Pressure Natriuresis and Autoregulation of Inner Medullary Blood Flow in Canine Kidney

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Abstract
We have evaluated the responses to changes in arterial pressure on regional blood flows in the renal medulla and sodium excretion simultaneously in denervated kidneys of six anesthetized sodium-replete dogs. Renal regional blood flow responses were determined using laser-Doppler needle flow probes and whole-kidney blood flow was assessed using an electromagnetic flow probe. The responses to stepwise reductions in renal arterial pressure (140 to 70 mm Hg) were examined first with a laser-Doppler needle probe inserted in the outer medulla and then repeated after advancing the same probe in the inner medulla. There were no differences in the control values of total renal blood flow (4.4±0.7 to 4.5±0.5 mL·min −1 ·g −1 ), glomerular filtration rate (0.89±0.7 to 0.94±0.9 mL·min −1 ·g −1 ), sodium excretion (3.6±0.6 to 3.4±0.5 μmol·min −1 ·g −1 ), and urinary excretion rate of nitric oxide metabolites (nitrate/nitrite, 1.6±0.2 to 1.5±0.2 nmol·min −1 ·g −1 ) at the start of both experimental periods. During changes in renal arterial pressure, inner medullary blood flow exhibited efficient autoregulation similar to that in outer medullary blood flow. Usual excretory responses to reductions in renal arterial pressure as well as autoregulation of cortical and whole-kidney blood flows and glomerular filtration rate were observed in these dogs. The slopes of the relationship between arterial pressure and sodium excretion (0.046±0.007 to 0.044±0.009 μmol·min −1 ·g −1 ·mm Hg −1 ) or nitrate/nitrite excretion (0.014±0.003 to 0.013±0.003 nmol·min −1 ·g −1 ·mm Hg −1 ) were similar in both experimental periods. These data indicate that blood flow to the inner medulla is efficiently autoregulated as in outer medulla and cortex of the kidney in anesthetized dogs and demonstrate further that the arterial pressure-induced natriuretic responses do not require associated changes in medullary blood flow.