High Salt-Induced Weakness of Anti-Oxidative Function of Natriuretic Peptide Receptor-C and Podocyte Damage in the Kidneys of Dahl Rats

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High Salt-Induced Weakness of Anti-Oxidative Function of Natriuretic Peptide Receptor-C and Podocyte Damage in the Kidneys of Dahl Rats

Hypertension represents a global health challenge, with salt-sensitive hypertension contributing significantly to cardiovascular morbidity and mortality. The pathophysiology of salt sensitivity involves complex interactions between genetic predisposition, renal sodium handling, and oxidative stress. This study investigates the role of renal natriuretic peptide receptor-C (NPR-C) in salt-sensitive hypertension, focusing on its anti-oxidative properties and association with podocyte injury in Dahl salt-sensitive (DS) rats.

Experimental Design and Animal Model

Dahl salt-sensitive (DS) and salt-resistant (DR) rats were subjected to a 6-week dietary regimen comprising either a normal-salt (NS; 0.4% NaCl) or high-salt (HS; 8% NaCl) diet. Blood pressure, sodium metabolism, renal oxidative stress markers, and podocyte integrity were systematically evaluated. Additional experiments utilized C-ANP4–23, an NPR-C agonist, to assess its therapeutic potential in mitigating HS-induced renal damage in DS rats.

Key Measurements

  • Systolic Blood Pressure (SBP): Tail-cuff method for non-invasive monitoring.
  • Sodium and Creatinine Levels: Plasmatic sodium (PLNa), urinary sodium excretion (UVNa), and serum creatinine (Scr) measured via spectrophotometric assays.
  • Atrial Natriuretic Peptide (ANP): Plasma, cardiac, and renal ANP concentrations quantified using ELISA.
  • Receptor Expression: Renal NPR-A and NPR-C expression analyzed by Western blot.
  • Oxidative Stress Markers: Malondialdehyde (MDA), lipofuscin, NADPH oxidase (Nox), and nitric oxide synthase (NOS) levels assessed in renal tissues.
  • Podocyte Integrity: Mitochondrial damage scored via transmission electron microscopy, desmin immunohistochemistry, and succinate dehydrogenase (SDHase) activity.

Impact of High-Salt Diet on Blood Pressure and Sodium Metabolism

Baseline SBP was comparable across all groups (116–120 mmHg). After 6 weeks, HS diet induced significant hypertension in DS rats (ΔSBP: +45.6 mmHg; P <0.001), while DR rats and NS-fed DS rats maintained normotensive levels. HS increased PLNa in both DS and DR rats, although urinary sodium excretion (UVNa) was more pronounced in DR rats (61.4 vs. 36.7 mmol/L; P <0.001), suggesting impaired sodium handling in DS rats. Serum creatinine (Scr) elevation was significantly higher in DS+HS (111.1 vs. 56.8 mmol/L; P <0.001), reflecting greater renal dysfunction in salt-sensitive animals.

Renal ANP Dynamics and NPR-C Downregulation

HS diet induced distinct tissue-specific ANP responses. Plasma ANP increased in DR+HS rats (126.4 vs. 92.2 pg/mL; P <0.001) but remained unchanged in DS+HS rats. Conversely, intrarenal ANP rose sharply in DS+HS (192.1 vs. 102.8 pg/mL; P <0.001), indicating compensatory renal ANP production. Western blot confirmed elevated renal ANP expression in DS+HS (t=−3.566, P=0.016), contrasting with DR rats.

Notably, NPR-C receptor expression decreased selectively in DS+HS kidneys (t=5.864, P=0.002), while NPR-A remained unaltered. This NPR-C downregulation coincided with oxidative stress and podocyte damage, suggesting a protective role of NPR-C in renal sodium handling and antioxidant defense.

Podocyte Injury and Mitochondrial Dysfunction

HS-induced podocyte damage was evident in DS rats through desmin immunostaining and ultrastructural analysis. Desmin staining scores increased by 3.1-fold (P=0.005) in DS+HS, reflecting cytoskeletal disruption. Transmission electron microscopy revealed mitochondrial swelling, cristae fragmentation, and vacuolization in podocytes (mitochondrial injury score: 4.2 vs. 0.8 in DS+HS vs. DS+NS; P=0.003). SDHase activity, a marker of mitochondrial function, decreased by 29.3% in DS+HS (P=0.017), corroborating mitochondrial impairment.

Oxidative Stress and NADPH Oxidase Activation

Oxidative stress markers surged in DS+HS kidneys:

  • MDA: Increased by 2.1-fold (P=0.009).
  • Lipofuscin: Elevated by 3.4-fold (P=0.001).
  • Nox Activity: Rose by 4.7-fold (P <0.001), while NOS remained unaffected.

Western blot further demonstrated upregulated Nox4 expression, a major isoform driving ROS production in podocytes. These changes correlated with NPR-C suppression, implicating NPR-C in redox homeostasis.

Therapeutic Effects of NPR-C Agonist C-ANP4–23

Administration of C-ANP4–23 (10 nmol/kg, twice weekly) attenuated HS-induced hypertension in DS rats:

  • SBP Reduction: From 166.2 mmHg (HS+vehicle) to 128.5 mmHg (HS+C-ANP4–23; P <0.05).
  • Sodium Excretion: UVNa increased by 21.9% (P <0.05), lowering PLNa (92.9 vs. 106.4 mmol/L; P <0.05).
  • Renal Protection: Scr normalized to near NS levels (65.3 vs. 112.1 mmol/L; P <0.05).

C-ANP4–23 also mitigated oxidative stress:

  • MDA and Lipofuscin: Reduced by 38% and 42%, respectively (P <0.05).
  • Nox Activity and Nox4 Expression: Declined by 34% and 28% (P <0.05).

These results highlight NPR-C activation as a viable strategy to counteract salt-sensitive hypertension via antioxidant and renal protective mechanisms.

Mechanistic Insights and Implications

The study elucidates a dual mechanism linking NPR-C dysfunction to salt-sensitive hypertension:

  1. Mitochondrial Injury in Podocytes: HS-induced oxidative stress disrupts mitochondrial integrity, impairing sodium excretion and glomerular filtration.
  2. NPR-C-Mediated Antioxidant Failure: NPR-C downregulation exacerbates Nox4-driven ROS production, perpetuating renal damage and hypertension.

C-ANP4–23 restores redox balance by enhancing NPR-C signaling, reducing Nox activity, and preserving podocyte mitochondria. This therapeutic approach addresses both oxidative stress and renal sodium handling, offering a targeted intervention for salt-sensitive hypertension.

Conclusion

High salt intake impairs NPR-C function in DS rats, leading to oxidative stress, podocyte damage, and hypertension. NPR-C agonism with C-ANP4–23 attenuates these effects, underscoring its potential as a novel therapy. Future studies should explore NPR-C signaling pathways and their modulation in human salt-sensitive hypertension.

DOI: https://doi.org/10.1097/CM9.0000000000000752

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