Receptors for atrial natriuretic peptide are decreased in the kidney of rats with streptozotocin-induced diabetes mellitus

Insulin Resistance and High Blood Pressure: Mechanistic Insight on the Role of the Kidney

The metabolic effects of insulin predominate in skeletal muscle, fat, and liver where the hormone binds to its receptor, thereby priming a series of cell-specific and biochemically diverse intracellular mechanisms. In the presence of a good secretory reserve in the pancreatic islets, a decrease in insulin sensitivity in the metabolic target tissues leads to compensatory hyperinsulinemia. A large body of evidence obtained in clinical and experimental studies indicates that insulin resistance and the related hyperinsulinemia are causally involved in some forms of arterial hypertension. Much of this involvement can be ascribed to the impact of insulin on renal sodium transport, although additional mechanisms might be involved. Solid evidence indicates that insulin causes sodium and water retention, and both endogenous and exogenous hyperinsulinemia have been correlated to increased blood pressure. Although important information was gathered on the cellular mechanisms that are triggered by insulin in metabolic tissues and on their abnormalities, knowledge of the insulin-related mechanisms possibly involved in blood pressure regulation is limited. In this review, we summarize the current understanding of the cellular mechanisms that are involved in the pro-hypertensive actions of insulin, focusing on the contribution of insulin to the renal regulation of sodium balance and body fluids.

The role of natriuretic peptides in diabetes and its complications

This review aimed to summarize recent findings on the role of natriuretic peptides (NPs) in diabetes and its important complications. Although the treatment of diabetes mellitus has benefited from recent advances, aggressive glycemic control can increase the risk of hypoglycemia and weight gain. Therefore, innovative therapies are required to address this issue. Natriuretic peptides (NPs) may have such novel therapeutic potential. NPs comprise a family of structurally related peptides, including atrial, brain, C-type, and dendroaspis. Each of these NPs has a wide range of specific functions to regulate and maintain cardiovascular, renal, and endocrine homeostasis. NPs exert their effects by interacting with three receptor subtypes including NPR-A, NPR-B, and NPR-C. The coronary NP system has been suggested to be involved in regulating water and salt balance, as well as vascular remodeling. In this review, we provide evidence that NPs play an important role in diabetes mellitus and its related complications including macrovascular and microvascular disorders. NPs hold promise as markers for early diagnosis, risk assessment, and intervention guidance in diabetes and its complications and may thus improve diabetes care.

Plasma NT-proBNP is independently associated with albuminuria in type 2 diabetes

OBJECTIVE

The aim of this study is to investigate the relationship between plasma NH2-terminal pro-brain natriuretic peptide (NT-proBNP) levels and diabetic nephropathy in type 2 diabetic patients.

METHODS

The records of 983 patients with type 2 diabetes hospitalized in the Endocrinology and Metabolism Department of Peking University People's Hospital between January 2009 and December 2012 were reviewed. Based on their urinary albumin-to-creatinine ratio (UACR), the subjects were divided into three groups, those with normal, micro- and macro-albuminuria. Clinical manifestations and biochemical parameters, including plasma NT-proBNP concentrations, were compared between the three groups. The association between plasma NT-proBNP levels and stages of diabetic nephropathy was also evaluated by ordinal and stepwise linear regression analyses.

RESULTS

The median plasma NT-proBNP concentration in the 983 patients with type 2 diabetes was 60.3 pg/ml, which is within the normal range of healthy subjects. One-way ANOVA analysis showed significant differences in plasma NT-proBNP concentrations among the three groups (p=0.000). Ordinal logistic regression analysis showed that plasma NT-proBNP concentration was an independent risk factor for advanced diabetic nephropathy (odds ratio=1.87,95% CI 1.52-2.29, p=0.000). Plasma NT-proBNP concentration was highly correlated with UACR (r=0.421, p=0.000) and had an independently positive association with UACR by stepwise linear regression analysis (p=0.000).

CONCLUSION

Plasma NT-proBNP levels correlated with the degree of diabetic nephropathy in patients with type 2 diabetes.

Heme Oxygenase Improves Renal Function by Potentiating Podocyte-Associated Proteins in Nω-Nitro-l-Arginine-Methyl Ester (l-NAME)-Induced Hypertension

BACKGROUND

Although heme-oxygenase (HO) is cytoprotective, its effects on podocyte regulators like podocalyxin, podocin, CD2-associated protein (CD2AP) in renal dysfunction in N (ω)-nitro-l-arginine-methyl ester (l-NAME) hypertension are largely unclear.

METHODS

Hypertension was induced in normotensive Sprague Dawley rats by administering l-NAME for 4 weeks. Enzyme immunoassay, enzyme-linked immunosorbent, histology/morphology, spectrophotometry, and western immunoblotting were used. HO was enhanced with heme-arginate (HA) or inhibited with chromium mesoporphyrin (CrMP).

RESULTS

Treatment with heme-arginate reduced several renal histo-pathological lesions including renal arteriolar thickening, glomerular abnormalities, tubular cast, tubular atrophy/fibrosis, and mononuclear cell infiltration in l-NAME-hypertensive rats. Similarly, HA abated the elevated levels of renal extracellular matrix/profibrotic proteins like collagen and fibronectin that deplete nephrin, a fundamental transmembrane protein that forms the scaffoldings of the podocyte slit diaphragm permitting small ions to filter, but not massive excretion of proteins, hence proteinuria. Correspondingly, HA enhanced the aberrant expression of nephrin alongside other important regulators of podocyte like podocalyxin, podocin, and CD2AP, and improved renal function by reducing albuminuria/proteinuria, while increasing creatinine clearance. The renoprotection by HA were accompanied by significant reduction of inflammatory/oxidative mediators including nuclear factor-kappaB, macrophage inflammatory protein-1-alpha, macrophage chemoattractant protein-1, tumor necrosis factor-alpha, interleukin (IL)-6, IL1β, 8-isoprostane, endothelin-1, and aldosterone. These were associated with increased levels of adiponectin, HO-1, HO activity, cyclic guanosine monophosphate, and atrial natriuretic peptide (ANP), whereas the HO inhibitor, CrMP annulled the renoprotection and exacerbated renal dysfunction.

CONCLUSIONS

HA improves renal function by attenuating histopathological lesions, suppressing inflammatory/oxidative mediators, abating profibrotic/extracellular matrix proteins, and reducing albuminuria/proteinuria, while concomitantly potentiating the HO-adiponectin-ANP axis, enhancing nephrin, podocin, podocalyxin, CD2AP and increasing creatinine clearance. Our study underscores the benefit of potentiating the HO-adiponectin-ANP against nephropathy.

Increased plasma concentrations of midregional proatrial natriuretic Peptide is associated with risk of cardiorenal dysfunction in type 1 diabetes

BACKGROUND

To examine possible associations between midregional proatrial natriuretic peptide (MR-proANP) and diabetic complications at baseline and risk of mortality and end-stage renal disease (ESRD) during follow-up in type 1 diabetes.

METHODS

Observational study including 667 patients, with plasma MR-proANP measured at baseline. Complications were defined as micro- (n = 168) or macroalbuminuria (n = 190) (urinary albumin excretion rate (UAER) 30-299 or ≥ 300 mg/24h), previous cardiovascular disease (CVD) (n = 143), cardiac autonomic dysfunction (heart rate variability < 11 beats/min) (n = 369), and retinopathy (n = 523). Adjustments included gender, age, systolic blood pressure, estimated glomerular filtration rate (eGFR), UAER, HbA1c, total cholesterol, 24-hour urinary sodium excretion (24h-U(Na)), body mass index, daily insulin dose, antihypertensive treatment, and smoking in linear regression analyses and analysis of covariance models. Development of ESRD (dialysis, renal transplantation, or GFR/eGFR < 15 ml/min/1.73 m(2)) and mortality was recorded through national registers.

RESULTS

The cohort included 293 (44%) females, aged 55 ± 13 years. Plasma MR-proANP (median (interquartile)) was 74.7 (49.2-116.8) pmol/L. Adjusted, MR-proANP correlated positively with age and UAER and negatively with eGFR, 24h-U(Na), total cholesterol, and HbA1c (P < 0.05). Moreover, MR-proANP levels increased with albuminuria degree and were higher in patients with previous CVD (P ≤ 0.001), but similar in patients with or without autonomic dysfunction or retinopathy (P ≥ 0.076). During follow-up (3.5 (3.1-4.0) years), higher MR-proANP concentrations predicted ESRD and mortality combined (n = 35) adjusted for gender, age, systolic blood pressure, eGFR, and previous CVD (hazard ratio per 1SD increase in logANP: 2.8 (1.6-4.7; P < 0.001)).

CONCLUSIONS

Increased plasma MR-proANP was associated with impaired renal function, increased albuminuria, and previous CVD. Moreover, MR-proANP concentrations were associated with increased risk of development of ESRD and mortality combined during follow-up.

Characteristics of dendroaspis natriuretic peptide and its receptor in streptozotocin-induced diabetic rats

Dendroaspis natriuretic peptide (DNP) shares a functionally important sequence homology with other natriuretic peptides. However, the characteristics of DNP and its receptor in the context of diabetes remafin to be fully elucidated. In the present study, alterations in the plasma levels and tissue contents of DNP and the properties of its receptor in diabetic rats, induced by streptozotocin (STZ) injection, were investigated. The plasma levels of DNP were 90.01 ± 4.12 and 196.68 ± 5.60 pg/ml in the control and STZ-induced diabetic rats, respectively. The tissue contents of DNP in the cardiac atrium, ventricle, renal cortex and inner medulla of the STZ-induced diabetic rats were also significantly increased compared with the control rats. Specific (125)I-DNP-binding sites were located predominantly in the glomeruli and inner medulla of the rat kidney. In the glomeruli of the kidney, the apparent dissociation constants (Kd) of (125)I-DNP in the control and STZ-induced diabetic rats were 0.41 ± 0.03 and 0.56 ± 0.06 nM, respectively. The maximum binding capacities (Bmax) of (125)I-DNP in control and STZ-induced diabetic rats were 2.98 ± 0.21 and 6.22 ± 1.06 fmol/mg protein, respectively. However, no differences were observed in the apparent Kd and Bmax of (125)I-DNP in the inner medulla of the kidney between the control and STZ-induced diabetic rats. In the glomerular and inner medullary kidney membranes, DNP stimulated the production of cyclic guanosine monophosphate (cGMP) in a dose-dependent manner. The magnitude of cGMP production in glomerular membranes was greater in the STZ-induced diabetic rats, whereas the magnitude of cGMP production in the inner medullary membranes was lower in the STZ-induced diabetic rats compared with the control rats. These results indicated that STZ-induced diabetes modulate DNP and its receptor, and also suggested that modulation of the DNP system is involved in the renal function of diabetic animals via the intracellular domain of the kidney NP receptor.

Hemin therapy improves kidney function in male streptozotocin-induced diabetic rats: role of the heme oxygenase/atrial natriuretic peptide/adiponectin axis

Diabetic nephropathy is characterized by elevated macrophage infiltration and inflammation. Although heme-oxygenase (HO) is cytoprotective, its role in macrophage infiltration and nephropathy in type 1 diabetes is not completely elucidated. Administering the HO inducer, hemin, to streptozotocin-diabetic rats suppressed renal proinflammatory macrophage-M1 phenotype alongside several proinflammatory agents, chemokines, and cytokines including macrophage inflammatory protein 1α (MIP-1α), macrophage-chemoattractant protein-1 (MCP-1), TNF-α, IL-1β, IL-6, nuclear factor-κB (NF-κB), and aldosterone, a stimulator of the inflammatory/oxidative transcription factor, NF-κB. Similarly, hemin therapy attenuated extracellular matrix/profibrotic proteins implicated in renal injury including fibronectin, collagen-IV, and TGF-β1 and reduced several renal histopathological lesions such as glomerulosclerosis, tubular necrosis, tubular vacuolization, and interstitial macrophage infiltration. Furthermore, hemin reduced markers of kidney dysfunction like proteinuria and albuminuria but increased creatinine clearance, suggesting improved kidney function. Correspondingly, hemin significantly enhanced the antiinflammatory macrophage-M2 phenotype, IL-10, adiponectin, HO-1, HO activity, and atrial natriuretic-peptide (ANP), a substance that abates TNF-α, IL-6, and IL-1β, with parallel increase of urinary cGMP, a surrogate marker of ANP. Contrarily, coadministering the HO inhibitor, chromium-mesoporphyrin with the HO-inducer, hemin nullified the antidiabetic and renoprotective effects, whereas administering chromium-mesoporphyrin alone abrogated basal HO activity, reduced basal adiponectin and ANP levels, aggravated hyperglycemia, and further increased MCP-1, MIP-1α, aldosterone, NF-κB, TNF-α, IL-6, IL-1β, proteinuria/albuminuria, and aggravated creatinine clearance, thus exacerbating renal dysfunction, suggesting the importance of the basal HO-adiponectin-ANP axis in renoprotection and kidney function. Collectively, these data suggest that hemin ameliorates diabetic nephropathy by selectively enhancing the antiinflammatory macrophage-M2 phenotype and IL-10 while concomitantly abating the proinflammatory macrophage-M1 phenotype and suppressing extracellular matrix/profibrotic factors with reduction of renal lesions including interstitial macrophage infiltration. Because aldosterone stimulate NF-κB, which activates cytokines like TNF-α, IL-6, IL-1β that in turn stimulate chemokines such as MCP-1 and MIP-1α to promote macrophage-M1 infiltration, the hemin-dependent potentiation of the HO-adiponectin-ANP axis may account for reduced macrophage infiltration and inflammatory insults in streptozotocin-diabetic rats.

Predictive value of serum NT-proBNP levels in type 2 diabetic people with diabetic nephropathy

OBJECTIVE

The serum N-terminal fragment of pro brain natriuretic peptide (NT-proBNP) level in type 2 diabetic subjects with or without diabetic nephropathy (DN) is still unclear. We aimed to evaluate the relationship between serum NT-proBNP levels and different stages of diabetic nephropathy, and identify probable factors predicting serum NT-proBNP level.

SUBJECTS AND METHODS

This cross-sectional study included 20 normoalbuminuric (Group-I), 28 microalbuminuric (Group-II), 20 macroalbuminuric type 2 diabetic patients (Group-III), and 20 healthy volunteers (Group-IV). Serum NT-proBNP levels were measured with highly sensitive and specific immunoassay.

RESULTS

Mean NT-proBNP levels were 32 ± 55, 91 ± 95, 331 ± 297, 42 ± 34 pg/ml for Groups I-IV, respectively. When patients with LVH were excluded, mean logNT-proBNP was still significantly higher in Group-III than all other groups. The three diabetic groups were similar in age, BMI, HbA1c, fasting serum glucose, and GFR. In a multivariate linear regression model, adjusting for factors significantly correlated with NT-proBNP levels, the patient group, presence of LVH, and hemoglobin remained as an independent predictor of serum NT-proBNP. These variables explained 68% of the variability of NT-proBNP (adjusted R(2)=0.683).

CONCLUSIONS

Mean serum NT-proBNP level of macroalbuminuric diabetic patients was higher than normoalbuminuric and microalbuminuric diabetic patients, and healthy control subjects even after exclusion of LVH. NT-proBNP may be a useful and predictive marker of diabetic nephropathy.

Diabetic neuropathy and heart failure: role of neuropeptides

Cardiovascular autonomic neuropathy (CAN), in which patients present with damage of autonomic nerve fibres, is one of the most common complications of diabetes. CAN leads to abnormalities in heart rate and vascular dynamics, which are features of diabetic heart failure. Dysregulated neurohormonal activation, an outcome of diabetic neuropathy, has a significant pathophysiological role in diabetes-associated cardiovascular disease. Key players in neurohormonal activation include cardioprotective neuropeptides and their receptors, such as substance P (SP), neuropeptide Y (NPY), calcitonin-gene-related peptide (CGRP), atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). These neuropeptides are released from the peripheral or autonomic nervous system and have vasoactive properties. They are further implicated in cardiomyocyte hypertrophy, calcium homeostasis, ischaemia-induced angiogenesis, protein kinase C signalling and the renin-angiotensin-aldosterone system. Therefore, dysregulation of the expression of neuropeptides or activation of the neuropeptide signalling pathways can negatively affect cardiac homeostasis. Targeting neuropeptides and their signalling pathways might thus serve as new therapeutic interventions in the treatment of heart failure associated with diabetes. This review discusses how neuropeptide dysregulation in diabetes might affect cardiac functions that contribute to the development of heart failure.

Glycaemic control with insulin prevents the reduced renal dopamine D1 receptor expression and function in streptozotocin-induced diabetes

BACKGROUND

It was demonstrated in streptozotocin (STZ)-induced diabetic rats that the D(1) receptor agonist failed to promote sodium excretion as a result of reduced renal D(1) receptor expression and decreased receptor G protein coupling. The present study examined the influence of glycaemic control with insulin on the renal D(1) receptor dysfunction in STZ-induced type 1 diabetes.

METHODS

Renal function, blood pressure, the natriuretic response to 5% volume expansion (VE) and the effects of the D(1) receptor agonist fenoldopam on natriuresis and on Na(+)/K(+)-ATPase activity in renal tubules were evaluated in uninephrectomized and sham-operated Wistar rats treated with STZ and compared with controls and STZ-treated rats made euglycaemic with insulin. D(1) receptor immunohistochemistry and protein abundance by western blot were also determined in all groups.

RESULTS

Treatment of sham and uninephrectomized rats with STZ caused a 4-fold increase in glucose plasma levels compared to controls and euglycaemic diabetic rats. A blunted natriuretic response to VE was observed in both sham and uninephrectomized hyperglycaemic diabetic rats, and this was accompanied by failure of fenoldopam to increase natriuresis and to inhibit renal Na(+)/K(+)-ATPase activity. In contrast, in both sham and uninephrectomized euglycaemic diabetic rats, the natriuretic response to VE, the fenoldopam-induced natriuresis and the accompanied inhibition of Na(+)/K(+)-ATPase activity were similar to those of the corresponding controls. D(1) receptor immunodetection and protein abundance were reduced in hyperglycaemic diabetic rats, but not in euglycaemic diabetic animals.

CONCLUSIONS

We conclude that the renal expression and natriuretic response to D(1) receptor activation is compromised in both sham and uninephrectomized rats with STZ-induced diabetes. These abnormalities were prevented by lowering glucose blood levels with insulin, thus providing evidence for the involvement of hyperglycaemia in the disturbances that underlie the compromised dopamine-sensitive natriuresis and increase of blood pressure in type 1 diabetes.

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.

Atrial natriuretic peptide in hypoxia

A growing number of mammalian genes whose expression is inducible by hypoxia have been identified. Among them, atrial natriuretic peptide (ANP) synthesis and secretion is increased during hypoxic exposure and plays an important role in the normal adaptation to hypoxia and in the pathogenesis of cardiopulmonary diseases, including chronic hypoxia-induced pulmonary hypertension and vascular remodeling, and right ventricular hypertrophy and right heart failure. This review discusses the roles of ANP and its receptors in hypoxia-induced pulmonary hypertension. We and other investigators have demonstrated that ANP gene expression is enhanced by exposure to hypoxia and that the ANP so generated protects against the development of hypoxic pulmonary hypertension. Results also show that hypoxia directly stimulates ANP gene expression and ANP release in cardiac myocytes in vitro. Several cis-responsive elements of the ANP promoter are involved in the response to changes in oxygen tension. Further, the ANP clearance receptor NPR-C, but not the biological active NPR-A and NPR-B receptors, is downregulated in hypoxia adapted lung. Hypoxia-sensitive tyrosine kinase receptor-associated growth factors, including fibroblast growth factor (FGF) and platelet derived growth factor (PDGF)-BB, but not hypoxia per se, inhibit NPR-C gene expression in pulmonary arterial smooth muscle cells in vitro. The reductions in NPR-C in the hypoxic lung retard the clearance of ANP and allow more ANP to bind to biological active NPR-A and NPR-B in the pulmonary circulation, relaxing preconstricted pulmonary vessels, reducing pulmonary arterial pressure, and attenuating the development of hypoxia-induced pulmonary hypertension and vascular remodeling.

Distribution of atrial natriuretic peptide and its effects on contraction and intracellular calcium in ventricular myocytes from streptozotocin-induced diabetic rat

The distribution of atrial natriuretic peptide (ANP) in blood plasma and cardiac muscle and its effects on ventricular myocyte contraction and intracellular free calcium concentration [Ca2+]i in the streptozotocin (STZ)-induced diabetic rat have been investigated. Blood plasma concentration and heart atrial and ventricular contents of ANP were significantly increased in STZ-treated rats compared to age-matched controls. STZ treatment increased the number of ventricular myocytes immunolabeled with antibodies against ANP. In control myocytes the percentage of cells that labeled positively and negatively were 17% versus 83%, respectively. However, in myocytes from STZ-treated rat the percentages were 52% versus 53%. Time to peak (TPK) shortening was significantly and characteristically prolonged in myocytes from STZ-treated rats (360+/-5 ms) compared to controls (305+/-5 ms). Amplitude of the Ca2+ transient was significantly increased in myocytes from STZ-treated rats compared to controls (0.39+/-0.02 versus 0.29+/-0.02 fura-2 RU in controls) and treatment with ANP reduced the amplitude of the Ca2+ transient to control levels. ANP may have a protective role in STZ-induced diabetic rat heart.

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.

C-type natriuretic peptide as a podocyte hormone and modulation of its cGMP production by glucose and mechanical stress

BACKGROUND

High glucose and mechanical strain resulting from capillary hypertension are relevant risk factors affecting glomerular cells in diabetes. Altered activity of the natriuretic peptide (NP) system acting via vasorelaxing cyclic guanosine 5' monophosphate (cGMP) has been proposed to be one of the reasons for diabetes-dependent impairment of kidney function. Podocytes possess the NP receptors (NPRs) coupled to particular guanylyl cyclase. We investigated whether mechanical stress and high ambient glucose influence cGMP generation in podocytes stimulated with NPs. Additionally, the C-type natriuretic peptide (CNP) system has been characterized in these cells.

METHODS

Conditionally immortalized mouse podocytes were stimulated with NP for 15 minutes and cGMP was determined by enzymatic immunoassay. The mRNA expression for CNP and CNP-specific NPR-B was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, cGMP synthesis was measured in cells exposed to mechanical stress and to 30 mmol/L glucose for 3 days.

RESULTS

Production of cGMP upon stimulation of the NPR-B receptor with CNP and inhibition by an antagonist HS-142-1 was dose-dependent. RT-PCR showed that podocytes express mRNA not only for NPR-B but also for CNP. Mechanical stress reduced the cGMP response by 50%, both to atrial natriuretic peptide (ANP) and to CNP. Conversely, upon high glucose, the CNP-induced production of cGMP was elevated twofold in stretched and in control cells. Furthermore, as compared to ANP, stimulation with CNP caused a larger increase in cGMP levels in stretched as well as in nonstretched cells.

CONCLUSION

Expression of CNP together with potent NPR-B receptors suggests that in podocytes, CNP may act in an autocrine and/or paracrine manner. Furthermore, in a diabetic kidney, high glucose and mechanical stress may modulate the CNP-dependent cGMP production in podocytes in an opposite manner.

Down-regulation does not mediate natriuretic peptide-dependent desensitization of natriuretic peptide receptor (NPR)-A or NPR-B: guanylyl cyclase-linked natriuretic peptide receptors do not internalize

Natriuretic peptide receptor A (NPR-A/GC-A) and B (NPR-B/GC-B) are members of the transmembrane guanylyl cyclase family that mediate the effects of natriuretic peptides via the second messenger, cGMP. Despite numerous reports of these receptors being down-regulated in response to various pathological conditions, no studies have actually measured desensitization and receptor internalization in the same cell line. Furthermore, the ligand-dependent trafficking properties of NPR-A remain controversial, whereas nothing is known about the trafficking of NPR-B. In this report, we tested whether down-regulation explains the ligand-dependent desensitization of NPR-A and NPR-B and characterized their trafficking properties using a combination of hormone-binding and antibody-based assays. Quantitative partition analysis indicated that (125)I-atrial natriuretic peptide (ANP) was rapidly released into the medium after 293T cells stably expressing NPR-A were warmed from 4 degrees to 37 degrees C. High-performance liquid chromatography fractionation of medium supplemented with the protease inhibitor phosphoramidon indicated that the (125)I-ANP was mostly intact. In contrast, (125)I-ANP purified from medium bathing cells expressing NPR-C, a receptor known to internalize natriuretic peptides, was degraded. Cleavable biotinylation and noncleavable biotinylation assays indicated that neither NPR-A nor NPR-B was internalized or degraded in response to natriuretic peptide binding. In contrast, agonist-dependent internalization of a G protein-coupled receptor was clearly apparent in the same cell line. Finally, we show that NPR-A and NPR-B are desensitized in cells in which they are not internalized. We suggest that mechanisms other than receptor down-regulation account for the desensitization of NPR-A and NPR-B that occurs in response to various physiological and pathological stimuli.

Calcium-dependent dephosphorylation mediates the hyperosmotic and lysophosphatidic acid-dependent inhibition of natriuretic peptide receptor-B/guanylyl cyclase-B

C-type natriuretic peptide binding to natriuretic peptide receptor-B (NPR-B) stimulates cGMP synthesis, which regulates vasorelaxation, cell proliferation, and bone growth. Here, we investigated the mechanistic basis for hyperosmotic and lysophosphatidic acid-dependent inhibition of NPR-B. Whole cell cGMP measurements and guanylyl cyclase assays indicated that acute hyperosmolarity decreased NPR-B activity in a reversible, concentration- and time-dependent manner, whereas chronic exposure had no effect. Acute hyperosmolarity elevated intracellular calcium in a concentration-dependent fashion that paralleled NPR-B desensitization. A calcium chelator, but not a protein kinase C inhibitor, blocked both calcium elevations and desensitization. Hyperosmotic medium stimulated NPR-B dephosphorylation, and the receptor was rapidly rephosphorylated and resensitized when the hypertonic media was removed. Lysophosphatidic acid also inhibited NPR-B in a calcium- and phosphorylation-dependent process, consistent with calcium being a universal regulator of NPR-B. The absolute requirement of dephosphorylation in this process was demonstrated by showing that a receptor with glutamates substituted at all known NPR-B phosphorylation sites is unresponsive to hyperosmotic stimuli. This is the first study to measure the phosphorylation state of an endogenous guanylyl cyclase and to link intracellular calcium elevations with its dephosphorylation.

B-type natriuretic peptide prevents acute hypertrophic responses in the diabetic rat heart: importance of cyclic GMP

Stimulation of cardiomyocyte guanosine 3',5'-cyclic monophosphate (cyclic GMP) via endothelial-derived nitric oxide (NO) is an important mechanism by which bradykinin and ACE inhibitors prevent hypertrophy. Endothelial NO dysfunction and cardiac hypertrophy are morbid features of diabetes not entirely prevented by ACE inhibitors. In cardiomyocyte/endothelial cell cocultures, bradykinin efficacy is abolished by high-glucose-induced endothelial NO dysfunction. We now demonstrate that antihypertrophic actions of natriuretic peptides, which stimulate cyclic GMP independently of NO, are preserved in cardiomyocytes despite high-glucose-induced endothelial dysfunction. Further, streptozotocin-induced diabetes significantly impairs the effectiveness of acute antihypertrophic strategies in isolated rat hearts. In hearts from citrate-treated control rats, angiotensin II-stimulated [(3)H]phenylalanine incorporation and atrial natriuretic peptide and beta-myosin heavy chain mRNA expression were prevented by B-type natriuretic peptide (BNP), bradykinin, the ACE inhibitor ramiprilat, and the neutral endopeptidase inhibitor candoxatrilat. These antihypertrophic effects were accompanied by increased left ventricular cyclic GMP. In age-matched diabetic hearts, the antihypertrophic and cyclic GMP stimulatory actions of bradykinin, ramiprilat, and candoxatrilat were absent. However, the blunting of hypertrophic markers and accompanying increases in cyclic GMP stimulated by BNP were preserved in diabetes. Thus BNP, which increases cyclic GMP independently of NO, is an important approach to prevent growth in the diabetic myocardium, where endothelium-dependent mechanisms are compromised.

Increased cGMP phosphodiesterase activity mediates renal resistance to ANP in rats with bile duct ligation

BACKGROUND

Liver disease resulting from common bile duct ligation (CBDL) causes abnormal sodium metabolism that is manifested by resistance to the natriuretic action of atrial natriuretic peptide (ANP). This resistance is corrected both in vitro and in vivo by zaprinast, a selective inhibitor of a guanosine cyclic-3'-5'-monophosphate (cGMP)-specific phosphodiesterase (PDE5). Several other PDEs with affinity for cGMP are expressed in kidney and could also be involved in this response.

METHODS

We measured cGMP hydrolysis in inner medullary collecting duct (IMCD) cell homogenates from kidneys of sham-operated and CBDL rats and quantitated the amount of PDE5 protein by Western blotting and immunoprecipitation studies. We also characterized ANP responsiveness in vivo of kidneys of anesthetized sham and CBDL rats by measuring sodium excretion before and after volume expansion (VE).

RESULTS

Kinetic analysis of PDE5 activity in homogenates of IMCD cells isolated from kidneys of sham-operated rats indicated a Vmax of 85.3 +/- 1.7 versus 157 +/- 2.9 pmol/mg/min from CBDL rats (P < 0.01), without a difference in Km. Enzyme activity was inhibited competitively by 1,3-dimethyl-6-(2-propoxy-5-methanesulfonylamidophenyl)pyrazol[3,4d]-pyrimidin-4-(5H)-one (DMPPO), a potent and specific inhibitor of PDE5, with an apparent Ki of 4.5 +/- 0.7 and 4.9 +/- 0.7 nmol/L and an IC50 of 6.1 +/- 0.8 and 8.7 +/- 0.7 nmol/L in sham and CBDL rats, respectively (P = NS). DMPPO exhibited very poor inhibitory activity against the calcium-calmodulin-dependent PDE1 in IMCD homogenates from sham rats (Ki 1.3 +/- 0.1 micromol/L and IC50 1.9 +/- 0.2 micromol/L). Western analysis using an antiserum made against bovine lung PDE5 revealed a twofold increase in PDE5 protein in cytosolic extracts from IMCD of CBDL rat kidneys compared with sham-operated controls, and immunoprecipitation studies indicated that the increase in PDE5 protein accounted for the observed increase in cGMP hydrolysis. DMPPO (10 nmol/L) normalized the blunted ANP-dependent cGMP accumulation by IMCD cells from CBDL rats in vitro. Intrarenal infusion of DMPPO (0.5 nmol/min) in CBDL rats corrected both the impaired natriuretic response to VE and the blunted VE-related increase in urinary cGMP excretion from the infused, but not the contralateral kidney.

CONCLUSION

These results demonstrate that renal resistance to ANP in CBDL rats is accompanied by heightened activity of PDE5, which is due largely to an increase in PDE5 protein. Other PDEs could contribute only a minor part to the enhanced cGMP hydrolysis observed in kidneys of CBDL rats. This PDE5-dependent ANP resistance may represent an important contributor to the sodium retention of liver disease.

Elevated atrial natriuretic peptides and early renal failure in type 2 diabetic Goto-Kakizaki rats

The present investigation was designed to determine if atrial natriuretic peptides (ANPs) are increased in a spontaneous model of non-obese type 2 diabetes, the Goto-Kakizaki (GK) rat. Four peptide hormones originating from the ANP prohormone were increased twofold (P < .05) to sixfold (P < .01) in the circulation of GK rats compared with nondiabetic Wistar rats from which the GK colony was originally derived. Thus, ANP, long-acting natriuretic peptide (LANP), vessel dilator, and kaliuretic peptide were (mean +/- SE) 497 +/- 78, 1,285 +/- 105, 457 +/- 45, and 385 +/- 87 pg/mL in GK rats, versus 78 +/- 23, 542 +/- 77, 137 +/- 26, and 134 +/- 33 pg/mL, respectively, in Wistar rats. In evaluating the cause of the increased ANPs, the blood volume of GK rats (16.2 +/- 0.4 mL) was significantly (P < .01) increased compared with Wistar rats (9.5 +/- 0.3 mL). The ventricles of GK rats were not dilated when examined by transthoracic echocardiography, but the venous system was markedly distended. GK rats had a 48% to 79% decrease in renal function (ie, increased serum creatinine and blood urea nitrogen [BUN]) compared with Wistar rats. These results indicate that circulating ANPs are increased in the GK spontaneously diabetic rat secondary to (1) increased blood volume, which leads to increased synthesis and release of ANPs, and (2) renal failure, which results in a delayed metabolic processing of these peptides. The early combined increases of the four atrial peptides collectively may contribute to the hyperfiltration that occurs in early diabetes mellitus.

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.

Cyclic-3',5'-nucleotide phosphodiesterase isozymes in cell biology and pathophysiology of the kidney

Investigations of recent years revealed that isozymes of cyclic-3', 5'-nucleotide phosphodiesterase (PDE) are a critically important component of the cyclic-3',5'-adenosine monophosphate (cAMP) protein kinase A (PKA) signaling pathway. The superfamily of cyclic-3', 5'-phosphodiesterase (PDE) isozymes consists of at least nine gene families (types): PDE1 to PDE9. Some PDE families are very diverse and consist of several subtypes and numerous PDE isoform-splice variants. PDE isozymes differ in molecular structure, catalytic properties, intracellular regulation and location, and sensitivity to selective inhibitors, as well as differential expression in various cell types. A number of type-specific "second-generation" PDE inhibitors have been developed. Current evidence indicates that PDE isozymes play a role in several pathobiologic processes in kidney cells. In rat mesangial cells, PDE3 and PDE4 compartmentalize cAMP signaling to the PDE3-linked cAMP-PKA pathway that modulates mitogenesis and PDE4-linked cAMP-PKA pathway that modulates generation of reactive oxygen species. Administration of selective PDE isozyme inhibitors in vivo suppresses proteinuria and pathologic changes in experimental anti-Thy-1.1 mesangial proliferative glomerulonephritis in rats. Increased activity of PDE5 (and perhaps also PDE9) in glomeruli and in cells of collecting ducts in sodium-retaining states, such as nephrotic syndrome, accounts for renal resistance to atriopeptin; diminished ability to excrete sodium can be corrected by administration of the selective PDE5 inhibitor zaprinast. Anomalously high PDE4 activity in collecting ducts is a basis of unresponsiveness to vasopressin in mice with hereditary nephrogenic diabetes insipidus. Apparently, PDE isozymes apparently also play an important role in the pathogenesis of acute renal failure of different origins. Administration of PDE isozyme-selective inhibitors suppresses some components of immune responses to allograft transplant and improves preservation and survival of transplanted organ. PDE isozymes are a target for action of numerous novel selective PDE inhibitors, which are key components in the design of novel "signal transduction" pharmacotherapies of kidney diseases.

Extrarenal resistance to atrial natriuretic peptide in rats with experimental nephrotic syndrome

Nephrotic syndrome is associated with resistance to the renal actions of atrial natriuretic peptide (ANP). We performed experiments in anesthetized, acutely nephrectomized rats 21-28 days after injection of adriamycin (7-8 mg/kg i.v.) or 9-14 days after injection of anti-Fx1A antiserum (5 ml/kg i.p.) (passive Heymann nephritis; PHN) to test whether extrarenal resistance also occurred. Proteinuria was significantly elevated in both models compared with controls before study. ANP infusion (1 microgram.kg-1.min-1) caused arterial pressure to decrease similarly in control rats, adriamycin-treated rats, and rats with PHN (by 8.2 +/- 1.0, 9.4 +/- 2.3, and 9.0 +/- 2.0%, respectively; all P < 0.05 vs. both baseline and vehicle-infused control rats). In control rats, hematocrit increased progressively to a maximal value 9.5 +/- 0.9% over baseline as a result of the infusion, an increase corresponding to a reduction in plasma volume of 16.1 +/- 0.9%. The ANP-induced increase in hematocrit was preserved in adriamycin-treated rats (9.2 +/- 1.3%) but was markedly blunted in rats with PHN (2.4 +/- 1.3%; P < 0.0001 vs. ANP infusion in control rats). ANP infusion increased plasma ANP levels to the same extent in the three groups, whereas plasma guanosine 3',5'-cyclic monophosphate was significantly lower in rats with PHN compared with both control and adriamycin-treated rats. Infusion of a subpressor dose of angiotensin II (ANG II, 2.5 ng.kg-1.min-1) fully restored the ANP-induced increase in hematocrit in rats with PHN. This study demonstrates that 1) the hemoconcentrating and hypotensive actions of ANP are preserved in adriamycin-treated rats, 2) the effect of ANP on hematocrit and fluid distribution is blunted in rats with PHN while its hypotensive action is preserved, and 3) low-level ANG II infusion normalizes the hemoconcentrating effect of exogenously infused ANP in rats with PHN. Thus deficient ANG II generation in rats with PHN, but not adriamycin nephrosis, may contribute to extrarenal ANP resistance.

Protein kinase C activation down-regulates natriuretic peptide receptor C expression via transcriptional and post-translational pathways

Natriuretic peptide receptor C (NPR-C) mRNA expression and ANP-binding activity via NPR-C are significantly down-regulated in HeLa cells with phorbol myristate acetate (PMA) treatment. Stabilization of the NPR-C mRNA by PMA indicated that down-regulation of its mRNA was mediated through negative transcriptional regulation. Despite the significant loss of the mRNA, reduction of NPR-C-specific ANP-binding activity after PMA exposure (4 h) was accompanied by a slight decrease in total NPR-C protein (with a 5% loss) and was also produced in the presence of actinomycin D or cycloheximide. The inhibitory effect of a long PMA exposure (18 h) paralleled with a decrease in total NPR-C protein is suggested to be dependent on reduction of de novo NPR-C synthesis. PMA-induced transcriptional and post-translational down-regulation of NPR-C was effectively reversible in the presence of the protein kinase C inhibitor GF109203X. These findings demonstrate that protein kinase C activation down-regulated NPR-C expression through transcriptional and post-translational pathways and that immediate functional receptor loss was mediated via a post-translational mechanism, such as enhanced receptor internalization.