Increased atrial natriuretic peptide mRNA expression in the kidney of diabetic rats

Mid-regional pro-atrial natriuretic peptide levels before and after hemodialysis predict long-term prognosis

BACKGROUND

Mid-regional pro-atrial natriuretic peptide (MR-proANP) is a strong prognostic biomarker in cardiovascular disease but there is limited data for its use among patients undergoing dialysis.

METHODS

This was a cohort study of patients receiving maintenance hemodialysis from two Danish centers. Blood sampling and echocardiography were performed before and after a dialysis session. We calculated the area under the curve (AUC) for the receiver operating characteristics for diagnosing heart failure and Cox regressions for cardiovascular events and all-cause mortality.

RESULTS

Of the 306 patients, 284 (93%) had MR-proANP measurements both before and after dialysis. Median concentration was 642 pmol/L (IQR 419-858) before and 351 pmol/L (IQR 197-537) after dialysis, a mean decrease of 330 pmol/L (43%, CI 296-364, P < 0.001). MR-proANP concentration both before and after dialysis was negatively correlated to left ventricular ejection fraction with no difference in predictive ability for heart failure, AUC before and after dialysis were 0.60 (CI 0.50-0.70) and 0.61 (CI 0.51-0.71) (P = 0.40). Median follow-up was 32 months (IQR 31-33), during which 99 patients (32%) had a cardiovascular event and 110 (36%) died. A doubling of MR-proANP concentration was associated with a hazard ratio (HR) of 1.6 (CI 1.3-1.9) before and 1.7 (CI 1.4-2.0) after dialysis for mortality and a HR of 1.5 (CI 1.2-1.9) before and 1.4 (CI 1.2-1.7) after dialysis for cardiovascular events (all P < 0.001).

CONCLUSION

The MR-proANP concentration is elevated among patients undergoing hemodialysis and decreases during dialysis. MR-proANP concentration both before, after and intra-dialysis change strongly predicted cardiovascular events and all-cause mortality.

Diabetes-induced loss of gastric ICC accompanied by up-regulation of natriuretic peptide signaling pathways in STZ-induced diabetic mice

Our previous study demonstrated that natriuretic peptides (NPs) play an inhibitory role in regulation of gastric smooth muscle motility. However, it is not clear whether NPs are involved in diabetics-induced loss of gastric interstitial cell of Cajal (ICC). The present study was designed to investigate the relationship between diabetics-induced loss of gastric ICC and natriuretic peptide signaling pathway in streptozotocin (STZ)-induced diabetic mice. The results showed that the protein expression levels of c-Kit and membrane-bound stem cell factor (mSCF) in gastric smooth muscle layers were decreased in STZ-induced diabetic mice. However, both mRNA and protein expression levels of natriuretic peptide receptor (NPR)-A, B and C were increased in the same place of the diabetic mice. The amplitude of spontaneous contraction in gastric antral smooth muscles was inhibited by C-type natriuretic peptide (CNP) dose-dependently and the inhibitory effect was potentiated in diabetic mice. Pretreatment of the cultured gastric smooth muscle cells (GSMCs) with different concentration of CNP can significantly decrease the mSCF expression level. 8-Bromoguanosine-3',5'-cyclomo-nophosphate (8-Br-cGMP), a membrane permeable cGMP analog, mimicked the effect of CNP but not cANF (a specific NPR-C agonist). Methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay showed that high concentration of cANF (10(-6) mol/L) inhibited cell proliferation in cultured GSMCs. These findings suggest that up-regulation of NPs/NPR-A, B/cGMP and NPs/NPR-C signaling pathways may be involved in diabetes-induced loss of gastric ICC.

Atrial natriuretic peptide signal pathway upregulated in stomach of streptozotocin-induced diabetic mice

AIM

To investigate atrial natriuretic peptide (ANP) secretion from gastric mucosa and the relationship between the ANP/natriuretic peptide receptor type A (NPR-A) pathway and diabetic gastroparesis.

METHODS

Male imprinting control region (ICR) mice (4 wk old) were divided into two groups: control mice, and streptozotocin-induced diabetic mice. Eight weeks after injection, spontaneous gastric contraction was recorded by using physiography in control and streptozotocin-induced diabetic mice. The ANP-positive cells in gastric mucosa and among dispersed gastric epithelial cells were detected by using immunohistochemistry and flow cytometry, respectively. ANP and natriuretic peptide receptor type A (NPR-A) gene expression in gastric tissue was observed by using the reverse transcriptase polymerase chain reaction.

RESULTS

The frequency of spontaneous gastric contraction was reduced from 12.9 +/- 0.8 cycles/min in the control group to 8.4 +/- 0.6 cycles/min in the diabetic mice (n = 8, P < 0.05). However, the amplitude of contraction was not significantly affected in the diabetic group. The depletion of interstitial cells of Cajal in the gastric muscle layer was observed in the diabetic mice. ANP-positive cells were distributed in the gastric mucosal layer and the density index of ANP-positive cells was increased from 20.9 +/- 2.2 cells/field in control mice to 51.8 +/- 2.9 cells/field in diabetic mice (n = 8, P < 0.05). The percentage of ANP-positive cells among the dispersed gastric epithelial cells was increased from 10.0% +/- 0.9% in the control mice to 41.2% +/- 1.0% in the diabetic mice (n = 3, P < 0.05). ANP and NPR-A genes were both expressed in mouse stomach, and the expression was significantly increased in the diabetic mice.

CONCLUSION

These results suggest that the ANP/NPR-A signaling pathway is upregulated in streptozotocin-induced diabetic mice, and contributes to the development of diabetic gastroparesis.

Local action of endogenous renal tubular atrial natriuretic peptide

Up-regulation of atrial natriuretic peptide (ANP) mRNA in the kidneys in several disorders has been demonstrated; however, evidence that ANP synthesized by the kidney exerts a local function has never been produced. Therefore, we investigated whether endogenous ANP could modulate high glucose-stimulated TGF-beta1, collagen type I and nuclear factor-kappaB (NF-kappaB) in NRK-52E cells using transfection of ANP and ANP small interfering RNA (siANP). NRK-52E cells were grown with or without transfection with ANP plasmid; cells were also transfected with ANP siRNA or control siRNA. These cells were then stimulated with a high glucose concentration to modulate ANP, TGF-beta1, collagen type I, NF-kappaB and IkappaB-alpha, and the results showed that ANP, TGF-beta1, collagen type I and NF-kappaB significantly increased in untransfected cells, and the transfection of ANP significantly attenuated high glucose-activated TGF-beta1, collagen I and NF-kappaB expression. ANP siRNA knocked-down ANP but significantly increased TGF-beta1 and collagen I under normal glucose conditions; ANP siRNA decreased IkappaB-alpha but strongly enhanced high glucose-activated TGF-beta1, collagen type I and NF-kappaB. In contrast, medium from ANP-transfected cells attenuated high glucose-activated TGF-beta1 and collagen type I expression in NRK-52E cells transfected with siANP. In conclusion, our results demonstrated that siANP increased activation of TGF-beta1, collagen type I and NF-kappaB in NRK-52E cells under high glucose conditions, and medium from ANP-transfected cells attenuated high glucose-activated TGF-beta1 and collagen type I. This is the first study to demonstrate the auto/paracrine action of endogenous ANP in renal tubular cells on the attenuation of hyperglycemia-activated TGF-beta1 and NF-kappaB expression. J. Cell. Physiol. 219: 776-786, 2009. (c) 2009 Wiley-Liss, Inc.

Atrial natriuretic peptide attenuates high glucose-activated transforming growth factor-beta, Smad and collagen synthesis in renal proximal tubular cells

Atrial natriuretic peptide, besides its role in the regulation of volume homeostasis, has been noted to exert cytoprotective effects in several cell types from hypoxia. The present study was performed to explore the effect of ANP on high glucose-activated transforming growth factor-beta1 (TGF-beta1), Smad and collagen synthesis in renal proximal epithelial cells. Cultured NRK-52E cells were divided into five groups: (1) normal glucose (5.5 mM), (2) high glucose (35 mM), (3) D-mannitol (29.5 mM), (4) high glucose plus ANP (10(-6)-10(-9) M), and (5) high glucose plus ANP (10(-6) M) and guanylate cyclase inhibitor LY83583 (10(-7) M) groups. Messenger RNA levels of TGF-beta1, Smad2, and collagens were measured by RT-PCR. ELISA, immunocytochemistry and Western blotting were used to detect protein levels of TGF-beta1, Smad2, phospho-Smad 2/3 and collagen type 1. We found high glucose to significantly increase mRNA levels of TGF-beta1, Smad 2, collagen types I and III and protein levels of TGF-beta1, phospho-Smad 2/3 and collagen type 1, but mannitol did not affect their expression. The addition of ANP significantly attenuated high glucose-enhanced mRNA and protein levels of TGF-beta1, Smad and collagens. LY83583 blocked the influence of ANP on high glucose-activated TGF-beta1, Smad and collagen synthesis. This is the first study to demonstrate that activation of TGF-beta1, Smad and collagen synthesis stimulated by high glucose can also be inhibited by exogenous ANP in renal tubular epithelial cells.

Urodilatin: a better natriuretic peptide?

The kidney natriuretic peptide urodilatin (ie, ularitide) decreases pulmonary capillary wedge pressure (PCWP) but does not cause diuresis in persons with congestive heart failure (CHF). Thirty-three percent of patients with CHF treated with 30 ng/kg/min ularitide develop hypotension with systolic blood pressures below 90 mmHg. Nesiritide and atrial natriuretic peptide lower PCWP and cause hypotension. They do not produce diuresis or natriuresis in patients with CHF. The best natriuretic peptide for treating CHF is the cardiac hormone vessel dilator which decreases PCWP and decreases systemic and pulmonary vascular resistance while simultaneously increasing cardiac output and cardiac index. What makes the vessel dilator markedly better than atrial natriuretic peptide, nesiritide, and ularitide for treatment of CHF is that it enhances sodium excretion fivefold and causes a fivefold enhanced diuresis in patients with CHF with its biologic effects lasting over 6 hours compared with less than 30 minutes for the above peptides.

The atrial natriuretic peptide (ANP) system in the pronephros and mesonephros of Bufo bufo larvae

In this study on the excretory apparatus of the Bufo bufo larvae, the ultrastructural features and the atrial natriuretic peptide (ANP)-system were examined using cytochemical and immunocytochemical methods. The early embryonic kidney, the pronephros, is replaced by a later stage, the mesonephros. The pronephros degenerates at the time of metamorphosis and the mesonephros becomes the functional kidney in the adult. Both these organs are targets for ANP, demonstrated by the presence of the specific receptors, indirectly highlighted by the cytochemical localization of the guanylate cyclase in the presence of exogenous atrial natriuretic peptide. This study concluded that the mesonephros produces ANP and thus clusters of cells containing ANP-like granules, positive to the anti-α ANP immunolocalization, were present along the mesonephric proximal tubule. The atrial natriuretic peptide system carries out an important osmoregulatory role in the excretory apparatus.

Alterations in atrial natriuretic peptide and its receptor levels in long-term, streptozotocin-induced, diabetes in rats

Diabetes mellitus (DM) shows a markedly increased incidence of cardiovascular pathology that leads to hypertension, endothelial macro- and microangiopathy, diabetic nephropathy, and myocardial infarction. Atrial natriuretic peptide (ANP), is a 28 amino acid peptide hormone synthesized mainly by the heart atria and ventricles. It has potent diuretic and natriuretic properties. In this article the effect of long-term DM on blood plasma, kidney, and heart atrial and ventricular ANP concentrations were evaluated in streptozotocin (STZ)-induced 8-month diabetic and control rats by using radioimmunoassay (RIA). Moreover, ANP receptors in STZ-induced, 8-month diabetic rat kidneys were studied by receptor autoradiography. In addition, the expression of ANP concentrations in the kidney of diabetic and control rats was evaluated by means of immunohistochemistry. Body weight loss and increased blood glucose levels were used as indices of DM in the STZ-induced diabetic rats. Our results showed significantly higher ANP concentrations in diabetic plasma (P < 0.05), kidney (P < 0.01), heart atria (P < 0.05), and ventricles (P < 0.01) compared to controls. We also demonstrated a significant decrease in ANP receptors in the outer cortex (P < 0.05), juxtaglomerular medulla (P < 0.05), and papilla (P < 0.05) of 8-month diabetic rat kidneys compared to controls. The observed increase in ANP levels in plasma and kidney could play a role in the development of diabetic nephropathy: probably by reducing the levels of ANP receptors in diabetic kidney. Furthermore, the role of ANP in the STZ-induced diabetic heart merits additional study.

Gentamicin decreases guanylyl cyclase activity in rat glomerulus

BACKGROUND

Effects of gentamicin (GM) on the local natriuretic peptide (NP) and nitric oxide (NO) systems in the kidney were investigated.

METHODS

Male Sprague-Dawley rats (180-200 g) were intramuscularly injected with GM (100 mg/kg/day) for 5 days. The expression of NO synthase (NOS) isoforms was determined by Western blot analysis, and that of NPs by real-time polymerase chain reaction. The activity of guanylyl cyclase was also determined by the amount of guanosine 3',5'-cyclic monophosphate (cGMP) generated in responses to atrial natriuretic peptide (ANP) or sodium nitroprusside (SNP).

RESULTS

GM treatment resulted in renal failure in association with increases in urinary flow and the fractional excretion of sodium. Accordingly, the expression of inducible NOS was increased in the cortex, while that of endothelial NOS remained unchanged. The urinary excretion of NO metabolites was increased. The expression of ANP, brain natriuretic peptide and C-type natriuretic peptide mRNA was increased in the kidney. The cGMP production provoked by either ANP or SNP was decreased in the glomerulus, but not in the papilla.

CONCLUSION

GM-induced nephropathy may be causally related with decreased guanylyl cyclase activities in the glomerulus.

Heart specific up-regulation of genes for B-type and C-type natriuretic peptide receptors in diabetic mice

BACKGROUND

Diabetes may cause cardiomyopathy characterized by cardiac fibrosis. Recent studies of genetically modified mice have elucidated a role of the natriuretic peptides (NP), type-A and type-B (ANP and BNP), and their common receptor [natriuretic peptide receptor (NPR), type-A] in development of cardiac fibrosis. The role of NP type-C (CNP) and NPR type-B (NPR-B) in the heart is less well established. In this study we examined if diabetes alters heart expression of the genes encoding the NP and its receptors.

MATERIALS AND METHODS

Cardiac mRNA was quantified by real-time PCR in diabetic streptozotocin (STZ)-treated and ob/ob-mice and nondiabetic control mice.

RESULTS

The ob/ob-mice with type-II diabetes displayed highly significant increases of the cardiac mRNA expression of NPR-B and NPR-C while the expression levels of NPR-A, ANP, BNP, and CNP mRNA were similar in ob/ob-mice and controls. Mice with STZ-induced type-I diabetes also showed an increase of heart NPR-B mRNA expression at 12 weeks, but not at 3, 6 or 9 weeks after STZ-treatment. The ANP and NPR-C mRNA expressions were only altered after 3 weeks, whereas BNP, CNP and NPR-A mRNA expressions were not altered in STZ-treated-mouse hearts at any of the time points.

CONCLUSIONS

The results show that diabetes in mice confers increased NPR-B gene expression in the heart, suggesting that increased NPR-B signalling may affect development of diabetic cardiomyopathy.

Alterations in atrial natriuretic peptide and its receptors in streptozotocin-induced diabetic rat kidneys

In this study the effect of diabetes mellitus on atrial natriuretic peptide (ANP) receptors in streptozotocin- (STZ-) induced diabetic rat kidneys was studied. Moreover, plasma ANP concentration was evaluated in diabetic and control rats by using radioimmunoassay. In addition, the expression of ANP in the kidneys of control and diabetic rats was evaluated by immunohistochemistry. Body-weight loss and increased glucose levels were used as indices of diabetes mellitus in the STZ-induced rats. There was a significant loss in the body weight of the diabetic rats compared to controls. The efficacy of STZ administration was confirmed by rising blood glucose levels, which were significantly higher in diabetic rats compared to controls. Plasma ANP concentration was significantly greater in the diabetic rats in comparison with controls. Moreover, our immunohistochemical results show that the expression of ANP in diabetic rats was higher than that in age-matched controls. ANP was observed in the cells lining the proximal convoluted tubules in the cortex. The distribution and levels of ANP receptors in the kidneys of diabetic rats and age-matched controls were investigated using quantitative receptor autoradiography. Our results demonstrate significant decrease in ANP receptors in the kidneys of the diabetic rats compared to controls. The significant decrease was found in the juxtaglomerular medulla, inner medulla, and the papillae. The decrease in ANP receptors observed in the diabetic kidneys could have pathological consequences resulting in renal resistance to ANP in diabetes.

Atrial natriuretic hormone prohormone gene expression in cardiac and extra-cardiac tissues of diabetic Goto-Kakizaki rats

The present investigation was designed to determine if the mechanism for the increased atrial natriuretic peptides within the circulation of diabetic animals involves atrial natriuretic hormone prohormone (proANH) gene expression upregulation. The tissue specificity of this potential upregulation of the proANH gene was investigated in a spontaneous model of type 2 diabetes, i.e. the Goto-Kakizaki (GK) rat with comparison to age-matched non-diabetic Wistar rats from which the GK colony was originally derived. Reverse transcription-polymerase chain reaction revealed that proANH gene expression was increased 3.1-fold in the left heart ventricle, 5-fold in lung, 2-fold in kidney, 3-fold within mucosa and 1.8-fold within muscle of gastric antrum (p < 0.05 for each) of GK rats compared to Wistar rats. There was no significant increase in proANH gene expression in atria and right ventricle of the heart of GK rats compared to Wistars. These results indicate that steady-state ANH prohormone mRNA levels increase within the left ventricle and extracardiac tissues in type 2 diabetic animals. This enhanced gene expression is a functional increase with its expressed proteins (4 peptide hormones; ANPs) increasing 2-6 fold within the circulation of GKs. The greater increase in proANH messenger RNA in the extracardiac tissues compared to the amount of increase within the heart and the greater tissue mass of these combined extra cardiac tissues suggests the majority of the increase in ANPs within the circulation of diabetics is secondary to increased synthesis in extracardiac tissues. This also suggests that there is a systemic regulatory mechanism of proANH gene expression not only within the heart but also within the lung, gastrointestinal tract and kidney. Diabetes is the first disease in which there is more upregulation of ANH prohormone in extracardiac tissues compared to upregulation within the heart itself.

Natriuretic peptides and acute renal failure

Atrial natriuretic peptides (ANPs) are a family of peptide hormones, e.g., ANP, long-acting natriuretic peptide, vessel dilator, and kaliuretic peptide synthesized by the ANP gene. Brain natriuretic peptide (BNP) and C-type natriuretic peptide are also members of this family but are synthesized by separate genes. Within the kidney, the ANP prohormone's posttranslational processing is different from that of other tissues, resulting in an additional four amino acids added to the NH2 terminus of ANP (e.g., urodilatin). Each of these natriuretic and diuretic peptides increases within the circulation with acute renal failure (ARF). Renal transplantation but not hemodialysis returns their circulating concentrations to those of healthy individuals. BNP and adrenomedullin, a 52-amino acid natriuretic peptide, have beneficial effects on glomerular hypertrophy and glomerular injury but do not improve tubular injury (i.e., acute tubular necrosis). Vessel dilator ameliorates acute tubular necrosis with regeneration of the brush borders of proximal tubules. Vessel dilator decreases mortality in ARF from 88 to 14% at day 6 of ARF, even when given 2 days after renal failure has been established.

The cellular localization of increased atrial natriuretic peptide mRNA and immunoreactivity in diabetic rat kidneys

Increased intrarenal atrial natriuretic peptide (ANP) mRNA expression has been reported in several disorders. To further investigate the action of renal ANP, we need to elucidate the exact site of its alteration in diseased kidneys. ANP mRNA and ANP were detected by in situ hybridization and immunohistochemistry in the kidneys from five normal and five diabetic rats. Renal ANP mRNA in eight normal and nine diabetic rats was measured by RT-PCR with Southern blot hybridization. In normal and diabetic rats, the distribution of ANP mRNA and ANP-like peptide was mainly located in proximal, distal, and collecting tubules. However, diabetic rats had significant enhancement of ANP mRNA and ANP-immunoreactive staining in the proximal straight tubules, medullary thick ascending limbs, and medullary collecting ducts. ANP mRNA in the outer and inner medulla of nine diabetic rats increased 5.5-fold and 3.5-fold, but only 1.8-fold in the renal cortex. This preliminary study showed that ANP mRNA and ANP immunoreactivity in proximal straight tubules, medullary thick ascending limb, and medullary collecting ducts apparently increased in diabetic kidneys. These findings imply that ANP synthesis in these nephrons may involve in adaptations of renal function in diabetes.

The effect of water deprivation on the expression of atrial natriuretic peptide and its receptors in the spinifex hopping mouse, Notomys alexis

This study investigated the mRNA expression of the atrial natriuretic peptide (ANP) system (peptide and receptors) during water deprivation in the spinifex hopping mouse, Notomys alexis, a native of central and western Australia that is well adapted to survive in arid environments. Initially, ANP, NPR-A and NPR-C cDNAs (partial for receptors) were cloned and sequenced, and were shown to have high homology with those of rat and mouse. Using a semi-quantitative multiplex PCR technique, the expression of cardiac ANP mRNA and renal ANP, NPR-A, and NPR-C mRNA was determined in 7- and 14-day water-deprived hopping mice, in parallel with control mice (access to water). The levels of ANP mRNA expression in the heart remained unchanged, but in the kidney ANP mRNA levels were increased in the 7-day water-deprived mice, and were significantly decreased in the 14-day water-deprived mice. NPR-A mRNA levels were significantly higher in 7-day water-deprived mice while no change for NPR-A mRNA expression was observed in 14-day water-deprived mice. No variation in NPR-C mRNA levels was observed. This study shows that water deprivation differentially affects the expression of the ANP system, and that renal ANP expression is more important than cardiac ANP in the physiological adjustment to water deprivation.

Cyclic GMP signaling in podocytes

Natriuretic peptides (NP), together with nitric oxide (NO) are powerful relaxing factors acting via a common second messenger, cyclic GMP (cGMP). Together with other vasoactive modulators, these vasorelaxing factors play an essential role in regulating the function of kidney glomeruli. The presence of NP receptors in podocytes has been well documented. Recently, also mRNA for soluble guanylate cyclase, the NO receptor, has been shown in these cells. Stimulation of podocytes with atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP), and NO donors results in considerable upregulation of cellular cGMP synthesis. The podocyte foot processes contain a highly organized network of microfilaments adhering to the glomerular basement membrane (GBM). Changes in podocyte cytoskeleton accompanied by detachment of the cells from the GBM are closely associated with many glomerulopathies. The contractile apparatus in the podocyte foot processes seems to be an obvious target for the cyclic GMP signaling cascade. However, little is known about implications of the cGMP synthesis in these cells. We briefly review the current art regarding generation and modulation of cyclic GMP levels in podocytes. We discuss also the possible targets for this secondary messenger as well as its functional role in podocytes.

Increased expression of atrial natriuretic peptide in the kidney of rats with bilateral ureteral obstruction

BACKGROUND

Whether the postobstructive diuresis can be related to an altered regulation of local atrial natriuretic peptide (ANP) in the kidney was investigated.

METHODS

Three groups of rats had both of their ureters obstructed for 48 hours. The kidneys were taken without releasing the obstruction in one group [bilateral ureteral obstruction (BUO)]. The obstruction was released in the other two groups and the animals were kept for 4 and 24 hours thereafter to collect urinary data (BUR-4 and BUR-24, respectively). Plasma and urine ANP levels were measured by radioimmunoassay. The mRNA expression of ANP, natriuretic peptide receptor-A (NPR-A), and NPR-C was determined by reverse transcription-polymerase chain reaction. ANP receptors were also quantitated by in vitro autoradiography. The activity of guanylyl cyclase was determined by the amount of cGMP generated in response to ANP.

RESULTS

Urinary volume and sodium excretion increased in BUR-4, along with the ANP mRNA expression in the kidney and the urinary ANP excretion. The ANP excretion positively correlated with the urinary volume and sodium excretion. The mRNA expression of both NPR-A and NPR-C was decreased by BUO, the latter being far more prominently affected. The maximal binding capacity of radiolabeled ANP was decreased in the glomerulus and papilla in BUO. Not only the urinary parameters but also the mRNA expression of ANP, NPR-A, and NPR-C were comparable between BUR-24 and control rats. ANP-stimulated cGMP generation was reduced in the glomerulus and papilla in BUO animals, which was rapidly resumed following the release of the obstruction.

CONCLUSIONS

Postobstructive diuresis may be due partially to an increased ANP activity in the kidney.

Up-regulation of adrenal cortical and medullary atrial natriuretic peptide and gene expression in rats with deoxycorticosterone acetate-salt treatment

Our previous study demonstrated that human adrenal medulla is a site of atrial natriuretic peptide (ANP) synthesis. To further evaluate the role of adrenal ANP in body fluid homeostasis, we investigated the changes in adrenal ANP in rats receiving deoxycorticosterone acetate (DOCA)-salt treatment. In situ hybridization and immunohistochemical study showed that adrenal ANP messenger RNA (mRNA) and ANP-like immunoreactivities (ANP-LI) were mainly localized in the zona glomerulosa and medulla of vehicle-treated rats. DOCA-salt treatment activated ANP mRNA and peptide expression in all adrenal zones, especially in the zona fasciculata/reticularis from 12 h to the entire 8-day study period. Using a semiquantitative RT-PCR technique, the relative quantities of ANP mRNA in the adrenals of the DOCA-salt-treated group were significantly increased from 1 to 8 days, whereas the adrenal weights of DOCA-salt-treated rats were significantly decreased from day 2 to day 8. Our results are the first to indicate that ANP is synthesized not only in the adrenal medulla but also in the adrenal cortex and their syntheses are markedly increased in DOCA-salt-treated rats. These results imply that adrenal ANP may participate in the intraadrenal regulation of adrenal function on water-electrolyte homeostasis in an autocrine or paracrine manner.

Increased nitric oxide synthase mRNA expression in the renal medulla of water-deprived rats

Increased nitric oxide synthase mRNA expression in the renal medulla of water-deprived rats.

BACKGROUND

Experiments were performed to investigate whether renal nitric oxide synthase (NOS) mRNA and protein expression are responsive to the alteration of body volume.

METHODS

Four days of water deprivation (WD) was initiated in 16 male Wistar rats, and 16 normal rats (NC) served as the control group. Neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS) mRNAs and immunoreactivity were measured by reverse transcription-polymerase chain reaction (RT-PCR) followed by Southern blot hybridization and immunohistochemistry, respectively. Plasma angiotensin II, vasopressin, and atrial natriuretic peptide (ANP) concentrations were measured by radioimmunoassay.

RESULTS

The four-day WD increased plasma sodium and osmolality levels, but severely decreased daily urine sodium excretion and urine volume. Plasma angiotensin II and vasopressin concentrations were increased, but the plasma ANP level was significantly decreased in WD rats. nNOS, eNOS, and iNOS mRNA levels were increased by 5.2-, 3.3-, and 3. 4-fold in the outer medulla and 1.7-, 1.5-, and 1.8-fold in the inner medulla, whereas no significant difference was found in the renal cortex of WD rats as compared with NC rats. Additionally, immunohistochemistry revealed that the immunostaining intensity of nNOS, eNOS, and iNOS was clearly enhanced in the medullary thick ascending limb, proximal straight tubule, inner medullary collecting duct, and proximal convoluted tubule in WD rats. Kidney angiotensin II content as well as renin mRNA levels in renal cortex, outer medulla, and inner medulla in WD rats were apparently increased.

CONCLUSIONS

Our results indicate that the increases of nNOS, eNOS, and iNOS synthesis in the kidney, particularly in the renal medulla, may have a role in the adaptation of renal function to volume depletion in the face of an increase of systemic and intrarenal vasoconstrictive substances.

Distribution and characterization of natriuretic peptide receptors in the kidney of the toad, Bufo marinus

The location and characteristics of atrial natriuretic peptide binding sites in the kidney of the toad, Bufo marinus, were determined. Specific (125)I-rANP binding sites were observed on glomeruli and blood vessels, but little if any binding was observed over regions corresponding to the renal tubules. (125)I-rANP binding in tissue sections and/or isolated membranes was completely displaced in the presence of 1 microM rat ANP, frog ANP, and porcine C-type natriuretic peptide (membranes only); however, residual binding remained after incubation with 1 microM of the NPR-C ligand, C-ANF, indicating the presence of two distinct binding sites. Electrophoresis of kidney membranes cross-linked to (125)I-rANP identified specific bands at approximately 70 and 140 kDa which correspond to the monomeric mass of NPR-C and the guanylate cyclase receptors, respectively. In addition, rat ANP, frog ANP, and porcine CNP stimulated a significant increase in cGMP production rates in membrane preparations, while C-ANF had no stimulatory effect. Two partial cDNA clones generated using primers based on conserved regions of vertebrate natriuretic peptide receptors showed high homology to an NPR-C and the natriuretic peptide guanylate cyclase receptors (NPR-GC), respectively. This study provides evidence that the kidney of B. marinus contains both NPR-C and NPR-GC and that the glomerulus is potentially the principal site of ANP regulation in the kidneys.

Mesangial cells from diabetic NOD mice constitutively express increased density of atrial natriuretic peptide C receptors

BACKGROUND

Experimental evidence shows that natriuretic peptides (NPs) play a pathophysiological role in the glomerular hemodynamic abnormalities that occur in diabetes mellitus.

METHODS

In this study, the cGMP response to NPs and the different subtypes of NP receptors were examined in mesangial cells derived from a genetic model of diabetes, the nonobese diabetic (NOD) mouse. Multiple mesangial cell lines were derived from diabetic (D-NOD) and nondiabetic (ND-NOD) adult mice and were studied at different passages.

RESULTS

cGMP accumulation after stimulation by atrial NP (ANP) or C-type NP (CNP) was markedly inhibited in D-NOD cells irrespective of the glucose concentration (6 or 20 mM) in the culture medium. In contrast, NP receptor density measured from [125I]-ANP saturation binding curves was 7.5 times greater in D-NOD than in ND-NOD cells. No change in KD (200 pM in both cell lines) was observed. Competitive inhibition studies showed that 4-23 C-ANP, which is specific of clearance receptors (NPR-C), displaced 90% of the maximum fraction bound, suggesting the predominance of NPR-C in both cell lines. Further identification was obtained from RNase protection assay and reverse transcription-polymerase chain reaction, which also demonstrated the higher expression of NPR-C mRNA in D-NOD cells. In contrast, NPR-A mRNA was not modified. Increased expression of NPR-C in D-NOD cells was associated with an increase of ANP internalization rate at 37 degrees C, indicating that these receptors were functional.

CONCLUSIONS

These studies demonstrate that the constitutive overexpression of NPR-C in D-NOD mesangial cells is associated with a decreased response of cGMP to ANP or CNP treatment. This could be due to the lesser availability of the peptides for binding to NPR-A or NPR-B or to an inhibitory effect on NP-dependent guanylate cyclase activity via the activation of NPR-C.

Increased renal ANP synthesis, but decreased or unchanged cardiac ANP synthesis in water-deprived and salt-restricted rats

BACKGROUND

Experiments were performed to examine the effect of water deprivation and salt restriction on ANP synthesis in the kidneys and hearts of normal rats.

METHODS

A 4-day water deprivation (WD) and 7-day salt restriction (SR; 0.01% NaCl) were performed in 12 and 14 rats, respectively. Atrial natriuretic peptide (ANP) mRNA expression in the kidney was assessed with reverse transcription-polymerase chain reaction coupled with Southern blot hybridization, while the ANP mRNA in the hearts was measured by Northern blot hybridization. ANP and angiotensin II concentrations in the extracted plasma were measured by radioimmunoassay. The molecular form of renal ANP-like protein was characterized by reverse phase-high-performance liquid chromatography (RP-HPLC).

RESULTS

Renal outer and inner medullary ANP mRNA showed a respective 11-fold and ninefold increase in WD rats, and an eightfold and fivefold increase in SR rats as compared to corresponding control groups. Inversely, cardiac atrial ANP mRNA and plasma ANP were decreased in WD rats, whereas they did not change in the SR group. Plasma angiotensin II concentration increased in conjunction with the decrease of urine sodium excretion in both groups. RP-HPLC analysis revealed a 45% extraction of ANP in the WD rat kidneys, whereas only 3% ANP in the control kidneys migrated in a molecular form similar to cardiac atrial proANP.

CONCLUSIONS

Our results demonstrate that water deprivation and salt restriction markedly enhance renal ANP mRNA, whereas water deprivation suppresses cardiac atrial ANP mRNA and plasma ANP concentrations. The current study indicates that renal ANP and cardiac atrial ANP appear to be two distinct systems regulated by different mechanisms and possibly exhibiting different intra-renal paracrine and systemic endocrine functions.