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

Different ATP effects on natriuretic peptide receptor subtypes in LLC-PK1 and NIH-3T3 cells

We have observed different ATP interactions in two guanylate cyclase (GC)-coupled natriuretic peptide (NP) receptor subtypes, designated NPR-A and NPR-B. The NPR-A is selectively expressed by LLC-PK1 epithelial cells and the NPR-B by NIH-3T3 fibroblast cells. In LLC-PK1 membranes, ATP-Mg2+ potentiated ANP-stimulated GC activity (ANP-s-GC). In contrast, in NIH-3T3 membranes, ATP-Mg2+ inhibited ANP-s-GC but enhanced CNP-stimulated GC activity (CNP-s GC). ATP in the presence of Mn2+ inhibited LLC-PK1 and NIH-3T3 membrane ANP-s-GC and CNP-s-GC. These are the first data suggesting that the ATP-Mg2+ produces different effects between membrane NPR-A and -B subtypes. We have also demonstrated that GC of NPR-B is sensitive to methylene blue.

Drugs acting on the intracellular signalling system

Cellular response to extracellular messages is a basic process to maintain and to support cell life. Several signalling molecules important as sites of therapeutic drug action are involved in the response. Recent studies on life sciences have elucidated molecular properties of intracellular signalling factors and mechanisms of cascading. Novel drugs acting on signalling molecules and possessing new sites and mechanisms of action have been found. This article summarizes the properties (subtypes, structures, functions) of signalling factors (receptors, ion channels, GTP binding proteins, second messenger-generating enzymes, second messenger-metabolizing enzymes, second messengers protein kinases, protein phosphatases) and lists in Tables A-H drugs that act on signalling molecules and which should find clinical use.

Proteolytic cleavage of atrial natriuretic factor receptor in bovine adrenal membranes by endogenous metalloendopeptidase. Effects on guanylate cyclase activity and ligand-binding specificity

Atrial natriuretic factor (ANF) is a peptide hormone from the heart atrium with potent natriuretic and vasorelaxant activities. The natriuretic activity of ANF is, in part, mediated through the adrenal gland, where binding of ANF to the 130-kDa ANF receptor causes suppression of aldosterone secretion. Incubation of bovine adrenal membranes at pH < 5.6 caused a rapid and spontaneous cleavage of the 130-kDa ANF receptor, yielding a 65-kDa polypeptide that could be detected by photoaffinity labeling by 125I-labeled N alpha 4-azidobenzoyl-ANF(4-28) followed by SDS/PAGE under reducing conditions. Within 20 min of incubation at pH 4.0, essentially all the 130-kDa receptor was converted to a 65-kDa ANF binding protein. This cleavage reaction was completely inhibited by inclusion of 5 mM EDTA. When SDS/PAGE was carried out under non-reducing conditions, the apparent size of the ANF receptor remained unchanged at 130 kDa, indicating that the 65-kDa ANF-binding fragment was still linked to the remaining part(s) of the receptor polypeptide through a disulfide bond(s). The disappearance of the 130-kDa receptor was accompanied by a parallel decrease in guanylate cyclase activity in the membranes. Inclusion of EDTA in the incubation not only prevented cleavage of the 130-kDa receptor, but also protected guanylate cyclase activity, indicating that proteolysis, but not the physical effects of the acidic pH, causes inactivation of guanylate cyclase. The 130-kDa ANF receptor in adrenal membranes was competitively protected from photoaffinity labeling by ANF(1-28) or ANF(4-28), but not by atriopeptin I [ANF(5-25)] or C-ANF [des-(18-22)-ANF(4-23)-NH2]. On the contrary, the 65-kDa ANF-binding fragment generated after incubation at pH 4.0 was protected from labeling by any of the above peptides, indicating broader binding specificity. After incubation in the presence of EDTA, the 130-kDa ANF receptor, which was protected from proteolysis, retained binding specificity identical to that of the 130-kDa receptor in untreated membranes. The results indicate that the broadening of selectivity is caused by cleavage, but not by the physical effect of acidic pH. Spontaneous proteolysis of ANF receptor by an endogenous metalloendopeptidase, occurring with concomitant inactivation of guanylate cyclase activity and broadening of ligand-binding selectivity, may be responsible for the generation of low-molecular-mass receptors found in the adrenal gland and other target organs of ANF. The proteolytic process may play a role in desensitization or down-regulation of the ANF receptor.

Discovery of a potent atrial natriuretic peptide antagonist for ANPA receptors in the human neuroblastoma NB-OK-1 cell line

The effects of seven competitive atrial natriuretic peptide (ANP) receptor antagonists were compared on cultured human neuroblastoma NB-OK-1 cells expressing exclusively ANPA receptors, by evaluating their capacity to inhibit [125I]ANP binding and to suppress ANP-stimulated cyclic GMP elevation. In ANP analogues with a shortened Cys7-Cys18 bridge, Asp13 and a hydrophobic Tic residue at position 16 expressed antagonistic activity, while Ala16 provoked lower antagonistic potency and Phe16 induced receptor activation. The binding affinity of A71915 ([Arg6, Cha8]ANP-(6-15)-D-Tic-Arg-Cys-NH2), the most potent antagonist (with a pKi of 9.18 and a pA2 of 9.48) was only 22 times less lower than that of the agonist ANP-(1-28).

Pulmonary arterial brain natriuretic peptide concentration and cardiopulmonary hemodynamics during exercise in patients with essential hypertension

Brain natriuretic peptide (BNP) is secreted through the coronary sinus of the human heart. The purpose of this study was to determine whether BNP secretion from the heart is stimulated by exercise and to examine the relationship between pulmonary arterial BNP concentrations and hemodynamic measurements, especially cardiopulmonary hemodynamics, during exercise in patients with essential hypertension. The exercise protocol consisted of three fixed workloads (25, 50, 75 W) on a bicycle ergometer in the supine position. The mean pulmonary arterial BNP level at rest was 14.8 +/- 4.1 pg/mL, and BNP values gradually increased with higher stages of exercise. At the maximum exercise stage, the BNP value increased to 40.9 +/- 6.5 pg/mL. Close correlations of pulmonary arterial pressure (PAP) and pulmonary arterial wedge pressure (PAWP) with pulmonary arterial BNP level were observed at four points at rest and during each stage of exercise. In contrast, heart rate, mean blood pressure, cardiac index (CI), and stroke index (SI) were not correlated with BNP values. Results suggest that cardiac secretion of BNP was increased during exercise in essential hypertensive subjects, and the observed increase of BNP may be related to elevated PAP and PAWP. The enhancement of BNP secretion during exercise in these patients may reflect increased redistribution of blood to the cardiopulmonary compartment.

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

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

HPLC and PDMS analysis of cleavage products aid in the design of small, stable ANP analogues

The degradation of a prototypical small analogue of atrial natriuretic peptide (ANP) has been studied using HPLC and mass spectrometric techniques. These studies revealed that removal of the N-terminal amino acid was the primary catabolic event in vitro. Based on this information the N-terminus was remanufactured to provide a family of more stable analogues. Additional stabilization was provided through modification of the C-terminal tripeptide. Through dramatically more stable in vitro, these new analogues do not appear to have longer in vivo half-lives.

Kinetic analysis of internalization, recycling and redistribution of atrial natriuretic factor-receptor complex in cultured vascular smooth-muscle cells. Ligand-dependent receptor down-regulation

The kinetics of internalization, sequestration and metabolic degradation of atrial natriuretic factor (ANF)-receptor complex were studied in rat thoracic aortic smooth-muscle (RTASM) cells. These parameters were directly determined by measuring 125I-ANF binding to total, intracellular and cell-surface receptors. Pretreatment of cells with the lysosomotropic agent chloroquine and the energy depleter dinitrophenol led to an increase in the intracellular 125I-ANF radioactivity. After 60 min incubation at 37 degrees C, cell-associated 125I-ANF radioactivity fell rapidly in chloroquine-treated cells (> 85%) compared with the controls (< 45%). 125I-ANF radioactivity increased to a peak of 65% of the initial level within 15 min in chloroquine-treated cells compared with only 22% in the control cells. During the initial incubation period at 37 degrees C, chloroquine inhibited the release of both intact and degraded 125I-ANF in a time-dependent manner. However, at later incubation times, the effect of chloroquine was diminished and release of both degraded and intact ligand was resumed. Extracellular unlabelled ANF did not affect the release of degraded 125I-ANF but it accelerated the release of intact ANF by a retroendocytotic mechanism. After the endocytosis, about 30-40% of ANF receptors were restored to the cell surface from the internalized pool of receptors. The restoration was blocked by chloroquine or dinitrophenol but not by cycloheximide. Exposure of RTASM cells to unlabelled ANF resulted in a time- and concentration-dependent loss of ANF receptors. Unlabelled ANF (10 nM) induced a loss of more than 52% of 125I-ANF binding, and a complete loss occurred at micromolar concentrations. It is inferred that ANF-induced down-regulation of its receptor resulted primarily from an increased rate in internalization and metabolic degradation of ligand-receptor complex by receptor-mediated endocytotic mechanisms.

Molecular cloning of a retina-specific membrane guanylyl cyclase

We have isolated and characterized cDNA clones encoding the human retinal guanylyl cyclase (retGC), a novel member of the membrane guanylyl cyclase gene family. Like other membrane guanylyl cyclases, the 1101 aa retGC is predicted to have a hydrophobic amino-terminal signal sequence followed by a large extracellular domain, a single membrane spanning domain, a kinase homology domain, and a guanylyl cyclase catalytic domain. In contrast to other membrane guanylyl cyclases, such as natriuretic peptide receptors, retGC has a relatively high basal level of activity when expressed in human 293 cells. cGMP production by retGC is unaffected by any of the known natriuretic peptides. In situ hybridization analysis of a variety of rhesus monkey tissues showed retGC transcripts to be localized exclusively along the retinal outer nuclear layer, corresponding to the nuclei of the rod and cone photoreceptor cells. Our results suggest that retGC may synthesize cGMP required for recovery of the dark state after phototransduction.

Atrial natriuretic peptide binds to ANP-R1 receptors in neuroblastoma cells or is degraded extracellularly at the Ser-Phe bond

ANP-R1 receptors for atrial natriuretic peptide (ANP) showed the following rank order of affinity in intact human neuroblastoma cells NB-OK-1: human ANP-(99-126) approximately human ANP-(102-126) approximately rat ANP-(99-126) (K1 17-32 pM) > human ANP-(103-126) > porcine brain natriuretic peptide (BNP). Analogues truncated at the C-terminal extremity or devoid of a disulphide bridge, such as rat ANP-(103-123), rat C-ANP-(102-121), rat ANP-(111-126), rat ANP-(99-109) and rat [desCys105,Cys121]ANP-(104-126) and chicken C-type natriuretic peptide, were not recognized. The occupancy of these high affinity ANP-R1 receptors led to marked cyclic GMP accumulation in the presence of 3-isobutyl 1-methylxanthine. An ectoenzymic activity, partly shed in the incubation medium, provoked the stepwise release of Phe-Arg-[125I]Tyr, Arg-[125I]Tyr and [125I]Tyr from rat [125I]ANP-(99-126), at an optimal pH of 7.0. Its inhibition by 1,10-phenanthroline, EDTA and bacitracin but not by thiorphan suggests the contribution of at least one neutral metalloendopeptidase, distinct from EC 3.4.24.11, for which ANP showed high affinity.

Natriuretic peptide receptor mRNAs in the rat and human heart

Functional studies indicate that atrial natriuretic peptide (ANP), a member of the natriuretic peptide family, has direct effects on cardiac muscle cells. However, conventional ligand-binding studies designed to establish the presence of natriuretic peptide-binding sites in the heart have yielded conflicting results. There are discrepancies also between the latter and the receptor distribution predicted from the pattern of the mRNA transcripts localized by in situ hybridization. Here we have employed the technique of cDNA amplification with the polymerase chain reaction to confirm the presence of natriuretic peptide A, B, and C receptor mRNAs in rat and human cardiac tissue. In the rat heart, the distribution of the A and B receptor transcripts appears to be relatively homogeneous; in contrast, the C type mRNA is concentrated principally in the atria, with no difference between the left and right sides of the heart. A and B receptor DNA products were obtained after amplification of left, but not right, ventricular cDNA from the heart of a 16-yr-old male with cystic fibrosis; the yield of C receptor DNA was similar for both ventricles. If these mRNA transcripts are translated into functional receptors in the rat and human heart, ANP and the other natriuretic peptides may have direct effects on cardiac function, including regulation of natriuretic peptide release via a short feedback loop, modulation of contractility of the heart, or activation of cardiac reflexes.

The natriuretic peptides and their receptors

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

Effects of natriuretic peptides and neutral endopeptidase 24.11 inhibition in isolated perfused rat lung

We have studied the acute effect of brain natriuretic peptide (BNP) and atrial natriuretic peptide (ANP) on pulmonary vascular tone in normoxia and acute hypoxia in the absence and presence of a specific inhibitor of neutral endopeptidase 24.11 (NEI, UK 73, 967, candoxatrilat; Pfizer) in the isolated and blood-perfused rat lung preparation. Baseline pulmonary artery pressure (Ppa) was 16.4 +/- 0.3 mm Hg in lungs from normoxic control rats and 22.5 +/- 0.3 mm Hg in lungs from rats kept in hypoxia (FIO2 = 10%) for 7 days. Acute hypoxic pulmonary vasoconstriction (HPV delta Ppa) was similar in normoxic control rats (9.5 +/- 0.6 mm Hg) and chronically hypoxic rats (9.8 +/- 0.9 mm Hg). NEI at 0.07 and 0.2 mg had no effect on baseline Ppa or HPV delta Ppa. Synthetic BNP at 10 nM had no effect on baseline Ppa but produced a 2.8 +/- 0.2 mm Hg reduction in HPV delta Ppa alone and 2.7 +/- 0.2 mm Hg reduction in the presence of 0.07 mg NEI in normoxic control rats. In contrast, ANP at 10 nM produced a significantly greater decrease in HPV delta Ppa in the presence of 0.07 mg NEI (4.8 +/- 0.3 mm Hg, p < 0.05) compared with ANP alone (2.9 +/- 0.4 mm Hg), and similar results were also observed in chronically hypoxic rats. Thus, BNP has a vasodilator effect similar to that of ANP in the pulmonary circulation. Inhibition of neutral endopeptidase 24.11 augments the effects of ANP on HPV but does not influence the pulmonary vascular responses to BNP.

Atrial natriuretic peptide effects in AtT-20 pituitary tumour cells

Whether atrial natriuretic peptide (ANP)-evoked inhibition of corticotrophin-releasing factor (CRF)-stimulated ACTH secretion was also manifest in ACTH secreting AtT-20 pituitary tumour cells was investigated. ANP stimulated increases in cGMP accumulation at concentrations of the peptide above 10(-8) M which indicates the presence of the ANP receptors on these cells. CRF stimulated a concentration-dependent increase in ACTH secretion from AtT-20 cells which was unaffected by ANP, 8-bromo-cGMP, or sodium nitroprusside (SNP). Calcium stimulated a concentration-dependent increase in ACTH secretion from electrically permeabilised cells which was unaffected by co-incubation with cGMP but potentiated by cAMP. These results reveal the presence of ANP receptors on AtT-20 cells but suggest that an incomplete expression of the stimulus-secretion coupling mechanisms for ANP, at some point after cGMP production, prevents the effects of natriuretic peptides upon ACTH secretion being manifest in these cells.

C-type natriuretic peptide inhibits growth factor-dependent DNA synthesis in smooth muscle cells

We have examined the ability of C-type natriuretic peptide (CNP) to interact with guanylate cyclase-coupled natriuretic peptide receptors by measuring its ability to stimulate intracellular guanosine 3',5'-cyclic monophosphate (cGMP) accumulation in cultured bovine aortic endothelial (BAE) and bovine aortic smooth muscle (BASM) cells. Our experiments indicate that CNP is unable to stimulate the production of cGMP in BAE cells, whereas both atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) markedly elevate cGMP levels in these cells (ANP = BNP >> CNP). In contrast, CNP is the most effective of the three peptides with respect to the stimulation of cGMP levels in BASM cells, fetal human vascular smooth muscle cells, and rat A10 cells (CNP >> ANP > BNP), with the maximal level of stimulation being approximately 5- to 10-fold over that observed for ANP. We have also shown that CNP is able to inhibit serum- and growth factor-induced DNA synthesis in BASM cells. Low concentrations of CNP (20 x 10(-9) M) inhibit up to 80% of the [3H]-thymidine incorporation induced by basic fibroblast growth factor, platelet derived growth factor, epidermal growth factor (EGF), and heparin binding EGF-like growth factor. These data indicate that, although CNP has been detected only in the central nervous system and not in the circulation, it may possess multiple effects on vascular tissue.

Molecular biology of the natriuretic peptides and their receptors

After the description in the past 5 years of BNP and CNP, interest in the natriuretic peptide family has dramatically increased. Molecular characterization of the receptors for this hormone family has identified a heterogeneity in the receptor subtypes not previously alluded to by pharmacological or biochemical studies. Much has been published on the physiology of ANP, but the major roles for BNP and CNP remain to be elucidated. Some experiments indicate that ANP and BNP may act synergistically, especially during cardiac stress; however, the high level of structural diversity of BNP among species and the ability of porcine BNP, but not human BNP, to activate human NPR-B suggest that an as yet unidentified receptor may exist that specifically recognizes BNP. Localization studies have implied that CNP is the most prominent neuropeptide in the natriuretic peptide family, and the restriction of its receptor, NPR-B, to the nervous system suggests that CNP and NPR-B may act in the brain to coordinate the central aspects of body fluid homeostasis. Of the three known NPRs, two, NPR-A and NPR-B, are capable of synthesizing their own second messenger, cGMP. The domain within these receptors that has high homology to protein kinases has been demonstrated to be essential for regulating this activity. No kinase activity has been measured in these proteins, but it is possible that this region is important for ATP regulation of guanylyl cyclase activity. This possibility raises interesting parallels with receptor-mediated cAMP signaling within cells. Seven transmembrane receptors, once activated by ligand, associate with G proteins to affect the activity of adenylyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

C-type natriuretic Peptide stimulates prolactin secretion by a hypothalamic site of action

The most recently discovered member of the family of natriuretic peptides, C-type natriuretic peptide (CNP), exerts many pharmacologic actions similar to its structural homolog A-type natriuretic peptide (ANP). Like ANP it failed to significantly alter prolactin release from dispersed, rat anterior pituitary cells incubated under static or dynamic conditions. Unlike ANP, however, which inhibits prolactin secretion in vivo by a hypothalamic action, CNP injection into the third cerebroventricle significantly stimulated prolactin secretion in ovariectomized, conscious rats. The effect was highly significant 15 min after injection and transient, lasting 30 min in animals injected with 2 nmole CNP. In a companion group of rats, significant inhibition of plasma prolactin levels was observed after central administration of similar doses of ANP. These results suggest differing hypothalamic actions of the CNP and ANP perhaps mediated by multiple natriuretic peptide receptors present in the tissue. Further, they provide additional support for unique roles exerted within the central nervous system by these structural homologs.

Precursor structure, expression, and tissue distribution of human guanylin

Heat-stable enterotoxins (STa) are small, cysteine-rich peptides secreted by Escherichia coli that are able to induce diarrhea through the stimulation of an intestine-specific receptor-guanylyl cyclase known as STaR. A 15-amino acid peptide, guanylin, was recently purified from rat jejunum and proposed to be a potential endogenous activator of this receptor. We describe here the cloning and characterization of human and mouse cDNAs encoding precursor proteins of 115 and 116 amino acids, respectively, having guanylin present at their C termini. Expression of the human cDNA in mammalian cells leads to the secretion of proguanylin, an inactive 94-amino acid protein. Guanylin generated by either trypsin or acid treatment of proguanylin was purified and found to bind to, and activate, STaR. Northern blot and in situ hybridization show high-level expression of guanylin mRNA restricted to the intestine, with localization to Paneth cells at the base of the small intestinal crypts. These results demonstrate that guanylin is an endogenous activator of STaR.

Receptor guanylyl cyclases

Three different guanylyl cyclase cell receptors are known, but others will likely be discovered within the next few years. The general function of these receptors appear to relate to the regulation of fluid volume or fluid movement. New receptors, or possibly the currently known receptors, therefore, may be discovered in areas of the body where fluid volume regulation is important. Such fluids whose volume or composition might be regulated by guanylyl cyclase receptors include synovial fluid, uterine/oviductal luminal fluid, follicular fluid, aqueous humor, cerebral spinal fluid, seminiferous tubule luminal fluid, epididymal luminal fluid, seminal plasma, and airway luminal fluid. The function of the heterodimeric forms of guanylyl cyclase appear to relate to a primary regulation of nitric oxide (or similar molecules) concentrations, which are in turn regulated by a Ca2+/calmodulin-sensitive nitric oxide synthase.

The role of the C-terminal region of rat brain natriuretic peptide in receptor selectivity

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) have different C-terminal tail structures compared with the rather conservative ring structures which consist of 17 amino acid residues. To examine the different effects of the tail structures of ANP and BNP on their interaction with receptors, we synthesized several peptide analogs and measured their biological actions in three different assay systems. Deletion of the C-terminal tail from rat BNP did not effect the vasorelaxation activity against rat aorta, but it promoted cGMP production in cultured rat aortic smooth muscle cells (RASMC). Deletion of the C-terminal tail from rat ANP diminished both vasorelaxant and cGMP producing activities. In a binding competition assay with RASMC and [125I]rat ANP-(1-28), the competition activities of both ANP and BNP were greatly reduced by C-terminal deletion. In addition, we obtained agonists with novel receptor selectivity.

Interaction of natriuretic peptides and cGMP production via the same receptor in mouse astrocytes

Atrial and brain natriuretic peptides have been found previously to bind to specific receptors on cultured mouse astrocytes and to stimulate cyclic guanosine 5-monophosphate (cGMP) production with similar dose dependency although brain natriuretic peptide (BNP) shows a greater maximal stimulatory effect. The present study provides evidence that both peptides work through the same pathway. No additive or synergistic effect was observed when astrocytes were exposed to both peptides. However, human ANF(99-126) at high concentrations partially inhibited porcine BNP induced cGMP production to the level seen with ANF alone. ANF could be viewed as a partial agonist of pBNP competing for the same effector sites. Differences in structure between human ANF(99-126) and porcine BNP may account for the difference in cGMP response. The interaction between the two peptides and the cGMP response does not reflect receptor binding affinities and is likely to be a post-binding event.

Developmental changes in the effect of atrial natriuretic peptide on tissue cyclic GMP content and particulate guanylate cyclase activity of aorta, kidney and lung of rats

To investigate possible developmental changes in the physiological effect of atrial natriuretic peptide (ANP) after birth, we studied the effect of ANP on the slice cGMP content and the particulate guanylate cyclase activity of aorta, kidney and lung in neonate, 2-week-old and adult rats of both sexes. Incubation with human ANP(99-126) (hANP) increased significantly the slice cGMP content of aorta, kidney and lung in three ages of rats. The hANP-stimulated fraction of cGMP contents of kidney decreased, that of lung increased with development, whereas that of aorta showed no significant change. Consistently, the hANP-responsive particulate guanylate cyclase activity decreased in kidney, increased in lung during development, without significant developmental change in aorta. These results indicate a differential change in the effect of hANP on the slice cGMP content among tissues during development. The developmental change in the effect of hANP on slice cGMP content is probably caused by the ontogenetic change in activation of ANP receptor-linked guanylate cyclase.

C-type natriuretic peptide is a potent stimulator of cyclic GMP production in cultured mouse astrocytes

The effect of C-type natriuretic peptide (CNP), a novel member of the natriuretic peptide family, on cyclic GMP (cGMP) generation was studied in primary cultures of mouse astrocytes. CNP stimulated cGMP production by mouse astrocytes in a dose-dependent fashion, with an EC50 of 32 nM and a maximal stimulatory concentration of greater than 1 microM, which induced a rise of cGMP level from a baseline of 1.0 +/- 0.1 pmol/mg of protein to 196.2 +/- 22.0 pmol/mg of protein. Compared with our previously reported atrial and brain natriuretic peptide-induced cGMP responses, CNP had a lower EC50 and was 10-20 times more efficacious in its maximal effect on cGMP stimulation. These data lend support to the concept of a significant role of CNP in neuromodulation/neurotransmission.

Characterization of ANF-R2 receptor-mediated inhibition of adenylate cyclase

We have characterized the ANF-R2 receptor-mediated inhibition of adenylate cyclase with respect to its modulation by several regulators. ANF (99-126) inhibits adenylate cyclase activity only in the presence of guanine nucleotides. The maximal inhibition (approximately 45%) was observed in the presence of 10-30 microM GTP gamma S, and at higher concentrations, the inhibitory effect of ANF was completely abolished. ANF-mediated inhibition was not dependent on the presence of monovalent cations, however Na+ enhanced the degree of inhibition by about 60%, whereas K+ and Li+ suppressed the extent of inhibition by about 50%. On the other hand, divalent cation, such as Mn2+ decreased the degree of inhibition in a concentration dependent manner, with an apparent Ki of about 0.7 mM, and at 2 mM; the inhibition was completely abolished. In addition, proteolytic digestion of the membranes with trypsin (40 ng/ml) resulted in the attenuation of ANF-mediated inhibition of adenylate cyclase. Other membrane disrupting agents such as neuraminidase and phospholipase A2 treatments also inhibited completely, the ANF-mediated inhibition of enzyme activity. N-Ethylmaleimide (NEM), phorbol ester and Ca(2+)-phospholipid dependent protein kinase (C-kinase) which have been shown to interact with inhibitory guanine nucleotide regulating protein (Gi) also resulted in the attenuation of ANF-mediated inhibition of adenylate cyclase activity. These results indicate that in addition to the Gi, the phospholipids and glycoproteins may also play an important role in the expression of ANF-R2 receptor-mediated inhibition of adenylate cyclase.

Atrial natriuretic peptide increases cyclic guanosine monophosphate immunoreactivity in the carotid body

The mammalian carotid body is a peripheral arterial chemoreceptor organ involved in the regulation of respiration, and in the modulation of blood pressure through reflex control of peripheral vascular resistance and cardiac output. In addition to its responsiveness to blood gases, the organ is also sensitive to hyperosmotic solutions, and we have recently shown that a systemic hormonal regulator of natriuresis and diuresis, atrial natriuretic peptide, is a potent inhibitor of chemoreceptor activity evoked by hypoxia in the cat carotid body. The present study demonstrates atrial natriuretic peptide immunoreactivity in type I cells of the carotid body, and shows further that a biologically active atrial natriuretic peptide fragment, atriopeptin III, increases cyclic guanosine monophosphate immunoreactivity in type I cells in a dose-dependent manner. Moreover, double-labeling techniques demonstrate co-existence of atrial natriuretic peptide immunoreactivity with the atriopeptin III-enhanced cyclic guanosine monophosphate reaction product. These findings indicate the probable existence of atrial natriuretic peptide receptors coupled to membrane-bound guanylate cyclase on the parenchymal type I cells. Our findings support the view that cyclic guanosine monophosphate functions as a second messenger in this organ, and may serve as a functional activity marker in identifying type I cells which respond to atrial natriuretic peptide.

Renal receptors and effects of atrial natriuretic factor in compensatory renal hypertrophy

In the present study we investigated the in vivo and in vitro renal responsiveness to ANF, and the adaptation of ANF receptors in compensatory renal hypertrophy in the rat. One week after left nephrectomy (UNx), plasma levels of immunoreactive ANF, blood pressure (MAP), hematocrit (Hct), and urine flow rate (V) were unaltered compared to control (C) rats. Baseline GFR and potassium excretion (UKV) were significantly higher, and sodium excretion (UNaV) tended to be elevated in UNx rats. Administered ANF led to similar dose-related decreases in MAP and increases in Hct in UNx and C rats. However, at each dose of infused ANF, absolute values and the increase in GFR and UNaV were higher in UNx than in C rats. Hypertrophied (H) kidneys were removed from UNx and perfused in vitro to determine distribution and density of ANF receptors, responsiveness to ANF, and receptor-mediated organ clearance of 125I-ANF1-28. The density of ANF receptors in cortex, outer medulla, and papilla of H kidneys was not significantly different from that in C kidneys. In H isolated kidneys, ANF led to dose-related increases in GFR, V, UNaV, and UKV that were indistinguishable (P greater than 0.05) from those in C kidneys. Receptor-mediated organ clearance of 125I-ANF1-28 in isolated H kidneys was 2.8 +/- .02 ml/min, a value not significantly different (P greater than 0.05) from that in C kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of A-, B-, and C-type natriuretic peptide genes in failing and developing human ventricles. Correlation with expression of the Ca(2+)-ATPase gene

Brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) are novel natriuretic peptides, originally isolated from porcine brain. Similar to atrial natriuretic peptide (ANP), BNP is also synthesized in and secreted from cardiocytes, but CNP is not expressed at significant levels in normal adult myocardium. Previous studies have indicated that the serum level and ventricular expression of the ANP gene were augmented in patients with heart failure. Recently, the serum level of BNP was also reported to increase in human heart failure. To examine whether or not the expression of these natriuretic peptides is regulated in ventricular myocardium in a concordant manner, we performed Northern blot analysis using total cellular RNA isolated from the diseased left ventricles of 30 cardiac transplant recipients with end-stage heart failure, seven ventricles from organ donors (control group), and two ventricles of artificially aborted 17- and 19-week-old fetuses. The levels of mRNAs encoding both BNP and ANP increased significantly (p less than 0.01) in the left ventricular myocardium from the patients with end-stage heart failure as compared with the control group. The levels of BNP mRNA correlated positively with those of ANP mRNA (r = 0.73, p less than 0.01) and negatively with those of sarcoplasmic reticulum Ca(2+)-ATPase mRNA (r = -0.66, p less than 0.01) in the left ventricular myocardium from the patients with heart failure. There was also a negative correlation between the levels of ANP and the sarcoplasmic reticulum Ca(2+)-ATPase mRNAs (r = -0.65, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma levels of brain natriuretic peptide in patients with cirrhosis

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

Phenotype-related alteration in expression of natriuretic peptide receptors in aortic smooth muscle cells

To elucidate the physiological and pathophysiological roles of the natriuretic peptide family in vascular smooth muscle cells, in which the natriuretic peptide family is implicated in growth inhibition as well as vasorelaxation, we have examined the phenotype-related expression of three kinds of natriuretic peptide receptors in rat aortic smooth muscle cells. The expression of natriuretic peptide receptors at the mRNA level was studied by Northern blot hybridization, and the expression at the protein level was determined by the cGMP production method and receptor binding assay. In intact aortic media, atrial natriuretic peptide (ANP)-A receptor mRNA and ANP-B receptor mRNA were detected, and the potency of cGMP production by ANP was at least two orders of magnitude stronger than that by C-type natriuretic peptide. Clearance receptor mRNA was undetectable, and only a small amount of the clearance receptor was detected by the binding assay in intact aortic media. By contrast, in cultured aortic smooth muscle cells at the first, fifth, and 17th passages, the ANP-B receptor mRNA level markedly increased; meanwhile, the expression of the ANP-A receptor mRNA became undetectable. C-type natriuretic peptide was one order of magnitude more potent than ANP in cGMP production in cultured aortic smooth muscle cells. The clearance receptor density and its mRNA level increased tremendously in these cultured cells. These results demonstrate that the marked phenotype-related alteration occurs in the expression of natriuretic peptide receptors in rat aortic smooth muscle cells.

Inhibitory effects of ATP and other nucleotides on atrial natriuretic peptide (ANP) binding to R1-type ANP receptors in human neuroblastoma NB-OK-1 cell membranes

ATP dose-dependently inhibited rat 125I-ANP-(99-126) binding to membranes from the human neuroblastoma cell line NB-OK-1 by increasing the KD value for the hormone without altering the Bmax value. After a 20 min preincubation with 37.5 pM 125I-ANP-(99-126) and 0.5 mM ATP, followed by the addition of 0.3 microM unlabelled ANP-(99-126), the proportion of rapidly dissociating receptors was 4-times higher than in the absence of ATP. The other nucleotides ADP, AMP, AMP-PNP, ATP gamma S, GTP, GDP, GMP, GMP-PNP and GTP gamma S were also inhibitory but with a lower potency and/or efficacy. Binding equilibrium data were satisfactorily simulated by a computer program based on partially competitive binding of ANP-(99-126) and the nucleotides, and this, together with the data on dissociation kinetics, strongly suggests that several nucleotides, when added at concentrations up to 1 mM, form a ternary ANP-receptor-nucleotide complex.

Conservation of the kinaselike regulatory domain is essential for activation of the natriuretic peptide receptor guanylyl cyclases

The natriuretic peptide receptors, NPR-A and NPR-B, are two members of the newly described class of receptor guanylyl cyclases. The kinaselike domain of these proteins is an important regulator of the guanylyl cyclase activity. To begin to understand the molecular nature of this type of regulation, we made complete and partial deletions of the kinase domain in NPR-A and NPR-B. We also made chimeric proteins in which the kinase domains of NPR-A and NPR-B were exchanged or replaced with kinase domains from structurally similar proteins. Complete deletion of the kinase homology domain in NPR-A and NPR-B resulted in constitutive activation of the guanylyl cyclase. Various partial deletions of this region produced proteins that had no ability to activate the enzyme with or without hormone stimulation. The kinase homology domain can be exchanged between the two subtypes with no effect on regulation. However, structurally similar kinaselike domains, such as from the epidermal growth factor receptor or from the heat-stable enterotoxin receptor, another member of the receptor guanylyl cyclase family, were not able to regulate the guanylyl cyclase activity correctly. These findings suggest that the kinaselike domain of NPR-A and NPR-B requires strict sequence conservation to maintain proper regulation of their guanylyl cyclase activity.

Conservation of the kinaselike regulatory domain is essential for activation of the natriuretic peptide receptor guanylyl cyclases

The natriuretic peptide receptors, NPR-A and NPR-B, are two members of the newly described class of receptor guanylyl cyclases. The kinaselike domain of these proteins is an important regulator of the guanylyl cyclase activity. To begin to understand the molecular nature of this type of regulation, we made complete and partial deletions of the kinase domain in NPR-A and NPR-B. We also made chimeric proteins in which the kinase domains of NPR-A and NPR-B were exchanged or replaced with kinase domains from structurally similar proteins. Complete deletion of the kinase homology domain in NPR-A and NPR-B resulted in constitutive activation of the guanylyl cyclase. Various partial deletions of this region produced proteins that had no ability to activate the enzyme with or without hormone stimulation. The kinase homology domain can be exchanged between the two subtypes with no effect on regulation. However, structurally similar kinaselike domains, such as from the epidermal growth factor receptor or from the heat-stable enterotoxin receptor, another member of the receptor guanylyl cyclase family, were not able to regulate the guanylyl cyclase activity correctly. These findings suggest that the kinaselike domain of NPR-A and NPR-B requires strict sequence conservation to maintain proper regulation of their guanylyl cyclase activity.

A ring-reversed analog of atrial natriuretic peptide retains receptor binding, guanylate cyclase stimulation

We have prepared an atrial natriuretic peptide analog, ANP[13-27][1-12], in which the connectivity of the disulfide-linked ring has been reversed by formally cleaving the ring and cyclizing the N- and C-terminal tails. This analog, which retains many of the spatial relationships of the native molecule, binds to both ANP-A and ANP-C receptor subtypes, and triggers the production of cyclic-GMP by ANP-A. ANP-C binding of ANP[13-27][1- 12] is roughly equipotent to that of ANP itself, although the ring cleavage falls within the putative ANP-C binding domain. ANP[13-27][1-8], a truncated analog in which much of this binding domain has been removed, surprisingly maintains a high affinity for ANP-C; however, this peptide has lost the ability to activate the ANP-A-linked guanylate cyclase.

Characterization of natriuretic peptide receptors in cultured cells

To elucidate physiological and clinical implications of the natriuretic peptide family, the expression of receptors for natriuretic peptides has been examined in cultured cells (a rat pheochromocytoma cell line [PC12], bovine endothelial cells, rat aortic smooth muscle cells, human mesangial cells, and a porcine kidney epithelial cell line [LLC-PK1]) by Northern blot analysis and cyclic GMP production method for the ANP-A and ANP-B receptors and by Northern blot analysis and binding assay for the clearance receptor. The ANP-A receptor was predominantly expressed in PC12 cells, bovine endothelial cells, and LLC-PK1 cells but was barely expressed in rat aortic smooth muscle cells and human mesangial cells. By contrast, the ANP-B receptor was the major subtype of the biologically active receptors in rat aortic smooth muscle cells and human mesangial cells. Only a small amount of the ANP-B receptor was detected in PC12 cells, bovine endothelial cells, and LLC-PK1 cells. The clearance receptor was abundantly expressed in rat aortic smooth muscle cells and human mesangial cells and was also present in bovine endothelial cells, but it was undetectable in PC12 cells and LLC-PK1 cells. These results demonstrate that the expression of three natriuretic peptide receptors varies from cell to cell, which is relevant to cell- or tissue-specific action of the natriuretic peptide family.

Autoradiographic visualization of the natriuretic peptide receptor-B in rat tissues

Natriuretic peptide receptor-B (NPRB) was visualized in rat tissues by in vitro autoradiography, using its putative physiological agonist C-type natriuretic peptide (CNP). In initial studies, we determined that atrial natriuretic peptide (ANP) is not a suitable ligand for labeling the NPRB: in tissues reported to contain NPRB transcripts, CNP did not inhibit [125I]ANP binding except to NPRC sites. Therefore, to visualize the NPRB we used 125I[Tyr(o)]-CNP as a radioligand with an excess of NPRC-blocking peptide: C-ANP. With this approach we detected the highest number of NPRB-like sites in the pars intermedia of the pituitary gland. A large number of these sites were present in pituitary neural and anterior lobes, area postrema, adrenal medulla and cortex. A moderate NPRB population was observed in the subfornical organ, plexiform layer of the olfactory bulb and kidney. Low concentrations of NPRB were noted in the cerebellum and cerebrum but not in the choroid plexus and pia-arachnoid. Saturation experiments performed on cerebellum sections revealed a very low concentration (Bmax 4.8 fmol/mg protein) of high affinity (Kd 1.2 nM) NPRB-like sites. This study is the first demonstration of 125I[Tyr(o)]-CNP binding sites with characteristics of the NPRB in intact tissues.

C-type natriuretic peptide-22 differentiates between natriuretic peptide receptors in rat choroid plexus and subfornical organ

Atrial natriuretic peptide (ANP) receptors in the rat choroid plexus and subfornical organ were characterized with C-type natriuretic peptide-22 (CNP-22) and 125I-Tyr0-CNP-22. The receptor autoradiographic method we used revealed that 125I-Tyr0-CNP-22 specifically bound to the choroid plexus, but only slightly to the subfornical organ, areas densely labeled with 125I-alpha-rat ANP (125I-rANP). CNP-22 significantly inhibited 125I-rANP binding to the choroid plexus; however, the peptide did not affect 125I-rANP binding to the subfornical organ. Thus, the characteristics of ANP receptors in the rat choroid plexus and subfornical organ are probably different.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

Atrial natriuretic factor: its (patho)physiological significance in humans

The first human studies using relatively high-doses of ANF revealed similar effects as observed in the preceding animal reports, including effects on systemic vasculature (blood pressure fall, decrease in intravascular volume), renal vasculature (rise in GFR, fall in renal blood flow), renal electrolyte excretion (rises in many electrolytes), and changes in release of a number of different hormones. Whether all these changes are the result of direct ANF effects or secondary to a (single) primary event of the hormone remains to be determined. Certainly, it has been proven that more physiological doses of ANF fail to induce short-term changes in many of these parameters leaving only a rise in hematocrit, natriuresis and an inhibition of the RAAS as important detectable ANF effects in humans. This leads us to hypothesize that ANF is a "natriuretic" hormone with physiological significance. The primary function in humans is to regulate sodium homeostasis in response to changes in intravascular volume (cardiac atrial stretch). Induction of excess renal sodium excretion and extracellular volume shift appear to be the effector mechanisms. The exact mechanism of the natriuresis in humans still needs to be resolved. It appears however, that possibly a small rise in GFR, a reduction in proximal and distal tubular sodium reabsorption, as well as an ensuing medullary washout, are of importance. The pathophysiological role of ANF in human disease is unclear. One may find elevated plasma irANF levels and/or decreased responses to exogenous ANF in some disease states. Whether these findings are secondary to the disease state rather than the cause of the disease remains to be resolved. Therapeutic applications for ANF, or drugs that intervene in its production or receptor-binding, seem to be multiple. Most important could be the antihypertensive effect, although areas such as congestive heart failure, renal failure, liver cirrhosis and the nephrotic syndrome cannot be excluded. Although the data that have been gathered to date allowed us to draw some careful conclusions as to the (patho)physiological role of ANF, the exact place of ANF in sodium homeostatic control must still be better defined. To achieve this, we will need more carefully designed low-dose ANF infusion, as well as ANF-breakdown inhibitor studies. Even more promising, however, is the potential area of studies open to us when ANF-receptor (ant)agonists become available for human use.

Atrial natriuretic peptide receptor subtypes in rat neuronal and astrocyte glial cultures

We have compared the levels and subtypes of atrial natriuretic peptide (ANP) receptors in astrocyte glial and neuronal cultures prepared from the hypothalamus and brain stem of 1-day-old rats. Astrocyte glial cultures contain approximately twice the number of ANP receptors, as measured by 125I-ANP specific binding, compared with neuronal cultures. Rat ANP-(99-126), rat brain natriuretic peptide (BNP32), C-type natriuretic peptide (CNP-22), atriopeptin I, and [des-Gln18,Ser19,Gly20,Leu21, Gly22]atrial natriuretic factor-(4-23)-NH2[C-ANF-(4-23)] all competed strongly for 125I-ANP binding in both culture types, with inhibitory constant values ranging from 0.47 to 8.07 nM. The presence of ANP-C receptors (clearance type) in both cell types is indicated from the strong competition of 125I-ANP specific binding by C-ANF-(4-23). The potency profiles for stimulation of guanosine 3',5'-cyclic monophosphate levels by these peptides were ANP = BNP much greater than CNP-22 greater than atriopeptin I in astrocyte glia and CNP-22 much greater than BNP32 greater than ANP greater than atriopeptin I in neuronal cultures. These results indicate that both types of culture contain guanylate cyclase-coupled ANP receptors, with astrocytes containing predominantly the ANP-A subtype and neurons predominantly the ANP-B subtype.

C-type natriuretic peptide is a potent activator of guanylate cyclase in endothelial cells from brain microvessels

An exposure of endothelial cells from rat brain microvessels to C-type natriuretic peptide (CNP) resulted in a rapid and large increase in cGMP formation. The action of CNP did not require inhibitors of phosphodiesterases to be observed and occurred at nanomolar concentrations. Other natriuretic peptides (ANP and BNP) also stimulated cGMP formation in endothelial cells from brain microvessels but with a potency that was at least 100 times less than that of CNP. In contrast, endothelial cells from the aorta showed large cGMP responses to low concentrations of ANP and BNP but were unresponsive to CNP up to concentrations as large as 100 nM. It is concluded that endothelial cells from brain microvessels and from aorta express different receptors subtypes for natriuretic peptides. Endothelial cells from brain microvessels express CNP specific ANPB receptors; aortic endothelial cells express ANP (and BNP) specific ANPA receptors. CNP may play an important role in the regulation of water and electrolyte movements across the blood brain barrier.

Signal transduction pathways in keratinocytes

Mammalian cells do not live as isolated organisms, but are instead organized into complex, highly specialized tissue organs composed of a homogeneous or a mixed cell population. In order to maintain tissue homeostasis in physiological and pathophysiological conditions, intercellular communication is an absolute requirement. This review will summarize our current knowledge as to how an extracellular signal is transduced via a specific receptor to the interior of the cell and how this signal will induce special cell functions. Attention will be paid to the major signal transduction pathways known to be active in keratinocytes, namely the adenylate cyclase, guanylate cyclase, tyrosine kinase, and phospholipase C systems. Finally, examples will be given of how interactions between these signal transduction pathways can take place and how 'signal cross-talk' might regulate keratinocyte function.

C-type natriuretic peptide inhibits thrombin- and angiotensin II-stimulated endothelin release via cyclic guanosine 3',5'-monophosphate

We examined the inhibitory effect of porcine C-type natriuretic peptide (CNP) on endothelin-1 secretion stimulated by thrombin and angiotensin II (Ang II) in cultured porcine endothelial cells. The results were compared with the effects of atrial (ANP) and brain (BNP) natriuretic peptides. Thrombin and Ang II produced a concentration-dependent stimulation of immunoreactive endothelin-1 secretion, and porcine CNP-22 potently inhibited this stimulated secretion in a concentration-dependent manner. CNP-22 had a stronger inhibitory effect than either porcine ANP(1-28) or porcine BNP-26. In addition, CNP potently increased the cellular level of cyclic guanosine 3',5'-monophosphate (GMP), with the inhibition of immunoreactive endothelin-1 secretion in response to thrombin and Ang II being paralleled by the increase in the cyclic GMP level. The increase of cyclic GMP produced by CNP was also greater than that due to porcine ANP(1-28) or porcine BNP-26. The immunoreactive endothelin-1 in the culture medium had two components on high-performance liquid chromatography; the major one corresponded to endothelin-1 (1-21) and the minor one to big endothelin-1 (porcine 1-39). Treatment with CNP did not affect this profile. Our results suggest that CNP probably inhibits the endothelin-1 secretion stimulated by thrombin and Ang II through a cyclic GMP-dependent process. The increase of cyclic GMP levels and the inhibition of immunoreactive endothelin-1 secretion produced by CNP appear to be greater than those produced by ANP or BNP.

Phorbol ester and atrial natriuretic peptide receptor response on vascular smooth muscle

At least two types of receptors for natriuretic peptides have been reported: biologically active receptors coupled with guanylate cyclase (atrial natriuretic peptide [ANP]-B receptors) and clearance receptors (ANP-C receptors). To elucidate the role of protein kinase C (PKC) in the regulation of ANP-B receptors, vascular smooth muscle cells in culture were treated with phorbol ester. Incubation with receptor agonists and phorbol ester led to the desensitization of receptor-mediated cyclic guanosine monophosphate (ANP-B receptor response) in rat vascular smooth muscle cells. Although a PKC inhibitor and downregulation of PKC by long-term incubation of cells with phorbol esters blocked the phorbol ester-induced desensitization of the ANP-B receptor response, they did not block the ANP-induced desensitization of the ANP-B receptor response. In addition, when desensitization by phorbol esters was observed, ANP was still capable of desensitization. These observations suggest that the mechanism for regulating ANP-B receptor sensitivity may be both PKC-dependent and PKC-independent and mediated by phorbol esters and ANP, respectively.