Atrial natriuretic peptide binding, cross-linking, and stimulation of cyclic GMP accumulation and particulate guanylate cyclase activity in cultured cells

Guanylyl cyclase/natriuretic peptide receptor-A: Identification, molecular characterization, and physiological genomics

The natriuretic peptides (NPs) hormone family, which consists mainly of atrial, brain, and C-type NPs (ANP, BNP, and CNP), play diverse roles in mammalian species, ranging from renal, cardiac, endocrine, neural, and vascular hemodynamics to metabolic regulations, immune responsiveness, and energy distributions. Over the last four decades, new data has transpired regarding the biochemical and molecular compositions, signaling mechanisms, and physiological and pathophysiological functions of NPs and their receptors. NPs are incremented mainly in eliciting natriuretic, diuretic, endocrine, vasodilatory, and neurological activities, along with antiproliferative, antimitogenic, antiinflammatory, and antifibrotic responses. The main locus responsible in the biological and physiological regulatory actions of NPs (ANP and BNP) is the plasma membrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), a member of the growing multi-limbed GC family of receptors. Advances in this field have provided tremendous insights into the critical role of Npr1 (encoding GC-A/NPRA) in the reduction of fluid volume and blood pressure homeostasis, protection against renal and cardiac remodeling, and moderation and mediation of neurological disorders. The generation and use of genetically engineered animals, including gene-targeted (gene-knockout and gene-duplication) and transgenic mutant mouse models has revealed and clarified the varied roles and pleiotropic functions of GC-A/NPRA in vivo in intact animals. This review provides a chronological development of the biochemical, molecular, physiological, and pathophysiological functions of GC-A/NPRA, including signaling pathways, genomics, and gene regulation in both normal and disease states.

Molecular Signaling Mechanisms and Function of Natriuretic Peptide Receptor-A in the Pathophysiology of Cardiovascular Homeostasis

The discovery of atrial, brain, and C-type natriuretic peptides (ANP, BNP, and CNP) and their cognate receptors has greatly increased our knowledge of the control of hypertension and cardiovascular homeostasis. ANP and BNP are potent endogenous hypotensive hormones that elicit natriuretic, diuretic, vasorelaxant, antihypertrophic, antiproliferative, and antiinflammatory effects, largely directed toward the reduction of blood pressure (BP) and cardiovascular diseases (CVDs). The principal receptor involved in the regulatory actions of ANP and BNP is guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), which produces the intracellular second messenger cGMP. Cellular, biochemical, molecular, genetic, and clinical studies have facilitated understanding of the functional roles of natriuretic peptides (NPs), as well as the functions of their receptors, and signaling mechanisms in CVDs. Transgenic and gene-targeting (gene-knockout and gene-duplication) strategies have produced genetically altered novel mouse models and have advanced our knowledge of the importance of NPs and their receptors at physiological and pathophysiological levels in both normal and disease states. The current review describes the past and recent research on the cellular, molecular, genetic mechanisms and functional roles of the ANP-BNP/NPRA system in the physiology and pathophysiology of cardiovascular homeostasis as well as clinical and diagnostic markers of cardiac disorders and heart failure. However, the therapeutic potentials of NPs and their receptors for the diagnosis and treatment of cardiovascular diseases, including hypertension, heart failure, and stroke have just begun to be expanded. More in-depth investigations are needed in this field to extend the therapeutic use of NPs and their receptors to treat and prevent CVDs.

Engineered hypoxia-responding Escherichia coli carrying cardiac peptide genes, suppresses tumor growth, angiogenesis and metastasis in vivo

Development of engineered non-pathogenic bacteria, capable of expressing anti-cancer proteins under tumor-specific conditions, is an ideal approach for selectively eradicating proliferating cancer cells. Herein, using an engineered hypoxia responding nirB promoter, we developed an engineered Escherichia coli BW25133 strain capable of expressing cardiac peptides and GFP signaling protein under hypoxic condition for spatiotemporal targeting of mice mammary tumors. Following determination of the in vitro cytotoxicity profile of the engineered bacteria, selective accumulation of bacteria in tumor microenvironment was studied 48 h after tail vein injection of 108 cfu bacteria in animals. For in vivo evaluation of antitumoral activities, mice with establishment mammary tumors received 3 consecutive intravenous injections of transformed bacteria with 4-day intervals and alterations in expression of tumor growth, invasion and angiogenesis specific biomarkers (Ki-67, VEGFR, CD31and MMP9 respectively), as well as fold changes in concentration of proinflammatory cytokines were examined at the end of the 24-day study period. Intravenously injected bacteria could selectively accumulate in tumor site and temporally express GFP and cardiac peptides in response to hypoxia, enhancing survival rate of tumor bearing mice, suppressing tumor growth rate and expression of MMP-9, VEGFR2, CD31 and Ki67 biomarkers. Applied engineered bacteria could also significantly reduce concentrations of IL-1β, IL-6, GC-SF, IL-12 and TNF-α proinflammatory cytokines while increasing those of IL-10, IL-17A and INF-γ. Overall, administration of hypoxia-responding E. coli bacteria, carrying cardiac peptide expression construct could effectively suppress tumor growth, angiogenesis, invasion and metastasis and enhance overall survival of mice bearing mammary tumors.

Guanylyl cyclase/natriuretic peptide receptor-A signaling antagonizes phosphoinositide hydrolysis, Ca(2+) release, and activation of protein kinase C

Thus far, three related natriuretic peptides (NPs) and three distinct sub-types of cognate NP receptors have been identified and characterized based on the specific ligand binding affinities, guanylyl cyclase activity, and generation of intracellular cGMP. Atrial and brain natriuretic peptides (ANP and BNP) specifically bind and activate guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), and C-type natriuretic peptide (CNP) shows specificity to activate guanylyl cyclase/natriuretic peptide receptor-B (GC-B/NPRB). All three NPs bind to natriuretic peptide receptor-C (NPRC), which is also known as clearance or silent receptor. The NPRA is considered the principal biologically active receptor of NP family; however, the molecular signaling mechanisms of NP receptors are not well understood. The activation of NPRA and NPRB produces the intracellular second messenger cGMP, which serves as the major signaling molecule of all three NPs. The activation of NPRB in response to CNP also produces the intracellular cGMP; however, at lower magnitude than that of NPRA, which is activated by ANP and BNP. In addition to enhanced accumulation of intracellular cGMP in response to all three NPs, the levels of cAMP, Ca²⁺ and inositol triphosphate (IP₃) have also been reported to be altered in different cells and tissue types. Interestingly, ANP has been found to lower the concentrations of cAMP, Ca²⁺, and IP₃; however, NPRC has been proposed to increase the levels of these metabolic signaling molecules. The mechanistic studies of decreased and/or increased levels of cAMP, Ca²⁺, and IP₃ in response to NPs and their receptors have not yet been clearly established. This review focuses on the signaling mechanisms of ANP/NPRA and their biological effects involving an increased level of intracellular accumulation of cGMP and a decreased level of cAMP, Ca²⁺, and IP₃ in different cells and tissue systems.

All-trans retinoic acid and sodium butyrate enhance natriuretic peptide receptor a gene transcription: role of histone modification

The objective of the present study was to delineate the mechanisms of GC-A/natriuretic peptide receptor-A (GC-A/NPRA) gene (Npr1) expression in vivo. We used all-trans retinoic acid (ATRA) and histone deacetylase (HDAC) inhibitor, sodium butyrate (NaBu) to examine the expression and function of Npr1 using gene-disrupted heterozygous (1-copy; +/-), wild-type (2-copy; +/+), and gene-duplicated heterozygous (3-copy; ++/+) mice. Npr1(+/-) mice exhibited increased renal HDAC and reduced histone acetyltransferase (HAT) activity; on the contrary, Npr1(++/+) mice showed decreased HDAC and enhanced HAT activity compared with Npr1(+)(/+) mice. ATRA and NaBu promoted global acetylation of histones H3-K9/14 and H4-K12, reduced methylation of H3-K9 and H3-K27, and enriched accumulation of active chromatin marks at the Npr1 promoter. A combination of ATRA-NaBu promoted recruitment of activator-complex containing E26 transformation-specific 1, retinoic acid receptor α, and HATs (p300 and p300/cAMP response element-binding protein-binding protein-associated factor) at the Npr1 promoter, and significantly increased renal NPRA expression, GC activity, and cGMP levels. Untreated 1-copy mice showed significantly increased systolic blood pressure and renal expression of α-smooth muscle actin (α-SMA) and proliferating cell nuclear antigen (PCNA) compared with 2- and 3-copy mice. Treatment with ATRA and NaBu synergistically attenuated the expression of α-SMA and PCNA and reduced systolic blood pressure in Npr1(+/-) mice. Our findings demonstrate that epigenetic upregulation of Npr1 gene transcription by ATRA and NaBu leads to attenuation of renal fibrotic markers and systolic blood pressure in mice with reduced Npr1 gene copy number, which will have important implications in prevention and treatment of hypertension-related renal pathophysiological conditions.

Mutations in Tyr808 reveal a potential auto-inhibitory mechanism of guanylate cyclase-B regulation

In this study, Tyr⁸⁰⁸ in GC-B (guanylate cyclase-B), a receptor of the CNP (C-type natriuretic peptide), has been shown to be a critical regulator of GC-B activity. In searching for phosphorylation sites that could account for suppression of GC-B activity by S1P (sphingosine-1-phosphate), mutations were introduced into several candidate serine/threonine and tyrosine residues. Although no novel phosphorylation sites that influenced the suppression of GC-B were identified, experiments revealed that mutations in Tyr⁸⁰⁸ markedly enhanced GC-B activity. CNP-stimulated activities of the Y808F and Y808A mutants were greater than 30-fold and 70-fold higher, respectively, than that of WT (wild-type) GC-B. The Y808E and Y808S mutants were constitutively active, expressing 270-fold higher activity without CNP stimulation than WT GC-B. Those mutations also influenced the sensitivity of GC-B to a variety of inhibitors, including S1P, Na₃VO₄ and PMA. Y808A, Y808E and Y808S mutations markedly weakened S1P- and Na₃VO₄-dependent suppression of GC-B activity, whereas Y808E and Y808S mutations rather elevated cGMP production. Tyr⁸⁰⁸ is conserved in all membrane-bound GCs and located in the niche domain showing sequence similarity to a partial fragment of the HNOBA (haem nitric oxide binding associated) domain, which is found in soluble GC and in bacterial haem-binding kinases. This finding provides new insight into the activation mechanism of GCs.

Guanylyl cyclase/natriuretic peptide receptor-A signaling antagonizes the vascular endothelial growth factor-stimulated MAPKs and downstream effectors AP-1 and CREB in mouse mesangial cells

Along with its natriuretic, diuretic, and vasodilatory properties, atrial natriuretic peptide (ANP), and its guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) exhibit an inhibitory effect on cell growth and proliferation. However, the signaling pathways mediating this inhibition are not well understood. The objective of this study was to determine the effect of ANP-NPRA system on mitogen-activated protein kinases (MAPKs) and the downstream proliferative transcription factors involving activating protein-1 (AP-1) and cAMP-response element binding protein (CREB) in agonist-stimulated mouse mesangial cells (MMCs). We found that ANP inhibited vascular endothelial growth factor (VEGF)-stimulated phosphorylation of MAPKs (Erk1, Erk2, JNK, and p38), to a greater extent in NPRA-transfected cells (50-60 %) relative to vector-transfected cells (25-30 %). The analyses of the phosphorylated transcription factors revealed that ANP inhibited VEGF-stimulated activation of CREB, and the AP-1 subunits (c-jun and c-fos). Gel shift assays demonstrated that ANP inhibited VEGF-stimulated AP-1 and CREB DNA-binding ability by 67 and 62 %, respectively. The addition of the protein kinase G (PKG) inhibitor, KT-5823, restored the VEGF-stimulated activation of MAPKs, AP-1, and CREB, demonstrating the integral role of cGMP/PKG signaling in NPRA-mediated effects. Our results delineate the underlying mechanisms through which ANP-NPRA system exerts an inhibitory effect on MAPKs and down-stream effector molecules, AP-1, and CREB, critical for cell growth and proliferation.

Interactive roles of Ets-1, Sp1, and acetylated histones in the retinoic acid-dependent activation of guanylyl cyclase/atrial natriuretic peptide receptor-A gene transcription

Cardiac hormones atrial and brain natriuretic peptides activate guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), which plays a critical role in reduction of blood pressure and blood volume. Currently, the mechanisms responsible for regulating the Npr1 gene (coding for GC-A/NPRA) transcription are not well understood. The present study was conducted to examine the interactive roles of all-trans retinoic acid (ATRA), Ets-1, Sp1, and histone acetylation on the transcriptional regulation and function of the Npr1 gene. Deletion analysis of the Npr1 promoter and luciferase assays showed that ATRA enhanced a 16-fold Npr1 promoter activity and greatly stimulated guanylyl cyclase (GC) activity of the receptor protein in both atrial natriuretic peptide (ANP)-dependent and -independent manner. As confirmed by gel shift and chromatin immunoprecipitation assays, ATRA enhanced the binding of both Ets-1 and Sp1 to the Npr1 promoter. The retinoic acid receptor α (RARα) was recruited by Ets-1 and Sp1 to form a transcriptional activator complex with their binding sites in the Npr1 promoter. Interestingly, ATRA also increased the acetylation of histones H3 and H4 and enhanced their recruitment to Ets-1 and Sp1 binding sites within the Npr1 promoter. Collectively, the present results demonstrate that ATRA regulates Npr1 gene transcription and GC activity of the receptor by involving the interactive actions of Ets-1, Sp1, and histone acetylation.

Regulation of natriuretic peptide receptor-A gene expression and stimulation of its guanylate cyclase activity by transcription factor Ets-1

ANP (atrial natriuretic peptide) exerts its biological effects by binding to GC (guanylate cyclase)-A/NPR (natriuretic peptide receptor)-A, which generates the second messenger cGMP. The molecular mechanism mediating Npr1 (coding for GC-A/NPRA) gene regulation and expression is not well understood. The objective of the present study was to elucidate the mechanism by which Ets-1 [Ets (E twenty-six) transformation-specific sequence] contributes to the regulation of Npr1 gene transcription and expression. Chromatin immunoprecipitation and gel-shift assays confirmed the in vivo and in vitro binding of Ets-1 to the Npr1 promoter. Overexpression of Ets-1 enhanced significantly Npr1 mRNA levels, protein expression, GC activity and ANP-stimulated intracellular accumulation of cGMP in transfected cells. Depletion of endogenous Ets-1 by siRNA (small interfering RNA) dramatically decreased promoter activity by 80%. Moreover, methylation of the Npr1 promoter region (-356 to +55) reduced significantly the promoter activity and hypermethylation around the Ets-1 binding sites directly reduced Ets-1 binding to the Npr1 promoter. Collectively, the present study demonstrates that Npr1 gene transcription and GC activity of the receptor are critically controlled by Ets-1 in target cells.

Differential patterning of cGMP in vascular smooth muscle cells revealed by single GFP-linked biosensors

Here, we report the design of unprecedented, non-FRET based cGMP-biosensors, named FlincGs, to assess the dynamics of nitric oxide (NO) and atrial natriuretic peptide (ANP) induced synthesis of intracellular cGMP, [cGMP](i). Regulatory fragments of PKG I alpha, PKG I beta, and an N-terminal deletion mutant of PKG I alpha were fused to circular permutated EGFP to generate alpha-, beta-, and delta-FlincG, with high dynamic ranges and apparent K(D,cGMP) values of 35 nM, 1.1 microM, and 170 nM, respectively. All indicators displayed significant selectivity for cGMP over cAMP, and 1.5- to 2.1-fold increases in fluorescence intensity at 510 nm when excited at 480 nm. Surprisingly, FlincGs displayed an additional excitation peak at 410 nm. delta-FlincG permitted ratiometric (480/410 nm) measurements, with a cGMP-specific 3.5-fold ratio change. In addition, delta-FlincG presented cGMP association and dissociation kinetics sufficiently fast to monitor rapid changes of [cGMP](i) in intact cells. In unpassaged, adenoviral transfected vascular smooth muscle (VSM) cells, delta-FlincG had an EC(50,cGMP) of 150 nM, and revealed transient global cGMP elevations to sustained physiological NO (EC(50,DEA/NO) = 4 nM), and the decay phase depended on the activity of PDE-5. In contrast, ANP elicited sustained submembrane elevations in [cGMP](i), which were converted to global cGMP elevations by inhibition of PDE-5 by sildenafil. These results indicate that FlincG is an innovative tool to elucidate the dynamics of a central biological signal, cGMP, and that NO and natriuretic peptides induce distinct cGMP patterning under the regulation of PDE-5, and therefore likely differentially engage cGMP targets.

Atrial natriuretic peptide-initiated cGMP pathways regulate vasodilator-stimulated phosphoprotein phosphorylation and angiogenesis in vascular endothelium

Nitric oxide (NO)- and atrial natriuretic peptide (ANP)-initiated cGMP signaling cascades are important in the maintenance of cardiovascular homeostasis. The molecular signaling mechanisms downstream of cGMP are not well understood, however. We have used small interfering RNA (siRNA) approaches to specifically knock down a series of signaling proteins in bovine aortic endothelial cells, and we have combined biochemical analyses with physiological assays to investigate cGMP-mediated signal transduction pathways. Activation of particulate guanylate cyclase (GC-A) by ANP leads to a substantial, dose-dependent, rapid, and sustained increase in intracellular cGMP. In contrast, stimulation of soluble guanylate cyclase by NO yields only a weak and transient increase in cGMP. ANP-induced cGMP production is selectively suppressed by siRNA-mediated knockdown of GC-A. ANP greatly enhances the phosphorylation at Ser-239 of the vasodilator-stimulated phosphoprotein (VASP), a major substrate of cGMP-dependent protein kinase (PKG) that significantly influences actin dynamics. Moreover, the ANP-induced phosphorylation of VASP at Ser-239 is accompanied by increased actin stress fiber formation and enhanced endothelial tube formation. siRNA-mediated knockdown of GC-A, VASP, or PKG abolishes ANP-induced VASP Ser-239 phosphorylation, stress fiber formation, and endothelial tube formation. We have demonstrated similar findings in human umbilical vein endothelial cells, where ANP substantially enhances intracellular cGMP content, phosphorylation of VASP at Ser-239, and endothelial tube formation. Taken together, our findings suggest that ANP-mediated cGMP signal transduction pathways regulate PKG phosphorylation of VASP Ser-239 in endothelial cells, resulting in reorganization of the actin cytoskeleton and enhancement of angiogenesis.

Small cytoplasmic domain peptides of natriuretic peptide receptor-C attenuate cell proliferation through Gialpha protein/MAP kinase/PI3-kinase/AKT pathways

The present studies were undertaken to investigate the effect of C-atrial natriuretic peptide (ANP)(4-23) and several peptide fragments containing 12 amino acids from different regions of the cytoplasmic domain of natriuretic peptide receptor (NPR)-C on cell proliferation in the absence or presence of angiotensin (ANG) II, endothelin (ET)-1, and arginine vasopressin (AVP) in A-10 vascular smooth muscle cells (VSMC). The peptide fragments used have either complete G(i) activator sequences K(461)-H(472) (peptide 1) and H(481)-H(492) (peptide 3) or partial G(i) activator sequences R(469)-K(480) (peptide 2) and I(465)-H(472) (peptide Y) with truncated COOH or NH(2) terminus, respectively. The other peptide used had no structural specificity (Q(473)-K(480), peptide X) or was the scrambled peptide control for peptide 1 (peptide Z). ANG II, ET-1 and AVP significantly stimulated DNA synthesis in these cells as determined by [(3)H]thymidine incorporation that was inhibited by peptides 1, 2, and 3 and not by peptides X, Y, and Z in a concentration-dependent manner, with an apparent K(i) between 1 and 10 nM. In addition, C-ANP(4-23), which interacts with NPR-C, also inhibited DNA synthesis stimulated by vasoactive peptides; however, the inhibition elicited by C-ANP(4-23) was not additive with the inhibition elicited by peptide 1. On the other hand, basal DNA synthesis in these cells was not inhibited by C-ANP(4-23) or the peptide fragments. Furthermore, vasoactive peptide-induced stimulation of DNA synthesis was inhibited by PD-98059 and wortmannin, and this inhibition was potentiated by peptide 1. In addition, peptide 1 also inhibited vasoactive peptide-induced phosphorylation of ERK1/2 and AKT and enhanced expression of G(i)alpha proteins. These data suggest that C-ANP(4-23) and small peptide fragments containing 12 amino acids irrespective of the region of the cytoplasmic domain of NPR-C inhibit proliferative responses of vasoactive peptides through G(i)alpha protein and MAP kinase/phosphatidylinositol 3-kinase/AKT pathways.

Inhibition and down-regulation of gene transcription and guanylyl cyclase activity of NPRA by angiotensin II involving protein kinase C

The objective of this study was to investigate the role of protein kinase C (PKC) in the angiotensin II (Ang II)-dependent repression of Npr1 (coding for natriuretic peptide receptor-A, NPRA) gene transcription. Mouse mesangial cells (MMCs) were transfected with Npr1 gene promoter-luciferase construct and treated with Ang II and PKC agonist or antagonist. The results showed that the treatment of MMCs with 10 nM Ang II produced a 60% reduction in the promoter activity of Npr1 gene. MMCs treated with 10nM Ang II exhibited 55% reduction in NPRA mRNA levels, and subsequent stimulation with 100 nM ANP resulted in 50% reduction in guanylyl cyclase (GC) activity. Furthermore, the treatment of MMCs with Ang II in the presence of PKC agonist phorbol ester (100 nM) produced an almost 75% reduction in NPRA mRNA and 70% reduction in the intracellular accumulation of cGMP levels. PKC antagonist staurosporine completely reversed the effect of Ang II and phorbol ester. This is the first report to demonstrate that ANG II-dependent transcriptional repression of Npr1 gene promoter activity and down-regulation of GC activity of translated protein, NPRA is regulated by PKC pathways.

Mediators and modulators of pulmonary arterial hypertension

Pulmonary hypertension (PH), defined as a mean pulmonary arterial (PA) pressure of >25 mmHg at rest or >30 mmHg during exercise, is characterized by a progressive and sustained increase in pulmonary vascular resistance that eventually leads to right ventricular failure. Clinically, PH may result from a variety of underlying diseases (Table 1 and Refs. 50, 113, 124). Pulmonary arterial hypertension (PAH) may be familial (FPAH) or sporadic (idiopathic, IPAH), formerly known as primary pulmonary hypertension, i.e., for which there is no demonstrable cause. More often, PAH is due to a variety of identifiable diseases including scleroderma and other collagen disorders, liver disease, human immunodeficiency virus, and the intake of appetite-suppressant drugs such as phentermine and fenfluramine (72). Other, more common, causes of PAH include left ventricular failure (perhaps the most common cause), valvular lesions, chronic pulmonary diseases, sleep-disordered breathing, and prolonged residence at high altitude. This classification, now widely accepted, was first proposed at a meeting in Evian, France, in 1998, and modified in Venice, Italy, in 2003 (124).

Biology of natriuretic peptides and their receptors

Increasing evidence suggests that natriuretic peptides (NPs) play diverse roles in mammals, including renal hemodynamics, neuroendocrine, and cardiovascular functions. Collectively, NPs are classified as hypotensive hormones; the main actions of NPs are implicated in eliciting natriuretic, diuretic, steroidogenic, antiproliferative, and vasorelaxant effects, important factors in the control of body fluid volume and blood pressure homeostasis. One of the principal loci involved in the regulatory actions of NPs is their cognate plasma membrane receptor molecules, which are activated by binding with specific NPs. Interaction of NPs with their receptors plays a central role in physiology and pathophysiology of hypertension and cardiovascular disorders. Gaining insight into the intricacies of NPs-specific receptor signaling pathways is of pivotal importance for understanding both hormone-receptor biology and the disease states arising from abnormal hormone receptor interplay. During the last decade there has been a surge in interest in NP receptors; consequently, a wealth of information has emerged concerning molecular structure and function, signaling mechanisms, and use of transgenics and gene-targeted mouse models. The objective of this present review is to summarize and document the previous findings and recent discoveries in the field of the natriuretic peptide hormone family and receptor systems with emphasis on the structure-function relationship, signaling mechanisms, and the physiological and pathophysiological significance in health and disease.

Atrial natriuretic peptide-C receptor-induced attenuation of adenylyl cyclase signaling activates phosphatidylinositol turnover in A10 vascular smooth muscle cells

Atrial natriuretic peptide (ANP)-C receptor activation has been shown to inhibit adenylyl cyclase (AC) activity as well as to stimulate phospholipase C (PLC) signaling pathways. The present studies were undertaken to investigate whether ANP-C receptor-mediated decreased cAMP levels contribute to the activation of PLC signaling. C-ANP(4-23) [des(Gln(18),Ser(19), Glu(20),Leu(21),Gly(22))ANP(4-23)-NH(2)], a ring-deleted peptide of ANP that interacts specifically with ANP-C receptor, stimulated inositol 1,4,5-tris-phosphate (IP(3)) production (PLC activity) in A10 vascular smooth muscle cells in a concentration- and time-dependent manner. The maximal stimulation observed was about 75% at 2 h of treatment, with an apparent EC(50) of about 20 to 30 nM. Pertussis toxin treatment of the cells completely abolished the C-ANP(4-23)-mediated stimulation of IP(3) production. Forskolin (FSK), a stimulator of adenylyl cyclase, dibutyryl cAMP (db cAMP), and isoproterenol (ISO), a beta-adrenergic agonist that stimulates adenylyl cyclase activity and cAMP levels, inhibited IP(3) production by about 35, 30, and 50%, respectively, whereas dideoxyadenosine (DDA), an inhibitor of adenylyl cyclase activity, and oxotremorine stimulated IP(3) production by about 90 and 80%, respectively, in these cells, suggesting a functional interaction between these two signaling pathways. Treatment of the cells with antisense oligonucleotide of ANP-C receptor that attenuated ANP-C receptor-mediated inhibition of adenylyl cyclase resulted in a complete attenuation of C-ANP(4-23)-induced stimulation of IP(3) formation, whereas FSK, db cAMP, and ISO-mediated decrease and oxotremorine and endothelin-1 (ET-1)-induced increase in IP(3) production was not affected by this treatment. Furthermore, C-ANP(4-23)-induced increase in IP(3) formation was significantly potentiated by DDA and inhibited by FSK and db cAMP, whereas ET-1-induced increase in IP(3) production was not affected by FSK. In addition, N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H-89), an inhibitor of protein kinase A, completely abolished C-ANP(4-23) and not ET-1-induced stimulation of IP(3) production. These results indicate that ANP-C receptor activation by C-ANP(4-23) and resulting decrease in cAMP levels may be responsible for the activation of phosphatidylinositol (PI) turnover signaling, suggesting a cross-talk between ANP-C receptor-mediated adenylyl cyclase and PLC signaling pathways.

Expression of brain natriuretic peptide in the rat heart studies during heart growth and in relation to sympathectomy

Brain natriuretic peptide (BNP) might be of importance during heart development and is described to be increasingly expressed in congestive heart failure and to affect the progress of this condition. However, details in the normal expression of BNP are still unclear in various parts of the adult and growing heart, including the conduction system. In this study, we investigated the expression of BNP in relation to that of atrial natriuretic peptide (ANP) in the growing as well as in the adult rat heart. The effects of chemical sympathectomy in adult rats were also examined. Contrary to previous BNP immunohistochemical studies, the BNP antiserum was preabsorbed with an excess of ANP before staining to abolish the crossreactivity with ANP. There was a pronounced BNP immunoreaction in the auricles, the trabeculated ventricular walls, and the peripheral parts of the conduction system at 0-1 days postnatally. The degree of immunoreaction gradually decreased with increasing age. A similar developmental pattern was seen concerning ANP expression, but the magnitude of the latter clearly exceeded that for BNP. Immunoreaction for BNP was never detected in the atrioventricular (AV) node and AV bundle at any stage. In contrast to the situation for ANP previously observed, no obvious changes in BNP immunoreaction patterns were observed in response to sympathectomy. This is the first study to thoroughly demonstrate the expression of BNP in the various regions of the rat heart during growth and in the normal and sympathectomized adult stage. The observations are related to possible functions of natriuretic peptides in the growing and adult heart.

A-type natriuretic peptide receptor in the spontaneously hypertensive rat kidney

Renal NPR-A binding characteristics was examined in SHR. Renal ANP binding sites of NPR-A showed a lower maximal binding capacity and higher affinity in SHR than in WKY at all intrarenal sites. Despite the lower B(max) in SHR, both ANP(1-28) and ANP(5-25) stimulate similar or greater cGMP production in isolated glomeruli. Studies on guanylate cyclase from glomerular and papillary membranes have reported an increased basal and stimulated guanylate cyclase activity in SHR. The present study provides further evidences for altered NPR-A receptors in SHR kidney, which might act as a negative feedback in response to hypertension.

Immunocytochemistry of cGMP in the Cerebellum of the Immature, Adult, and Aged Rat: the Involvement of Nitric Oxide. A Micropharmacological Study

In this study we describe the localization of formaldehyde-fixed cGMP-immunoreactivity (cGMP-IR) in rat cerebellar tissue slices incubated in vitro. In the absence of phosphodiesterase inhibition, cGMP-immunofluorescence was of low intensity in tissue slices prepared from immature cerebella. Addition of isobutylmethylxanthine (IBMX) to the incubation medium resulted in the appearance of cGMP-IR in clusters of astrocytes in the internal granular layer. Addition of N-methyl-d-aspartate (NMDA), kainic acid, atrial natriuretic factor (ANF), or sodium nitroprusside (SNP) gave an intense cGMP-IR in Bergmann fibres, Bergmann cell bodies, and astrocytes in the internal granular layer. Astrocytes in the white matter showed cGMP-IR after incubation of the slice in the presence of ANF or nitroprusside, but not after NMDA or kainic acid. In addition, after SNP stimulation of cGMP production, cGMP-IR was found in fibres which were not positive for glial fibrillary acidic protein (GFAP). In the adult cerebellar slice, intense basal cGMP-immunostaining was observed in Bergmann fibres, Bergmann cell bodies, and astrocytes in the granular layer. No cGMP-IR was observed in Purkinje cells. Stimulation of the cGMP-content in the glial structures by NMDA, ANF, or SNP, was suggested by the immunocytochemical results. However, when measured biochemically, only the effect of SNP was statistically significant, and immunocytochemistry showed that SNP clearly stimulated cGMP synthesis in neuronal cell structures. In the cerebellum of the aged rat a reduced cGMP-IR was found compared to the adult, in the same structures which showed cGMP-IR in the adult. Basal cGMP-immunostaining was reduced in the presence of haemoglobin, methylene blue, by inhibiting nitric oxide synthesis with NG-monomethyl-l-arginine (NGMAr), or by depletion of external Ca2+. Also the stimulatory effect of NMDA and of ANF (partly) on the cGMP-IR was inhibited by these compounds. cGMP-IR after stimulation of guanylate cyclase by SNP was reduced by the concomitant presence of haemoglobin or methylene blue, but not by NGMAr, or by omission of Ca2+. Our results point to an important role for cGMP in the functioning of glial tissue in the cerebellum and also suggest a role for nitric oxide as an intercellular mediator in the functioning of glutamate and ANF in the cerebellum.

Dendroaspis natriuretic peptide-like immunoreactivity and its regulation in rat aortic vascular smooth muscle

Dendroaspis natriuretic peptide (DNP) is a recently isolated 38 amino acid peptide that shares structural and functional properties with the other members of the natriuretic peptide family. The present study demonstrates the presence of DNP-like immunoreactivity in sections of rat aorta, carotid artery and renal vasculature and tubules. DNP-like immunoreactivity was detected in culture aortic vascular smooth muscle cells and medium and is regulated by endothelin-1, angiotensin II and sodium nitroprusside but not by transforming growth factor-beta. Our observations indicate that DNP elicits a marked inhibitory effect on DNA synthesis in culture rat aortic vascular smooth muscle cells.

Cytoplasmic domain of natriuretic peptide receptor C constitutes Gi activator sequences that inhibit adenylyl cyclase activity

We have recently demonstrated that a 37-amino acid peptide corresponding to the cytoplasmic domain of the natriuretic peptide receptor C (NPR-C) inhibited adenylyl cyclase activity via pertussis toxin (PT)-sensitive G(i) protein. In the present studies, we have used seven different peptide fragments of the cytoplasmic domain of the NPR-C receptor with complete, partial, or no G(i) activator sequence to examine their effects on adenylyl cyclase activity. The peptides used were KKYRITIERRNH (peptide 1), RRNHQEESNIGK (peptide 2), HRELREDSIRSH (peptide 3), RRNHQEESNIGKHRELR (peptide 4), QEESNIGK (peptide X), ITIERRNH (peptide Y), and ITIYKKRRNHRE (peptide Z). Peptides 1, 3, and 4 have complete G(i) activator sequences, whereas peptides 2 and Y have partial G(i) activator sequences with truncated carboxyl or amino terminus, respectively. Peptide X has no structural specificity, whereas peptide Z is the scrambled peptide control for peptide 1. Peptides 1, 3, and 4 inhibited adenylyl cyclase activity in a concentration-dependent manner with apparent K(i) between 0.1 and 1 nm; however, peptide 2 inhibited adenylyl cyclase activity with a higher K(i) of about 10 nm, and peptides X, Y, and Z were unable to inhibit adenylyl cyclase activity. The maximal inhibitions observed were between 30 and 40%. The inhibition of adenylyl cyclase activity by peptides 1-4 was absolutely dependent on the presence of guanine nucleotides and was completely attenuated by PT treatment. In addition, the stimulatory effects of isoproterenol, glucagon, and forskolin on adenylyl cyclase activity were inhibited to different degrees by these peptides. These results suggest that the small peptide fragments of the cytoplasmic domain of the NPR-C receptor containing 12 or 17 amino acids were sufficient to inhibit adenylyl cyclase activity through a PT-sensitive G(i) protein. The peptides having complete structural specificity of G(i) activator sequences at both amino and carboxyl termini were more potent to inhibit adenylyl cyclase activity as compared with the peptides having a truncated carboxyl terminus, whereas the truncation of the amino-terminal motif completely attenuates adenylyl cyclase inhibition.

Ligand binding-dependent limited proteolysis of the atrial natriuretic peptide receptor: juxtamembrane hinge structure essential for transmembrane signal transduction

The atrial natriuretic peptide (ANP) receptor is a 130-kDa transmembrane protein containing an extracellular ANP-binding domain, a single transmembrane sequence, an intracellular kinase-homologous domain, and a guanylate cyclase (GCase) domain. We observed that the receptor, when bound with ANP, was rapidly cleaved by endogenous or exogenously added protease to yield a 65-kDa ANP-binding fragment. No cleavage occurred without bound ANP. This ligand-induced cleavage abolished GCase activation by ANP. Cleavage occurred in an extracellular, juxtamembrane region containing six closely spaced Pro residues and a disulfide bond. Such structural features are shared among the A-type and B-type ANP receptors but not by ANP clearance receptors. The potential role of the hinge structure was examined by mutagenesis experiments. Mutation of Pro(417), but not other Pro residues, to Ala abolished GCase activation by ANP. Elimination of the disulfide bond by Cys to Ser mutations yielded a constitutively active receptor. Pro(417), and Cys(423) and Cys(432) forming the disulfide bond are strictly conserved among GCase-coupled receptors, while other residues are largely variable. The conserved Pro(417) and the disulfide bond may represent a consensus signaling motif in the juxtamembrane hinge structure that undergoes a marked conformational change upon ligand binding and apparently mediates transmembrane signal transduction.

Modulation of endocardial natriuretic peptide receptors in right ventricular hypertrophy

Natriuretic peptide (NP) receptors (NPRs) located at the endocardial endothelium are suggested to be involved in regulating myocardial contractility. However, the characteristics and modulation of NPRs in relation to cardiac failure are not well defined. This study examined the properties of NPRs in ventricular endocardium using quantitative receptor autoradiography, RT-PCR, Southern blot analysis, and activation of particulate guanylyl cyclase (GC) by NPs. In control rats, specific 125I-labeled rat atrial NP (rANP)(1-28) binding sites were localized in right (RV) and left ventricular (LV) endocardium. Binding affinities of 125I-rANP(1-28) were remarkably higher in RV than LV endocardium. Radioligand binding at these sites was mostly inhibited by des[Gln18,Ser19,Gly20,Leu21, Gly22]ANP(4-23), a specific NP clearance receptor ligand. mRNAs for all three recognized NPRs were detected in endocardial cells by RT-PCR and confirmed by Southern blot analysis. Production of cGMP by particulate GC in endocardial cell membranes was stimulated by NPs with a rank order of potency of C-type NP(1-22) >> brain NP (BNP)(1-26) > ANP(1-28). We also examined the modulation of these NPRs during cardiac hypertrophy induced by monocrotaline (MCT). In MCT-treated rats with pulmonary hypertension, specific (125)I-rANP(1-28) binding to hypertrophied RV endocardium almost disappeared and cGMP production by NPs was significantly decreased. In rats with pulmonary hypertension, plasma levels of ANP and BNP were increased by fivefold compared with controls. The results indicate that there is a differential distribution of NPRs in the cardiac chambers, with the most abundant binding sites in RV endocardium, that NPR-B is the predominant GC-coupled NPR in ventricular endocardium, and that endocardial NPRs are downregulated with ventricular hypertrophy. Downregulation of NPRs may be associated with an increment of endogenous NP production caused by mechanical overload in hypertrophied ventricle.

Higher proportions of type C than of types A and B natriuretic peptide receptors exist in the rat ciliary body

We investigate the interaction of atrial natriuretic peptide (ANP) brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) with their receptors (NPRA, NPRB and NPRC), as well as the proportion and localization of those receptors in the rat ciliary body. Binding assays and affinity cross-linking experiments demonstrated the presence of the NPRC receptor type. However, the three natriuretic peptides stimulate the guanylate cyclase activity in the ciliary body membranes suggesting the presence of the NPRA and NPRB receptor type. Microautoradiographic data show that the NPRs are localized in the whole ciliary body. Our results indicated that NPRC is the most prominent receptor type in this tissue.

Interaction of isatin with type-A natriuretic peptide receptor: possible mechanism

The effect of isatin on rat brain particulate guanylate cyclase (GC) was investigated. The enzyme was stimulated by atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and urodilatin, but not by C-type natriuretic peptide (CNP). Their effects were not additive, pointing to action via the GC-A receptor. Isatin, in dose-dependent manner, abolished this stimulation. The non-hydrolysable ATP analogue, adenylylimidodiphosphate, potentiated the effects of submaximal doses of ANP, BNP and urodilatin on this particulate GC-A, and attenuated or abolished sensitivity to isatin. These results suggest that isatin antagonises the generation of second messenger by GC-A; this sensitivity might be regulated at an ATP binding site, possibly a protein kinase-like domain.

Atrial natriuretic peptide-C receptor and membrane signalling in hypertension

Atrial natriuretic peptide (ANP) regulates a variety of physiological parameters, including the blood pressure and intravascular volume, by interacting with its receptors present on the plasma membrane. ANP receptors are of three subtypes: ANP-A, -B and -C receptors. ANP-A and ANP-B receptors are guanylyl cyclase receptors, whereas ANP-C receptors are coupled to adenylyl cyclase inhibition or phospholipase C activation through inhibitory guanine nucleotide-regulating protein. Unlike other G protein-coupled receptors, ANP-C receptors have a single transmembrane domain and a short cytoplasmic domain of 37 amino acids, the cytoplasmic domain has a structural specificity like those of other single-transmembrane-domain receptors and 37 amino-acid cytoplasmic domain peptide is able to exert is inhibitory effect on adenylyl cyclase. The activation of ANP-C receptor by C-ANP(4-23) (a ring-deleted peptide of ANP) and C-type natriuretic peptide inhibits the mitogen-activated protein kinase activity stimulated by endothelin-3, platelet-derived growth factor and phorbol-12 myristate 13-acetate. C-ANP also inhibits mitogen-induced stimulation of DNA synthesis, indicating that the ANP-C receptor plays a role in cell proliferation through an inhibition of mitogen-activated protein kinase and suggesting that the ANP-C receptor might also be coupled to other signal transduction mechanism(s) or that there might be an interaction of the ANP-C receptor with some other signalling pathways. ANP receptor binding is decreased in most organs in hypertensive subjects and hypertensive animals. This decrease is consistent with there being fewer guanylyl cyclase-coupled receptors in the kidney and vasculature and selective inhibition of the ANP-C receptor in the thymus and spleen. Platelet ANP-C receptors are decreased in number in hypertensive patients and spontaneously hypertensive rats. ANP-A, -B and -C receptors are decreased in number in deoxycorticosterone acetate-salt-treated kidneys and vasculature; however, the responsiveness of adenylyl cyclase to ANP is augmented in the vasculature and heart and is attenuated completely in platelets. These alterations in ANP receptor subtypes may be related to the pathophysiology of hypertension. Several hormones such as angiotensin II, ANP and catecholamines, the levels of which are increased in hypertension, downregulate or upregulate ANP-C receptors and ANP-C receptor-mediated inhibition of adenylyl cyclase. It can be suggested that the antihypertensive action of several types of drugs such as angiotensin converting enzyme inhibitors, angiotensin type 1 receptor antagonists and beta2-adrenergic antagonists may partly be attributed to their ability to modulate the expression and function of the ANP-C receptor.

Inhibition of human neutrophil beta2-integrin-dependent adherence by hyperbaric O2

Animal and clinical investigations have reported that exposure to hyperbaric O(2) improved the outcome of some reperfusion injuries. Animal studies have suggested that this may be due to an inhibition of leukocyte adherence to injured endothelium. This investigation tested the hypothesis that exposure to hyperbaric O(2) would inhibit beta2-integrin-dependent adherence of human neutrophils. Subjects were exposed to O(2) at partial pressures of up to 3 atmospheres absolute (ATA; 1 ATA = 0.1 MPa) for 45 min, and neutrophil binding to nylon columns and to fibrinogen-coated surfaces was measured. Exposure to O(2) at 2.8 or 3.0 ATA inhibited beta2-integrin-dependent neutrophil adherence but had no effect on the cell-surface expression of beta2-integrins, respiratory burst in response to phorbol ester, or non-beta2-integrin-dependent adherence to plastic plates coated with a fibronectin-like protein. beta2-Integrin adherence was restored by incubating blood with 8-bromoguanosine 3',5'-cyclic monophosphate (cGMP) and hyperbaric O(2) inhibited synthesis of cGMP by neutrophils stimulated with N-formyl-Met-Leu-Phe (FMLP). In studies of cell fractions, the activity of membrane guanylate cyclase was found to be increased by incubation with FMLP as well as by atrial natriuretic peptide (ANP) plus ATP. Hyperbaric O(2) had no effect on the basal activity of soluble or membrane-bound guanylate cyclase. However, hyperbaric O(2) inhibited the function of both the extracellular binding domain of membrane guanylate cyclase as well as intracellular catalytic activity. There are approximately 7,300 membrane guanylate cyclase molecules per cell, based on binding studies with ANP, with a dissociation constant of approximately 450 pM. Hyperbaric O(2) inhibits the function of human neutrophil beta2-integrins by a process linked to impaired synthesis of cGMP.

Cyclic GMP accumulation in pulmonary microvascular endothelial cells measured by intact cell prelabeling

Cyclic GMP accumulation in cultured rat pulmonary microvascular endothelial cells (RPMVEC) was studied with a new prelabeling method developed using intact platelets and smooth muscle cells (1). [3H]-hypoxanthine was used to radiolabel the cellular guanine nucleotide pool. Neutral alumina and Dowex-50 double column chromatography was used to purify and quantitate the levels of [3H]-cyclic GMP. Changes in cyclic GMP metabolism in short and long term RPMVEC cultures were studied using rat atrial naturetic factor 8-33 (ANF) and sodium nitroprusside (SNP) in the presence and absence of cyclic nucleotide (CN) phosphodiesterase (PDE) inhibitors. In RPMVEC exogenous hypoxanthine was incorporated into both low (65% uptake) and high (34% uptake) passage cells in a time-dependent manner reaching maximum incorporation near 8 hours. Basal cyclic GMP values in both groups were 0.003% of the total cellular tritium (9 x 10(6) and 4 x 10(6) cpm/10(6) cells, respectively). ANF treatment of prelabeled RPMVEC resulted in a 10- to 12-fold increase in [3H]-cyclic GMP in the absence of CN PDE inhibitors (EC50 = 5.4 nM). However, incubation with SNP showed no changes in cellular cyclic GMP accumulation. Several relatively selective CN PDE inhibitors had no effect on ANF or SNP induced cyclic GMP accumulation in RPMVEC. The ANF induced cGMP accumulation was verified by radioimmunoassay. These studies confirm the utility of the hypoxanthine prelabeling technique to monitor intact microvascular EC cyclic GMP accumulation. Cultured RPMVEC show little or no functional soluble guanylate cyclase or cyclic GMP PDE activity.

Thrombin inhibits atrial natriuretic peptide receptor activity in cultured bovine endothelial cells

Thrombin and the atrial natriuretic peptide (ANP) possess a number of functionally antagonistic properties in vascular endothelial cells. Thus, regulatory interactions that modulate the activity of one or the other could have important sequelae with regard to cardiovascular homeostasis. Thrombin treatment effected a dose- and time-dependent reduction in ANP receptor activity (maximal 70% to 80% inhibition) in cultured bovine aortic endothelial cells. This resulted from a decrease in total receptor number as well as a modest reduction in the affinity of the receptor for its ligand. The inhibition was largely confined to the type C receptor population, in that thrombin had no effect on maximal type A receptor-linked cGMP accumulation. The protein kinase C-activating phorbol ester 12-O-tetradecanoylphorbol 13-acetate effected a similar reduction in binding activity; however, suppression of protein kinase C activity did not reverse the thrombin effect. Pretreatment of endothelial cells with cycloheximide did not completely prevent the thrombin-dependent inhibition, and thrombin did not effect a reduction in type C receptor mRNA levels, findings that argue for a postsynthetic inhibitory locus. The inhibition of receptor activity was effectively irreversible in that suspension of protein synthesis blocked the recovery of receptor density on the cell surface. Reduction in type C receptor density was accompanied by modest increases in the stability of ANP in the culture medium and enhancement of the cellular cGMP response to the peptide, particularly at low ligand concentrations. These findings demonstrate a potentially important interaction between these two agonist systems in regulating endothelial cell function within the vascular wall.

Regulation of human erythrocyte Na+/H+ exchange by soluble and particulate guanylate cyclase

Guanylate cyclase activity in human erythrocytes is investigated by evaluating the intracellular guanosine 3',5'-cyclic monophosphate (cGMP) content in the presence of various agents that exert specific effects on soluble or particulate guanylate cyclase. The increase in the intraerythrocyte cGMP content by the soluble guanylate cyclase activators nitroprusside and NaNO2 suggests the presence of this enzyme in human erythrocytes. The effects of four different atrial natriuretic peptide (ANP) fragments on the intraerythrocyte cGMP content is also studied. ANP II and ANP III increase the intraerythrocyte cGMP content, whereas ANP I and des-Ser5,des-Ser6-ANP III are ineffective. Thus our data show that human erythrocytes possess particulate guanylate cyclase together with the soluble enzyme. The ANP fragments ANP II and ANP III also activate the erythrocyte Na+/H+ exchange. Nitroprusside, M & B 22948 (an inhibitor of cGMP phosphodiesterase), and the cGMP analogues dibutyryl cGMP and 8-bromoguanosine 3',5'-cyclic monophosphate also increase the erythrocyte Na+/H+ exchange rate. The latter data also suggest that the erythrocyte Na+/H+ exchange is regulated by cGMP.

Lactational changes in the rat exocrine pancreas somatostatin receptors and modulation of guanylate cyclase

Hyperplasia of the pancreatic tissue during late lactation (third week) and lasting for at least the first two weeks after weaning has been observed by several authors. Since the tetradecapeptide somatostatin (SS) inhibits pancreatic growth and its plasma levels are elevated during these periods, the aim of the present study was to determine the possible implication of the somatostatinergic system in the pancreatic changes cited above. Thus, the present study investigated 125I-Tyr(11)-somatostatin (125I-Tyr(11)-SS) binding and the effects of SS on guanylate cyclase activity as well as pancreatic somatostatin-like immunoreactivity (SSLI) levels in pancreatic acinar membranes from control, lactating and weaning rats. SS receptors were identified using 125I-Tyr(11)-SS and isolated pancreatic acinar membranes in vitro. There was an increase in the number of SS receptors after the third week of lactation (244 +/- 6 vs. 155 +/- 12 fmol/mg protein, P < 0.01) and the first two weeks after weaning (327 +/- 8 vs. 164 +/-10 fmol/mg protein, P < 0.001). No change in the affinity of the receptor site was detected at either study time. In addition, SS-stimulated guanylate cyclase activity was markedly increased at the third week of lactation (119%) and at the second week after weaning (158%) when compared with the control group. In contrast, basal guanylate cyclase activity was not modified at either study period. Thus, SS-stimulated guanylate cyclase activity is increased in pancreatic acinar membranes at late lactation and at the second week after beginning weaning probably due to an increase in the number of SS receptors. Significant decreases in SSLI content were observed at the third week of lactation (69%) and the second week after weaning (37%) when compared with the respective controls. The present results suggest that pancreatic acinar cell growth observed at the third week of lactation and the second week after weaning is associated with up-regulation of SS receptors which would represent a mechanism promoted by the cell that would negatively regulate the mitogenic activity of the increased number of pancreatic growth factors observed during both periods.

The mechanism of relaxation induced by atrial natriuretic peptide in the porcine renal artery

1. The mechanisms underlying the relaxation of the porcine renal artery induced by atrial natriuretic peptide (ANP) were investigated, using front-surface fluorimetry with fura-2 and receptor-coupled permeabilization by alpha-toxin. 2. ANP decreased the cytosolic Ca2+ concentration ([Ca2+]i) and tension during the contraction induced by a high external K+ solution, in a concentration-dependent manner. This ANP-induced decrease in [Ca2+]i during the contraction induced by high K+ solution was composed of two phases, an initial rapid phase, followed by a maintenance phase. The initial rapid decrease in [Ca2+]i, but not the maintained decrease in [Ca2+]i, was inhibited when the tissue was treated with thapsigargin, a selective Ca2+ pump inhibitor of the sarcoplasmic reticulum. When the tissues were treated with thapsigargin and external Ca2+ was replaced by Ba2+, which cannot be transported by the Ca2+ pump, ANP did not induce a decrease in [Ba2+]i, even though the elevation of tension induced by Ba2+ was strongly inhibited. 3. In the absence of extracellular Ca2+, ANP inhibited the release of Ca2+ from the intracellular store induced by noradrenaline (NA). 4. The [Ca2+]i (abscissa scale)-tension (ordinate scale) relationship observed during the contraction induced by various concentrations of high external K+ solution was shifted downwards by the addition of 10(-8) M ANP, indicating that, at any given [Ca2+]i, the tension generated by high K+ solution was considerably inhibited by the addition of 10(-8) M ANP. The [Ca2+]i-tension curve of the contraction obtained by the cumulative application of external Ca2+ (0-3.75 mM) during depolarization with 118 mM K+ solution was shifted to the left by 3 x 10(-7) M NA. This NA-induced [Ca2+]i-tension relationship was shifted to the right by 10(-8) M ANP, indicating that the ANP-induced reduction of Ca(2+)-sensitivity operates during the contraction induced by NA. 5. In alpha-toxin-permeabilized preparations, ANP induced relaxation of tissues precontracted with a mixture of 3 x 10(-7) M Ca2+, 10(-5) M guanosine 5'-triphosphate (GTP) and 10(-6) M NA. Thus a component of ANP-induced relaxation took place by way of a reduction in the Ca2+ sensitivity of the myofilaments, independent of changes in [Ca2+]i. 6. These results indicate that ANP induces relaxation of the porcine renal artery by: (1) reducing [Ca2+]i mainly via the activation of the Ca2+ pumps located on the sarcoplasmic reticulum and sarcolemma, as well as via inhibition of agoinist-induced release of Ca2+ from the intracellular store; and (2) decreasing the Ca(2+)-sensitivity of the contractile elements.

Natriuretic peptide receptors on human trabecular meshwork cells

The effects of natriuretic peptides on cGMP formation and [125I]ANP binding in human trabecular meshwork cells were investigated. CNP at 1 microM stimulated cGMP formation approximately 18-25 fold, with a half maximal effective concentration approximately 20-30nM. BNP at 1 microM stimulated approximately 7 fold, while ANP stimulated cGMP formation 2-fold at 1 microM but had little or no effect at concentrations below 1 microM. Displacement binding of [125I]ANP to intact TM cells in the presence of unlabeled ANP indicated a single binding site with a dissociation constant approximately 0.15nM.c-ANP, which binds specifically to natriuretic peptide C receptors, displaced > 95% [125I]ANP binding to surface receptor sites with a half-maximal effective concentration comparable to that of ANP or BNP. c-ANP had no inhibitory effect on CNP stimulation of cGMP formation. The data suggest that human TM cells possess natriuretic peptide B receptors as the primary guanylyl cyclase-containing subtype and C receptors as the numerically predominant subtype of natriuretic peptide receptors.

Binding of CNP-22 and CNP-53 to cultured mouse astrocytes and effects on cyclic GMP

We examined the binding characteristics of the two endogenous forms of C-type natriuretic peptide (CNP-22 and CNP-53) and their effects on cyclic GMP (cGMP) accumulation in primary cultures of mouse astrocytes. CNP-22 and CNP-53 competitively inhibited the specific binding of [125I][Tyr0]CNP, with an IC50 value of 32 and 37 pM, respectively. They also induced cGMP production in a dose-dependent and similar fashion, with an EC50 of 32 nM and maximal cGMP responses of 189.6 +/- 21.6 pmol/mg protein for CNP-22, and 170.3 +/- 18.7 pmol/mg protein for CNP-53, respectively. The effect of CNP-53 could not be explained by conversion to CNP-22, because HPLC analysis did not show significant proteolytic conversion by astrocytes during the incubation. Our results suggest that CNP-53 could, in concert with other natriuretic peptides, have a neuromodulatory function and thereby contribute to the central regulation of hemodynamic and fluid homeostasis.

Atrial natriuretic peptide suppresses the transcription of its guanylyl cyclase-linked receptor

Atrial natriuretic peptide (ANP) treatment of rat aortic smooth muscle cells suppressed both 125I-ANP binding and ANP-dependent cGMP accumulation, suggesting reductions in the type C (NPR-C) and type A (NPR-A) natriuretic peptide receptor populations, respectively. NPR-A, but not NPR-C, mRNA levels were reduced in a dose-dependent fashion by ANP. The latter effect appeared to be due, at least in part, to suppression of NPR-A gene promoter activity. ANP effected a dose- and time-dependent reduction in a transiently transfected NPR-A luciferase reporter (-1575LUC). Analysis of 5' deletion mutants of the NPR-A promoter demonstrated that the ANP-dependent sequence lies between -1575 and -1290 relative to the transcription start site. Inhibition of the ANP promoter was also effected by brain natriuretic peptide, type C natriuretic peptide, and 8-bromo-cGMP, but not by the NPR-C-selective ligand cANF. In the case of 8-bromo-cGMP, the responsive element(s) was localized to the same 285-base pair region linked to the ANP effect above. These findings indicate that ANP autoregulates its own receptors in these cells and, at least in the case of NPR-A, it does so through suppression of receptor gene expression and receptor synthesis. This suppression may operate through a cGMP-dependent element located more than a kilobase upstream from the transcription start site.

Isatin is a potent endogenous antagonist of guanylate cyclase-coupled atrial natriuretic peptide receptors

Isatin (indole-2,3-dione) is an endogenous compound with anxiogenic properties. In the brain, highest levels (0.1 microgram/g) have been found in the rat hippocampus. In the present study, we show that isatin has little effect on a wide range of neurotransmitter and hormonal receptors but that it acts as an inhibitor of atrial natriuretic peptide (ANP) binding, with an IC50 of 4x 10(-7) M. It also inhibits ANP-activated particulate guanylate cyclase from rat kidney, heart and brain membranes in dose-dependent fashion, varying also with ANP concentration. These findings suggest that isatin is a new endogenous regulator of mammalian ANP activity, with potential implications for the control of both anxiety and natriuresis.

Defective ANF-R2/ANP-C receptor-mediated signalling in hypertension

In the present studies we have shown that atrial natriuretic factor (peptide) receptor of ANF-R2/ANP-C type is coupled to adenylyl cyclase/cAMP signal transduction system through Gi-regulatory protein and is implicated in mediating some of the physiological responses of atrial natriuretic factor or peptide (ANP). ANF-R2/ANP-C receptor-mediated adenylyl cyclase inhibition was altered in hypertension. This alteration was tissue specific. In heart, aorta, brain and adrenal, the extent of inhibition of adenylyl cyclase by ANP was enhanced in SHR as compared to age-matched WKY, whereas in platelets, the ANP-mediated inhibition was completely attenuated. The enhanced inhibition of adenylyl cyclase by ANP was also observed in heart and aorta from DOCA-salt hypertensive rats. In addition, the augmented inhibition of adenylyl cyclase by ANP was observed in 2 weeks and older SHR but not in 3-5 days old SHR. Similarly, in DOCA-salt hypertensive rats, the enhanced inhibition of adenylyl cyclase by ANP was observed after 2 weeks of DOCA-salt treatment when the blood pressure was also enhanced, however one week older SHR but not in 3-5 days old SHR. Similarly, in DOCA-salt hypertensive rats, the enhanced inhibition of adenylyl cyclase by ANP was observed after 2 weeks of DOCA-salt treatment when the blood pressure and augmented ANP-mediated inhibition of adenylyl of DOCA-salt treatment did not result in an augmented blood pressure and augmented ANP-mediated inhibition of adenylyl cyclase, suggesting that blood pressure increase may be responsible for the enhanced responsiveness of ANP to adenylyl cyclase inhibition. However, in genetic model of hypertension, the increased inhibition of adenylyl cyclase by ANP at 2 weeks of age (when the blood pressure is normal) may be implicated in the pathogenesis of hypertension. The augmented inhibition of adenylyl cyclase in cardiovascular tissues from SHR and DOCA-salt hypertensive rats may be due to the upregulation of ANF-R2/ANP-C receptors or due to the amplification of post-receptor signalling mechanisms.

Atrial natriuretic peptide blocks hemorrhagic brain edema after 4-hour delay in rats

BACKGROUND AND PURPOSE

Atrial natriuretic peptide (ANP) and arginine vasopressin regulate brain water and electrolytes. Treatment with ANP at the onset of a hemorrhagic injury reduces edema. Clinically, however, hemorrhagic masses form too rapidly for preventive treatment. Therefore, we measured the effect of ANP on brain edema after the hemorrhagic mass was formed.

METHODS

Adult rats had hemorrhagic lesions produced by the intracerebral injection of 0.4 U bacterial collagenase. Four hours later, an infusion of ANP (120 or 700 ng/kg per 20 hours) was begun into the peritoneum using an implanted miniosmotic pump. Twenty-four hours after the injury, brain water and electrolyte values were measured. The mechanism of ANP action was explored in other groups of rats that either had osmolality increased with mannitol or were injected with the cyclic GMP analogue, 8-bromo-cGMP.

RESULTS

Atrial natriuretic peptide given after a 4-hour delay significantly reduced brain water and sodium 24 hours after the injury (P < .05). However, neither mannitol nor 8-bromo-cGMP affected brain edema.

CONCLUSIONS

Delayed administration of ANP reduces brain edema secondary to a hemorrhagic mass. Because it is effective after the mass has formed, ANP may be useful in treatment of edema secondary to intracranial bleeding.

Natriuretic peptides inhibit DNA synthesis in cardiac fibroblasts

We have examined the effects of the natriuretic peptides on DNA synthesis in primary cultures of neonatal rat cardiac fibroblasts. Binding analysis using 125I-labeled atrial natriuretic peptide identified a single class of high-affinity binding sites (Kd = 0.03 +/- 0.01 nmol/L) in these cells. Of these sites, 80% appear to be of the natriuretic peptide C receptor subtype, with the remainder being A and B receptor subtypes. Northern blot analysis confirmed the presence of all three natriuretic peptide receptors in these cells. Atrial natriuretic peptide (10(-7) mol/L) effected a modest but consistent reduction in both agonist- and stretch-stimulated [3H]thymidine incorporation (17% to 41%). Moreover, brain natriuretic peptide (10(-7) mol/L), C-type natriuretic peptide (10(-7) mol/L), and des-[Gln18,Ser19,Gly20,Leu21,Gly22]-ANF 4-23-NH2 (10(-7) to 10(-6) mol/L) all proved capable of antagonizing growth factor-dependent [3H]thymidine incorporation (the inhibition ranged from 14% to 28%) and cell proliferation, suggesting that all three natriuretic peptide receptor subtypes are involved in the regulation of mitogenesis in these cultures. The inhibition by atrial natriuretic peptide was amplified by cotreatment with phosphodiesterase inhibitors. Similar reduction in [3H]thymidine incorporation was seen after treatment with 8-bromo-cGMP (10(-4) to 10(-3) mol/L) or nitroprusside (10(-4) to 10(-3) mol/L). These results suggest an important paracrine role for the natriuretic peptides in regulating fibroblast growth during cardiac hypertrophy.

Somatostatin binding capacity, guanylate cyclase and tyrosine phosphatase activities during pancreatic proliferation in the rat induced by gastrectomy

Gastrectomy increased pancreatic growth and this effect was associated with an increase in the number of somatostatin-14 (SS) receptors (146% of control) without altering their affinity. SS increased guanylate cyclase activity twofold in pancreatic acinar membranes from gastrectomized rats. The gastrectomy decreased pancreatic SS-like immunoreactivity (SS-LI) content (55% of control levels) and tyrosine phosphatase activity (74% of control levels). Administration of proglumide (20 mg/kg, IP), a gastrin/cholecystokinin (CCK) receptor antagonist, suppressed the inhibitory effect of gastrectomy on basal tyrosine phosphatase activity and SS-LI content, which returned to control levels. Furthermore, proglumide suppressed the increase of the number of SS receptors and of SS-stimulated guanylate cyclase activity induced by gastrectomy. All this suggests that pancreatic acinar cell growth is associated with upregulation of SS receptors, which could represent a mechanism promoted by the cell to negatively regulate the mitogenic activity of pancreatic growth factors such as CCK. In addition, the results also suggest that the negative regulation of tyrosine phosphatase activity may be important in the events involved in the pancreatic hyperplasia observed after gastrectomy.

Hormonal modulation of atrial natriuretic factor receptors in adrenal fasciculata cells from female rats

We recently reported that the hormonal status of female rats modified atrial natriuretic factor (ANF) receptors and the aldosterone-suppressant activity of ANF in adrenal glomerulosa cells; here we investigated if this was also true for adrenal fasciculata cells. Adrenal fasciculata cells from animals in different hormonal states contained guanylate cyclase linked ANF-R1 receptors but not ANF-R2 (clearance) receptors. The concentration of ANF-R1 receptors in cells from intact virgin rats was insignificantly higher than in cells from 13- to 15-day pregnant rats and significantly higher than in cells from ovariectomized (OVX), OVX beta-estradiol-treated, and OVX progesterone-treated rats. Under none of the hormonal states did ANF suppress adrenocorticotropic hormone (ACTH) stimulated corticosterone secretion. Data suggest that the interactions between ANF and ACTH on mineralocorticoid and glucocorticoid synthesis markedly differ.

Atrial natriuretic factor inhibits autophosphorylation of protein kinase C and A 240-kDa protein in plasma membranes of bovine adrenal glomerulosa cells: involvement of cGMP-dependent and independent signal transduction mechanisms

To clarify the intracellular signalling mechanisms of atrial natriuretic factor (ANF), we studied its effect on protein phosphorylation in plasma membranes of bovine adrenal cortical cells. ANF (1 x 10(-7) M) inhibited phosphorylation of the 78-kDa protein kinase C (PKC) and a 240-kDa protein in specific manner. In parallel experiments, cGMP (0.5 mM) inhibited phosphorylation of only the 78-kDa PKC but it did not affect phosphorylation of the 240-kDa protein. Phosphorylation of the 78-kDa PKC was enhanced in a Ca(2+)-/phospholipid-dependent manner. However, after prolonged preincubation of plasma membranes with Ca2+ (0.5 mM), the incorporation of 32P-radioactivity rapidly decreased in the 78-kDa PKC and subsequently increased in the 45- and 48-kDa protein bands due to Ca(2+)-dependent proteolytic degradation of 78-kDa PKC. Polyclonal antibodies against brain PKC were used to immunoblot and immunoprecipitate the 78-kDa PKC. Preincubation of plasma membranes with Ca2+ for varying times, followed by immunoblotting revealed a gradual loss of the immunoreactive 78-kDa PKC band in a time-dependent manner. Immunoprecipitation of phosphorylated 78-kDa PKC in plasma membranes showed that its phosphorylation was significantly inhibited in the presence of ANF as compared to control membranes, phosphorylated in the absence of ANF. The results in this present study document a new signal transduction mechanism of ANF at molecular level which possibly involves dephosphorylation of the 78-kDa PKC and a 240-kDa protein in a cGMP-dependent and -independent manner in bovine adrenal glomerulosa cell membranes.

Somatostatin activates particulate guanylate cyclase in cultured rat mesangial cells

Although the ability of somatostatin (ST) to relax cultured rat mesangial cells has recently been described, the intimate cellular mechanisms responsible for this effect have not been adequately clarified. The present experiments were designed to test the hypothesis that cyclic GMP (cGMP) could be involved in the genesis of this relaxation. ST increased cGMP synthesis by cultured rat mesangial cells, in basal conditions and in the presence of isobutylmethylxanthine or zaprinast. This effect was dose-dependent, with a threshold value of about 1 nM and a maximal response at ST concentrations between 0.1 and 1 microM. This increased cGMP synthesis was dependent on the stimulation by ST of a particulate guanylate cyclase, as the synthesis of cGMP by a particulate membrane fraction obtained from the cells increased in the presence of ST. When the cGMP-specific phosphodiesterase of mesangial cells was blocked with zaprinast, the ST-dependent relaxation, assessed both by morphological and biochemical criteria, significantly increased with respect to the experiments performed without zaprinast. These results support a role for cGMP in the ST-dependent relaxation of cultured rat mesangial cells. The increased cGMP synthesis appears to be the consequence of the activation of some form of particulate guanylate cyclase.

Modulation of guanylate cyclase-coupled atrial natriuretic factor receptor activity by mastoparan and ANF in murine Leydig tumor cells: role of G-proteins

Mastoparan potently stimulated catalytic activity of guanylate cyclase-coupled atrial natriuretic factor receptor (GC-A/ANF-R), both in the plasma membranes and intact Leydig tumor (MA-10) cells. In plasma membrane preparations, a maximum of 5-fold GC catalytic activity was stimulated by 100 microM mastoparan and the half maximum stimulation (EC50) was achieved at 40 microM concentration. Mastoparan potentiated GC activity by more than 40%, above the level, stimulated by ANF. Mas 7, an active analog of mastoparan, stimulated the GC activity in a similar manner to mastoparan whereas Mas 17, an inactive analog, did not enhance GC activity. In membranes prepared from mastoparan-treated intact MA-10 cells, GC catalytic activity was enhanced by more than 4-fold as compared with untreated control cells. Pretreatment of membranes with either anti-Gs alpha or anti-Gi alpha antibodies had no effect on mastoparan-stimulated GC activity, however, anti-Go alpha antibodies inhibited the stimulatory effect of mastoparan by almost 50%. Agents known to modulate the effect of mastoparan such as EGTA (Ca2+ chelator), W7 (calmodulin inhibitor) and staurosporine (protein kinase C inhibitor) had no effect on the mastoparan-stimulated GC activity. Mastoparan enhanced the ANF-stimulated GC activity in detergent solubilized membrane preparations without a significant change in ANF-binding capacity. The data establish a role for mastoparan in the ANF-dependent stimulation of GC-A/ANF-R catalytic activity, both in the plasma membrane preparations and intact Leydig tumor (MA-10) cells. Furthermore, these findings provide new evidence that mastoparan (isolated from wasp venom) potently stimulates guanylate cyclase activity of GC-A/ANF-R by activating G-proteins.

The control of endothelin-1 secretion

1. The human endothelin-1 (ET-1) gene, which is located on chromosome 6, contains cis-regulatory elements in the 5'-flanking region including the TPA-responsive element, nuclear factor 1 binding element and GATA motif. 2. The expression of preproendothelin-1 (PPET-1) mRNA is regulated by a mechanism involving receptor mediated mobilization of intracellular Ca2+ and activation of protein kinase C in endothelial cells. 3. Activation of protein kinase C results in the synthesis of c-Jun protein and the rapid dephosphorylation of c-Jun protein. Consequently, the binding activity of c-Jun protein to the TPA-responsive element increases, and this causes the induction of PPET-1 mRNA. 4. The microtubular system seems to play some important roles in ET-1 secretion, especially in the process of transferring the synthesized ET-1 to the cell surface of the endothelial cells. 5. The secretion of ET-1 from endothelial cells is also regulated by intracellular Ca2+ released from the Ca2+ store and by Ca2+-calmodulin complex. The phosphorylation of the myosin light chain, elicited by myosin light chain kinase and activated by Ca2+-calmodulin complex, facilitates the formation of filamentous myosin and actin which probably participate in ET-1 secretion especially in transporting the ET-1-containing vesicles towards the cell membrane in the stimulated endothelial cells. 6. Many cultured cells, other than endothelial cells, also secret ET-1 into the culture medium and this secretion can be stimulated by a variety of agents.

Lack of biologic activity or specific binding of amino-terminal pro-ANP segments in the rat

In addition to atrial natriuretic peptide (ANP99-126) itself, linear peptide fragments from its N-terminal prohormone segment (pro-ANP) have been reported to have biological activity. In vivo, diuresis and natriuresis, as well as hypotension have been observed. In vitro, sodium uptake into medullary collecting duct cells was inhibited, and tone of vascular smooth muscle was reduced, associated with activation of guanylate cyclase. Such previous studies have used heterologous peptides and species, e.g., human pro-ANP1-30 or pro-ANP31-67, tested in rat, pig, or dog. The present experiments were designed to test whether rat pro-ANP1-30 or pro-ANP31-67 were natriuretic and hypotensive in rats, whether the two peptides showed specific binding to plasma membranes from rat kidney cortex or aorta, and whether they affected particulate guanylate cyclase activity in rat glomerular membranes. To extend in vitro results from the literature, the effect of human pro-ANP31-67 on transport in the rat medullary collecting duct in vivo was also tested. Although rat ANP99-126, as expected, increased diuresis and natriuresis, associated with inhibition of transport in the medullary collecting duct, in identically treated rats human pro-ANP31-67 was without effect. Similarly, only the ANP99-126 infusion resulted in reduction of arterial blood pressure. Furthermore, no diuretic, natriuretic, or hypotensive responses were observed in rats infused with either rat pro-ANP31-67 or pro-ANP1-30. In plasma membranes from rat kidney cortex or aorta, neither of the rat prosegments showed specific binding, or interference with ANP99-126 receptors. Finally, in contrast to ANP99-126, neither of the prosegments was able to increase basal guanylate cyclase activity in rat glomerular membranes. Therefore, under our experimental conditions we were unable to replicate the earlier results. This study thus does not support a regulatory role for pro-ANP fragments in blood volume or blood pressure homeostasis.

Differential regulation of natriuretic peptide receptor activity in vascular cells

We studied the regulation of the individual natriuretic peptide receptor subtypes by 12-O-tetradecanoylphorbol 13-acetate (TPA) and forskolin in cultured bovine aortic endothelial and smooth muscle cells. In cultured endothelial cells, 10(-8) mol/L TPA caused a reduction in atrial natriuretic peptide (ANP) receptor binding activity that was seen as early as 2 hours after the treatment and reached a maximum (88 +/- 3% of control) after 24 hours, whereas the inhibition of ANP-stimulated cyclic GMP (cGMP) accumulation peaked at 2 hours (62 +/- 13% of control) and returned to control levels after 12 hours. After 24 hours of forskolin (10(-4) mol/L) treatment, ANP binding activity fell to 47 +/- 6%, and cGMP accumulation was 52 +/- 11% of control. Northern blot analysis revealed that 10(-8) mol/L TPA suppressed natriuretic peptide C receptor transcript levels, and forskolin increased levels modestly after 24 hours of treatment. Natriuretic peptide A receptor transcript levels remained unchanged by either treatment. In cultured smooth muscle cells, 10(-8) mol/L TPA suppressed ANP binding activity and ANP-stimulated cGMP formation in a fashion similar to that seen in endothelial cells. TPA treatment also resulted in an inhibition of C-type natriuretic peptide-stimulated cGMP production (59 +/- 7% of control); however, this response persisted for as long as 24 hours after addition of the agonist. Treatment with 10(-4) mol/L forskolin produced a time-dependent inhibition of ANP binding activity and did not inhibit cGMP production stimulated by either ANP or C-type natriuretic peptide. In contrast to the effects seen with endothelial cells, TPA caused a dose-dependent stimulation of natriuretic peptide C receptor mRNA, whereas forskolin was inhibitory in smooth muscle cells. These results indicate that the effects of the kinase activators are a function of the individual receptor subtype as well as the cell in which it is expressed and imply a considerable degree of flexibility in the response to regulatory stimuli.