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. 2018 Aug 7;7(15):e009501.
doi: 10.1161/JAHA.118.009501.

Nitric Oxide Synthase Inhibition Induces Renal Medullary Hypoxia in Conscious Rats

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Free PMC article

Nitric Oxide Synthase Inhibition Induces Renal Medullary Hypoxia in Conscious Rats

Tonja W Emans et al. J Am Heart Assoc. .
Free PMC article

Abstract

Background Renal hypoxia, implicated as crucial factor in onset and progression of chronic kidney disease, may be attributed to reduced nitric oxide because nitric oxide dilates vasculature and inhibits mitochondrial oxygen consumption. We hypothesized that chronic nitric oxide synthase inhibition would induce renal hypoxia. Methods and Results Oxygen-sensitive electrodes, attached to telemeters, were implanted in either renal cortex (n=6) or medulla (n=7) in rats. After recovery and stabilization, baseline oxygenation ( pO 2) was recorded for 1 week. To inhibit nitric oxide synthase, N-ω-nitro-l-arginine (L-NNA; 40 mg/kg/day) was administered via drinking water for 2 weeks. A separate group (n=8), instrumented with blood pressure telemeters, followed the same protocol. L-NNA rapidly induced hypertension (165±6 versus 108±3 mm Hg; P<0.001) and proteinuria (79±12 versus 17±2 mg/day; P<0.001). Cortical pO 2, after initially dipping, returned to baseline and then increased. Medullary pO 2 decreased progressively (up to -19±6% versus baseline; P<0.05). After 14 days of L-NNA, amplitude of diurnal medullary pO 2 was decreased (3.7 [2.2-5.3] versus 7.9 [7.5-8.4]; P<0.01), whereas amplitudes of blood pressure and cortical pO 2 were unaltered. Terminal glomerular filtration rate (1374±74 versus 2098±122 μL/min), renal blood flow (5014±336 versus 9966±905 μL/min), and sodium reabsorption efficiency (13.0±0.8 versus 22.8±1.7 μmol/μmol) decreased (all P<0.001). Conclusions For the first time, we show temporal development of renal cortical and medullary oxygenation during chronic nitric oxide synthase inhibition in unrestrained conscious rats. Whereas cortical pO 2 shows transient changes, medullary pO 2 decreased progressively. Chronic L-NNA leads to decreased renal perfusion and sodium reabsorption efficiency, resulting in progressive medullary hypoxia, suggesting that juxtamedullary nephrons are potentially vulnerable to prolonged nitric oxide depletion.

Keywords: hypoxia; nitric oxide synthase; oxygen consumption; renal oxygenation; telemetry.

Figures

Figure 1
Figure 1
Proteinuria, renal blood flow (RBF), and sodium reabsorption efficiency (TNa/QO 2) after 2 weeks of nitric oxide synthase (NOS) inhibition (n=21) compared with controls (n=7). Proteinuria was measured from urine collected in metabolic cages for 24 hours. Hemodynamics were measured under isoflurane anesthesia. ***P≤0.001 vs control, unpaired Student t test.
Figure 2
Figure 2
Dark‐ and light‐phase analysis of urine and water/food intake. Rats were individually housed in metabolic cages for 24 hours. Urine was collected during the dark‐phase (active) and during the lights‐phase (resting) period. Data are expressed as mean±SEM for (A) water intake, (B) food intake, (C) urine volume, (D) creatinine excretion, (E) Na+ excretion, (F) K+ excretion, (G) creatinine clearance, (H) urine osmolality, and (I) osmolyte excretion. *P<0.05; **P<0.001 vs control, ANOVA for repeated‐measures post hoc Tukey test. NOS indicates nitric oxide synthase.
Figure 3
Figure 3
Cortical and medullary oxygenation (pO 2) and mean arterial pressure (MAP) during 2 weeks of nitric oxide synthase (NOS) inhibition. N‐ω‐nitro‐l‐arginine (40 mg/kg/day) was added to drinking water on day 0 after at least 1 week of baseline measurements. Telemetric recordings of cortical (closed circles, n=6) and medullary (open circles, n=7) oxygenation (pO 2) were recorded continuously. Values are expressed as a percentage of the baseline period before NOS inhibition. MAP was determined by telemetry (black dots, n=8) in another subset of animals. Data points represent the mean of 6‐hour averages±SEM. ANOVA for repeated measures with Tukey post hoc was performed: *P≤0.05; **P≤0.01 vs baseline in MAP or medulla; P≤0.05 vs baseline in cortex.
Figure 4
Figure 4
Circadian rhythmicity during nitric oxide synthase (NOS) inhibition in mean arterial pressure (MAP; A), heart rate (HR; B), cortical oxygenation (pO 2; C), and medullary pO 2 (D), and relation between MAP and pO 2 in cortex (E) and medulla (F). All data are plotted as 15‐minutes mean values as recorded over 4 days in each rat, before (open circles, mean±SEM) and during 1 to 4 (light gray circles, mean only), 6 to 9 (dark gray circles, mean only), and 11 to 14 days of NOS inhibition (closed circles, mean±SEM). Dashed line represents circadian‐rhythm–adjusted mean before NOS inhibition. Data in (A through D) were analyzed by cosinor analysis (period=24 hours). ZT indicates Zeitgeber Time.

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