The unique role of nitric oxide (NO) in the regulation of renal medullary function is supported by the evidence summarized in this review. The impact of reduced production of NO within the renal medulla on the delivery of blood to the medulla and on the long-term regulation of sodium excretion and blood pressure is described. It is evident that medullary NO production serves as an important counterregulatory factor to buffer vasoconstrictor hormone-induced reduction of medullary blood flow and tissue oxygen levels. When NO synthase (NOS) activity is reduced within the renal medulla, either pharmacologically or genetically [Dahl salt-sensitive (S) rats], a super sensitivity to vasoconstrictors develops with ensuing hypertension. Reduced NO production may also result from reduced cellular uptake of l-arginine in the medullary tissue, resulting in hypertension. It is concluded that NO production in the renal medulla plays a very important role in sodium and water homeostasis and the long-term control of arterial pressure.

The present study examined whether chronic increased oxidative stress within the medulla of the kidney lowers medullary blood flow and leads to hypertension. Optical fibers were implanted into the renal cortex and medulla of uninephrectomized Sprague-Dawley rats (Harlan Sprague-Dawley, Madison, Wis) for the daily measurement of blood flow to these regions using laser-Doppler flowmetry techniques, while arterial pressure was measured from an indwelling aortic catheter. A renal medullary interstitial catheter was implanted for the continuous delivery of the superoxide dismutase (SOD) inhibitor, diethyldithiocarbamic acid (DETC), at a dose of 7.5 mg/kg/d. Renal interstitial superoxide (O ₂ ⁻ ) levels were determined by perfusing an O ₂ ⁻ sensitive fluorescent dye, dihydroethidium, through a microdialysis probe implanted into the medulla. Urine samples (24 hours) were collected for measurements of isoprostane excretion. The results indicate that medullary DETC infusions increased tissue O ₂ ⁻ concentrations in the renal medulla (93.4±22.3,n=8, saline and 867.3±260.2, n=8, DETC; fluorescence units) and increased urinary 8-isoprostane excretion (4.1±0.4 ng/d, n=9, saline and 8.8±1.6 ng/d, n=10, DETC). Mean arterial pressure increased 24 hours after the start of intrarenal DETC infusion and remained nearly 20 mm Hg above control pressure throughout the 5 days of medullary SOD inhibition. During chronic medullary DETC infusion, medullary blood flow was significantly reduced (42.7%), whereas cortical blood flow was unchanged. Intravenous infusion of the same dose of DETC produced no changes in renal medullary or cortical blood flow or arterial blood pressure. The present experiments indicate that an increase in superoxide concentration within the renal medulla selectively reduces medullary blood flow resulting in chronic hypertension.