Oxygen-radical regulation of renal blood flow following suprarenal aortic clamping

Objective: Renal insufficiency continues to be complication that can affect patients after treatment for suprarenal aneurysms and renal artery occlusive disease. One proposed mechanism of renal injury after suprarenal aortic clamping (above the superior mesenteric artery) and reperfusion (SMA-SRACR) is the loss of microvascular renal blood flow with subsequent loss of renal function. This study examines the hypothesis that the loss of medullary and cortical microvascular blood flow following SMA-SRACR is due to oxygen-derived free radical down-regulation of endogenous medullary and cortical nitric oxide synthesis. Methods. Anesthetized male Sprague-Dawley rats (about 350 g) either had microdialysis probes or laser Doppler fibers inserted into the renal cortex (depth of 2 mm) and into the renal medulla (depth of 4 mm). Laser Doppler blood flow was continuously monitored. The microdialysis probes were connected to a syringe pump and perfused in vivo at 3 mu L/min with lactated Ringer's solution. The animals were subjected to SMA-SRACR (or sham) for 30 minutes, followed by 60 minutes of reperfusion. Laser Doppler blood flow after the 30 minutes of SMA-SRACR followed by 60 minutes of reperfusion was compared with the time zero (basal) and with the corresponding sham group and reported as percent change compared with the time zero baseline. The microdialysis fluid was collected at time zero (basal) and compared with the dialysis fluid collected after 30 minutes of SNIA-SRACR followed by 60 minutes of reperfusion as well as the corresponding sham group. The microdialysis dialysate was analyzed for total nitric oxide (mu M) and prostaglandin E-2 (PGE(2)), 6-keto-PGF(1 alpha) (PGI(2) metabolite), and thromboxane B-2 synthesis. The data are reported as percent change compared with the baseline time zero. The laser Doppler blood flow and microdialysis groups were treated with either saline carrier, N-omega-nitro-L-arginine methyl ester hydrochloride (L-NAME) (30 mg/kg, nitric oxide synthesis inhibitor), L-arginine (400 mg/kg, nitric oxide precursor), superoxide dismutase (SOD, 10,000 U/kg, oxygen-derived free radical scavenger), L-NAME + SOD, or L-arginine + SOD. SOD was given 30 minutes before the reperfusion, and the other drugs were given 15 minutes before reperfusion. The renal cortex and medulla were separated and analyzed for inducible nitric oxide synthase (iNOS), cyclooxygenase-2, prostacyclin synthase, and PGE(2) synthase content by Western blot. Results: Superior mesenteric artery-SRACR caused a marked decrease in medullary and cortical blood flow with a concomitant decrease in endogenous medullary and cortical nitric oxide synthesis. These changes were further accentuated by L-NAME treatment but restored toward sham levels by L-arginine treatment after SMA-SRACR. The kidney appeared to compensate for these changes by increasing cortical and medullary PGE(2) synthesis and release. SOD treatment restored renal cortical and medullary nitric oxide synthesis and blood flow in the ischemia-reperfusion group and in the ischemia-reperfusion group treated with L-NAME. Conclusions. These data show that nitric oxide is important in maintaining renal cortical and medullary blood flow and nitric oxide synthesis. These data also support the hypothesis that the loss of medullary and cortical microvascular blood flow following SRACR is due in part to oxygen-derived free radical downregulation of endogenous medullary and cortical nitric oxide synthesis. (J Vasc Surg 2006;43:577-86.) Clinical Relevance: This study suggests that clinically relevant cortical and medullary vasodilators (nitric oxide and vasodilator prostanoids) are required to maintain microvascular renal cortical and medullary blood flow. This study combines in vivo techniques of microdialysis and laser Doppler flow probes to show that oxygen-derived free radicals are one of the mediators that downregulate endogenous cortical and medullary nitric oxide synthesis, contributing to decreased cortical and medullary blood flow that occurs in the SMA-SRACR model. Prevention or inhibition of oxygen-derived free radical production during SMA-SRACR should be one of the treatment strategies that could help maintain renal microvascular blood flow during the treatment of complex aortic pathology that requires SMA-SRACR.