Nitric oxide, cGMP, and hormone regulation of active sodium transport

Ouabain Reverts CUS-Induced Disruption of the HPA Axis and Avoids Long-Term Spatial Memory Deficits

Ouabain (OUA) is a cardiotonic steroid that modulates Na+, K+ -ATPase activity. OUA has been identified as an endogenous substance that is present in human plasma, and it has been shown to be associated with the response to acute stress in both animals and humans. Chronic stress is a major aggravating factor in psychiatric disorders, including depression and anxiety. The present work investigates the effects of the intermittent administration of OUA (1.8 μg/kg) during the chronic unpredictable stress (CUS) protocol in a rat's central nervous system (CNS). The results suggest that the intermittent OUA treatment reversed CUS-induced HPA axis hyperactivity through a reduction in (i) glucocorticoids levels, (ii) CRH-CRHR1 expression, and by decreasing neuroinflammation with a reduction in iNOS activity, without interfering with the expression of antioxidant enzymes. These changes in both the hypothalamus and hippocampus may reflect in the rapid extinction of aversive memory. The present data demonstrate the ability of OUA to modulate the HPA axis, as well as to revert CUS-induced long-term spatial memory deficits.

Influence of Nitric Oxide-Cyclic GMP and Oxidative STRESS on Amyloid-β Peptide Induced Decrease of Na,K-ATPase Activity in Rat Hippocampal Slices

Amyloid-β peptide (Aβ) has been shown to cause synaptic dysfunction and can render neurons vulnerable to excitotoxicity and oxidative stress. Na,K-ATPase plays an important role to maintain cell ionic equilibrium and it can be modulated by N-methyl-D-aspartate (NMDA)-nitric oxide (NO)-cyclic GMP pathway. Disruption of NO synthase (NOS) activity and reactive oxygen species (ROS) production could lead to changes in Na,K-ATPase isoforms' activities that may be detrimental to the cells. Our aim was to evaluate the signaling pathways of Aβ in relation to NMDA-NOS-cyclic GMP versus oxidative stress on α₁-/α_2,3-Na,K-ATPase activities in rat hippocampal slices. Aβ_1-40 induced a concentration-dependent increase of NOS activity and increased cyclic guanosine monophosphate (cGMP), TBARS (thiobarbituric acid reactive substances), and 3-Nitrotyrosine (3-NT)-modified protein levels in rat hippocampal slices. The increase in NOS activity and cyclic GMP levels induced by Aβ_1-40 was completely blocked by MK-801 (inhibitor of NMDA receptor) and L-NAME (inhibitor of NOS) pre-treatment but changes in TBARS levels were only partially blocked by both compounds. The Aβ treatment also decreased Na,K-ATPase activity which was reverted by N-nitro-L-arginine methyl ester hydrochloride (L-NAME) but not by MK-801 pre-treatment. The decrease in enzyme activity induced by Aβ was isoform-specific since only α₁-Na,K-ATPase was affected. These findings suggest that the activation of NMDA-NOS signaling cascade linked to α_2,3-Na,K-ATPase activity may mediate an adaptive, neuroprotective response to Aβ in rat hippocampus.

Fluoride Exposure Induces Inhibition of Sodium-and Potassium-Activated Adenosine Triphosphatase (Na+, K+-ATPase) Enzyme Activity: Molecular Mechanisms and Implications for Public Health

In this study, several lines of evidence are provided to show that Na + , K + -ATPase activity exerts vital roles in normal brain development and function and that loss of enzyme activity is implicated in neurodevelopmental, neuropsychiatric and neurodegenerative disorders, as well as increased risk of cancer, metabolic, pulmonary and cardiovascular disease. Evidence is presented to show that fluoride (F) inhibits Na + , K + -ATPase activity by altering biological pathways through modifying the expression of genes and the activity of glycolytic enzymes, metalloenzymes, hormones, proteins, neuropeptides and cytokines, as well as biological interface interactions that rely on the bioavailability of chemical elements magnesium and manganese to modulate ATP and Na + , K + -ATPase enzyme activity. Taken together, the findings of this study provide unprecedented insights into the molecular mechanisms and biological pathways by which F inhibits Na + , K + -ATPase activity and contributes to the etiology and pathophysiology of diseases associated with impairment of this essential enzyme. Moreover, the findings of this study further suggest that there are windows of susceptibility over the life course where chronic F exposure in pregnancy and early infancy may impair Na + , K + -ATPase activity with both short- and long-term implications for disease and inequalities in health. These findings would warrant considerable attention and potential intervention, not to mention additional research on the potential effects of F intake in contributing to chronic disease.

Distribution, projections, and association with calbindin baskets of motor neurons, interneurons, and sensory neurons in guinea-pig distal colon

Normal gut function relies on the activity of the enteric nervous system (ENS) found within the wall of the gastrointestinal tract. The structural and functional organization of the ENS has been extensively studied in the guinea pig small intestine, but less is known about colonic circuitry. Given that there are significant differences between these regions in function, observed motor patterns and pathology, it would be valuable to have a better understanding of the colonic ENS. Furthermore, disorders of colonic motor function, such as irritable bowel syndrome, are much more common. We have recently reported specialized basket-like structures, immunoreactive for calbindin, that likely underlie synaptic inputs to specific types of calretinin-immunoreactive neurons in the guinea-pig colon. Based on detailed immunohistochemical analysis, we postulated the recipient neurons may be excitatory motor neurons and ascending interneurons. In the present study, we combined retrograde tracing and immunohistochemistry to examine the projections of circular muscle motor neurons, myenteric interneurons, and putative sensory neurons. We focused on neurons with immunoreactivity for calbindin, calretinin and nitric oxide synthase and their relationship with calbindin baskets. Retrograde tracing using indocarbocyanine dye (DiI) revealed that many of the nerve cell bodies surrounded by calbindin baskets belong to motor neurons and ascending interneurons. Unique functional classes of myenteric neurons were identified based on morphology, neuronal markers and polarity of projection. We provide evidence for three groups of ascending motor neurons based on immunoreactivity and association with calbindin baskets, a finding that may have significant functional implications.

Inducible nitric oxide synthase inhibitor 1400W increases Na+ ,K+ -ATPase levels and activity and ameliorates mitochondrial dysfunction in Ctns null kidney proximal tubular epithelial cells

Nitric oxide (NO) has been shown to play an important role in renal physiology and pathophysiology partly through its influence on various transport systems in the kidney proximal tubule. The role of NO in kidney dysfunction associated with lysosomal storage disorder, cystinosis, is largely unknown. In the present study, the effects of inducible nitric oxide synthase (iNOS)-specific inhibitor, 1400W, on Na⁺ ,K⁺ -ATPase activity and expression, mitochondrial integrity and function, nutrient metabolism, and apoptosis were investigated in Ctns null proximal tubular epithelial cells (PTECs). Ctns null PTECs exhibited an increase in iNOS expression, augmented NO and nitrite/nitrate production, and reduced Na⁺ ,K⁺ -ATPase expression and activity. In addition, these cells displayed depolarized mitochondria, reduced adenosine triphosphate content, altered nutrient metabolism, and elevated apoptosis. Treatment of Ctns null PTECs with 1400W abolished these effects which culminated in the mitigation of apoptosis in these cells. These findings indicate that uncontrolled NO production may constitute the upstream event that leads to the molecular and biochemical alterations observed in Ctns null PTECs and may explain, at least in part, the generalized proximal tubular dysfunction associated with cystinosis. Further studies are needed to realize the potential benefits of anti-nitrosative therapies in improving renal function and/or attenuating renal injury in cystinosis.

The Influence of Na(+), K(+)-ATPase on Glutamate Signaling in Neurodegenerative Diseases and Senescence

Decreased Na(+), K(+)-ATPase (NKA) activity causes energy deficiency, which is commonly observed in neurodegenerative diseases. The NKA is constituted of three subunits: α, β, and γ, with four distinct isoforms of the catalytic α subunit (α1-4). Genetic mutations in the ATP1A2 gene and ATP1A3 gene, encoding the α2 and α3 subunit isoforms, respectively can cause distinct neurological disorders, concurrent to impaired NKA activity. Within the central nervous system (CNS), the α2 isoform is expressed mostly in glial cells and the α3 isoform is neuron-specific. Mutations in ATP1A2 gene can result in familial hemiplegic migraine (FHM2), while mutations in the ATP1A3 gene can cause Rapid-onset dystonia-Parkinsonism (RDP) and alternating hemiplegia of childhood (AHC), as well as the cerebellar ataxia, areflexia, pescavus, optic atrophy and sensorineural hearing loss (CAPOS) syndrome. Data indicates that the central glutamatergic system is affected by mutations in the α2 isoform, however further investigations are required to establish a connection to mutations in the α3 isoform, especially given the diagnostic confusion and overlap with glutamate transporter disease. The age-related decline in brain α2∕3 activity may arise from changes in the cyclic guanosine monophosphate (cGMP) and cGMP-dependent protein kinase (PKG) pathway. Glutamate, through nitric oxide synthase (NOS), cGMP and PKG, stimulates brain α2∕3 activity, with the glutamatergic N-methyl-D-aspartate (NMDA) receptor cascade able to drive an adaptive, neuroprotective response to inflammatory and challenging stimuli, including amyloid-β. Here we review the NKA, both as an ion pump as well as a receptor that interacts with NMDA, including the role of NKA subunits mutations. Failure of the NKA-associated adaptive response mechanisms may render neurons more susceptible to degeneration over the course of aging.

Chitosan treatment abrogates hypercholesterolemia-induced erythrocyte's arginase activation

This study aimed to evaluate the protective effect of chitosan (CS) against hypercholesterolemia (HC) induced arginase activation and disruption of nitric oxide (NO) biosynthesis using erythrocytes as cellular model. Human erythrocytes were isolated and classified into eight groups. Next, cells were treated with l-arginine (l-ARG), N^ω-nitro-l-arginine methyl ester (l-NAME), CS or CS + l-ARG in the presence of normal plasma or cholesterol enriches plasma. Then, erythrocytes were incubated at 37 °C for 24 h. The present results revealed that, HC induced significant increase of cholesterol inclusion into erythrocytes membrane compared to control. Moreover, HC caused significant decrease in nitric oxide synthase (NOS) activity similar to l-NAME; however, arginase activity and arginase/NOS ratio significantly increased compared to control. On contrast, treatment of HC with, l-arginine, CS or CS plus l-arginine prevents HC induced cholesterol loading into erythrocytes membrane, NOS inhibition and arginase activation. This study suggested that CS could be protective agent against HC induced disruption of erythrocyte’s oxidative status and arginase activation.

Hyponatremia in patients with systemic lupus erythematosus

The aim of this study was to determine whether decreased serum sodium concentration could be associated with the disease activity in SLE. We retrospectively analyzed the data of the two independent cohorts of children and adults with SLE in two centers. Hyponatremia was associated with serum chloride (p = 0.004), albumin (p = 0.002) and SLE disease activity index (SLEDAI) (p = 0.026) in children with SLE. Serum sodium levels were correlated negatively with ESR (p =0.001) and positively with serum albumin levels (p < 0.0001) and C3 (p = 0.008) in children with SLE and those levels were correlated negatively with serum interleukin-6 levels (p = 0.003) in adults with SLE. Independent risk factors for the development of hyponatremia were the decreased serum C3 levels (OR 1.069, p = 0.031), the decreased serum chloride levels (OR 2.054, p = 0.006) and increased erythrocyte sedimentation rate (ESR) (OR 1.066, p = 0.03) in children with SLE and increased C-reactive protein (CRP) (OR 1.480, p = 0.023) in combined cohorts with SLE by multiple logistic regression analyses. Our study firstly showed that hyponatremia could reflect a disease activity and severe inflammation of SLE.

Nitric Oxide Increases the Expression of Aquaporin-4 Protein in Rat Optic Nerve Astrocytes through the Cyclic Guanosine Monophosphate/Protein Kinase G Pathway

AIMS

Nitric oxide (NO) is associated with neuroinflammation in the central nervous system. We determined whether NO increases the expression of aquaporin-4 (AQP4) in optic nerve astrocytes of rats.

METHODS

Isolated astrocytes were incubated under normoxic or hypoxic conditions with or without glucose (5.5 mM). The astrocytes were also exposed to different concentrations of S-nitroso-N-acetyl-DL-penicillamine (SNAP, 1.0-100 μM), an NO donor. The expression of AQP4 was determined by Western blot analyses, and NO formation was measured by the Griess reaction. The changes in astrocytic cellular volumes were determined by flow cytometry.

RESULTS

Hypoxia and glucose deprivation increased AQP4 expression and NO formation. Inhibition of NO synthetase (NOS) significantly suppressed these changes. SNAP caused a significant increase in AQP4 expression, and the increase was significantly suppressed by carboxy-PTIO, a scavenger of NO. Incubation with 8-Br-cyclic guanosine monophosphate (cGMP) mimicked the effects of SNAP, while the addition of either 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ; inhibitor of soluble guanylate cyclase) or KT5823 (protein kinase G inhibitor) suppressed the SNAP-induced increase in AQP4 significantly. SNAP also caused a significant increase in astrocytic cellular volume through the AQP4 channels.

CONCLUSIONS

NO increased the AQP4 expression of optic nerve astrocytes through the cGMP/protein kinase G pathway and enlarged their volume.

Altered KLOTHO and NF-κB-TNF-α Signaling Are Correlated with Nephrectomy-Induced Cognitive Impairment in Rats

Renal insufficiency can have a negative impact on cognitive function. Neuroinflammation and changes in klotho levels associate with chronic kidney disease (CKD) and may play a role in the development of cognitive impairment (CI). The present study evaluates the correlation of cognitive deficits with neuroinflammation and soluble KLOTHO in the cerebral spinal fluid (CSF) and brain tissue of nephrectomized rats (Nx), with 5/6 renal mass ablation. Nx and sham Munich Wistar rats were tested over 4 months for locomotor activity, as well as inhibitory avoidance or novel object recognition, which started 30 days after the surgery. EMSA for Nuclear factor-κB and MILLIPLEXMAP or ELISA kit were used to evaluate cytokines, glucocorticoid and KLOTHO levels. Nx animals that showed a loss in aversive-related memory and attention were included in the CI group (Nx-CI) (n=14) and compared to animals with intact learning (Nx-M n=12 and Sham n=20 groups). CSF and tissue samples were collected 24 hours after the last behavioral test. The results show that the Nx-groups have increased NF-κB binding activity and tumor necrosis factor-alpha (TNF-α) levels in the hippocampus and frontal cortex, with these changes more pronounced in the Nx-CI group frontal cortex. In addition, the Nx-CI group showed significantly increased CSF glucocorticoid levels and TNF-α /IL-10 ratio compared to the Sham group. Klotho levels were decreased in Nx-CI frontal cortex but not in hippocampus, when compared to Nx-M and Sham groups. Overall, these results suggest that neuroinflammation mediated by frontal cortex NF-κB, TNF-α and KLOTHO signaling may contribute to Nx-induced CI in rats.

Effects of intermittent fasting on age-related changes on Na,K-ATPase activity and oxidative status induced by lipopolysaccharide in rat hippocampus

Chronic neuroinflammation is a common characteristic of neurodegenerative diseases, and lipopolysaccharide (LPS) signaling is linked to glutamate-nitric oxide-Na,K-ATPase isoforms pathway in central nervous system (CNS) and also causes neuroinflammation. Intermittent fasting (IF) induces adaptive responses in the brain that can suppress inflammation, but the age-related effect of IF on LPS modulatory influence on nitric oxide-Na,K-ATPase isoforms is unknown. This work compared the effects of LPS on the activity of α1,α2,3 Na,K-ATPase, nitric oxide synthase gene expression and/or activity, cyclic guanosine monophosphate, 3-nitrotyrosine-containing proteins, and levels of thiobarbituric acid-reactive substances in CNS of young and older rats submitted to the IF protocol for 30 days. LPS induced an age-related effect in neuronal nitric oxide synthase activity, cyclic guanosine monophosphate, and levels of thiobarbituric acid-reactive substances in rat hippocampus that was linked to changes in α2,3-Na,K-ATPase activity, 3-nitrotyrosine proteins, and inducible nitric oxide synthase gene expression. IF induced adaptative cellular stress-response signaling pathways reverting LPS effects in rat hippocampus of young and older rats. The results suggest that IF in both ages would reduce the risk for deficits on brain function and neurodegenerative disorders linked to inflammatory response in the CNS.

Hydrogen sulfide targets EGFR Cys797/Cys798 residues to induce Na(+)/K(+)-ATPase endocytosis and inhibition in renal tubular epithelial cells and increase sodium excretion in chronic salt-loaded rats

AIMS

The role of hydrogen sulfide (H2S) in renal sodium and water homeostasis is unknown. We investigated whether H2S promoted Na(+)/K(+)-ATPase endocytosis via the H2S/EGFR/gab1/PI3K/Akt pathway in renal tubular epithelial cells.

RESULTS

H2S decreased Na(+)/K(+)-ATPase activity and induced its endocytosis in renal tubular epithelial cells, which was abrogated by small interfering RNA (siRNA) knockdown of epidermal growth factor receptor (EGFR) and gab1, a dominant-negative mutant of Akt and PI3K inhibitors. H2S increased EGFR, gab1, PI3K, and Akt phosphorylation in both renal tubular epithelial cells and kidneys of chronic salt-loaded rats. These increases were abrogated by siRNA knockdown of EGFR, but not of c-Src. Radiolabeled H2S exhibited transient, direct binding to EGFR and directly activated EGFR. Some disulfide bonds in EGFR intracellular kinase domain were susceptible to H2S-induced cleavage. Mutations of EGFR Cys797 (human) or Cys798 (rat) residues increased EGFR activity and prevented H2S-induced Na(+)/K(+)-ATPase endocytosis. H2S also inhibited sodium hydrogen exchanger-3 (NHE3) activity in renal tubular epithelial cells. H2S treatment increased sodium excretion in chronic and acute salt-loaded rats and decreased blood pressure in chronic salt-loaded rats.

INNOVATION AND CONCLUSION

H2S directly targets some disulfide bonds in EGFR, which activates the EGFR/gab1/PI3K/Akt pathway and subsequent Na(+)/K(+)-ATPase endocytosis and inhibition in renal tubular epithelial cells. EGFR Cys797/Cys798 residues are essential for an intrinsic inhibitory mechanism and for H2S actions in renal tubular epithelial cells. Other pathways, including NHE3, may be involved in mediating the renal effects of H2S. Our results reveal a new renal sodium homeostasis mechanism, which may provide for novel treatment approaches for diseases related to renal sodium homeostasis dysfunction.

Erythrocyte nitric oxide synthase as a surrogate marker for mercury-induced vascular damage: the modulatory effects of naringin

In this study, endothelial nitric oxide synthase activity and nitric oxide (NO) production by human erythrocytes in the presence and absence of mercuric chloride (HgCl2 ), L-arginine (L-ARG), N ω- nitro-L-arginine methyl ester (L-NAME), and naringin (NAR) were investigated. In addition, the levels of reduced glutathione (GSH) and related enzymes were estimated in erythrocytes hemolysate. The protein carbonyl content (PCC) and thiobarbituric acid-reactive substances (TBARS) levels were also determined. The results of this study revealed that the treatment of erythrocytes with either HgCl2 or L-NAME induced a significant decrease in NOS activity and nitrite levels compared with control cells. Furthermore, mercury exposure significantly increased the levels of PCC and TBARS but reduced the GSH level. The activities of glucose-6-phosphate dehydrogenase, glutathione reductase, glutathione peroxidase, and glutathione-S-transferase (GST) were inhibited. The exposure of erythrocytes to HgCl2 in combination with L-ARG, NAR, or both ameliorated the investigated parameters compared with erythrocytes incubated with HgCl2 alone. These results indicate that mercury exposure decreased both erythrocyte NOS activity and nitrite production, and that these parameters might be indicative of mercury exposure. The data also suggest that concomitant treatment with NAR can restore NO bioavailability through either its metal-chelating properties or its antioxidant activity.

C-peptide replacement therapy as an emerging strategy for preventing diabetic vasculopathy

Lack of C-peptide, along with insulin, is the main feature of Type 1 diabetes mellitus (DM) and is also observed in progressive β-cell loss in later stage of Type 2 DM. Therapeutic approaches to hyperglycaemic control have been ineffective in preventing diabetic vasculopathy, and alternative therapeutic strategies are necessary to target both hyperglycaemia and diabetic complications. End-stage organ failure in DM seems to develop primarily due to vascular dysfunction and damage, leading to two types of organ-specific diseases, such as micro- and macrovascular complications. Numerous studies in diabetic patients and animals demonstrate that C-peptide treatment alone or in combination with insulin has physiological functions and might be beneficial in preventing diabetic complications. Current evidence suggests that C-peptide replacement therapy might prevent and ameliorate diabetic vasculopathy and organ-specific complications through conservation of vascular function, as well as prevention of endothelial cell death, microvascular permeability, vascular inflammation, and neointima formation. In this review, we describe recent advances on the beneficial role of C-peptide replacement therapy for preventing diabetic complications, such as retinopathy, nephropathy, neuropathy, impaired wound healing, and inflammation, and further discuss potential beneficial effects of combined C-peptide and insulin supplement therapy to control hyperglycaemia and to prevent organ-specific complications.

Mitigation of lead-induced neurotoxicity by the naringin: erythrocytes as neurons substitute markers

This study aimed to investigate the effect of lead (Pb) on neuronal nitric oxide synthase (nNOS) activity using erythrocytes as neurons surrogate markers. Moreover, the protective effect of naringin (NAR) against lead acetate (PbAc)-induced neurotoxicity was investigated. Human erythrocytes were incubated with L-arginine (L-Arg), Nω-nitro-L-Arginine methyl ester ( L-NAME), NAR, PbAc, PbAc+L-Arg, PbAc+NAR, or PbAc+L-Arg+NAR. The present results revealed that incubation of erythrocytes with PbAc inhibited NOS activity and decreased nitrite levels as an index for nitric oxide (NO) production to values similar that of L-NAME as known NOS inhibitor. Likewise, PbAc induced a significant decrease in activities of ATPases and acetylcholinesterase compared to control cells. Furthermore, PbAc exposure significantly increased protein carbonyl content (PCC) and malondialdehyde (MDA) levels while significantly decrease the levels of reduced glutathione (GSH). On the contrary, incubation of erythrocytes with PbAc in the presence of L-Arg+NAR synergistically ameliorated the investigated parameters compared to erythrocytes incubated with PbAc alone. These data suggest that NAR can restore NO bioavailability in a situation of Pb-induced cellular damage. This attributed to antioxidant activity and restoration NOS activity.

Novel regulation of cardiac Na pump via phospholemman

As the only quantitatively significant Na efflux pathway from cardiac cells, the Na/K ATPase (Na pump) is the primary regulator of intracellular Na. The transmembrane Na gradient it establishes is essential for normal electrical excitability, numerous coupled-transport processes and, as the driving force for Na/Ca exchange, thus setting cardiac Ca load and contractility. As Na influx varies with electrical excitation, heart rate and pathology, the dynamic regulation of Na efflux is essential. It is now widely recognized that phospholemman, a 72 amino acid accessory protein which forms part of the Na pump complex, is the key nexus linking cellular signaling to pump regulation. Phospholemman is the target of a variety of post-translational modifications (including phosphorylation, palmitoylation and glutathionation) and these can dynamically alter the activity of the Na pump. This review summarizes our current understanding of the multiple regulatory mechanisms that converge on phospholemman and govern NA pump activity in the heart. The corrected Fig. 4 is reproduced below. The publisher would like to apologize for any inconvenience caused. [corrected].

Nitric oxide regulates cardiac intracellular Na⁺ and Ca²⁺ by modulating Na/K ATPase via PKCε and phospholemman-dependent mechanism

In the heart, Na/K-ATPase regulates intracellular Na⁺ and Ca^2 + (via NCX), thereby preventing Na⁺ and Ca^2 + overload and arrhythmias. Here, we test the hypothesis that nitric oxide (NO) regulates cardiac intracellular Na⁺ and Ca^2 + and investigate mechanisms and physiological consequences involved. Effects of both exogenous NO (via NO-donors) and endogenously synthesized NO (via field-stimulation of ventricular myocytes) were assessed in this study. Field stimulation of rat ventricular myocytes significantly increased endogenous NO (18 ± 2 μM), PKCε activation (82 ± 12%), phospholemman phosphorylation (at Ser-63 and Ser-68) and Na/K-ATPase activity (measured by DAF-FM dye, western-blotting and biochemical assay, respectively; p < 0.05, n = 6) and all were abolished by Ca^2 +-chelation (EGTA 10 mM) or NOS inhibition l-NAME (1 mM). Exogenously added NO (spermine-NONO-ate) stimulated Na/K-ATPase (EC50 = 3.8 μM; n = 6/grp), via decrease in K_m, in PLM^WT but not PLM^KO or PLM^3SA myocytes (where phospholemman cannot be phosphorylated) as measured by whole-cell perforated-patch clamp. Field-stimulation with l-NAME or PKC-inhibitor (2 μM Bis) resulted in elevated intracellular Na⁺ (22 ± 1.5 and 24 ± 2 respectively, vs. 14 ± 0.6 mM in controls) in SBFI-AM-loaded rat myocytes. Arrhythmia incidence was significantly increased in rat hearts paced in the presence of l-NAME (and this was reversed by l-arginine), as well as in PLM^3SA mouse hearts but not PLM^WT and PLM^KO. We provide physiological and biochemical evidence for a novel regulatory pathway whereby NO activates Na/K-ATPase via phospholemman phosphorylation and thereby limits Na⁺ and Ca^2 + overload and arrhythmias. This article is part of a Special Issue entitled “Na⁺ Regulation in Cardiac Myocytes”.

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We tested whether nitric oxide regulates intracellular Na⁺ and Ca^2 + in the heart.

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Nitric oxide increased Na/K ATPase activity via PKCε-induced phospholemman phosphorylation.

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Inhibiting nitric oxide pathway resulted in Na⁺ and Ca^2 + overload and contributed to arrhythmia development in the heart.

Age-related changes in nitric oxide activity, cyclic GMP, and TBARS levels in platelets and erythrocytes reflect the oxidative status in central nervous system

Aging is associated with an increased susceptibility to neurodegenerative disorders which has been linked to chronic inflammation. This process generates oxygen-reactive species, ultimately responsible for a process known as oxidative stress, leading to changes in nitric oxide (NO), and cyclic guanosine monophosphate (cyclic GMP) signaling pathway. In previous studies, we showed that human aging was associated with an increase in NO Synthase (NOS) activity, a decrease in basal cyclic GMP levels in human platelets, and an increase in thiobarbituric acid-reactant substances (TBARS) in erythrocytes. The aim of the present work was to evaluate NOS activity, TBARS and cyclic GMP levels in hippocampus and frontal cortex and its correlation to platelets and erythrocytes of 4-, 12-, and 24-month-old rats. The result showed an age-related decrease in cyclic GMP levels which was linked to an increase in NOS activity and TBARS in both central areas as well as in platelets and erythrocytes of rats. The present data confirmed our previous studies performed in human platelets and erythrocytes and validate NOS activity and cyclic GMP in human platelet as well as TBARS in erythrocytes as biomarkers to study age-related disorders and new anti-aging therapies.

In vivo regulation of renal expression of (pro)renin receptor by a low-sodium diet

Effects of low salt (LS) on (pro)renin receptor (PRR) expression are not well established. We hypothesized that LS enhances renal PRR expression via the cGMP-protein kinase G (PKG) signaling pathway. Sprague-Dawley rats were fed a normal-salt (NS) or LS diet associated with intrarenal cortical administration of vehicle (V), the nitric oxide (NO) synthase inhibitor nitro-l-arginine methyl ester (l-NAME), the NO donor S-nitroso-N-acetyl-dl-penicillamine (SNAP), the cGMP analog 8-bromoguanosine (8-Br)-cGMP, the guanylyl cyclase inhibitor 1H-[1, 2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), or a PKG inhibitor (PKGi) for 6 days via osmotic minipump. We evaluated the effects of each treatment on renal interstitial fluid (RIF) levels of nitrate/nitrite and cGMP and renal PRR expression. There were no significant changes in blood pressure with any of the treatments. Urinary sodium excretion was significantly lower in rats given a LS diet. Compared with NS + V, RIF nitrate/nitrite and cGMP levels increased in LS + V rats. In NS groups, RIF nitrate/nitrite and cGMP levels did not change with l-NAME, ODQ, or PKGi and increased in response to SNAP. 8-Br-cGMP increased RIF cGMP but not RIF nitrate/nitrite. In LS groups, RIF nitrate/nitrite decreased with l-NAME and did not change with ODQ or PKGi whereas RIF cGMP decreased with l-NAME, ODQ, and PKGi. PRR mRNA and protein increased in LS + V. In NS rats, PRR mRNA and protein increased in response to 8-Br-GMP and were not affected by any of other treatments. In LS rats, PRR mRNA and protein decreased significantly in response to l-NAME, ODQ, and PKGi. We conclude that LS intake enhances renal expression of PRR via cGMP-PKG signaling pathway.

Mechanisms underlying the inhibitory effects of uroguanylin on NHE3 transport activity in renal proximal tubule

We previously demonstrated that uroguanylin (UGN) significantly inhibits Na(+)/H(+) exchanger (NHE)3-mediated bicarbonate reabsorption. In the present study, we aimed to elucidate the molecular mechanisms underlying the action of UGN on NHE3 in rat renal proximal tubules and in a proximal tubule cell line (LLC-PK(1)). The in vivo studies were performed by the stationary microperfusion technique, in which we measured H(+) secretion in rat renal proximal segments, through a H(+)-sensitive microelectrode. UGN (1 μM) significantly inhibited the net of proximal bicarbonate reabsorption. The inhibitory effect of UGN was completely abolished by either the protein kinase G (PKG) inhibitor KT5823 or by the protein kinase A (PKA) inhibitor H-89. The effects of UGN in vitro were found to be similar to those obtained by microperfusion. Indeed, we observed that incubation of LLC-PK(1) cells with UGN induced an increase in the intracellular levels of cAMP and cGMP, as well as activation of both PKA and PKG. Furthermore, we found that UGN can increase the levels of NHE3 phosphorylation at the PKA consensus sites 552 and 605 in LLC-PK(1) cells. Finally, treatment of LLC-PK(1) cells with UGN reduced the amount of NHE3 at the cell surface. Overall, our data suggest that the inhibitory effect of UGN on NHE3 transport activity in proximal tubule is mediated by activation of both cGMP/PKG and cAMP/PKA signaling pathways which in turn leads to NHE3 phosphorylation and reduced NHE3 surface expression. Moreover, this study sheds light on mechanisms by which guanylin peptides are intricately involved in the maintenance of salt and water homeostasis.

Influence of N-methyl-D-aspartate receptors on ouabain activation of nuclear factor-κB in the rat hippocampus

It has been shown that ouabain (OUA) can activate the Na,K-ATPase complex and mediate intracellular signaling in the central nervous system (CNS). Inflammatory stimulus increases glutamatergic transmission, especially at N-methyl-D-aspartate (NMDA) receptors, which are usually coupled to the activation of nitric oxide synthase (NOS). Nuclear factor-κB (NF-κB) activation modulates the expression of genes involved in development, plasticity, and inflammation. The present work investigated the effects of OUA on NF-κB binding activity in rat hippocampus and the influence of this OUA-Na,K-ATPase signaling cascade in NMDA-mediated NF-κB activation. The findings presented here are the first report indicating that intrahippocampal administration of OUA, in a concentration that did not alter Na,K-ATPase or NOS activity, induced an activation of NF-κB, leading to increases in brain-derived neurotrophic factor (Bdnf), inducible NOS (iNos), tumor necrosis factor-α (Tnf-α), and B-cell leukemia/lymphoma 2 (Bcl2) mRNA levels. This response was not linked to any significant signs of neurodegeneration as showed via Fluoro-Jade B and Nissl stain. Intrahippocampal administration of NMDA induced NF-κB activation and increased NOS and α(2/3) -Na,K-ATPase activities. NMDA treatment further increased OUA-induced NF-κB activation, which was partially blocked by MK-801, an antagonist of NMDA receptor. These results suggest that OUA-induced NF-κB activation is at least in part dependent on Na,K-ATPase modulatory action of NMDA receptor in hippocampus. The interaction of these signaling pathways could be associated with biological mechanisms that may underlie the basal homeostatic state linked to the inflammatory signaling cascade in the brain.

Defective nitric oxide production impairs angiotensin II-induced Na-K-ATPase regulation in spontaneously hypertensive rats

Angiotensin (ANG) II via ANG II type 1 receptors (AT1R) activates renal sodium transporters including Na-K-ATPase and regulates sodium homeostasis and blood pressure. It is reported that at a high concentration, ANG II either inhibits or fails to stimulate Na-K-ATPase. However, the mechanisms for these phenomena are not clear. Here, we identified the signaling molecules involved in regulation of renal proximal tubular Na-K-ATPase at high ANG II concentrations. Proximal tubules from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were incubated with low concentrations of ANG II (pM), which activated Na-K-ATPase in both the groups; however, the stimulation was more robust in SHR. A high concentration of ANG II (μM) failed to stimulate Na-K-ATPase in WKY rats. However, in SHR ANG II (μM) continued to stimulate Na-K-ATPase, which was sensitive to the AT1R antagonist candesartan. In the presence of N(G)-nitro-l-arginine methyl ester (l-NAME), a nitric oxide (NO) synthase (NOS) inhibitor, ANG II (μM) caused stimulation of Na-K-ATPase in proximal tubules of WKY rats while having no further stimulatory effect in SHR. ANG II (μM), via AT1R, increased proximal tubular NO levels in WKY rats but not in SHR. In SHR, NOS was uncoupled as incubation of proximal tubules with ANG II and l-arginine, a NOS substrate, caused superoxide generation only in SHR and not in WKY rats. The superoxide production in SHR was sensitive to l-NAME. There was exaggerated proximal tubular AT1R-G protein coupling and NAD(P)H oxidase activation in response to ANG II (μM) in proximal tubules of SHR compared with WKY rats. In SHR, inhibition of NADPH oxidase restored NOS coupling and ANG II-induced NO accumulation. In conclusion, at a high concentration ANG II (μM) activates renal NO signaling, which prevents stimulation of Na-K-ATPase in WKY rats. However, in SHR ANG II (μM) overstimulates NADPH oxidase, which impairs the NO system and leads to continued Na-K-ATPase activation.

Sodium MRI in a rat migraine model and a NEURON simulation study support a role for sodium in migraine

INTRODUCTION

Increased lumbar cerebrospinal fluid (CSF) sodium has been reported during migraine. We used ultra-high field MRI to investigate cranial sodium in a rat migraine model, and simulated the effects of extracellular sodium on neuronal excitability.

METHODS

Behavioral changes in the nitroglycerin (NTG) rat migraine model were determined from von Frey hair withdrawal response and photography. Central sensitization was measured by counting cFos-immunoreactive cells in the trigeminal nucleus caudalis (TNC). Sodium was quantified in vivo by ultra-high field sodium MRI at 21 Tesla. Effects of extracellular sodium on neuronal excitability were modeled using NEURON software.

RESULTS

NTG decreased von Frey withdrawal threshold (p=0.0003), decreased eyelid vertical height:width ratio (p<0.0001), increased TNC cFos stain (p<0.0001), and increased sodium between 7.5 and 17% in brain, intracranial CSF, and vitreous humor (p<0.05). Simulated neurons exposed to higher sodium have more frequent and earlier spontaneous action potentials, and corresponding earlier sodium and potassium currents.

CONCLUSIONS

In the rat migraine model, sodium rises to levels that increase neuronal excitability. We propose that rising sodium in CSF surrounding trigeminal nociceptors increases their excitability and causes pain and that rising sodium in vitreous humor increases retinal neuronal excitability and causes photosensitivity.

L-NAME treatment enhances exercise-induced content of myocardial heat shock protein 72 (Hsp72) in rats

BACKGROUND/AIM

Nitric oxide (NO) modulates the expression of the chaperone Hsp72 in the heart, and exercise stimulates both NO production and myocardial Hsp72 expression. The main purpose of the study was to investigate whether NO interferes with an exercise-induced myocardial Hsp72 expression.

METHODS

Male Wistar rats (70-100 days) were divided into control (C, n=12), L-NAME-treated (L, n=12), exercise (E, n=13) and exercise plus L-NAME-treated (EL, n=20) groups. L-NAME was given in drinking water (700 mg·L(-1)) and the exercise was performed on a treadmill (15-25 m·min(-1), 40-60 min.day(-1)) for seven days. Left ventricle (LV) protein Hsp content, NOS and phosphorylated-NOS (p-NOS) isoforms were measured using Western blotting. The activity of NOS was assayed in LV homogenates by the conversion of [(3)H]L-arginine to [(3)H]L-citrulline.

RESULTS

Hsp72 content was increased significantly (223%; p < 0.05) in the E group compared to the C group, but exercise alone did not alter the NOS content, p-NOS isoforms or NOS activity. Contrary to our expectation, L-NAME enhanced (p < 0.05) the exercise-induced Hsp72 content (EL vs. C, L and E groups = 1019%, 548% and 457%, respectively). Although the EL group had increased stimulatory p-eNOS(Ser1177) (over 200%) and decreased inhibitory p-nNOS(Ser852) (ñ50%) compared to both the E and L groups (p < 0.05), NOS activity was similar in all groups.

CONCLUSIONS

Our results suggest that exercise-induced cardiac Hsp72 expression does not depend on NO. Conversely, the in vivo L-NAME treatment enhances exercise-induced Hsp72 production. This effect may be due to an increase in cardiac stress.

Altered reactivity of gastric fundus smooth muscle in the mouse with targeted disruption of the kinin B1 receptor gene

Relaxing action of sodium nitroprusside (SNP) was significantly reduced in the stomach fundus of mice lacking the kinin B(1) receptor (B(1)(-/-)). Increased basal cGMP accumulation was correlated with attenuated SNP induced dose-dependent relaxation in B(1)(-/-) when compared with wild type (WT) control mice. These responses to SNP were completely blocked by the guanylate cyclase inhibitor ODQ (10 microM). It was also found that Ca(2+)-dependent, constitutive nitric oxide synthase (cNOS) activity was unchanged but the Ca(2+)-independent inducible NOS (iNOS) activity was greater in B(1)(-/-) mice than in WT animals. Zaprinast (100 microM), a specific phosphodiesterase inhibitor, increased the nitrergic relaxations and the accumulation of the basal as well as the SNP-stimulated cGMP in WT but not in B(1)(-/-) stomach fundus. From these findings it is concluded that the inhibited phosphodiesterase activity and high level of cGMP reduced the resting muscle tone, impairing the relaxant responses of the stomach in B(1)(-/-) mice. In addition, it can be suggested that functional B(2) receptor might be involved in the NO compensatory mechanism associated with the deficiency of kinin B(1) receptor in the gastric tissue of the transgenic mice.

Na+/K+-ATPase-mediated signal transduction and Na+/K+-ATPase regulation

A number of studies suggest that Na(+)/K(+)-ATPase in caveolae interacts with neighboring membrane proteins and organizes cytosolic cascades of signaling proteins to send messages to intracellular organelles in different tissues, mostly in cardiac myocytes. Low concentration of ouabain binding to Na(+)/K(+)-ATPase activates Src/epidermal growth factor receptor complex to initiate multiple signal pathways, which include PLC/IP3/CICR, PI3K, reactive oxygen species (ROS), PLC/DG/PKC/Raf/MEK/ERK1/2, and Ras/Raf/MEK/ERK1/2 pathways. In cardiac myocytes, the resulting downstream events include the induction of some early response proto-oncogenes, activation of transcription factors, activator protein-1, and nuclear factor-kappaB, the regulation of a number of cardiac growth-related genes, and the stimulation of protein synthesis and myocyte hypertrophy and apoptosis. Conversely, several factors acting through signal pathways, such as protein kinases, Ca(2+), ROS, etc., can modulate the activity of the Na(+)/K(+)-ATPase.

Apolipoprotein E genotype is related to nitric oxide production in platelets

The presence of the epsilon4 allele of apolipoprotein E (APOE) is considered a risk factor for sporadic Alzheimer's disease (AD). Our recent data demonstrated that the systemic modulation of oxidative stress in platelets and erythrocytes is disrupted in aging and AD. In this study, the relationship between APOE genotype and oxidative stress markers, both in AD patients and controls, was evaluated. The AD group showed an increase in the content of thiobarbituric acid-reactive substances (TBARS) and in the activities of nitric oxide synthase (NOS) and Na, K-ATPase, when compared to controls. Both groups had a similar cGMP content and superoxide dismutase activity. APOE epsilon4 allele carriers showed higher NOS activity than non-carriers. These results suggest a possible influence of APOE genotype on nitric oxide (NO) production that might enhance the effects of age-related specific factor(s) associated with neurodegenerative disorders.

Age-related changes in cerebellar phosphatase-1 reduce Na,K-ATPase activity

We evaluated whether changes in protein content and activity of PP-1 and PP-2A were the mechanism underneath the basal age-related reduction in alpha(2/3)-Na,K-ATPase activity in rats cerebella and whether this occurred through the cyclic GMP-PKG pathway. PP1 activity, but not its expression, increased with age, whereas PP-2 was not changed. The activity of alpha(2/3)-Na,K-ATPase varied with age, and there was a negative association between the PP-1 and alpha(2/3)-Na,K-ATPase activities. In young rats, the inhibition of PP-1 and PP-2A by okadaic acid (OA) increased in a dose-dependent manner alpha(1)- and alpha(2/3)-Na,K-ATPase, but had no effect on Mg-ATPase activity. A direct stimulation of PKG with 8-Br-cyclic GMP did not surmount the effect of OA. This analogue of cyclic GMP inhibited PP-1 activity only, indicating that at least part of the increase in alpha(1)- and alpha(2/3)-Na,K-ATPase activity induced by OA was mediated by the cyclic GMP-PKG-PP-1 cascade. Taking into account that PP1 inhibition increased alpha(2/3)-Na,K-ATPase activity, we propose that an age-related increase in PP-1 activity due to a decrease in cyclic GMP-PKG modulation plays a role for the age-related reduction of alpha(2/3)-Na,K-ATPase activity in rat cerebellum.

Endothelin-1 impairs alveolar epithelial function via endothelial ETB receptor

RATIONALE

Endothelin-1 (ET-1) is increased in patients with high-altitude pulmonary edema and acute respiratory distress syndrome, and these patients have decreased alveolar fluid reabsorption (AFR).

OBJECTIVES

To determine whether ET-1 impairs AFR via activation of endothelial cells and nitric oxide (NO) generation.

METHODS

Isolated perfused rat lung, transgenic rats deficient in ETB receptors, coincubation of lung human microvascular endothelial cells (HMVEC-L) with rat alveolar epithelial type II cells or A549 cells, ouabain-sensitive 86Rb+ uptake.

MEASUREMENTS AND MAIN RESULTS

The ET-1-induced decrease in AFR was prevented by blocking the endothelin receptor ETB, but not ETA. Endothelial-epithelial cell interaction is required, as direct exposure of alveolar epithelial cells (AECs) to ET-1 did not affect Na,K-ATPase function or protein abundance at the plasma membrane, whereas coincubation of HMVEC-L and AECs with ET-1 decreased Na,K-ATPase activity and protein abundance at the plasma membrane. Exposing transgenic rats deficient in ETB receptors in the pulmonary vasculature (ET-B(-/-)) to ET-1 did not decrease AFR or Na,K-ATPase protein abundance at the plasma membrane of AECs. Exposing HMVEC-L to ET-1 led to increased NO, and the ET-1-induced down-regulation of Na,K-ATPase was prevented by the NO synthase inhibitor l-NAME, but not by a guanylate cyclase inhibitor.

CONCLUSIONS

We provide the first evidence that ET-1, via an endothelial-epithelial interaction, leads to decreased AFR by a mechanism involving activation of endothelial ETB receptors and NO generation leading to alveolar epithelial Na,K-ATPase down-regulation in a cGMP-independent manner.

Hemodynamic parameters during normal and hypertensive pregnancy in rats: evaluation of renal salt and water transporters

OBJECTIVE

To determine whether alterations in extracellular volume expansion observed during normal and hypertensive pregnancy run in parallel to changes in the mRNA expression of renal transporters.

METHODS

Wistar rats were divided into four groups: control (C, n = 5); pregnancy (P, n = 5); N(omega)-nitro-l-arginine methyl ester (L-NAME; 50 mg/kg/d)-treated control (H, n = 6); and pregnant rats (HP, n = 6). Hemodynamic studies were performed on day 14 of pregnancy, at which time we also analyzed of the sodium transporters (NHE3, Na/K/2Cl and Na/Cl), potassium channel (ROMK2) and water channel (AQP2).

RESULTS

As expected, P rats presented high cardiac output (CO) and normal blood pressure (BP), whereas H rats presented lower CO and elevated BP. A significant (threefold) increase in total vascular resistance and a decrease in stroke volume were observed in the HP group. Hypertension resulting from nitric oxide (NO) synthesis inhibition blunted systemic hemodynamic adaptations during pregnancy. Compared with C rats, mRNA expression of ROMK2 in P rats was lower, whereas that of AQP2 was higher. Expression of AQP2 was significantly higher in H than in C or HP groups. Expression of BSC and NHE3 was lower in the HP than in the P group. The NO inhibition also provoked renal transporter alterations in HP.

CONCLUSIONS

Our results suggest that tubule transporter variants may mediate the hemodynamic adaptations seen during pregnancy, although we cannot rule out the hypothesis that other factors are also mediating hemodynamic changes.

Natriuretic peptides stimulate the cardiac sodium pump via NPR-C-coupled NOS activation

Natriuretic peptides (NPs) and their receptors (NPRs) are expressed in the heart, but their effects on myocyte function are poorly understood. Because NPRs are coupled to synthesis of cGMP, an activator of the sarcolemmal Na+-K+ pump, we examined whether atrial natriuretic peptide (ANP) regulates the pump. We voltage clamped rabbit ventricular myocytes and identified electrogenic Na+-K+ pump current (arising from the 3:2 Na+:K+ exchange and normalized for membrane capacitance) as the shift in membrane current induced by 100 μmol/l ouabain. Ten nanomoles per liter ANP stimulated the Na+-K+ pump when the intracellular compartment was perfused with pipette solutions containing 10 mmol/l Na+ but had no effect when the pump was at near maximal activation with 80 mmol/l Na+ in the pipette solution. Stimulation was abolished by inhibition of cGMP-activated protein kinase with KT-5823, nitric oxide (NO)-activated guanylyl cyclase with 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ), or NO synthase with NG-nitro-l-arginine methyl ester (l-NAME). Since synthesis of cGMP by NPR-A and NPR-B is not NO dependent or ODQ sensitive, we exposed myocytes to AP-811, a highly selective ligand for the NPR-C “clearance” receptor. It abolished ANP-induced pump stimulation. Conversely, the selective NPR-C agonist ANP(4-23) reproduced stimulation. The stimulation was blocked by l-NAME. To examine NO production in response to ANP(4-23), we loaded myocytes with the NO-sensitive fluorescent dye diacetylated diaminofluorescein-2 and examined them by confocal microscopy. ANP(4-23) induced a significant increase in fluorescence, which was abolished by l-NAME. We conclude that NPs stimulate the Na+-K+ pump via an NPR-C and NO-dependent pathway.

Oxygen-induced Regulation of Na/K ATPase in cerebellar granule cells

Adjustment of the Na/K ATPase activity to changes in oxygen availability is a matter of survival for neuronal cells. We have used freshly isolated rat cerebellar granule cells to study oxygen sensitivity of the Na/K ATPase function. Along with transport and hydrolytic activity of the enzyme we have monitored alterations in free radical production, cellular reduced glutathione, and ATP levels. Both active K(+) influx and ouabain-sensitive inorganic phosphate production were maximal within the physiological pO(2) range of 3-5 kPa. Transport and hydrolytic activity of the Na/K ATPase was equally suppressed under hypoxic and hyperoxic conditions. The ATPase response to changes in oxygenation was isoform specific and limited to the alpha1-containing isozyme whereas alpha2/3-containing isozymes were oxygen insensitive. Rapid activation of the enzyme within a narrow window of oxygen concentrations did not correlate with alterations in the cellular ATP content or substantial shifts in redox potential but was completely abolished when NO production by the cells was blocked by l-NAME. Taken together our observations suggest that NO and its derivatives are involved in maintenance of high Na/K ATPase activity under physiological conditions.

Extracellular renal guanosine cyclic 3'5'-monophosphate modulates nitric oxide and pressure-induced natriuresis

This study addresses the hypothesis that NO- and pressure-induced natriuresis are inhibited when guanosine cyclic 3',5'-monophosphate (cGMP) is prevented from being transported outside its renal synthesizing cells in vivo. Rats received a renal interstitial (RI) infusion of NO donor S-nitroso-N-acetylpenicillamine (SNAP) or SNAP+organic anion transporter inhibitor probenecid (PB) or SNAP+PB+cGMP. SNAP alone increased U(Na)V (P<0.05 at 1 hour and P<0.005 at 2 hours). In contrast, SNAP failed to increase U(Na)V when coinfused with PB, but cGMP coinfused with SNAP+probenecid restored the natriuretic response. SNAP alone increased RI cGMP (P<0.05) during the second experimental period. PB abolished the increase in RI cGMP in response to SNAP (P<0.01), but cGMP levels were restored by coinfusion with cGMP. PB also abolished SNAP-induced increases in fractional excretion of Na(+) (FE(Na)) and lithium (FE(Li)) (both P<0.01). PB also abolished the rise in RI cGMP and natriuresis induced by raising renal perfusion pressure (RPP) from 100 to 160 mm Hg in rats subjected to a standard pressure-natriuresis protocol and the natriuretic response was rescued by coinfusion with cGMP. RI administration of phosphodiesterase type V (PDE V) reduced both RIcGMP and U(Na)V in parallel (both P<0.01) without altering RIcAMP. The data demonstrate that export of cGMP from its renal synthesizing cells into the extracellular RI compartment is critical for the natriuretic action of NO donor SNAP or increased RPP and that RI cGMP controls basal Na(+) excretion. Extracellular cGMP modulates NO- and pressure-induced natriuresis.

Beta3-adrenoceptor agonist stimulation of the Na+, K+ -pump in rat skeletal muscle is mediated by beta2- rather than beta3-adrenoceptors

BACKGROUND AND PURPOSE

In cardiac muscle, BRL 37344, a selective beta3-adrenoceptor agonist, activates the Na+, K+ -pump via NO signalling. This study investigated whether BRL 37344 also activates the Na+, K+ -pump via beta3-adrenoceptors in skeletal muscle.

EXPERIMENTAL APPROACH

Isolated rat soleus muscles were incubated between 1 and 60 min in buffer. Intracellular Na+, K+ content and Na+, K+ -pump activity were measured using flame photometry and ouabain-suppressible 86Rb+ uptake, respectively. Additional muscles were mounted on force transducers and stimulated (60 Hz for 2 s) every 10 min.

KEY RESULTS

BRL 37344 (10(-8) -10(-5) M) induced a concentration- and time-dependent reduction in intracellular Na+, and increased ouabain-suppressible 86Rb+ uptake by up to 112%. BRL 37344-induced reductions in intracellular Na+ were blocked by the beta1/beta2-adrenoceptor antagonist, nadolol (10(-7) M), and the beta2-adrenoceptor antagonist, ICI 118,551 (10(-7) -10(-5) M), but not by beta3- or beta1-adrenoceptor antagonists, SR 59230A (10(-7) M) and CGP 20712A (10(-7) -10(-5) M), respectively. Another beta3-adrenoceptor agonist, CL 316,243, did not alter intracellular Na+. BRL 37344-induced reductions in intracellular Na+ were not blocked by L-NAME, an NOS inhibitor, or ODQ, a guanylyl cyclase inhibitor. The NO donors, SNP and SNAP, did not alter intracellular Na+. BRL 37344 rapidly recovered force in muscles depressed by high [K+]o, an effect that was blocked by nadolol, but not L-NAME.

CONCLUSIONS AND IMPLICATIONS

In rat soleus muscle, the beta3-adrenoceptor agonist BRL 37344 stimulated the Na+, K+ -pump via beta2-adrenoceptors. A more selective beta3-adrenoceptor agonist did not affect Na+, K+ homeostasis in skeletal muscle. NO did not seem to mediate Na+, K+ -pump stimulation in skeletal muscle.

Changes in vascular reactivity following administration of isoproterenol for 1 week: a role for endothelial modulation

1. The aim of this study was to assess the effects of treatment with isoproterenol (ISO, 0.3 mg kg-1 day-1, s.c.) for 7 days on the vascular reactivity of rat-isolated aortic rings. Additionally, potential mechanisms underlying the changes that involved the endothelial modulation of contractility were investigated. 2. Treatment with ISO induced cardiac hypertrophy without changes in haemodynamic parameters. Aortic rings from ISO-treated rats showed an increase in the contraction response to phenylephrine (PHE) and serotonin, but did not change relaxations produced by acetylcholine or isoproterenol. Removal of the endothelium increased the responses to PHE in both groups. However, this procedure was less effective in ISO-treated as compared with control rats. Endothelial cell removal abolished the increase in the response to PHE in ISO-treated rats. The presence of Nomega-nitro-L-arginine methyl ester shifted the concentration-response curve to PHE to the left in both groups of rats. However, this effect was more pronounced in the ISO group. In addition, aminoguanidine (50 microM) potentiated the actions of PHE only in the ISO group. ISO treatment increased nitric oxide synthase (NOS) activity and neuronal NOS and endothelial NOS protein expression in the aorta. 3. Neither losartan (10 microM) nor indomethacin (10 microM) abolished the effects of ISO on the actions of PHE. Superoxide dismutase (SOD, 150 U ml-1) and L-arginine (5 mM), but neither catalase (300 U ml-1) nor apocynin (100 microM), blocked the effect of ISO treatment. In addition, we observed an increase in superoxide anion levels as measured by ethidium bromide fluorescence and of copper and zinc superoxide dismutase protein expression in ISO-treated rats. 4. In conclusion, our data suggest that ISO treatment alters the endothelial cell-mediated modulation of the contraction to PHE in rat aorta. The increased maximal response of PHE seems to be due to an increase in superoxide anion generation, which inactivates some of the basal NO produced and counteracts NO-mediated negative modulation even in the presence of high NO production and antioxidant defence.

Phosphodiesterase 5 inhibitor ameliorates renal resistance to atrial natriuretic peptide associated with obesity and hyperleptinemia

BACKGROUND

Abnormal neurohormonal regulation of renal sodium handling plays an important role in obesity-associated hypertension. We investigated the effect of experimental obesity on renal response to atrial natriuretic peptide (ANP).

METHODS

The effect of ANP was studied in three groups of rats: (1) lean controls, (2) animals made obese by a highly palatable diet, (3) rats treated with adipose tissue hormone, leptin, for 7 days to reproduce hyperleptinemia observed in obesity.

RESULTS

ANP administered at a dose of 50 pmol/kg min(-1) induced about a 3-fold lower increase in Na+ and cGMP excretion in obese and leptin-treated rats than in the control group. ANP decreased Na+,K+-ATPase activity in the renal medulla only in the control group. Natriuretic effect of exogenous cGMP was also impaired in obese and leptin-treated rats. In contrast, hydrolysis-resistant cGMP derivative, 8-bromo-cGMP exerted comparable natriuretic effects in all groups. Neutral endopeptidase inhibitor, phosphoramidon, and ANP clearance receptor antagonist, C-ANP, increased urinary ANP excretion in all groups to a similar level, but their natriuretic effect was impaired in obese and leptin-treated groups. A specific inhibitor of cGMP-degrading phosphodiesterase, zaprinast, had comparable natriuretic and Na+,K+-ATPase-lowering effects in all groups and restored normal sensitivity to ANP.

CONCLUSIONS

(1) Dietary-induced obesity is accompanied by impaired natriuretic effect of ANP, (2) ANP resistance in obesity may be accounted for by increased leptin level, (3) accelerated degradation of cGMP may contribute to ANP resistance associated with obesity and hyperleptinemia, suggesting that inhibiting cGMP-specific phosphodiesterases may be useful in the treatment of obesity-associated hypertension.

Angiotensin II type 2 receptor agonist directly inhibits proximal tubule sodium pump activity in obese but not in lean Zucker rats

We have reported recently that the renal angiotensin II type 2 (AT2) receptors are upregulated and involved in promoting natriuresis/diuresis in obese but not in lean Zucker rats. In the present study, we tested the hypothesis that there is an enhanced AT2 receptor signaling via NO/cGMP pathway leading to greater inhibition of the Na(+), K(+)-ATPase (NKA) activity in the proximal tubules (PT) of obese rather than lean Zucker rats. The AT2 agonist CGP42112 (0.1 to 100 nmol/L) inhibited (33% at 100 nmol/L) the NKA activity in the PTs of obese but not in lean Zucker rats. The AT2 antagonist PD123319 (1 micromol/L), not the angiotensin II type 1 antagonist losartan (1 micromol/L), significantly diminished the CGP42112-induced inhibition of the NKA activity in obese rats. The AT2 agonist (10 nmol/L)-induced NKA inhibition was abolished by the soluble guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo-[4,3-a] quinoxalin-1-one (10 micromol/L), the NO synthase inhibitor NG-nitro-L-arginine methyl ester (100 micromol/L), and the protein kinase G inhibitor K1388 (2 micromole/L). CGP42112 (10 nmol/L) caused an increase in serine phosphorylation of NKA alpha1-subunit in PT of obese rats. Measurement of cGMP and NO revealed that CGP42112 (0.1 to 100 nmol/L) increased cGMP and NO accumulation in the PTs of obese but not lean rats. The CGP42112-induced stimulation of NO and cGMP was blocked by PD123319 (1 micromol/L), NG-nitro-L-arginine methyl ester (100 micromol/L), and 1H-[1,2,4] oxadiazolo-[4,3-a] quinoxalin-1-one (10 micromol/L) but not by losartan (1 micromol/L). The data suggest that the AT2 receptor activation via stimulation of the NO/cGMP/protein kinase G pathway directly inhibits the tubular NKA activity that provides as a mechanism responsible for the AT2 receptor-mediated natriuresis in obese but not in lean Zucker rats.

Changes in ion transport in inflammatory disease

Ion transport is essential for maintenance of transmembranous and transcellular electric potential, fluid transport and cellular volume. Disturbance of ion transport has been associated with cellular dysfunction, intra and extracellular edema and abnormalities of epithelial surface liquid volume. There is increasing evidence that conditions characterized by an intense local or systemic inflammatory response are associated with abnormal ion transport. This abnormal ion transport has been involved in the pathogenesis of conditions like hypovolemia due to fluid losses, hyponatremia and hypokalemia in diarrhoeal diseases, electrolyte abnormalities in pyelonephritis of early infancy, septicemia induced pulmonary edema, and in hypersecretion and edema induced by inflammatory reactions of the mucosa of the upper respiratory tract. Components of membranous ion transport systems, which have been shown to undergo a change in function during an inflammatory response include the sodium potassium ATPase, the epithelial sodium channel, the Cystic Fibrosis Transmembrane Conductance Regulator and calcium activated chloride channels and the sodium potassium chloride co-transporter. Inflammatory mediators, which influence ion transport are tumor necrosis factor, gamma interferon, interleukins, transforming growth factor, leukotrienes and bradykinin. They trigger the release of specific messengers like prostaglandins, nitric oxide and histamine which alter ion transport system function through specific receptors, intracellular second messengers and protein kinases. This review summarizes data on in vivo measurements of changes in ion transport in acute inflammatory conditions and in vitro studies, which have explored the underlying mechanisms. Potential interventions directed at a correction of the observed abnormalities are discussed.