Production and Actions of Superoxide in the Renal Medulla
There is increasing evidence that oxidant stress is involved in the development of hypertension. Given the importance of renal medullary blood flow (MBF) in the long-term control of arterial blood pressure (AP), we hypothesized that renal medullary redox status may importantly contribute to the control of AP through alteration of MBF. The present study first characterized the biochemical pathways for superoxide (O2-.) production in the rat kidney. Using the dihydroethidium/DNA fluorescence spectrometry, the production of O2-. was monitored with the tissue homogenates from different kidney regions incubated in microtiter plates with either NADH or NAHPH, or xanthine, or succinic acid/antimycin A, which are the substrates of NADH/NADPH oxidase, xanthine oxidase and mitochondrial respiratory chain, respectively. The production of O2-. via NADH oxidase was greater (p<0.05) in the renal cortex and outer medulla (OM) than papilla. The mitochondrial enzyme activity for O2-.production was higher (p<0.05) in the OM than cortex and papilla. Overall, the renal OM exhibited greater enzyme activities for O2-. production. In anesthetized rats, renal medullary interstitial infusion of a membrane-permeable SOD mimetic, 4-hydroxy-tetramethyl piperidine-1-oxyl (TEMPOL, 30 μmol/kg/min) significantly increased MBF by 31%, as measured by laser-Doppler flowmetry and increased sodium excretion by 39%. Renal cortical blood flow and AP were unchanged. We conclude that (i) renal medullary O2-. may be primarily produced in the renal OM; (ii) both NADH oxidase and intramitochondrial enzymes are responsible for the O2-. production in this kidney region; (iii) O2-. exerts a tonic regulatory action on vascular tone in the renal medullary circulation.