PRESSURE NATRIURESIS AND AUTOREGULATION OF INNER MEDULLARY BLOOD-FLOWIN CANINE KIDNEY

We have evaluated the responses to changes in arterial pressure on reg ional blood flows in the renal medulla and sodium excretion simultaneo usly in denervated kidneys of six anesthetized sodium-replete dogs. Re nal regional blood flow responses were determined using laser-Doppler needle flow probes and whole-kidney blood flow was assessed using an e lectromagnetic flow probe. The responses to stepwise reductions in ren al arterial pressure (140 to 70 mm Hg) were examined first with a lase r-Doppler needle probe inserted in the outer medulla and then repeated after advancing the same probe in the inner medulla. There were no di fferences 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 mu mol . 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 pe ri ods. During changes in renal arterial pressure, inner medullary blood flow exhibited efficient autoregulation similar to that in outer medul lary blood flow. Usual excretory responses to reductions in renal arte rial pressure as well as autoregulation of cortical and whole-kidney b lood flows and glomerular filtration rate were observed in these dogs. The slopes of the relationship between arterial pressure and sodium e xcretion (0.046 +/- 0.007 to 0.044 +/- 0.009 mu mol . 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 e fficiently autoregulated as in outer medulla and cortex of the kidney in anesthetized dogs and demonstrate further that the arterial pressur e-induced natriuretic responses do not require associated changes in m edullary blood flow.