Relation Between Vasa Recta Blood Flow and Renal Interstitial Hydrostatic Pressure During Pressure Natriuresis

Pressure natriuresis may be mediated through increases in inner medullary vasa recta blood flow (Q_VR). By means of acute renal decapsulation to prevent increases in renal interstitial hydrostatic pressure (RIHP), the effect of increases in Q_VR in the presence and absence of changes in RIHP in the natriuretic and diuretic responses to increases in renal perfusion pressure (RPP) was evaluated. Blood flow in descending (Q_DVR) and ascending (Q_AVR) vasa recta was determined in the exposed papilla by fluorescence videomicroscopy in anesthetized euvolemic Munich Wistar rats. In rats with intact renal capsules (n=12), increases in RPP from 101±0.5 to 132±2.9 mm Hg caused significant increases in Q_DVR (from 4.7±0.9 to 5.5±0.9 nl/min, p<0.001) and Q_AVR (from 2.8±0.2 to 3.5±0.2 nl/min, p<0.001) in association with increases in RIHP (from 4.6±1.3 to 7.6±1.3 mm Hg, p<0.001), urine flow (from 16.2±2.6 to 20.2±3.2 μl · min⁻¹.g kidney wt⁻¹, p<0.01), and urinary sodium excretion (from 2.10±0.38 to 3.36±0.62 μeq.min⁻¹.g kidney wt⁻¹, p<0.001). Acute bilateral renal decapsulation (n=12) prevented the rise in RIHP (from 4.0±0.6 to 3.6±0.6 mm Hg), urine flow (from 13.0±2.1 to 14.2±1.5 μl · min⁻¹.g kidney wt⁻¹), and urinary sodium excretion (from 1.76±0.28 to 2.12±0.31 μeq.min⁻¹.g kidney wt⁻¹) as RPP increased from 100±0.3 to 134±2.2 mm Hg but not the increases in either Q_DVR (from 4.4±0.5 to 5.1±0.5 nl/min, p<0.01) or Q_AVR (from 2.6±0.3 to 3.3±0.3 nl/min, p<0.001). No changes in Q_DVR, Q_AVR, RIHP, urine flow, and fractional excretion of sodium were observed in a third group of time control decapsulated rats (n=7) in which RPP was held constant at 100 mm Hg. Glomerular filtration rate and total renal blood flow were stable in all three groups. These findings demonstrate that Q_VR increases in response to elevations in RPP but only results in natriuresis when associated with a concomitant rise in RIHP.