Effects of Renal Perfusion Pressure on Renal Medullary Hydrogen Peroxide and Nitric Oxide Production

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Studies were designed to determine the effects of increases of renal perfusion pressure on the production of hydrogen peroxide (H2O2) and NO2−+NO3− within the renal outer medulla. Sprague-Dawley rats were studied with either the renal capsule intact or removed to ascertain the contribution of changes of medullary blood flow and renal interstitial hydrostatic pressure on H2O2 and NO2−+NO3− production. Responses to three 30-minute step changes of renal perfusion pressure (from ≈85 to ≈115 to ≈145 mm Hg) were studied using adjustable aortic occluders proximal and distal to the left renal artery. Medullary interstitial H2O2 determined by microdialysis increased at each level of renal perfusion pressure from 640 to 874 to 1593 nmol/L, as did H2O2 urinary excretion rates, and these responses were significantly attenuated by decapsulation. Medullary interstitial NO2−+NO3− increased from 9.2 to 13.8 to 16.1 μmol/L, with parallel changes in urine NO2−+NO3−, but decapsulation did not significantly blunt these responses. Over the range of renal perfusion pressure, medullary blood flow (laser-Doppler flowmetry) rose ≈30% and renal interstitial hydrostatic pressure rose from 7.8 to 19.7 cm H2O. Renal interstitial hydrostatic pressure and the natriuretic and diuretic responses were significantly attenuated with decapsulation, but medullary blood flow was not affected. The data indicate that pressure-induced increases of H2O2 emanated largely from increased tubular flow rates to the medullary thick-ascending limbs of Henle and NO largely from increased medullary blood flow to the vasa recta. The parallel pressure–induced increases of H2O2 and NO indicate a participation in shaping the “normal” pressure-natriuresis relationship and explain why an imbalance in either would affect the blood pressure salt sensitivity.

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