Two distinct forms of receptors for atrial natriuretic factor in bovine adrenocortical cells. Purification, ligand binding, and peptide mapping

Characterization of ATP-stimulated guanylate cyclase activation in rat lung membranes

Many of the effects of ANP are mediated through the elevation of cellular cGMP levels by the activation of particulate guanylate cyclase. While the stimulation of this enzyme is receptor-mediated, the molecular mechanism of activation remains unknown. In this study we present evidence that ATP as well as its analogues adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S) and adenylylimidophosphate (AMPPNP) activates guanylate cyclase from rat lung membranes and markedly potentiates the effect of ANP on the enzyme. The order of potency is ATP gamma S greater than ATP greater than AMPPNP. The enzyme activation by adenine nucleotide and ANP together is much more than the sum of the individual activations, suggesting that ATP may be the physiological component essential for the ANP-stimulated guanylate cyclase activation. The ATP gamma S-stimulated guanylate cyclase activity diminishes in the presence of various kinds of detergents, suggesting either that the conformation of an ATP binding site in guanylate cyclase is altered by detergents or that protein-protein interaction may be involved in the activation of guanylate cyclase by ATP. Guanylate cyclase from rat lung membranes is poorly activated by ANP and/or ATP gamma S after removing the cytosolic and weakly membrane-associated proteins or factors by centrifugation. Pre-incubation of the membranes with ATP gamma S retains enzyme activation after membrane washing. These results suggest either that ATP gamma S stabilizes the conformation of nucleotide binding site in guanylate cyclase from denaturation by membrane washing, or that the stimulatory effect of ATP on guanylate cyclase activity may be mediated by accessory proteins or non-protein cofactors which are lost during membrane washing, but remain bound to membranes by ATP gamma S pretreatment.

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.

Native form of endothelin receptor in human placental membranes

Little is known about the native form of endothelin receptor. To clarify its functional and structural properties, we solubilized the receptor from human placenta in an active form using mild detergents CHAPS and digitonin and showed that it is able to bind 125I-endothelin-1 in a specific manner, with a pH optimum between 6 and 8 in contrast to a reported pH optimum of 4. The molecular weight of the receptor was estimated as 340,000 by gel filtration of the solubilized membrane in the presence of 0.2% (w/v) digitonin. When the solubilized membranes were labeled with 125I-endothelin-1 prior to gel filtration, the radioactive ligand also migrated in the position corresponding to a 340 kDa protein. These results indicated that the native form of endothelin receptor in human placenta is a 340 kDa protein.

Regional distribution of endothelin receptor in porcine cardiovascular tissues

To clarify effecting sites of endothelin (ET) in a circulation system, we have identified specific receptors for porcine ET(ET-1) and investigated the distribution in the porcine cardiovascular tissues. Scatchard analysis of 125I-porcine ET binding indicated the presence of a single class of high-affinity binding sites. The binding was highly specific for ET-1, because (1) none of the other various peptides or Ca2+-channel antagonists affected the binding, (2) the scission of disulfide bonds, the digestion of the C-terminal 6-amino acid residues, or nitrophenylsulfenylization of the C-terminal Trp21 of ET-1 markedly reduced the binding ability and, (3) ET-1 showed the highest affinity for the vascular receptor among three ET isopeptides. Cardiac atria possessed the highest density (2.7 pmol/mg protein) of ET receptors of all the tissues examined, including thoracic aorta, cardiac atria and cardiac ventriculi, basilar, renal, coronary and pulmonary branch arteries, coronary, renal and jugular veins, and small vessels of pia mater encephali. Small vessels, renal and coronary arteries also showed relatively high density (0.8-1.4 pmol/mg protein). Various veins examined also showed considerable density (0.45-0.74 pmol/mg protein). The apparent Kd of cardiac ET receptors (0.76 nM) was significantly greater than that of the receptors of the other tissue (0.06-0.14 nM). The extensive distribution and the local enrichment of ET receptor in a cardiovascular system strongly suggests that ET is one of the essential endogenous substances to control the tone of the vasculature.

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.

Regulation of particulate guanylate cyclase by atriopeptins: relation between peptide structure, receptor binding, and enzyme kinetics

Structural analogs of atriopeptins (APs) were compared for their ability to activate particulate guanylate cyclase and bind to specific receptors in rat adrenal membranes. All analogs tested increase Vmax without altering the concentration of substrate required for half-maximum activity or the positive coperativity exhibited by the enzyme. Maximum velocities (pmoles of cGMP produced per min per mg protein) achieved in the absence and presence of APs were 128.3 +/- 6.6 and 283.8 +/- 20.6 using Mn2+-GTP, and 53.7 +/- 3.7 and 149.9 +/- 7.6 using Mg2+-GTP as the substrate, respectively. Although all APs were equally efficacious in activating the enzyme, their rank potency was ANF (8-33) = AP III = AP II greater than AP I when either divalent cation was used as the cofactor. The EC50 for activation of guanylate cyclase by AP I was about 10(-7) M, while that for the other peptides was about 10(-8) M, using either divalent cation cofactor. 125I-labeled ANF bound to rat adrenal membranes with a KD of 5.10(-10) M. Although all APs were equally efficacious in competing with labeled ANF for receptor binding, their rank potency was identical to that for enzyme activation. The Ki for AP I was about 10(-8) M, while that for the other peptides was about 10(-10) M. These data suggest that the carboxy terminal Phe-Arg present in the AP analogs except AP I and critical for biological and receptor-binding activity are also important in coupling receptor-ligand interaction with guanylate cyclase activation. The correlation between the rank order potency for receptor binding, enzyme activation, and the reported physiological actions of APs support the suggestion of a functional coupling between these proteins.

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.

Production and characterization of monoclonal antibodies against particulate guanylate cyclase in porcine kidney

Three monoclonal antibodies (Ig G1 type) to particulate guanylate cyclase from porcine kidney cortex have been produced by fusing spleen cells from immunized BALB/c mouse with P3X63 myeloma cells. The antibodies were detected by their ability to bind immobilized antigen and by immunoprecipitation of enzyme activity. After subcloning by limiting dilution, hybridomas were injected intraperitoneally into mice to produce ascitic fluid. The antibodies recognized a 180,000 dalton protein in Lubrol-PX extract of porcine kidney cortex membrane, and when immobilized on Sepharose 4B, they co-precipitated both [125I]human atrial natriuretic peptide (ANP)-receptor complex and guanylate cyclase activity. The antibodies caused a greater increase in generation of cGMP than that of ANP.

Solubilization and identification of human placental endothelin receptor

Endothelin-1 (ET-1) receptor was identified on the membranes from human placenta and 66% of original binding activity in the membranes was solubilized with 0.75% (w/v) CHAPS. Binding studies of the solubilized membranes using 125I-ET-1 indicated the presence of a single class of high-affinity binding sites with an apparent Kd of 760 pM and a Bmax of 1.8 pmol/mg of protein. The binding was inhibited by addition of unlabeled ET-1 and ET-3 in dose dependent manner. The Ki values of solubilized membranes were 84 pM for ET-1 and 250 pM for ET-3, whereas particulate membranes had weaker affinities (Ki = 410 pM for ET-1, 2500 pM for ET-3). Calcium channel blockers such as nicardipine, verapamil and diltiazem did not affect the binding of 125I-ET-1. Affinity labeling of the particulate and solubilized membranes with CHAPS revealed a specific binding protein with a Mr of 32,000.

Stimulation of protein phosphorylation by atrial natriuretic factor in plasma membranes of bovine adrenal cortical cells

Atrial natriuretic factor (ANF) rapidly enhanced phosphorylation of plasma membrane proteins of bovine adrenal cortical cells. Pretreatment of the membranes with ANF (1 x 10(-8)M to 1 x 10(-7)M) resulted two- to four-fold in an incorporation of 32p-radioactivity from [gamma -32p]ATP as compared to the controls. The guanosine 3', 5' monophosphate (cGMP) which has been considered a second messenger of ANF also enhanced the phosphorylation of several proteins which were stimulated by ANF. However, the phosphorylation of certain proteins was stimulated differentially only by either ANF or cGMP. These results suggest that ANF-induced protein phosphorylation may play a role in transmembrane signalling pathway involving other second messenger(s) in addition to cGMP during the biological action of ANF.

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.

Dopamine D1 receptors of the calf parathyroid gland: identification of a ligand binding subunit with lower apparent molecular weight but similar primary structure to neuronal D1-receptors

The ligand binding subunit of the calf parathyroid D1 dopamine receptor was visualized by autoradiography following photoaffinity labeling with (+/-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4'-azidophenyl)-2,3,4,5- tetrahydro-1H-benzazepine ([125I]IMAB) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein comprising the D1 binding subunit migrated with an apparent Mr approximately equal to 62,000. Photoincorporation of [125I]IMAB into the Mr approximately equal to 62,000 polypeptide required the presence of protease inhibitors and was stereoselectively antagonized by dopaminergic agonists and antagonists with an appropriate pharmacological specificity for D1 receptors. The electrophoretic mobility of the [125I]IMAB-labeled receptor was not altered by the absence or presence of urea or thiol-reducing/oxidizing reagents. The Mr approximately equal to 62,000 protein representing the ligand binding subunit of bovine parathyroid D1 receptors corresponds to one of three D1 receptor binding subunits (Mr = 74,000, 62,000, and 51,000) identified in bovine brain. Peptide map comparisons of radiolabeled D1 receptors from calf parathyroid and brain following limited proteolytic digestion with Staphylococcus aureus V8 and papain revealed marked structural similarities. These data suggest that, despite tissue-specific differences in overall molecular weight, both parathyroid and neuronal D1 dopamine binding subunits appear to be pharmacologically and structurally homologous and that the molecular mechanism(s) responsible for the apparent lack of a one to one correspondence in the subunit composition of the D1 receptor in these tissues probably reflect(s) tissue-specific posttranslational modifications.

Identification of two types of specific endothelin receptors in rat mesangial cell

Two types of receptors specific for endothelin were identified using cross-linking technique in cultured rat mesangial cells. The molecular weights of these receptors were approximately 58,000 and 34,000 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The binding of radioiodinated-endothelin to its receptors was inhibited by excess of unlabeled endothelin, but not by nifedipine, nicardipine, verapamil, diltiazem, angiotensin II or [Arg8]-vasopressin. The endothelin binding proteins were solubilized with 1% digitonin and fractionated under non-denaturing conditions by gel filtration. Two endothelin binding peaks eluted at the positions corresponding to the molecular weights of 65,000 and 43,000. These observations indicate that there are two types of specific endothelin receptors in rat mesangial cells which are distinct from voltage dependent L-type calcium channel.

Production of a monoclonal antibody against C terminus of atrial natriuretic factor by in vitro immunization

Spleen cells, sensitized in vitro with ANF-KLH conjugate, were fused with a non-producing mouse myeloma cell line, X63-Ag8.653. One of the isolated hybridomas secreted a monoclonal antibody which exhibited an association constant of 7.25 X 10(-9) M for ANF and recognized an epitope at the C-terminal of the synthetic rat ANF(101-126). Cross reactivity experiments with Auriculin B, Atriopeptin I and Atriopeptin II suggests Phe124-Arg125-Tyr126 in the epitope recognized by this antibody. The epitope appears to be similar to the portion of the hormone molecule essential for ANF binding to the B-receptor (high m.wt. receptor). This antibody may be useful for the generation of anti-idiotypic antibodies for the B-receptors.

Effects of a bolus dose of atrial natriuretic factor in young and elderly volunteers

We assessed the haemodynamic and renal effects as well as the effects on plasma cGMP levels of a small i.v. dose (33 micrograms) of human atrial natriuretic factor (99-126; hANF) in two age groups of healthy volunteers. Binding properties of platelet ANF receptors were also measured. The elderly (four males, eight females, mean age 52.3 years) showed increased haemodynamic (decrease in blood pressure) and renal responses (diuresis, natriuresis, calciuresis) as well as greater increases in plasma cGMP levels and urinary cGMP excretion than the young subjects (four males, 12 females, mean age 26 years). Binding capacities and affinities of platelet ANF receptors were identical in both groups. These data indicate that the sensitivity to ANF increases with age and that this increased sensitivity is reflected in the reactivity of plasma cGMP levels but not in the properties of platelet ANF receptors. The data may be important for the therapeutic use of ANF, for the understanding of the physiological regulation of ANF action and may underline the necessity of using age-matched control subjects for clinical studies on the possible therapeutic effectiveness of ANF.

Opioid receptors in magnesium-digitonin-solubilized rat brain membranes are tightly coupled to a pertussis toxin-sensitive guanine nucleotide-binding protein

Opioid receptors solubilized in Mg2+-digitonin (2%, wt/vol) from Mg2+-pretreated rat brain membranes maintain, in addition to high-affinity opioid agonist binding, the modulation by guanine nucleotides. One of the modes of expression of the latter property is an attenuation of agonist binding by guanine nucleotides in the presence of Na+. To investigate the molecular basis of this modulation and to identify the G protein(s) involved, the soluble receptors were [32P]ADP-ribosylated by means of Bordetella pertussis toxin and subjected to molecular size exclusion chromatography. In addition, soluble extracts were chromatographed on lectin and hydrophobic affinity columns. The binding of 35S- and 3H-labelled analogues of GTP was also monitored in the species separated. The oligomeric G protein-coupled opioid receptors and the guanine nucleotide/pertussis toxin-sensitive species showed similar chromatographic properties in all three systems. This indicates that the biochemically functional G protein-opioid receptor complex formed in Mg2+-pretreated membranes in the absence of an agonist is stable in digitonin solution and to chromatographic separation. Further analysis showed that the guanine nucleotide modulation of opioid receptors is via the pertussis toxin substrates with Mr of 41,000 and 39,000, which are identified as Gi and Go alpha subunits, respectively.

A membrane form of guanylate cyclase is an atrial natriuretic peptide receptor

Atrial natriuretic peptide (ANP) is a polypeptide hormone whose effects include the induction of diuresis, natriuresis and vasorelaxation. One of the earliest events following binding of ANP to receptors on target cells is an increase in cyclic GMP concentration, indicating that this nucleotide might act as a mediator of the physiological effects of the hormone. Guanylate cyclase exists in at least two different molecular forms: a soluble haem-containing enzyme consisting of two subunits and a non-haem-containing transmembrane protein having a single subunit. It is the membrane form of guanylate cyclase that is activated following binding of ANP to target cells. We report here the isolation, sequence and expression of a complementary DNA clone encoding the membrane form of guanylate cyclase from rat brain. Transfection of this cDNA into cultured mammalian cells results in expression of guanylate cyclase activity and ANP-binding activity. The ANP receptor/guanylate cyclase represents a new class of mammalian cell-surface receptors which contain an extracellular ligand-binding domain and an intracellular guanylate cyclase catalytic domain.

Biochemical studies of soluble atrial natriuretic peptide (ANP) receptors from rat olfactory bulb and vascular smooth muscle cells

1. Aim. The biochemical characteristics of atrial natriuretic peptide receptors (ANP-R) derived from rat vascular smooth muscle (A-10 cell line) and central nervous system (CNS; olfactory bulb) tissue were compared. 2. Method and Results. ANP-Rs from each source were solubilized with 40 to 65% efficiency utilizing the nonionic detergent Lubrol-PX. Upon solubilization, the ANP-R from each source maintained the ability to bind 125I-ANP (99-126) with a high affinity; Scatchard analysis indicated that the VSMC ANP-R displayed a Kd for the radioligand of approximately 10 pM, whereas the olfactory receptor possessed a Kd of about 165 pM. The Bmax values for the soluble VSMC and olfactory ANP-Rs were 285 and 30 fmol/mg protein, respectively. Competition binding studies indicated that the VSMC ANP-R bound ANP(99-126), ANP(103-126), and ANP(103-123) with similar affinities, whereas the olfactory ANP-R was much more sensitive to changes in the COOH-terminal structure of the competing peptide. The soluble ANP-Rs from VSMC and olfactory were chromatographically indistinguishable on phenyl-, DEAE-, and wheat germ agglutinin-agarose columns. However, the ANP-Rs could be distinguished using GTP-agarose; the olfactory ANP-R was capable of binding to the resin, whereas the VSMC ANP-R was not. 3. Conclusions. Coupled with other studies, these data suggest that the A10 VSMC ANP-R observed in this study may not be coupled to guanylate cyclase and may represent a receptor serving a clearance function, whereas a significant proportion of the olfactory CNS ANP-R appears to be associated with GTP-binding proteins, likely particulate guanylate cyclase, and probably represents a coupled form of the receptor.

Molecular basis of signalling in the spermatozoon

The spermatozoon contains cell surface receptors for various egg-associated molecules, one of which has now been identified as the enzyme guanylate cyclase. A single membrane-spanning region divides the enzyme such that about one-half is extracellular and one-half is intracellular. A new paradigm for signal/transduction is established by these observations, in that a hormone or effector molecule binding to an extracellular site activates the catalytic domain of the same protein, resulting in the increased formation of a low molecular weight second messenger. Research on signalling mechanisms in the spermatozoon clearly apply to a diversity of cellular processes as well as to fertilization.

Atrial natriuretic factor inhibits angiotensin-induced aldosterone secretion: not through cGMP or interference with phospholipase C

ANF did not prevent the formation of [3H] inositol trisphosphate in response to AII but inhibited aldosterone secretion in calf adrenal glomerulosa cells. 8-bromo cGMP did not affect either inositol phosphate formation or aldosterone secretion. Changes in cytosolic Ca++ concentration induced by AII, as measured by Quin 2 fluorescence, were also unaffected by ANF. No difference in adrenal cell protein phosphorylation with AII or AII + ANF was observed. The results suggest that ANF may inhibit aldosterone secretion through a non-guanyl cyclase linked receptor system not involving the formation of phosphoinositide-derived second messengers. Interference with protein kinase C activity cannot be ruled out.

Mechanism of activation of particulate guanylate cyclase by atrial natriuretic peptide as deduced from radiation inactivation analysis

The interaction between the receptor (Rc) for atrial natriuretic peptide (ANP) and the effector enzyme particulate guanylate cyclase (GC) has been studied by radiation inactivation. Irradiation of bovine lung membranes produced an increase in GC activity at low radiation doses followed by a dose-dependent reduction at higher doses. This deviation from linearity in the inactivation curve disappeared when lung membranes were pretreated with ANP. Essentially identical results were also obtained with adrenal membranes. Based on these radiation inactivation data, the following dissociative mechanism of activation of particulate guanylate cyclase by ANP has been proposed: Rc.GC(inactive) + ANP----Rc.ANP + GC(active).

Cyclic nucleotide-mediated effects of ANF and VIP on flounder intestinal ion transport

The purpose of this study was to compare the effects of atriopeptin III (AP-III), vasoactive intestinal peptide (VIP), and ionomycin on Na and Cl influx and to correlate changes in transport with effects on intracellular adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 5'-cyclic monophosphate (cGMP) content of the tissue. In addition, the question of whether AP-III inhibits ion transport directly by acting on enterocyte receptors for AP-III or indirectly by stimulation of enteric nerves in the submucosa was also addressed. The results showed that AP-III, ionomycin, and bumetanide all inhibited the initial rate of Na and Cl influx, suggesting that they directly block Na-K-2Cl cotransport activity. VIP had no effect on unidirectional influx of Na and Cl. AP-III caused a fourfold increase in intracellular [cGMP] without any significant effect on [cAMP]. VIP stimulated [cAMP] by fourfold but had no effect on [cAMP]. Ionomycin had no effect on either [cAMP] or [cGMP]. Inhibition of transport by AP-III could not be blocked by tetrodotoxin (TTX), indicating that enteric nerves in the submucosa are not directly involved in mediating the effects of AP-III on epithelial ion transport. The observation that two classes of neuronal depolarizing agents (veratrine and scorpion venom) cause TTX-sensitive inhibition of basal ion transport establishes that NaCl absorption in flounder intestine is subject to regulation by enteric nerves located in the submucosa.

Porcine brain natriuretic peptide receptor in bovine adrenal cortex

The action of porcine brain natriuretic peptide (pBNP) on the steroidogenesis was investigated in cultured bovine adrenocortical cells. Porcine BNP induced a significant dose-dependent inhibition of both ACTH- and A II-stimulated aldosterone secretion. 10(-8) M and 10(-7) M pBNP also significantly inhibited ACTH-stimulated cortisol and dehydroepiandrosterone (DHEA) secretions. Binding studies of [125I]-pBNP to bovine adrenocortical membrane fractions showed that adrenal cortex had high-affinity and low-capacity pBNP binding sites, with a dissociation constant (Kd) of 1.70 x 10(-10) M and a maximal binding capacity (Bmax) of 19.9 fmol/mg protein. Finally, the 135 Kd radioactive band was specially visualized in the affinity labeling of bovine adrenal cortex with disuccinimidyl suberate (DSS). These results suggest that pBNP may have receptor-mediated suppressive actions on bovine adrenal steroidogenesis, similar to that in atrial natriuretic peptide (ANP).

Anti-idiotypic antibody as a probe for ANF receptor

Generation of anti-idiotypic antibodies (anti-Id) is a rapid and new approach to produce anti-receptor antibodies without isolation of the receptor. This report describes the production of polyclonal anti-ANF anti-Id antibodies. These antibodies could inhibit the binding of [125I]-ANF to its receptor on aortic smooth muscle cells. Immunoblot analysis of detergent Chaps-solubilized adrenal gland membranes indicated that these anti-Id antibodies could recognize an Mr 130,000 band under nonreducing condition and an Mr 70,000 band under reducing condition. In addition, these antibodies could slightly increase the production of cyclic GMP in aortic smooth muscle cells.

Atrial dipeptidyl carboxyhydrolase is a zinc-metallo proteinase which possesses tripeptidyl carboxyhydrolase activity

Atrial dipeptidyl carboxyhydrolase readily converts one atrial natriuretic peptide, atriopeptin II (Ser103-Arg125 peptide), to another, atriopeptin I (Ser103-Ser123 peptide), by selective removal of the C-terminal dipeptide, Phe-Arg. The atrial peptides possess natriuretic, diuretic, smooth muscle relaxant, and cardiodynamic properties and their existence has shown the mammalian heart to be an endocrine organ. After inactivating the bovine atrial enzyme with EDTA, activity is restored by the addition of Co+2, Zn+2 and Mn+2 but not by Cu+2, Mg+2, Ca+2, or Cd+2. The enzyme is thus likely to be a zinc-metallo proteinase. In addition to its dipeptidyl activity, the enzyme also displays tripeptidyl carboxyhydrolase activity with atriopeptin III (Ser103-Try126 peptide) as substrate. The hydrolytic products resulting from tripeptidyl cleavage are atriopeptin I and Phe-Arg-Tyr. However, with [mercaptopropionyl105,(D)Ala107]-atriopeptin III-NH2 peptide (a potent agonist of atriopeptin III) as substrate, the enzyme acts exclusively as a tripeptidyl carboxyhydrolase. To examine the basis for this shift in cleavage point, pentapeptides based on the C-terminal sequence of atriopeptin III were prepared; a C-terminal Tyr or Tyr-NH2 residue is not sufficient to cause the change in cleavage point. The amidated pentapeptide is not a substrate but is a competitive inhibitor of hydrolysis of the corresponding free-acid peptide.

Characterization and regulation by protein kinase C of renal glomerular atrial natriuretic peptide receptor-coupled guanylate cyclase

The nature and regulation of atrial natriuretic peptide (ANP)-sensitive guanylate cyclase in rat renal glomerular membranes was examined. By affinity crosslinking techniques, three bands with apparent molecular masses of 180, 130 and 64 kDa were specifically labeled with [125I]ANP. A specific antibody to the 180 kDa membrane guanylate cyclase of rat adrenocortical carcinoma recognized a 180 kDa band on Western blot analysis of solubilized, GTP-affinity purified glomerular membrane proteins. The same antibody completely inhibited ANP-stimulated guanylate cyclase activity in glomerular membrane fractions. Partially purified protein kinase C inhibited ANP-stimulated guanylate cyclase activity in glomerular membrane fractions. It is concluded that a 180 kDa ANP-sensitive guanylate cyclase is present in glomerular membranes, and that this enzyme is inhibited directly by protein kinase C.

Blunted cGMP response to ANF in vascular smooth muscle cells of SHR

Abnormalities in the coupling of atrial natriuretic factor (ANF) receptors with the guanosine 5'-cyclic monophosphate (cGMP) system in vascular smooth muscle cells (VSMCs) may play a role in the pathophysiology of hypertension in the spontaneously hypertensive rat (SHR). This concept was examined in cultured, aortic VSMCs (passages 6-10) from SHR, Wistar-Kyoto (WKY), and American Wistar (Wis) rats. Quiescent VSMCs of the SHR (serum deprived for 24 h) had higher ANF receptor density (Bmax) and lower affinity [i.e., increased equilibrium dissociation constant (Kd)] than cells from normotensive controls. Maximal binding (Bmax) (specific binding sites/cell) values for these cells were SHR 112,855 +/- 6,951, WKY 48,650 +/- 3,607, and Wis 36,122 +/- 2,607 (means +/- SE; P less than 0.001 for SHR vs. both WKY and Wis). The Kd values were (in nM) SHR 1.20 +/- 0.098, WKY 0.657 +/- 0.065, and Wis 0.37 +/- 0.037 (P less than 0.001 for SHR vs. both WKY and Wis). Despite their higher Bmax, VSMCs of the SHR showed a substantially lower maximal stimulation of cGMP accumulation in response to ANF: 987 +/- 29.3, 1,992 +/- 574.2, and 2,019 +/- 273.8 fmol.4 min-1.10(6) cells-1 for SHR, WKY, and Wis, respectively (P less than 0.01 for SHR vs. Wis and P less than 0.02 for SHR vs. WKY). Further experiments demonstrated that the poor response of SHR VSMCs to the ANF was not due to a population of receptors that did not couple to the particulate guanylate cyclase. Such findings demonstrate a dissociation of the cGMP response to ANF from the binding of the hormone to its receptors in VSMCs of the SHR compared with controls. This appears to represent a primary and innate defect in these cells that may contribute to the hypertensive process in the SHR.

Atrial natriuretic factor R1 receptor from bovine adrenal zona glomerulosa: purification, characterization, and modulation by amiloride

The atrial natriuretic factor (ANF) R1 receptor from bovine adrenal zona glomerulosa was solubilized with Triton X-100 and purified 13,000-fold, to apparent homogeneity, by sequential affinity chromatography on ANF-agarose and steric exclusion high-performance liquid chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining of the purified receptor preparation in the absence or presence of dithiothreitol revealed a single protein band of Mr 130,000. Affinity cross-linking of 125I-ANF to the purified receptor resulted in the labeling of the Mr 130,000 band. The purified receptor bound ANF with a specific activity of 6.8 nmol/mg of protein, corresponding to a stoichiometry of 0.9 mol of ANF bound/mol of Mr 130,000 polypeptide. Starting with 500 g of adrenal zona glomerulosa tissue, we obtained more than 500 pmol of purified receptor with an overall yield of 9%. The purified receptor showed a typical ANF-R1 pharmacological specificity similar to that of the membrane-bound receptor. The homogeneous Mr 130,000 receptor protein displayed high guanylate cyclase activity [1.4 mumol of cyclic GMP formed min-1 (mg of protein)-1] which was not stimulated by ANF. This finding supports the notion that the ANF binding and the guanylate cyclase activities are intrinsic components of the same polypeptide. Finally, the purified ANF-R1 receptor retained its sensitivity to modulation by amiloride, suggesting the presence of an allosteric binding site for amiloride on the receptor protein.

High affinity receptors for atrial natriuretic factor in PC12 cells

Specific receptors for atrial natriuretic factor (ANF) are characterized on PC12 cells (rat pheochromocytoma cell line). Radioiodinated synthetic ANF (Rat, 8-33) bound to a single class of high affinity binding sites with the Kd value of 6.7 X 10(-10) M. The Bmax value was 29 fmol/10(5) cells and receptor density was calculated as 194,000 +/- 20,000/cell. Photoaffinity labelling of ANF receptor specifically labelled two protein bands with apparent m.wt of 70,000 and 130,000. When the cells were incubated with the labelled ligand at 37 degrees C the ligand was internalized. The rate of internalization increased in the presence of increased ligand concentration. ANF receptors on PC12 cells are reported for the first time which would provide a unique model for study of ANF-receptor interaction.

Vasorelaxant effects of atriopeptins in human internal mammary artery

The vasorelaxant effects of the atriopeptins, AP-(103-126) and AP-(103-123) were compared in human internal mammary artery (IMA) rings which had been contracted with norepinephrine. AP-(103-126) completely relaxed the IMA with an IC50 within the physiologically relevant range (15 nM). However, relaxation with AP-(103-123) was greatly reduced compared to AP-(103-126), with complete relaxation observed in only half of the vessels tested and only at concentrations of 10-50 microM. These results indicate that the atriopeptin (AP) receptor in human arteries may be more similar to the AP receptor in rabbit rather than rat arteries.

Role of atrial natriuretic peptide in the natriuretic response to central volume expansion induced by head-out water immersion in sodium-retaining cirrhotic subjects

PURPOSE

It is possible that abnormalities in atrial natriuretic peptide may be involved in the pathogenesis of sodium retention in edema states. We performed a study in a group of 12 sodium-retaining cirrhotic subjects to determine the role of this peptide in mediating differences in the natriuretic response to central volume expansion induced by head-out water immersion.

PATIENTS AND METHODS

Each patient was maintained for seven days on a 20-mmol sodium intake, and then studied on both control and immersion days. On each day, measurements of the following were obtained: plasma atrial natriuretic peptide, hematocrit, electrolytes, creatinine, plasma renin activity, serum aldosterone, urinary cyclic guanosine monophosphate (cGMP), blood pressure, and pulse rate.

RESULTS

In six subjects, immersion resulted in a marked natriuresis sufficient to induce negative sodium balance by the third hour, and these subjects were termed "responders." In these six patients, baseline pre-immersion levels of plasma renin activity and serum aldosterone were all below 3 ng/liter/second and 4 nmol/liter, respectively. In the other six subjects, the natriuretic response to immersion was markedly blunted and insufficient to induce negative sodium balance, and these subjects were termed "non-responders." In these subjects, baseline pre-immersion levels of plasma renin activity and aldosterone were all above 3.5 ng/liter/second and 5 nmol/liter, respectively, and were significantly elevated compared with the responders, and compared with the normal range for control subjects consuming the same sodium intake. In both groups of cirrhotic subjects, baseline levels of plasma atrial natriuretic peptide and cGMP excretion were significantly and comparably elevated compared with the normal range for control subjects ingesting the same sodium intake. Despite the marked difference in the natriuretic response to immersion in both responders and non-responders, there was a significant and comparable further elevation of plasma atrial natriuretic peptide and urinary cGMP excretion during immersion, compared with the control day.

CONCLUSION

These results suggest that the relative resistance to the natriuretic action of atrial natriuretic peptide in the non-responders compared with the responders is mediated by anti-natriuretic factors acting at a level parallel with or beyond atrial natriuretic peptide release or coupling to its cGMP-linked receptors.

Membrane guanylate cyclase is a cell-surface receptor with homology to protein kinases

Guanylate cyclase has been strongly implicated as a cell-surface receptor on spermatozoa for a chemotactic peptide, and on various other cells as a receptor for atrial natriuretic peptides. Resact (Cys-Val-Thr-Gly-Ala-Pro-Gly-Cys-Val-Gly-Gly-Gly-Arg-Leu-NH2), the chemotactic peptide released by sea urchin Arbacia punctulata eggs, is specifically crosslinked to A. punctulata spermatozoan guanylate cyclase. After the binding of the peptide the state of guanylate cyclase phosphorylation modulates enzyme activity. We report here that the deduced amino-acid sequence of the spermatozoan membrane form of guanylate cyclase predicts an intrinsic membrane protein of 986 amino acids with an amino-terminal signal sequence. A single transmembrane domain separates the protein into putative extracellular and cytoplasmic-catalytic domains. The cytoplasmic carboxyl-terminal 95 amino acids contain 20% serine, the likely regulatory sites for phosphorylation. Unexpectedly, the enzyme is homologous to the protein kinase family.

[Guanidine nucleotide binding proteins as membrane signal transduction components and regulators of enzymatic effectors]

The vast majority of extracellular signals alters cell function by activating cell surface receptors. The transmembranous signalling process initiated by an activated receptor leads to the generation of an intracellular signal and eventually to a cellular response. In contrast to receptors that are permanently coupled to an enzyme or an ion channel representing the effector, a large number of surface receptors for hormones, neurotransmitters and receptors for exogenous chemical or physical stimuli reversibly interacts with membranous signal transduction components which, in turn, regulate intracellular messenger-generating effectors. The transducer molecules isolated so far form a family of guanine nucleotide-binding proteins (G- or N-proteins). All isolated G-proteins are composed of three different subunits (alpha, beta, gamma). The alpha-subunit, which is specific for the individual G-protein, binds and hydrolyzes GTP and is target of ADP-ribosylating bacterial toxins. Hormone-induced activation of a receptor causes interaction with the alpha-subunit of a G-protein and the exchange of bound GDP with GTP. The GTP-bound form of the alpha-subunit represents the active form of the G-protein, which is capable of stimulating or inhibiting the respective effector. The active state of the alpha-subunit is terminated by its inherent GTPase activity causing hydrolysis of bound GTP. The beta gamma-complexes of G-proteins are structurally very similar and functionally interchangeable; they appear to dissociate from the alpha-subunits during receptor activation of the G-protein. Possible functions of the beta gamma-complex are to anchor the non-activated G-protein in the membrane, to facilitate G-protein-receptor interaction, and to promote the inactive state of the alpha-subunit. G-protein-regulated effectors include enzymes, ion channels and probably transporters. The best studied G-protein-regulated enzyme is the retinal cyclic GMP-phosphodiesterase which is activated by bleached rhodopsin via the tissue-specific G-protein, termed transducin. The ubiquitously occurring membrane-bound adenylate cyclase is under dual control by families of stimulatory and inhibitory receptors, acting via G-proteins called Gs and Gi, respectively. Moreover, the receptor control of phospholipases A2 and C and probably of phospholipase D most likely involves G-proteins which have not yet been identified. Finally, the activity of NADPH oxidase of neutrophils and that of cyclic AMP phosphodiesterases in liver and fat cells may be regulated via G-proteins. Modulations of non-enzymatic effectors are reviewed elsewhere.

ANF receptors: distribution and regulation in central and peripheral tissues

Atrial natriuretic factor is a recently-discovered family of biologically active peptides produced in, stored and secreted by mammalian atria. ANF exerts a wide variety of actions in the periphery as well as within the central nervous system. In general, these actions are directed toward the maintenance of body fluid and electrolyte balance and regulation of arterial blood pressure. In a fashion similar to that of many other hormonal systems, the actions of ANF in various target tissues appear to be mediated by at least one class of specific receptors. However, while the biosynthesis and biological actions of ANF have been extensively investigated, little research has been focused on ANF receptor systems. In this article, we will provide an overview of current literature regarding the distribution and binding characteristics of receptor sites for ANF in peripheral and central target tissues. In addition, we will consider factors involved in the regulation and alteration of ANF receptor sites in various tissues. Finally, a brief discussion of the emerging concept of ANF and angiotensin II as mutual antagonists in body fluid homeostasis and cardiovascular regulation will be offered.

Multiple forms of atrial natriuretic factor receptor in human placenta

Multiple forms of atrial natriuretic factor receptor have been identified in human placental membranes. Atrial natriuretic factor binds specifically to placental membranes and the binding activity could be solubilized using non ionic detergent, Triton X-100. Binding to the detergent solubilized preparation was inhibited 80% by the addition of 0.5 M sodium chloride. Affinity cross-linking analysis indicated that this binding was associated with a single protein band of molecular weight 170-kDa. On the other hand, if sodium chloride was added together with a chelator, o-phenanthroline, ANF binding to this preparation was stimulated 300%. Binding under these conditions was not to the 170-kDa protein but was associated with a broad band in the region of 100/110-kDa and a minor band at 200-kDa. These observations clearly indicated that in human placental membranes, atrial natriuretic factor binds to distinctly different molecular species depending on the presence or absence of certain ions and chelators. The two types of binding could be conveniently assayed in the presence of each other by elimination or inclusion of sodium chloride and o-phenanthroline in the assay system.