NADPH oxidases are the major sources of reactive oxygen species in 
cardiovascular, neural, and kidney cells. The NADPH oxidase 5 (NOX5) gene is 
present in humans but not rodents. Because Nox isoforms in renal proximal 
tubules (RPTs) are involved in the pathogenesis of hypertension, we tested the 
hypothesis that NOX5 is differentially expressed in RPT cells from normotensive 
(NT) and hypertensive subjects (HT). We found that NOX5 mRNA, total NOX5 
protein, and apical membrane NOX5 protein were 4.2±0.7-fold, 5.2±0.7-fold, and 
2.8±0.5-fold greater in HT than NT. Basal total NADPH oxidase activity was 
4.5±0.2-fold and basal NOX5 activity in NOX5 immunoprecipitates was 6.2±0.2-fold 
greater in HT than NT (P=<0.001, n=6-14/group). Ionomycin increased total NOX 
and NOX5 activities in RPT cells from HT (P<0.01, n=4, ANOVA), effects that were 
abrogated by pre-treatment of the RPT cells with diphenylene-iodonium or 
superoxide dismutase. Silencing NOX5 using NOX5-siRNA decreased NADPH oxidase 
activity (-45.1±3.2% vs. mock-siRNA, n=6-8) in HT. D1-like receptor stimulation 
decreased NADPH oxidase activity to a greater extent in NT (-32.5±1.8%) than HT 
(-14.8±1.8). In contrast to the marked increase in expression and activity of 
NOX5 in HT, NOX1 mRNA and protein were minimally increased in HT, relative to 
NT; total NOX2 and NOX4 proteins were not different between HT and NT, while the 
increase in apical RPT cell membrane NOX1, NOX2, and NOX4 proteins in HT, 
relative to NT, was much less than those observed with NOX5. Thus, we 
demonstrate, for the first time, that NOX5 is expressed in human RPT cells and 
to greater extent than the other Nox isoforms in HT than NT. We suggest that the 
increased expression of NOX5, which may be responsible for the increased 
oxidative stress in RPT cells in human essential hypertension, is caused, in 
part, by a defective renal dopaminergic system.