Overproduction of nitric oxide intensifies brain infarction and cerebrovascular damage through reduction of claudin-5 and ZO-1 expression in striatum of ischemic brain

In Mild and Moderate Acute Ischemic Stroke, Increased Lipid Peroxidation and Lowered Antioxidant Defenses Are Strongly Associated with Disabilities and Final Stroke Core Volume

In acute ischemic stroke (AIS), there are no data on whether oxidative stress biomarkers have effects above and beyond known risk factors and measurements of stroke volume. This study was conducted in 122 mild-moderate AIS patients and 40 controls and assessed the modified ranking scale (mRS) at baseline, and 3 and 6 months later. We measured lipid hydroperoxides (LOOH), malondialdehyde (MDA), advanced oxidation protein products, paraoxonase 1 (PON1) activities and PON1 Q192R genotypes, high density lipoprotein cholesterol (HDL), sulfhydryl (-SH) groups), and diffusion-weighted imaging (DWI) stroke volume and fluid-attenuated inversion recovery (FLAIR) signal intensity. We found that (a) AIS is characterized by lower chloromethyl acetate CMPAase PON1 activity, HDL and -SH groups and increased LOOH and neurotoxicity (a composite of LOOH, inflammatory markers and glycated hemoglobin); (b) oxidative and antioxidant biomarkers strongly and independently predict mRS scores 3 and 6 months later, DWI stroke volume and FLAIR signal intensity; and (c) the PON1 Q192R variant has multiple effects on stroke outcomes that are mediated by its effects on antioxidant defenses and lipid peroxidation. Lipid peroxidation and lowered -SH and PON1-HDL activity are drug targets to prevent AIS and consequent neurodegenerative processes and increased oxidative reperfusion mediators due to ischemia-reperfusion injury.

Cardiac-specific overexpression of Claudin-5 exerts protection against myocardial ischemia and reperfusion injury

Claudin-5 has recently attracted increasing attention by its potential as a novel treatment target in the early stage of heart failure. However, whether Claudin-5 produces beneficial effects on myocardial ischemia and reperfusion (IR) injury has not been elucidated yet. In this study, we identified reduced levels of Claudin-5 in the hearts of mice subjected to acute myocardial IR injury and murine HL-1 cardiomyocytes subjected to hypoxia and reoxygenation (HR). We then constructed cardiac-specific Cldn5-overexpressing mice using an adeno-associated virus (AAV9) vector and demonstrated that Cldn5 overexpression ameliorated cardiac dysfunction and myocardial damage in mice subjected to myocardial IR injury. Moreover, Cldn5 overexpression attenuated myocardial oxidative stress (DHE and protein levels of Nrf2, HO-1, and NQO1), inflammatory response (levels of MPO, F4/80, Ly6C, and circulating inflammatory cells), mitochondrial dysfunction (protein levels of PGC-1α, NRF1, and TFAM), endoplasmic reticulum stress (protein levels of GRP78, ATF6, and CHOP and p-PERK), energy metabolism disorder (p-AMPK and ACC), and apoptosis (TUNEL assay and protein levels of Bax and Bcl2) in mice subjected to myocardial IR. Next, we generated Cldn5 knockdown cells by lentiviral shRNA and observed that Cldn5 knockdown inhibited cell viability and affected the expression or activation of these IR-related signalings in HL-1 cardiomyocytes subjected to HR. Mechanistically, SIRT1 was proved to be involved in regulating the expression of Claudin-5 by co-immunoprecipitation analysis and Sirt1 knockdown experiments. Our data demonstrated that targeting Claudin-5 may represent a promising approach for preventing and treating acute myocardial IR injury.

Tumor Necrosis Factor-stimulated Gene-6 (TSG-6) Secreted by BMSCs Regulates Activated Astrocytes by Inhibiting NF-κB Signaling Pathway to Ameliorate Blood Brain Barrier Damage After Intracerebral Hemorrhage

To investigate the influence of tumor necrosis factor-stimulated gene-6 (TSG-6) secreted by bone mesenchymal stem cells (BMSCs) on blood brain barrier (BBB) after intracerebral hemorrhage (ICH) and its related mechanisms. BMSCs and astrocytes were isolated and induced by TNF-α and LPS respectively. The effect of TSG-6 secreted by BMSCs on the proliferation and apoptosis of astrocytes and inflammatory response were assessed by CCK8, flow cytometry, and ELISA respectively. Then we studied the effects of TSG-6 secreted by BMSCs through the paracrine mechanism on the integrity of BBB after ICH via NF-κB signaling pathway in vitro and in vivo. We successfully isolated BMSCs and astrocytes. After LPS treatment of astrocytes, IL-1β, IL-6, and TNF-α showed an upward trend. TSG-6 secreted by TNF-α-activated BMSCs could antagonize the inflammatory response in activated astrocytes. Through the co-culture of astrocytes and BMSCs and the ICH animal model, we found that TSG-6 regulates activated astrocytes by inhibiting the NF-κB signaling pathway and ameliorates BBB damage. Furthermore, we found that TNF-α-activated BMSCs secreted exosomes containing TSG-6 and played an anti-inflammatory effect. TSG-6 secreted by BMSCs regulates activated astrocytes by inhibiting the NF-κB signaling pathway, thereby ameliorating BBB damage.

Microglia as the Critical Regulators of Neuroprotection and Functional Recovery in Cerebral Ischemia

Microglial activation is considered as the critical pathogenic event in diverse central nervous system disorders including cerebral ischemia. Proinflammatory responses of activated microglia have been well reported in the ischemic brain and neuroinflammatory responses of activated microglia have been believed to be the potential therapeutic strategy. However, despite having proinflammatory roles, microglia can have significant anti-inflammatory roles and they are associated with the production of growth factors which are responsible for neuroprotection and recovery after ischemic injury. Microglia can directly promote neuroprotection by preventing ischemic infarct expansion and promoting functional outcomes. Indirectly, microglia are involved in promoting anti-inflammatory responses, neurogenesis, and angiogenesis in the ischemic brain which are crucial pathophysiological events for ischemic recovery. In fact, anti-inflammatory cytokines and growth factors produced by microglia can promote neuroprotection and attenuate neurobehavioral deficits. In addition, microglia regulate phagocytosis, axonal regeneration, blood-brain barrier protection, white matter integrity, and synaptic remodeling, which are essential for ischemic recovery. Microglia can also regulate crosstalk with neurons and other cell types to promote neuroprotection and ischemic recovery. This review mainly focuses on the roles of microglia in neuroprotection and recovery following ischemic injury. Furthermore, this review also sheds the light on the therapeutic potential of microglia in stroke patients.

Human placenta mesenchymal stem cell protection in ischemic stroke is angiotensin converting enzyme-2 and masR receptor-dependent

Thromboembolic stroke remains a major cause of neurological disability and death. Current stroke treatments (aspirin, tissue plasminogen activator) are significantly limited by timing and risks for hemorrhage which have driven researchers to explore other approaches. Stem cell‐based therapy appears to be an effective option for ischemic stroke. Besides trans‐differentiation into neural cells, stem cells also provide acute protection via paracrine signaling pathways through which releasing neuroprotective factors. We previously reported that intraperitoneal administration of human placenta mesenchymal stem cell (hPMSC) therapy upon reperfusion significantly protected the brain against middle cerebral artery occlusion (MCAO)‐induced injury. In the present study, we specifically investigated the role of hPMSC‐derived angiotensin converting enzyme‐2 (ACE‐2) in protection of MCAO‐induced brain injury by measurement of brain tissue viability, cerebral blood flow, and neurological score. Here, we report for the first time that hPMSC expressing substantial amount of ACE‐2, which mediates hPMSC protection in the MCAO model. Strikingly, we found that the protective effects of hPMSC in MCAO‐induced brain injury could be attenuated by pretreatment of hPMSCs with MLN‐4760, a specific inhibitor of ACE‐2 activity, or by transfection of hPMSCs with ACE‐2‐shRNA‐lentivirus. The hPMSC‐derived ACE‐2 specific protective mechanism was further demonstrated by administration of PD123319, an Angiotensin type‐2 receptor antagonist, or A779, a MasR antagonist. Importantly, our study demonstrated that the protective effects of hPMSC in experimental stroke are ACE‐2/MasR dependent and this signaling pathway represents an innovative and highly promising approach for targeted stroke therapy.

Role of the Platelets and Nitric Oxide Biotransformation in Ischemic Stroke: A Translative Review from Bench to Bedside

Ischemic stroke remains the fifth cause of death, as reported worldwide annually. Endothelial dysfunction (ED) manifesting with lower nitric oxide (NO) bioavailability leads to increased vascular tone, inflammation, and platelet activation and remains among the major contributors to cardiovascular diseases (CVD). Moreover, temporal fluctuations in the NO bioavailability during ischemic stroke point to its key role in the cerebral blood flow (CBF) regulation, and some data suggest that they may be responsible for the maintenance of CBF within the ischemic penumbra in order to reduce infarct size. Several years ago, the inhibitory role of the platelet NO production on a thrombus formation has been discovered, which initiated the era of extensive studies on the platelet-derived nitric oxide (PDNO) as a platelet negative feedback regulator. Very recently, Radziwon-Balicka et al. discovered two subpopulations of human platelets, based on the expression of the endothelial nitric oxide synthase (eNOS-positive or eNOS-negative platelets, respectively). The e-NOS-negative ones fail to produce NO, which attenuates their cyclic guanosine monophosphate (cGMP) signaling pathway and-as result-promotes adhesion and aggregation while the e-NOS-positive ones limit thrombus formation. Asymmetric dimethylarginine (ADMA), a competitive NOS inhibitor, is an independent cardiovascular risk factor, and its expression alongside with the enzymes responsible for its synthesis and degradation was recently shown also in platelets. Overproduction of ADMA in this compartment may increase platelet activation and cause endothelial damage, additionally to that induced by its plasma pool. All the recent discoveries of diverse eNOS expression in platelets and its role in regulation of thrombus formation together with studies on the NOS inhibitors have opened a new chapter in translational medicine investigating the onset of acute cardiovascular events of ischemic origin. This translative review briefly summarizes the role of platelets and NO biotransformation in the pathogenesis and clinical course of ischemic stroke.

Association Between Ghrelin Gene Polymorphism and Cerebral Infarction

Background

The aim of this study was to explore the associations of ghrelin gene polymorphisms at rs26312, rs26802 and rs27647 with cerebral infarction.

Material/Methods

A total of 200 cerebral infarction patients in our hospital were enrolled as the disease group, while 200 healthy people were enrolled as the control group. Peripheral venous blood was collected from both groups, and the ghrelin gene polymorphisms at rs26312, rs26802, and rs27647 in nucleated cells were detected through sequencing.

Results

The genotype distribution at ghrelin gene loci rs26802 and rs27647 in the disease group was significantly different from that in the control group. The distribution of recessive model at ghrelin gene locus rs26802 in the disease group was different from that in the control group, in which the TG+GG frequency was evidently higher in the disease group. The AA genotype at ghrelin gene locus rs26312 was remarkably associated with the ghrelin gene expression level, and the expression level of ghrelin gene in the disease group was remarkably lower than that in the control group. The genotype at ghrelin gene locus rs26312 was associated with activated partial thromboplastin time (APTT), and APTT was significantly shorter in patients with GG genotype. The genotype at ghrelin gene locus rs26802 was associated with D-dimer, and the D-dimer level was significantly lower in patients with TG genotype. The genotype at ghrelin gene locus rs27647 was associated with prothrombin time (PT), and PT was obviously shorter in patients with TT genotype.

Conclusions

The ghrelin gene polymorphisms are remarkably associated with the occurrence of cerebral infarction.

Immune-Inflammatory, Metabolic, Oxidative, and Nitrosative Stress Biomarkers Predict Acute Ischemic Stroke and Short-Term Outcome

Immune-inflammatory, metabolic, oxidative, and nitrosative stress (IMO&NS) pathways and, consequently, neurotoxicity are involved in acute ischemic stroke (IS). The simultaneous assessment of multiple IMO&NS biomarkers may be useful to predict IS and its prognosis. The aim of this study was to identify the IMO&NS biomarkers, which predict short-term IS outcome. The study included 176 IS patients and 176 healthy controls. Modified Rankin scale (mRS) was applied within 8 h after IS (baseline) and 3 months later (endpoint). Blood samples were obtained within 24 h after hospital admission. IS was associated with increased white blood cell (WBC) counts, high sensitivity C-reactive protein (hsCRP), interleukin (IL-6), lipid hydroperoxides (LOOHs), nitric oxide metabolites (NOx), homocysteine, ferritin, erythrocyte sedimentation rate (ESR), glucose, insulin, and lowered iron, 25-hydroxyvitamin D [25(OH)D], total cholesterol, and high-density lipoprotein (HDL) cholesterol. We found that 89.4% of the IS patients may be correctly classified using the cumulative effects of male sex, systolic blood pressure (SBP), glucose, NOx, LOOH, 25(OH)D, IL-6, and WBC with sensitivity of 86.2% and specificity of 93.0%. Moreover, increased baseline disability (mRS ≥ 3) was associated with increased ferritin, IL-6, hsCRP, WBC, ESR, and glucose. We found that 25.0% of the variance in the 3-month endpoint (mRS) was explained by the regression on glucose, ESR, age (all positively), and HDL-cholesterol, and 25(OH)D (both negatively). These results show that the cumulative effects of IMO&NS biomarkers are associated with IS and predict a poor outcome at 3-month follow-up.

The impact of single and combined PPAR-α and PPAR-γ activation on the neurological outcomes following cerebral ischemia reperfusion

AIM

The neuronal damage and accompanied functional deficits induced by cerebral ischemia are among the most common causes of disabilities in adults. Activation of subtypes of peroxisome proliferator-activated receptors (PPARs); PPAR-α and PPAR-γ have shown neuroprotective effects in different neurodegenerative diseases including stroke. Thus, this study aimed to compare the effects of two different agonists: PPAR-α (fenofibrate) and PPAR-γ (pioglitazone) as well as the effect of their combination in ameliorating post-ischemia behavioral deficits.

METHODS

Male Wistar rats were either pretreated with vehicle, fenofibrate (100 mg/kg/day p.o), pioglitazone (10 mg/kg/day p.o) or their combination for 14 days prior to bilateral common carotid artery occlusion followed by reperfusion for 24 hoursh. The sensory motor functions of rats were assessed, then rats were sacrificed to determine infarct volume and histopathological changes as well as oxidative stress, inflammatory and apoptotic markers in the brain tissue.

KEY FINDINGS

Pre-treatment with fenofibrate and pioglitazone in addition to their combination improved neurobehavioral dysfunction, reduced cerebral infarct volume, attenuated inflammatory and apoptotic markers and ameliorated histopathological changes in I/R injured rats. The effect of pioglitazone in cerebral cortex was higher than its corresponding effect in fenofibrate while the combined administration of both drugs had additive neuroprotective effect and normalized inflammatory and apoptotic mediators in ischemic rats.

SIGNIFICANCE

The study compared the neuroprotective effects of PPAR-α and PPAR-γ agonists, and tested the impact of their combination. We concluded that no additional benefits on the functional outcomes might be gained upon their combination.

Effect of MicroRNA-126a-3p on Bone Marrow Mesenchymal Stem Cells Repairing Blood-brain Barrier and Nerve Injury after Intracerebral Hemorrhage

OBJECTIVE

Intracerebral hemorrhage (ICH) is a disease that threatens human health due to its high morbidity and mortality. On behalf of finding the better methods in the treatment of ICH, researchers pay more attention to a new technology which is finding effective genes to modify stem cells.

METHODS

In this study, we isolated, cultured and identified bone marrow mesenchymal stem cells (MSCs) in vitro. Further, the MSCs (transfected with lentivirus expressing microRNA-126a-3p (miR-126)) were injected into the type Ⅶ collagenase-induced ICH rats to investigate the recovery effects of blood-brain barrier (BBB) and nerve damage in vivo.

RESULTS

The MSCs surface marker molecules (CD29: 98.5%; CD90: 96.5%) were highly expressed, and the blood cell surface molecule was negatively expressed (CD45: 2%). Meanwhile, it was verified that miR-126 facilitated the differentiation of MSCs into vascular endothelial cells, owing to the rise of markers (CD31 and VE-cadherin). The modified neurological severity score, modified limb placing test score, brain water content and evans blue content were reduced after transplanted miR-126-modified MSCs. It was found that miR-126 accelerated the differentiation of MSCs into vascular endothelial cells via immunohistochemical staining in vivo. HE staining indicated the area of edema was obviously decreased compared with that in ICH + vector-MSCs group. MiR-126-modified MSCs alleviated the cell apoptosis in brain tissues by TUNEL assay. In addition, the mRNA and protein expression of protease activated receptor-1 and matrix metalloproteinase-9 were diminished, whilst the expression of zonula occludens-1 (ZO-1) and claudin-5 were enhanced in ICH+miR-126-MSCs group. Immunofluorescence assay revealed that miR-126-modified MSCs decreased the disruption of tight junction (ZO-1 and claudin-5).

CONCLUSIONS

All data illustrate that miR-126-modified MSCs repair BBB and nerve injury after ICH.

Evaluation of the neuroprotective and antioxidant effects of Dorema aucheri extract on cerebral ischaemia-reperfusion injury in rats

CONTEXT

The hydroalcoholic extract of Dorema aucheri Bilhar (Umbelliferae) (DA) leaves, a medicinal plant, has powerful antioxidant properties.

OBJECTIVE

This study evaluates the neuroprotective effects of pre-treatment with DA leaves extract against cerebral ischaemia-induced brain injury through alteration of the antioxidant capacity.

MATERIALS AND METHODS

The study was conducted in three groups of Wistar rats (N = 47) as follows; sham, control ischaemic and pre-treated ischaemic groups. Rats were administered a fresh hydroalcoholic extract of DA leaves at a dosage of 200 mg/kg/day for 14 days. Then, the middle cerebral artery (MCA) of the right hemisphere was occluded for 90 min to achieve cerebral ischaemia. After 24 h reperfusion, cerebral infarction and superoxide dismutase (SOD) and catalase activities, as well as malondialdehyde (MDA), glutathione, and NOx contents were determined in the right hemispheres.

RESULTS

Occlusion of the right MCA caused noticeable cerebral infarction (298 ± 21 mm³) in control ischaemic group, but pre-treatment with DA extract considerably attenuated it (92 ± 14 mm³) in the pre-treated ischaemic group. DA extract significantly decreased the levels of MDA by 28% and NOx by 11% in pre-treated ischaemic group compared to the control ischaemic group. DA extract also enhanced glutathione content by 7%, SOD activity by 16% and catalase activity by 46% in pre-treated ischaemic rats compared to control ischaemic rats.

DISCUSSION AND CONCLUSIONS

DA is able to improve the antioxidant capacity and injuries of ischaemic brain. It is proposed as a neuroprotectant following cerebral ischaemia to decrease the injuries of ischaemic stroke.

Targeting phosphodiesterase 4 as a potential therapeutic strategy for enhancing neuroplasticity following ischemic stroke

Sensorimotor recovery following ischemic stroke is highly related with structural modification and functional reorganization of residual brain tissues. Manipulations, such as treatment with small molecules, have been shown to enhance the synaptic plasticity and contribute to the recovery. Activation of the cAMP/CREB pathway is one of the pivotal approaches stimulating neuroplasticity. Phosphodiesterase 4 (PDE4) is a major enzyme controlling the hydrolysis of cAMP in the brain. Accumulating evidences have shown that inhibition of PDE4 is beneficial for the functional recovery after cerebral ischemia; i. subtype D of PDE4 (PDE4D) is viewed as a risk factor for ischemic stroke; ii. inhibition of PDE4 enhances neurological behaviors, such as learning and memory, after stroke in rodents; iii.PDE4 inhibition increases dendritic density, synaptic plasticity and neurogenesis; iv. activation of cAMP/CREB signaling by PDE4 inhibition causes an endogenous increase of BDNF, which is a potent modulator of neuroplasticity; v. PDE4 inhibition is believed to restrict neuroinflammation during ischemic stroke. Cumulatively, these findings provide a link between PDE4 inhibition and neuroplasticity after cerebral ischemia. Here, we summarized the possible roles of PDE4 inhibition in the recovery of cerebral stroke with an emphasis on neuroplasticity. We also made some recommendations for future research.

Fullerenol Nanoparticles Decrease Blood-Brain Barrier Interruption and Brain Edema during Cerebral Ischemia-Reperfusion Injury Probably by Reduction of Interleukin-6 and Matrix Metalloproteinase-9 Transcription

BACKGROUND

The present study aimed to examine the protective role of fullerenol nanoparticles against blood-brain barrier (BBB) interruption and brain edema during cerebral ischemia-reperfusion injury probably by reduction of interleukin-6 (IL-6) and matrix metalloproteinase-9 (MMP-9) transcription.

METHODS

The male Wistar rats (weighting 280-320 g) were randomly assigned into four groups as follows: sham, control ischemic, pretreated ischemic, and posttreated ischemic groups. Cerebral ischemia-reperfusion (IR) injury was performed by occlusion of middle cerebral artery (MCA) for 90 minutes followed by twenty-four hours reperfusion. Rats were administered fullerenol 5mg/kg, intraperitoneally, 30 minutes before induction of IR in pretreated ischemic group and immediately after termination of MCA occlusion in posttreated ischemic group. After twenty-four hours reperfusion, the method of Evans blue dye extravasation (EBE) and RT-PCR were used for determination of BBB permeability and mRNA expression levels of MMP-9 and IL-6, respectively. Neuronal deficit score (NDS) and edema of the ischemic hemispheres were also evaluated.

RESULTS

MCA occlusion increased NDS in control ischemic rats (3.16 ± 0.16) with concomitant increase in EBE (15.30 ± 3.98µg/g) and edema (3.53 ± 0.50%). Fullerenol in both pretreated and posttreated ischemic groups reduced NDS (36% and 68%, respectively), EBE (89% and 91%, respectively) and edema (53% and 81%, respectively). Although MCA occlusion increased the mRNA expression levels of MMP-9 and IL-6 in ischemic hemispheres, fullerenol in both treatment groups noticeably decreased the mRNA expression levels of these genes.

CONCLUSION

In conclusion, fullerenol nanoparticles can protect BBB integrity and attenuate brain edema after cerebral ischemia-reperfusion injury possibly by reduction of IL-6 and MMP-9 transcription.

The role of nitric oxide in stroke

Stroke is considered to be an acute cerebrovascular disease, including ischemic stroke and hemorrhagic stroke. The high incidence and poor prognosis of stroke suggest that it is a highly disabling and highly lethal disease which can pose a serious threat to human health. Nitric oxide (NO), a common gas in nature, which is often thought as a toxic gas, because of its intimate relationship with the pathological processes of many diseases, especially in the regulation of blood flow and cell inflammation. However, recent years have witnessed an increased interest that NO plays a significant and positive role in stroke as an essential gas signal molecule. In view of the fact that the neuroprotective effect of NO is closely related to its concentration, cell type and time, only in the appropriate circumstances can NO play a protective effect. The purpose of this review is to summarize the roles of NO in ischemic stroke and hemorrhagic stroke.

Kaempherol and Luteolin Decrease Claudin-2 Expression Mediated by Inhibition of STAT3 in Lung Adenocarcinoma A549 Cells

Claudin-2 is highly expressed in human lung adenocarcinoma tissues and may be a novel target for cancer chemotherapy because knockdown of claudin-2 decreases cell proliferation. We found that flavonoids including kaempferol, chrysin, and luteolin concentration-dependently decrease claudin-2 expression in lung adenocarcinoma A549 cells. Claudin-2 expression is up-regulated by mitogen-activated protein kinase kinase (MEK)/ extracellular signal-regulated kinase (ERK)/c-Fos and phosphoinositide 3-kinase (PI3K)/Akt/nuclear factor-κB (NF-κB) pathways, but these activities were not inhibited by kaempferol, chrysin, and luteolin. Promoter deletion assay using luciferase reporter vector showed that kaempferol and luteolin inhibit the function of transcriptional factor that binds to the region between −395 and −144 of claudin-2 promoter. The decrease in promoter activity was suppressed by mutation in signal transducers and activators of transcription (STAT)-binding site, which is located between −395 and −144. The phosphorylation level of STAT3 was not decreased, but the binding of STAT3 on the promoter region is suppressed by kaempferol and luteolin in chromatin immunoprecipitation assay. The inhibition of cell proliferation caused by kaempferol and luteolin was partially recovered by ectopic claudin-2 expression. Taken together, kaempferol and luteolin decreased claudin-2 expression and proliferation in A549 cells mediated by the inhibition of binding of STAT3 on the promoter region of claudin-2. The intake of foods and nutrients rich in these flavonoids may prevent lung adenocarcinoma development.

Nitric oxide reduces paracellular resistance in rat thick ascending limbs by increasing Na+ and Cl- permeabilities

About 50% of the Na⁺ reabsorbed in thick ascending limbs traverses the paracellular pathway. Nitric oxide (NO) reduces the permselectivity of this pathway via cGMP, but its effects on absolute Na⁺ ([Formula: see text]) and Cl^- ([Formula: see text]) permeabilities are unknown. To address this, we measured the effect of l-arginine (0.5 mmol/l; NO synthase substrate) and cGMP (0.5 mmol/l) on [Formula: see text] and [Formula: see text] calculated from the transepithelial resistance (R_t) and [Formula: see text]/[Formula: see text] in medullary thick ascending limbs. R_t was 7,722 ± 1,554 ohm·cm in the control period and 6,318 ± 1,757 ohm·cm after l-arginine treatment (P < 0.05). [Formula: see text]/[Formula: see text] was 2.0 ± 0.2 in the control period and 1.7 ± 0.1 after l-arginine (P < 0.04). Calculated [Formula: see text] and [Formula: see text] were 3.52 ± 0.2 and 1.81 ± 0.10 × 10^-5 cm/s, respectively, in the control period. After l-arginine they were 6.65 ± 0.69 (P < 0.0001 vs. control) and 3.97 ± 0.44 (P < 0.0001) × 10^-5 cm/s, respectively. NOS inhibition with N^ω-nitro-l-arginine methyl ester (5 mmol/l) prevented l-arginine's effect on R_t Next we tested the effect of cGMP. R_t in the control period was 7,592 ± 1,470 and 4,796 ± 847 ohm·cm after dibutyryl-cGMP (0.5 mmol/l; db-cGMP) treatment (P < 0.04). [Formula: see text]/[Formula: see text] was 1.8 ± 0.1 in the control period and 1.6 ± 0.1 after db-cGMP (P < 0.03). [Formula: see text] and [Formula: see text] were 4.58 ± 0.80 and 2.66 ± 0.57 × 10^-5 cm/s, respectively, for the control period and 9.48 ± 1.63 (P < 0.007) and 6.01 ± 1.05 (P < 0.005) × 10^-5 cm/s, respectively, after db-cGMP. We modeled NO's effect on luminal Na⁺ concentration along the thick ascending limb. We found that NO's effect on the paracellular pathway reduces net Na⁺ reabsorption and that the magnitude of this effect is similar to that due to NO's inhibition of transcellular transport.