds predominantly to a protein of apparent molecular mass of 66,000 daltons on the cell surface of cultured bovine aortic smooth muscle cells. A minor protein species of 180,000 Mr is also visualized after cross-linking. Endothelial cells, however, whose ANF binding parameters differ substantially from smooth muscle cells, also appear to have qualitatively identical 125I-ANF binding proteins. The identity of these putative proteins, as the ANF receptor, is confirmed by findings that covalent attachment of 125I-ANF is saturable, concentration-dependent, and competed by nanomolar concentrations of unlabeled ANF. Furthermore, other peptide hormones such as angiotensin II, glucagon, or insulin are ineffective in competing for 125I-ANF binding and cross-linking to the receptor.     

Comparison of binding and cyclic GMP accumulation by atrial natriuretic peptides in endothelial cells

Rat 125I-labeled atrial natriuretic factor (ANF (8-33)) was used to identify ANF receptors on cultured bovine aortic endothelial cells. Specific binding of 125I-ANF at 37 degrees C to confluent endothelial cells was saturable and of high affinity. Scatchard analysis of the equilibrium binding data indicated that endothelial cells contain a single class of binding sites with a Kd of 0.1 +/- 0.01 nM. This particular clone of endothelial cells had 16000 +/- 1300 receptors per cell. The order of potency for competing with 125I-ANF binding was human atrial natriuretic peptide (hANP) = atrial natriuretic factor (ANF (8-33)) greater than atriopeptin II greater than atriopeptin III greater than atriopeptin. The weakest competitor, atriopeptin I, had a K1 of 0.45 nM, which was only 6-fold higher than the K1 for hANP and ANF (8-33). ANF (8-33) and hANP in the presence of 0.5 mM isobutylmethyl-xanthine produced a 15-20-fold increase in cyclic GMP content at 10 pM and a maximal 500-fold elevation of cyclic GMP at 10 nM. The concentrations required to elicit a half-maximal increase in cyclic GMP for hANP, ANF (8-33), atriopeptin I, atriopeptin II and atriopeptin III were 0.30, 0.35, greater than 500, 4.0 and 5.0 nM, respectively. Although atriopeptin I acted as a partial agonist, it was unable to antagonize the effect of ANF (8-33) on cyclic GMP formation. These findings suggest that endothelial cells have multiple and functionally distinct ANF-binding sites.     

Atrial natriuretic factor receptor on cultured Leydig tumor cells: ligand binding and photoaffinity labeling

Atrial natriuretic factor (ANF) is a peptide hormone discovered recently from the heart atrium that possesses potent natriuretic and vasorelaxant activities. Recently we found that ANF markedly stimulates intracellular cGMP and almost completely inhibits cAMP accumulation in testicular interstitial tumor cells [Pandey, K. N., Kovacs, W. J., & Inagami, T. (1985) Biochem. Biophys. Res. Commun. 133, 800-806]. These actions of ANF suggest the presence of ANF receptors in testicular interstitial cells. In this study, cultured murine Leydig tumor cells have been shown to contain specific binding sites for ANF. Saturation binding studies indicated a single class of binding sites with a Kd of 5 X 10(-9) M at a density of 2 X 10(6) sites/cell. The binding of mono[125I]iodo-ANF (125I-ANF) was competed by unlabeled ANF in a dose-dependent manner. Hormones unrelated to ANF such as angiotensin I, bovine luteinizing hormone, and human chorionic gonadotropin were not able to compete against 125I-ANF. The binding of 125I-ANF was rapid, reaching maximum levels in 15 min at 4 degrees C. At 37 degrees C, the cell-bound 125I label was quickly decreased. Pretreatment of cells with NH4Cl, chloroquine, or NaN3 resulted in significant increases in maximum levels of the cell-bound 125I radioactivity. A photoaffinity reagent for ANF receptor was prepared by reacting ANF with succinimido 4-azidobenzoate, and resultant 4-azidobenzoyl- (AZB-) ANF was purified by high-performance liquid chromatography (HPLC). AZB-ANF was radioiodinated by use of chloramine T and purified again by HPLC.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification and characterization of atrial natriuretic factor receptors in the rat retina

The characteristics of atrial natriuretic factor (ANF) receptors where studied in rat retinal particulate preparations. Specific 125I-ANF binding to retinal particulate preparations was greater than 90% of total binding and saturable at a density (Bmax) of 40 +/- 8 fmol/mg protein with an apparent dissociation constant (Kd) of 6.0 +/- 2.0 pM (n = 3). Apparent equilibrium conditions were established within 30 min. The Kd value of 125I-ANF binding calculated by kinetic analysis was 4.0 pM. The Bmax of 60 +/- 10 fmol/mg protein and the Kd of 5 +/- 2 pM, calculated by competition analysis, were in close agreement with the values obtained from Scatchard plots or kinetic analysis. The 125I-ANF binding to retinal particulate preparations was not inhibited by 1 microM concentration of somatostatin, vasopressin, vasoactive intestinal peptide, adrenocorticotropin, thyrotropin releasing hormone, or leu-enkephalin. The rank order of potency of the unlabelled atrial natriuretic peptides for competing with specific 125I-ANF (101-126) binding sites was rANF (92-126) greater than rANF (101-126) greater than rANF (99-126) greater than rANF (103-126) greater than Tyro-Atriopeptin I greater than hANF (105-126) greater than rANF (1-126). Similar results have been obtained in peripheral tissues and mammalian brain, indicating that central and peripheral ANF-binding sites have somewhat similar structural requirements. Affinity cross-linking of 125I-ANF to retinal particulate preparations resulted in the labelling of two sites of molecular weight 140 and 66 kDa, respectively. This demonstration of specific high-affinity ANF receptors suggests that the peptide may act as a neurotransmitter or neuromodulator in the retina.     

Synthetic atrial natriuretic factor induces release (possibly receptor-mediated) of vasopressin from rat posterior pituitary

The synthetic fragment (Arg 101-Tyr 126) of atrial natriuretic factor (ANF) induces release of arginine vasopressin from the isolated posterior lobe of the rat hypophysis in vitro. At a physiological concentration (3 X 10(-10)M) ANF was three times more effective than 61 mM KCL. In vitro binding studies with 125I-ANF revealed the presence of high affinity receptor sites displaying a pK = 9.9, a Kd = 0.14 nM, a Bmax = 20 fmol/posterior lobe and and IC50 = 200 pM. These results suggest that arginine vasopressin release by synthetic atrial natriuretic factor may be receptor mediated.     

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

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

Association of the atrial natriuretic factor receptor with guanylate cyclase in solubilized rat glomerular membranes

The elution profile of solubilized rat glomerular membranes from a gel filtration column showed two peaks of 125I-ANF (atrial natriuretic factor) binding (367 +/- 21, 156 +/- 12 KDa). Over 85% of the total binding for the extract was in the 367 KDa peak. Guanylate cyclase activity was correlated with 125I-ANF specific binding. ANF activation of guanylate cyclase was also observed. As observed previously with particulate membrane, Scatchard-analysis of ANF binding data with the solubilized extract was consistent with a two-site model. Both affinities (Kd's), 4 pM and 1 nM, are within the range of blood concentrations reported for ANF. These observations suggest that most rat glomerular ANF receptors are large molecular complexes coupled with guanylate cyclase in the 300-350 KDa size range.     

entaaification of Atrial Natriuretic Factor Receptor of Neuroblastoma N4TG1 Cells: Binding Characteristics and Photoaffinity Labeling

We have found specific receptors for atrial natriuretic factor (ANF) in cultured neuroblastoma cells (N4TG1) of peripheral ganglionic origin. Scatchard analysis of the displacement binding revealed noninteracting, single-class binding sites with a KD of 1 X 10(-10) M and a density (Bmax) of 110,000-150,000 sites/cell. The cell-bound 125I-ANF was displaced by unlabeled ANF in a dose-dependent manner. Hormones unrelated to ANF such as angiotensins, adrenocorticotropic hormone, or arginine vasopressin were ineffective in displacing the cell-bound radioactivity. Using azidobenzoyl-125I-ANF as a photoaffinity ligand, an ANF receptor with an apparent Mr of 138,000 was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The addition of unlabeled ANF (1 microM) to the incubation medium completely abolished the labeling of this protein band, but atriopeptin I (1 microM) or angiotensins I, II, and III (each 1 microM) were not effective in inhibiting the affinity labeling. The treatment of the neuroblastoma cells with ANF stimulated intracellular cyclic GMP levels in a dose-dependent manner with an EC50 of 5 nM. ANF (1 X 10(-7) M) stimulated cyclic GMP accumulation in less than 5 min by 30-fold as compared to the controls.     

Identification and characterization of three distinct atrial natriuretic factor receptors. Evidence for tissue-specific heterogeneity of receptor subtypes in vascular smooth muscle, kidney tubular epithelium, and Leydig tumor cells by ligand binding, photoaffinity labeling, and tryptic proteolysis

Three distinct atrial natriuretic factor (ANF) receptors have been identified and characterized from rat thoracic aortic cultured vascular smooth muscle (RTASM) cells, kidney tubular epithelium (MDCK), and Leydig tumor (MA-10) cells. These include 1) a disulfide-linked 140-kDa protein found in RTASM cells, which was reduced by dithiothreitol (DTT) to 70 kDa, 2) a 120-135-kDa single polypeptide protein, specific to MDCK and MA-10 cells whose Mr was not reduced by DTT, and 3) a 66-70-kDa protein prevalent in both RTASM and MDCK cells, which was not reduced by DTT. After incubation of RTASM cells with 4-azidobenzoyl 125I-ANF, labeling of the 140-kDa protein was blocked by both full-length ANF(99-126) and truncated ANF103-123. In contrast, the labeling of the 120-kDa receptor in MDCK cells was blocked only by full-length ANF(99-126). However, labeling of the 68-70-kDa receptor in both RTASM and MDCK cells was blocked by full-length ANF(99-126) and truncated ANF(103-123). Binding of 125I-ANF(99-126) to RTASM and MDCK cells was rapid, specific, and saturable with a Kd of 1.5 x 10(-10) M and binding capacity (Bmax) of 2.1 x 10(5) sites/RTASM cell and Kd 4.5 x 10(-10) M and Bmax 5 x 10(4) sites/MDCK cell, respectively. Binding of 125I-ANF(99-126) to RTASM cells was displaced with both full-length ANF(99-126) and truncated ANF(103-123), however, binding to MDCK cells was efficiently displaced only with full-length ANF. Both ANF(99-126) and ANF(103-123) stimulated cGMP in RTASM cells but only ANF(99-126) elicited cGMP in MDCK cells. Tryptic proteolysis of the high Mr single chain receptor produced only a 68-kDa fragment, whereas disulfide-linked 140-kDa receptor yielded 52-, 38-, 26-, and 14-kDa fragments. These data provide direct biochemical evidence for three distinct ANF receptors which might be linked to diverse physiological functions of ANF such as natriuresis in the kidney, vasorelaxation in vascular smooth muscle, and steroidogenic responsiveness in Leydig cells.     

Atrial natriuretic factor and sodium nitroprusside increase cyclic GMP in cultured rat lung fibroblasts by activating different forms of guanylate cyclase.

We used cultured rat lung fibroblasts to evaluate the role of particulate and soluble guanylate cyclase in the atrial natriuretic factor (ANF)-induced stimulation of cyclic GMP. ANF receptors were identified by binding of 125I-ANF to confluent cells at 37 degrees C. Specific ANF binding was rapid and saturable with increasing concentrations of ANF. The equilibrium dissociation constant (KD) was 0.66 +/- 0.077 nM and the Bmax. was 216 +/- 33 fmol bound/10(6) cells, which corresponds to 130,000 +/- 20,000 sites/cell. The molecular characteristics of ANF binding sites were examined by affinity cross-linking of 125I-ANF to intact cells with disuccinimidyl suberate. ANF specifically labelled two sites with molecular sizes of 66 and 130 kDa, which we have identified in other cultured cells. ANF and sodium nitroprusside produced a time- and concentration-dependent increase in intracellular cyclic GMP. An increase in cyclic GMP by ANF was detected at 1 nM, and at 100 nM an approx. 100-fold increase in cyclic GMP was observed. Nitroprusside stimulated cyclic GMP at 10 nM and at 1 mM a 500-600-fold increase in cyclic GMP occurred. The simultaneous addition of 100 nM-ANF and 10 microM-nitroprusside to cells resulted in cyclic GMP levels that were additive. ANF increased the activity of particulate guanylate cyclase by about 10-fold, but had no effect on soluble guanylate cyclase. In contrast, nitroprusside did not alter the activity of particulate guanylate cyclase, but increased the activity of soluble guanylate cyclase by 17-fold. These results demonstrate that rat lung fibroblasts contain ANF receptors and suggest that the ANF-induced stimulation of cyclic GMP is mediated entirely by particulate guanylate cyclase.     

Does atrial natriuretic factor protect against right ventricular overload? II. Tissue binding

Previous studies have led us to hypothesize that the physiological significance of the diuretic and pulmonary vaso-relaxant effects of atrial natriuretic factor (ANF) is to protect the right heart. This study was designed to evaluate the relative importance of various peripheral tissues as sites of ANF action by tracing the temporal pattern of distribution of 125I-ANF and quantitating the specific binding sites. An in vivo approach, utilizing trace amount of 125I-ANF was adopted to simulate physiological conditions. 125I-ANF injected either intravenously or intra-arterially was quickly bound to peripheral tissues with less than 5% remaining in the circulation after 1 min. The relative binding capacity was greatest in the lung, followed by the kidney, right ventricle, adrenal gland, and left ventricle. The magnitude of specific ANF binding sites per gram of tissue weight followed a similar order. The data demonstrate that ANF released under all circumstances is quickly bound to the target organs, particularly the lung and the kidney, and suggest that these two organs could be the most important target organs of ANF. This evidence provides further support for the proposed hypothesis that a major evolutionary role of ANF is the protection of the right ventricle from mechanical loads.     

Functional heterogeneity of atrial natriuretic factor receptor in bovine adrenal zona glomerulosa is explained by an amiloride-sensitive high affinity molecular complex

The effects of amiloride on the molecular characteristics of the atrial natriuretic factor (ANF) receptor from bovine adrenal zona glomerulosa were studied by computer modeling of competitive binding data, by affinity labeling experiments, and by steric exclusion high performance liquid chromatography of solubilized receptor. The order of potency of a series of truncated ANF analogs in competing for 125I-ANF binding to bovine adrenal zona glomerulosa membranes was the same as that obtained for inhibition of aldosterone secretion. Deletion of amino acids at the COOH-terminal end drastically reduced the affinities of the peptides. Computer analysis of competition curves revealed that all ANF analogs tested show similar binding characteristics: shallow competition curves, discrimination of varying proportions of high and low affinity binding states, and sensitivity to amiloride which increases the proportion of the high affinity binding component. These results from binding studies are suggestive of potential heterogeneity of ANF binding sites. In contrast, results from affinity cross-linking experiments are consistent with the notion of a single receptor protein. Incubation of membranes with increasing concentrations of 125I-ANF-(99-126) up to 3 nM resulted in the labeling of a single band of Mr 130,000. The ability of ANF analogs to compete for the labeling of the Mr 130,000 band by 125I-ANF-(99-126) agreed well with their potency as inhibitors of 125I-ANF binding to intact membranes. Addition of amiloride caused a dose-dependent increase in the labeling of the Mr 130,000 band. A single Mr 130,000 band was also labeled in bovine aorta and LLC-PK1 cell membranes. In order to further investigate the molecular basis for the apparent heterogeneity of ANF binding we have prelabeled the membrane receptor with 125I-ANF-(99-126) prior to solubilization with octyl-beta-D-glucoside and chromatography on a Superose 6 steric exclusion column. The elution profile of the prelabeled receptor consistently showed two peaks of radioactivity with mean Stokes radii of 70 and 50 A. When amiloride was added to the incubation medium, the elution profile consisted almost exclusively of the 70-A peak. Quantitative analysis of the chromatographic profiles revealed that amiloride increases by 2-3 times the area of the 70-A peak. We conclude that the 70-A form represents a ternary complex of the receptor with an amiloride-sensitive effector protein.     

Affinity cross-linking of atrial natriuretic factor to its receptor in bovine adrenal zona glomerulosa

125I-Labeled atrial natriuretic factor (ANF) was covalently cross-linked to its binding sites in bovine adrenal zona glomerulosa membranes using the hydrophilic cross-linker bis(sulfosuccinimidyl) suberate. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence of 2-mercaptoethanol revealed that two protein bands with apparent Mr 68,000 and 114,000 were specifically labeled. The labeling of the two bands was prevented in a dose-dependent fashion by unlabeled ANF with a significant inhibition observed at 10(-10) M. High concentrations of angiotensin II and adrenocorticotropic hormone did not compete with 125I-ANF for binding and cross-linking. The dose-response curve for inhibition of covalent cross-linking of 125I-ANF by unlabeled ANF coincided with the dose-response curve for inhibition of binding to the receptor. No radioactive bands were observed in liver membranes. Experiments in which adrenal membranes were prepared and incubated in the presence of protease inhibitors showed no difference in the labeling pattern. Electrophoresis in the absence of reductant showed that the affinity-labeled species are not derived from larger disulfide-linked components. The apparent molecular weight of the two labeled species was not affected by a 100-fold variation in cross-linker concentration, and the labeling of both species increased in parallel. Possible relationships between the two labeled species are discussed.     

Increase in the number of atrial natriuretic hormone receptors in regenerating rat liver.

Forty-eight hours after partial (approximately 67%) hepatectomy the activity of the particulate guanylate cyclase was increased by 2-fold in the regenerating rat liver. This increase was not an artifact of membrane isolation procedures, and as determined by 125I-labeled Tyr-28 atrial natriuretic hormone-(1-28) ANF binding, was accompanied by a 2-fold increase in the number of ANF receptors. The Kd of the receptors in membranes of regenerating livers was not significantly different from the Kd of the receptors in livers of sham-operated rats. The linear synthetic descysteine analog of ANF, analog I, which binds only to the 66-kDa receptors, displaced approximately 40% of the specifically bound 125I-ANF in liver membranes from both hepatectomized and sham-operated (control) animals. Affinity cross-linking studies with 125I-ANF confirmed the increase in the 116-kDa ANF receptor in membranes of regenerating livers. In perfused livers derived from control and hepatectomized animals, the basal rates of cGMP production were not significantly different. However, atriopeptin II-stimulated cGMP production was twice as great in regenerating livers as compared with controls. These data demonstrate that the increase in particulate guanylate cyclase activity observed during liver regeneration is due to an increase in the 116-kDa ANF receptor-associated activity. Additionally, our data demonstrate that the regenerating rat liver may be a valuable model with which to study the role of the hepatic ANF receptor/particulate guanylate cyclase.     

Dynamics of atrial natriuretic factor-guanylate cyclase receptors and receptor-ligand complexes in cultured glomerular mesangial and renomedullary interstitial cells.

The dynamics of the guanylate cyclase receptor of atrial natriuretic factor (GCA-ANF receptor) were investigated in cultured glomerular mesangial and renomedullary interstitial cells from the rat. In these cells, the GCA-ANF receptor did not mediate internalization and lysosomal hydrolysis of 125I-ANF1-28 and did not undergo ligand-induced endocytosis. Glomerular mesangial cells were able, however, to mediate internalization and lysosomal hydrolysis of 125I-ANF1-28 via clearance ANF (C-ANF) receptors and to promote rapid receptor-mediated internalization and lysosomal hydrolysis of 125I-(Sar1) angiotensin II. Radioligand specifically bound to surface GCA-ANF receptors was rapidly dissociated at 37 degrees C (k(off) greater than 0.8 min-1), with a Q10(30-37 degrees C) greater than 6. The dissociation was markedly slower at subphysiological temperatures (Q10(4-30 degrees C), 2-3) or in the presence of 0.5 mM amiloride. The results demonstrate that the GCA-ANF receptor, contrary to C-ANF receptors and most other polypeptide hormone receptors, is a membrane resident protein that does not mediate internalization and lysosomal hydrolysis of ligand. The termination of the interaction of ANF with GCA-ANF receptors results from a physiological process that leads to rapid dissociation of receptor-ligand complexes. The unique dynamics of GCA-ANF receptor-ligand complexes are likely to contribute importantly to stimulus-response homeostasis of ANF.     

Amiloride potentiates atrial natriuretic factor inhibitory action by increasing receptor binding in bovine adrenal zona glomerulosa

The interaction of atrial natriuretic factor (ANF) with the diuretic amiloride was studied in bovine adrenal zona glomerulosa. Amiloride enhances 2 to 3-fold high affinity binding of [125I] ANF to zona glomerulosa membrane receptor with an ED50 of 10 microM. This effect is due to a recruitement of high affinity receptor sites and to an increase of their affinity from a Kd of 23 to 8 pM. This enhancing effect is almost equipotently elicited by guanabenz, while clonidine is 20-fold less potent and arginine is inactive. ATP reduces by 30 to 50% [125I] ANF binding with an IC50 of 50 microM. Amiloride and ATP opposite effects on [125I] ANF binding are mutually competitive. Low concentrations of amiloride (less than 100 microM) potentiate the inhibitory effect of ANF in hormone-stimulated steroid secretion with a 3-fold decrease in ANF IC50 at 10 microM amiloride. Higher concentrations of amiloride (greater than 100 microM) directly inhibit aldosterone secretion with an IC50 of 500 microM and a maximum of 80 to 100% reversal of stimulation by various secretagogues. These results indicate that amiloride synergistically potentiates ANF inhibitory action by altering ANF receptor binding properties. They also suggest a role for sodium transport and for phosphorylation-dephosphorylation mechanisms in the mode of action of ANF.     

Atrial natriuretic factor (ANF) binding sites in frog kidney and adrenal.

Atrial natriuretic factor (ANF) binding sites were localized and quantified in kidney and adrenal of the frog Rana temporaria by quantitative in vitro autoradiography. [125I]-rat ANF(99-126) binding was present in kidney glomeruli and in the outer layer of interrenal tissue in the adrenal gland. ANF binding exhibited positive cooperativity with a half-maximal binding concentration (EC50) of 102 +/- 16 pM in glomeruli and 93 +/- 19 pM in interrenal tissue (n = 8). The corresponding maximal binding capacities (Bmax) were 1.33 +/- 0.16 and 1.21 +/- 0.36 fmol/mm2. [125I]-Rat ANF(99-126) binding was competitively displaced by unlabeled ANF analogues with an intact disulfide bridge showing a lower affinity than the iodinated ligand. The presence of ANF binding in glomeruli and steroidogenic interrenal cells suggests physiological functions of ANF for the osmomineral regulation in the frog by influencing glomerular filtration rate and adrenal steroid secretion.     

Characterization of specific receptors for atrial natriuretic factor in bovine adrenal zona glomerulosa

We have recently shown that synthetic rat atrial natriuretic factor (ANF) directly inhibits mineralocorticoid and glucocorticoid secretion in cultured bovine adrenal cells with a potency of 100 pM. [125I]iodo-ANF was used in the present study to characterize potential receptor sites in bovine zona glomerulosa membranes. ANF binds to a class of high affinity binding sites with a pK of 10.2 and a density of 1.3 pmol/mg protein. Detailed competition curves with ANF document a class of high affinity sites with a pK of 10.2 and also a second class of lower affinity sites with a pK of 8.5. Nonspecific binding amounts to less than 10% of [125I]iodo-ANF binding at concentrations less than 100 pM. High affinity binding of [125I]iodo-ANF is reversible with a half-time of association of 15 minutes at 25 pM and a half-time of dissociation of 140 minutes. Monovalent cations Na, Li and K equipotently enhance [125I]iodo-ANF specific binding. Divalent cations Mg, Ca and Mn also increase [125I]iodo-ANF specific binding, with Mn being the most active cation. No effect of guanine nucleotide could be detected on ANF binding. The binding of [125I]iodo-ANF is very specific and is not inhibited by 1 microM angiotensin II, ACTH, VIP, somatostatin, Leu-enkephalin, dynorphin or by the N-terminal of POMC. The N-terminal fragment ANF-(1-16) is also completely inactive. Reduction of the disulfide bridge of ANF inactivates the peptide. This enabled the development of a highly specific radio-receptor assay for ANF with a minimum detectable dose of 2 femtomoles. The results document the specific receptor involved in the potent inhibitory effect of ANF on adrenal steroidogenesis and indicate that bovine adrenal zonal glomerulosa provide a highly sensitive system for studying the recently discovered atrial natriuretic factor.