Functional properties and dynamics of natriuretic peptide receptors

C-type natriuretic peptide regulates the molecular components of the rainbow trout gill epithelium tight junction complex

Freshwater (FW) fish experience passive paracellular loss of ions into the surrounding environment across water-exposed epithelia such as the gill. The mitigation of paracellular ion loss is thought to be regulated by proteins of the tight junction (TJ) complex and in particular, the large superfamily of claudin (cldn) TJ proteins plays an important role. Transcript and protein levels of TJ proteins in teleosts are known to be under endocrine control of several important osmoregulatory hormones and the current study was aimed at determining whether the osmoregulatory hormone, C-type natriuretic peptide (CNP), can alter paracellular permeability and TJ protein abundance in a primary cultured gill epithelium derived from rainbow trout. Natriuretic peptide receptors were detected in the cultured trout gill epithelium. It was found that (i) developing cultured gill epithelia "grown" in the presence of 10 nM CNP, and (ii) mature cultured gill epithelia exposed to 10 nM CNP for 48 h, exhibited augmented barrier properties. This occurred in association with reduced flux rates of a paracellular permeability marker (polyethylene glycol, molecular mass 400; PEG-400) and, reduced ion efflux (i.e. ion loss) when preparations were exposed to apical FW. Exposure to CNP altered mRNA abundance of cldn-3a, -5a, -6, - 8c, -20a, -25b, -28a, -32a and cgn, but differences in the transcriptional response were observed between chronic and acute CNP exposure. In contrast, chronic and acute exposure to CNP resulted in reduced cldn-10e/Cldn-10e abundance. Data suggest that CNP may play a role in regulating the molecular physiology of the TJ complex in the fish gill epithelium and contribute to the regulation of salt and water balance by influencing the paracellular permeability properties of this tissue.

Spatiotemporal expression patterns of natriuretic peptides in rat testis and epididymis during postnatal development

Natriuretic peptide (NP) family is composed of atrial, brain and C-type NP (NPPA, NPPB and NPPC). Here, we aimed to investigate NP expression in testis and epididymis during postnatal development. NPPA expression was observed in gonocytes at prepubertal period but in only spermatocytes in pachytene and leptotene/zygotene stage at pubertal period. In prepubertal and pubertal periods, we detected NPPB expression in only Leydig cells. However, NPPC expression was detected in all of the gonocytes and Sertoli cells, spermatocytes and some interstitial cells in prepubertal and pubertal periods. In postpubertal and mature periods, NPPA and NPPB staining were detected in Leydig cells, elongated and round spermatids but not in spermatogonia and spermatocytes. However, we observed NPPC expression in all cells of the seminiferous tubules and Leydig cells in the postpubertal and mature periods. Epididymal epithelium showed intense NPPC expression during postnatal period but weak NPPA and NPPB expression in prepubertal and pubertal periods. The expression of three NPs in the testis significantly increased after puberty. In conclusion, puberty had a significant effect on NP expression in testis. Unlike NPPA and NPPB, expression of NPPC in all cells of the seminiferous tubule suggests that NPPC is effective in each step of spermatogenesis.

Natriuretic peptides induce weak VASP phosphorylation at Serine 239 in platelets

Cyclic guanosine-3',5'-monophoshate (cGMP) is the common second messenger for the cardiovascular effects of nitric oxide (NO) and natriuretic peptides (NP; e.g. atrial NP [ANP]), which activate soluble and particulate guanylyl cyclases, respectively. The role of NO in regulating cGMP and platelet function is well documented, whereas there is little evidence supporting a role for NPs in regulating platelet reactivity. By studying platelet aggregation and secretion in response to a PAR-1 peptide, collagen and ADP, and phosphorylation of the cGMP-dependent protein kinase (PKG) substrate vasodilator-stimulated phosphoprotein (VASP) at serine 239, we evaluated the effects of NPs in the absence or presence of the non-selective cGMP and cAMP phosphodiesterase (PDE) inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Our results show that NPs, possibly through the clearance receptor (natriuretic peptide receptor-C) expressed on platelet membranes, increase VASP phosphorylation but only following PDE inhibition, indicating a small, localised cGMP synthesis. As platelet aggregation and secretion measured under the same conditions were not affected, we conclude that the magnitude of PKG activation achieved by NPs in platelets per se is not sufficient to exert functional inhibition of platelet involvement in haemostasis.

Association of natriuretic peptide receptor-C gene with ischemic stroke and hypertension in Chinese Han population

To investigate whether natriuretic peptide receptor-C (NPR3) gene polymorphisms were associated with ischemic stroke (IS) and hypertension (a conventional risk factor for stroke), we conducted a case-control study in Chinese Han population. We found that rs696831, located in intron 2, was associated with IS. In addition, we found that rs16890208 and rs700925, in linkage disequilibrium (LD) with each other in intron 3, were associated with hypertension. The A allele of the rs16890208, T allele of the rs700925, and the AT haplotype, derived from rs16890208 and rs700925, increased the risk of hypertension with odds ratios (ORs) of 1.74 (95% CI = 1.23-2.47), 1.72 (95% CI = 1.21-2.42), and 1.54 (95% CI = 1.14-2.08), respectively. Further, we found that rs11745562 and rs2270915, in LD with each other in intron 5 and exon 8, were associated with hypertension. The A allele of the rs11745562 and the G allele of the rs2270915 increased the risk of hypertension with ORs of 1.53 (95% CI = 1.07-2.19) and 1.55 (95% CI = 1.08-2.22), respectively. Therefore, we provided novel evidences that polymorphisms or haplotype in NPR3 gene may influence the risk of IS or hypertension independently in Chinese population.

Natriuretic peptides and cardiovascular damage in the metabolic syndrome: molecular mechanisms and clinical implications

Natriuretic peptides are endogenous antagonists of vasoconstrictor and salt- and water-retaining systems in the body's defence against blood pressure elevation and plasma volume expansion, through direct vasodilator, diuretic and natriuretic properties. In addition, natriuretic peptides may play a role in the modulation of the molecular mechanisms involved in metabolic regulation and cardiovascular remodelling. The metabolic syndrome is characterized by visceral obesity, hyperlipidaemia, vascular inflammation and hypertension, which are linked by peripheral insulin resistance. Increased visceral adiposity may contribute to the reduction in the circulating levels of natriuretic peptides. The dysregulation of neurohormonal systems, including the renin-angiotensin and the natriuretic peptide systems, may in turn contribute to the development of insulin resistance in dysmetabolic patients. In obese subjects with the metabolic syndrome, reduced levels of natriuretic peptides may be involved in the development of hypertension, vascular inflammation and cardio vascular remodelling, and this may predispose to the development of cardiovascular disease. The present review summarizes the regulation and function of the natriuretic peptide system in obese patients with the metabolic syndrome and the involvement of altered bioactive levels of natriuretic peptides in the pathophysiology of cardiovascular disease in patients with metabolic abnormalities.

Vasoactive peptides in cardiovascular (patho)physiology

Numerous vasoactive agents play an important physiological role in regulating vascular tone, reactivity and structure. In pathological conditions, alterations in the regulation of vasoactive peptides result in endothelial dysfunction, vascular remodeling and vascular inflammation, which are important processes underlying vascular damage in cardiovascular disease. Among the many vasoactive agents implicated in vascular (patho)biology, angiotensin II (Ang II), endothelin (ET), serotonin and natriuretic peptides appear to be particularly important because of their many pleiotropic actions and because they have been identified as potential therapeutic targets in cardiovascular disease. Ang II, ET-1, serotonin and natriuretic peptides mediate effects via specific receptors, which belong to the group of G-protein-coupled receptors. ET, serotonin and Ang II are primarily vasoconstrictors with growth-promoting actions, whereas natriuretic peptides, specifically atrial, brain and C-type natriuretic peptides, are vasodilators with natriuretic effects. Inhibition of vasoconstrictor actions with drugs that block peptide receptors, compounds that inhibit enzymes that generate vasoactive peptides or agents that increase levels of natriuretic peptides are potentially valuable therapeutic tools in the management of cardiovascular diseases. This review focuses on ET, natriuretic peptides and serotonin. The properties and distribution of these vasoactive agents and their receptors, mechanisms of action and implications in cardiovascular (patho)physiology will be discussed.

Combined endopeptidase inhibition and adrenomedullin in sheep with experimental heart failure

Adrenomedullin and the natriuretic peptides exert vasodilator, natriuretic, and aldosterone-inhibitory actions, making augmentation of both systems potential therapeutic strategies in heart failure. Adrenomedullin and an endopeptidase inhibitor (SCH32615) were administered separately and in combination in 8 sheep with heart failure. Compared with the control condition, SCH32615 (5 mg bolus+1 mg/kg per hour infusion for 3 hours) reduced arterial pressure, left atrial pressure, and peripheral resistance and increased cardiac output, urinary volume, sodium, creatinine, and cAMP excretion. Plasma atrial and brain natriuretic peptide and cGMP concentrations were increased, whereas aldosterone tended to fall. Adrenomedullin (50 ng/kg per minute infusion for 3 hours) induced directionally similar but significantly greater changes in all hemodynamic variables compared with SCH32615. Urinary cAMP, sodium, and creatinine excretion rose, whereas urinary volume was maintained. Circulating adrenomedullin, cAMP, renin, and angiotensin II levels were increased, aldosterone was reduced, and natriuretic peptide levels were unchanged. Coadministration of adrenomedullin and SCH32615 produced hemodynamic effects greater than those achieved during adrenomedullin administration alone. Despite the larger falls in blood pressure, renal function (urinary volume, sodium excretion, and creatinine clearance) was improved to a level similar to that during SCH32615 administration. Elevations in plasma adrenomedullin and cAMP were greater than those during adrenomedullin administration alone, whereas increments in natriuretic peptides were similar to those during SCH32615 alone. Plasma renin and angiotensin II were increased and aldosterone levels were reduced. In conclusion, cotreatment with adrenomedullin and an endopeptidase inhibitor has beneficial hemodynamic and renal effects in heart failure beyond those of either agent separately.

Fluid balance in patients with chronic renal failure assessed with N-terminal proatrial natriuretic peptide, atrial natriuretic peptide and ultrasonography

The N-terminal proatrial natriuretic peptide (proANP) has become an important parameter for assessing the prognosis of patients with cardiac disease. Its use for evaluating the hydration status in patients with chronic renal failure, however, is still under investigation. The present study comprised 12 haemodialysis (HD) and 17 pre-dialysis patients. In the HD patients, the inferior vena cava diameter during quiet expiration (IVCe) was estimated by ultrasonography and plasma concentrations of N-terminal proANP, atrial natriuretic peptide (ANP) and cyclic guanosine monophosphate (cGMP) were measured before and 4 h after termination of HD. In the pre-dialysis patients venous blood samples were taken during rest to measure plasma N-terminal proANP and ANP and serum creatinine. Normal values for N-terminal proANP and ANP were obtained from 18 healthy volunteers. The plasma concentrations of N-terminal proANP and ANP in healthy volunteers were 328 +/- 92 and 11.4.0 +/- 3.1 pM L-1, respectively. In pre-dialysis patients, serum creatinine ranged from 110 to 447 microM L-1 and was significantly correlated to plasma N-terminal proANP (r = 0.60, P < 0.05) but not to ANP. This may indicate that N-terminal proANP is more dependent on renal function for its clearance than ANP, which is probably cleared by extrarenal mechanisms as well. In HD patients, IVCe was significantly correlated to the three hormones before HD, most strongly to N-terminal proANP. After dialysis, IVCe was significantly correlated to ANP and cGMP but was not correlated to N-terminal proANP. This may suggest that proANP takes a longer time than other hormones to reflect changes in intravascular volume. In conclusion, N-terminal proANP is a hormone closely related to degree of renal function. Furthermore, it is a sensitive marker reflecting the interdialytic hydration status in HD patients, as indicated by its high correlation to IVCe, a standard method which is used frequently nowadays to assess the body hydration. However N-terminal proANP could not reflect the acute changes in fluid volume induced by HD, probably because it is slowly metabolized.

Practical implications of current natriuretic peptide research

Since the original discovery of atrial natriuretic peptide (ANP) nearly 20 years ago and the subsequent realisation of the existence of a family of natriuretic peptides, there has been considerable progress in the elucidation of the physiological and pathophysiological significance of these peptides. This review has examined two potentially important practical aspects arising from natriuretic peptide research - the significance of measurement of plasma levels of ANP and of brain natriuretic peptide BNP for cardiovascular disease and the therapeutic potential of targeting the natriuretic peptide system. Several situations where the measurement of plasma ANP and BNP may be of benefit in the overall assessment and prognosis of cardiac disease have been discussed. The measurement of plasma levels of these peptides appears to have limited value as a specific diagnostic tool and is unlikely to replace well-established procedures to assess cardiac function. Nevertheless, given the strong negative predictive value, the value of the measurement of plasma natriuretic peptides particularly BNPs, in people with suspected heart disease, rests on the evidence that a normal value indicates a low risk of cardiac impairment. Moreover, a consistently elevated plasma level of BNP after myocardial infarction is associated with a distinctly poor prognosis. In turn, this may help to select those with high plasma levels for subsequent detailed investigation of cardiac dysfunction. This may be an important option, especially where the facilities for the more invasive cardiological procedures are not available. Intriguingly, recent research also suggests the possibility that plasma levels of natriuretic peptides may have an important role in guiding more effective therapy for heart failure. The potent cardiovascular and renal effects of ANP and BNP provide an important therapeutic potential for hypertension and for conditions associated with volume overload. A number of approaches which have been used to enhance endogenous activity of these peptides have been highlighted. The use of the native peptides ANP and BNP may well be valuable in some circumstances, such as in critically ill individuals with congestive heart failure or renal failure. However, the limitations of the use of peptides, especially for long-term treatment, are obvious. In view of this, considerable effort has been devoted to the development of orally active agents to enhance endogenous natriuretic peptides by inhibition of breakdown by neutral endopeptidase. This research has led to the development of vasopeptidase inhibitors - dual inhibitors of both endopeptidase and angiotensin-converting enzyme - to enhance endogenous natriuretic peptide function on a background of reduced angiotensin II activity. The broad spectrum of action and the potentially important target-organ protection of these inhibitors offer potential benefits which may well go beyond existing treatment of hypertension and of conditions associated with overt volume overload.

The molecular basis of hypertension

More than 50 million Americans display blood pressures outside the safe physiological range. Unfortunately for most individuals, the molecular basis of hypertension is unknown, in part because pathological elevations of blood pressure are the result of abnormal expression of multiple genes. This review identifies a number of important blood pressure regulatory genes including their loci in the human, mouse, and rat genome. Phenotypes of gene deletions and overexpression in mice are summarized. More detailed discussion of selected gene products follows, beginning with proteins involved in ion transport, specifically the epithelial sodium channel and sodium proton exchangers. Next, proteins involved in vasodilation/natriuresis are discussed with emphasis on natriuretic peptides, guanylin/uroguanylin, and nitric oxide. The renin angiotensin aldosterone system has an important role antagonizing the vasodilatory cyclic GMP system.

Natriuretic peptides in fish physiology

Natriuretic peptides exist in the fishes as a family of structurally-related isohormones including atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP) and ventricular natriuretic peptide (VNP); to date, brain natriuretic peptide (or B-type natriuretic peptide, BNP) has not been definitively identified in the fishes. Based on nucleotide and amino acid sequence similarity, the natriuretic peptide family of isohormones may have evolved from a neuromodulatory, CNP-like brain peptide. The primary sites of synthesis for the circulating hormones are the heart and brain; additional extracardiac and extracranial sites, including the intestine, synthesize and release natriuretic peptides locally for paracrine regulation of various physiological functions. Membrane-bound, guanylyl cyclase-coupled natriuretic peptide receptors (A- and B-types) are generally implicated in mediating natriuretic peptide effects via the production of cyclic GMP as the intracellular messenger. C- and D-type natriuretic peptide receptors lacking the guanylyl cyclase domain may influence target cell function through G(i) protein-coupled inhibition of membrane adenylyl cyclase activity, and they likely also act as clearance receptors for circulating hormone. In the few systems examined using homologous or piscine reagents, differential receptor binding and tissue responsiveness to specific natriuretic peptide isohormones is demonstrated. Similar to their acute physiological effects in mammals, natriuretic peptides are vasorelaxant in all fishes examined. In contrast to mammals, where natriuretic peptides act through natriuresis and diuresis to bring about long-term reductions in blood volume and blood pressure, in fishes the primary action appears to be the extrusion of excess salt at the gills and rectal gland, and the limiting of drinking-coupled salt uptake by the alimentary system. In teleosts, both hypernatremia and hypervolemia are effective stimuli for cardiac secretion of natriuretic peptides; in the elasmobranchs, hypervolemia is the predominant physiological stimulus for secretion. Natriuretic peptides may be seawater-adapting hormones with appropriate target organs including the gills, rectal gland, kidney, and intestine, with each regulated via, predominantly, either A- or B-type (or C- or D-type?) natriuretic peptide receptors. Natriuretic peptides act both directly on ion-transporting cells of osmoregulatory tissues, and indirectly through increased vascular flow to osmoregulatory tissues, through inhibition of drinking, and through effects on other endocrine systems.

Kinin B2 receptors mediate blockade of atrial natriuretic peptide natriuresis induced by glucose or feeding in fasted rats

We have shown previously that the kininogen-derived peptides bradykinin, prokinins, and PU-D1, given intravenously or into the duodenal lumen, block the atrial natriuretic peptide (ANP)-induced diuretic-natriuretic effect in fasting, anesthetized rats infused with isotonic glucose. HOE-140, an inhibitor of bradykinin B2 receptors, completely suppresses this ANP blockade. When intravenous glucose infusion is omitted, the above-described inhibition of ANP does not take place. Therefore, to clarify the role of glucose and/or feeding in this phenomenon, we used fasted, anesthetized rats to test how the ANP excretory response was affected by (1) short-term feeding before anesthesia, (2) 1 mL of isotonic glucose introduced into the stomach, and (3) the interaction of HOE-140 with these treatments. In addition, we tested the effects of 1 mL of intragastric glucose administration and HOE-140 on urinary excretion in awake rats. In anesthetized rats, both glucose administration and feeding significantly inhibited the diuretic-natriuretic effect of ANP for up to 90 minutes. Similarly, intragastric glucose delayed spontaneous sodium and water excretion for 90 minutes in awake rats. In all 3 cases, pretreatment with HOE-140 (2.5 microg IV) fully prevented the inhibition of ANP excretory action, ruling out osmotic effects as the cause of reduced diuresis. These results indicate that the presence of glucose in the digestive tract triggers an inhibitory effect on ANP renal actions that requires activation of kinin B2 receptors, providing strong support to our hypothesis that during the early prandial period, gastrointestinal signals elicit a transient blockade of renal excretion with a mechanism involving the kallikrein-kinin system.

A novel promoter variant of the natriuretic peptide clearance receptor gene is associated with lower atrial natriuretic peptide and higher blood pressure in obese hypertensives

OBJECTIVE AND DESIGN

The clearance receptor for natriuretic peptides (NPRC), a candidate gene for essential hypertension, is highly expressed in adipose tissue, where is nutritionally regulated. The objectives of the present study were to sequence the human 5'-flanking regulatory region of NPRC, to identify allelic variants and their frequencies, and to study the genotype/phenotype correlation in hypertensive patients.

METHODS AND RESULTS

Using polymerase chain reaction (PCR) and direct automated sequencing, a biallelic (A/C) polymorphism was detected at position -55 in a conserved promoter element named P1. The novel C(-55) variant makes the promoter sequence identical to the mouse gene and introduces a second Hgal site in the amplified DNA, allowing the genotyping of a large number of subjects. In a random sample of 232 white Caucasians the C(-55) allele was more commonly found (81.7% of all alleles) with 155 CC (66.8%), 69 AC (29.7%) and only eight AA (3.5%) genotypes. Atrial natriuretic peptide (ANP) levels were determined in 84 patients with essential hypertension. In the presence of obesity (body mass index (BMI) > or = 30 kg/m2) the homozygous CC hypertensives (n = 21) had significantly lower plasma ANP (33.6 +/- 11.1 pg/ml) compared with the AC patients (n = 11; 46.8 +/- 15.9 pg/ml; P = 0.01), whereas systolic blood pressure (SBP) and mean blood pressure (MBP) had the opposite association (SBP 163.9 +/- 18.7 versus 150.9 +/- 12.9 and MBP 123.3 +/- 12 versus 114.5 +/- 5.9 mmHg; P< 0.05). The difference in ANP levels were also present when overweight patients (BMI > or = 27 kg/m2) were considered.

CONCLUSION

A common 'ancestral' C(-55) variant of the NPRC P1 promoter is associated with lower ANP levels and higher SBP and MBP in obese hypertensives. The C(-55) variant, in the presence of increased adiposity, might reduce plasma ANP through increased NPRC-mediated ANP clearance, contributing to higher blood pressure.

Comparative binding study of rat natriuretic peptide receptor-A

The natriuretic peptide receptor-A (NPR-A) is involved in blood pressure and body fluid regulation in order to help maintain cardiovascular homeostasis. It has been shown that these biological effects are mediated through the natriuretic peptide family of hormones, which bind NPR-A according to the rank order ANP>BNP>>CNP. Previous studies performed with rat kidney papillary tissue suggested the existence of an heterologous NPR-A population since two binding components were obtained for pBNP32, one of high affinity (pK 9.4 +/- 0.1) and the other of lower affinity (pK 7.5 +/- 0.1), while in the same preparation rANP28 binding displayed the expected affinity (pK 10.22 +/- 0.01) and was best fitted with a model involving a single class of binding sites. This apparent heterogeneity of NPR-A in rat kidney papillae could be explained by the presence of two receptor isoforms or of monomeric and oligomeric forms of the same receptor. To investigate the NPR-A binding heterogeneity, we have cloned the rat NPR-A from PC12 cells and compared its pharmacological profile with that of the papillae. Our results with rat NPR-A transfected Cos-P cells show an equivalent pharmacological profile as with the rat tissue, i.e. a high affinity for rANP28 (pK 10.4 +/- 0.1) and two distinctive affinities for pBNP32 (pK 9.74 +/- 0.05 and 7.8 +/- 0.1). Although multiple receptor glycoforms were sometimes detectable by western blotting, only one molecular form was obtained by cross-linking with 125I-rANP28. It thus appears that NPR-A alone can account for the two binding components found in the rat papillae and that a single molecular form of the protein is implicated. We therefore propose that the oligomerization state of the receptors could be responsible for the apparent binding heterogeneity of rat NPR-A.

A peptide released by pepsin from kininogen domain 1 is a potent blocker of ANP-mediated diuresis-natriuresis in the rat

A 20-amino acid peptide, KYEIKEGDCPVQSGKTWQDC (PU-D1), released by pepsin hydrolysis of LMW kininogen domain 1 was tested for its ability to antagonize the diuretic and natriuretic effect of ANP(103-125) in anesthetized rats. A single dose of 10.8 or 21.6 pmol (25 or 50 ng) PU-D1 given intravenously or into the duodenal lumen suppressed the diuresis-natriuresis induced by 209 pmol (500 ng) ANP by 43% to 59% and 69% to 96%, respectively. None of the doses tested (2.16 to 432 pmol, 5 ng to 1 microg) modified systemic blood pressure. Strikingly, a single IV dose of 10.8 pmol PU-D1 blocked the action of ANP for more than 3 hours. ANP blockade by PU-D1 was annulled completely by the bradykinin (BK) B2 receptor inhibitor Hoe 140. On a molar basis, PU-D1 is more effective than BK and kinins of 15, 16, and 18 amino acids for blocking the ANP-mediated diuresis-natriuresis. As with BK and other kinins, the inhibitory effect of Pu-D1 on ANP is obtained only within a small range of picomol doses. A single dose of 2.16 or 4.32 pmol PU-D1 or 47 pmol (50 ng) BK is ineffective against ANP if injected alone. However, when both substances are administered concomitantly at these subthreshold doses, they totally suppress ANP-induced diuresis-natriuresis. These results raise the question of whether PU-D1, released from kininogen domain 1, either alone or associated with BK, may interact with ANP in the regulation of urinary water and electrolyte excretion in physiological and pathological conditions.