Oxygen availability limits renal NADPH-dependent superoxide production

Renal oxygen tension is substantially lower in the medulla than in the cortex and is reduced in hypertensive rats, a model of oxidative stress. Expression of NADPH oxidase, the primary source for superoxide anion (O(2)(-)center dot) in the kidney, is elevated in hypertension. Because molecular oxygen (O(2)) is required for O(2)(-)center dot formation, we tested the hypothesis that renal NADPH oxidase activity is limited by low O2. O(2)(-center dot) production by rat kidney tissue or cultured cells exposed to levels of Po(2) that mimics those in the kidney was assessed by lucigenin-enhanced chemiluminescence. NADPH-dependent O(2)(-)center dot production by kidney homogenates decreased reversibly by 60 - 90% after graded reductions of ambient O2 from 10 to 0% ( 76 to 2 mmHg Po(2)). The NADPH-dependent O(2)(-)center dot production by the kidney homogenate was reduced by decreasing PO(2) below similar to 30 mmHg. The response of tissue homogenates to low PO2 was not different between normotensive and hypertensive rats. Similarly, NADPH-dependent O(2)(-)center dot production was lower during 2% O(2) compared with 10% O(2) in rat proximal tubule cells (-57 +/- 1%), vascular smooth muscle (-42 +/- 5%), cardiomyocytes (-57 +/- 1%), and mouse inner medulla collecting duct cells (-58 +/- 3%). We conclude that O(2)(-)center dot production by NADPH oxidase is dependent on availability of O(2). Therefore, O(2)(-)center dot generation may be limited in the kidney, both in the normal renal medulla and in the cortex of hypertensive kidneys.