Elevated Superoxide Production in Response to Angiotensin II by Proximal Tubules of Rats Consuming a Moderately Enriched Fructose Diet

Abstract

Dietary fructose consumption has increased dramatically over the past four decades in part due to increased consumption of sugary beverages. Rats ingesting a diet that includes 20 % fructose in the drinking water develop salt‐sensitive hypertension. This is in part due to an increased sensitivity of the proximal nephron to angiotensin II (Ang II), which stimulates Na reabsorption in this segment at physiological concentrations via increases in reactive oxygen species such as superoxide (O2−). Thus we hypothesized that dietary fructose enhances the ability of Ang II to stimulate O2− production by proximal tubules. To test this hypothesis, we first measured the effect of Ang II on O2− production by proximal tubule suspensions from rats fed either a control diet or one in which 20% fructose was included in the drinking water. Ang II (10 nM) increased O2− production by 351 ± 85 relative light units (RLU)/mg protein/s in tubules from fructose‐fed rats but only by 131 ± 55 RLU/mg/s in tubules from control animals (Δ 220 ± 101 RLU/mg/s; p < 0.01, n=8). To test whether the increase was due to activation of NADPH oxidase (NOX), we tested the ability of apocynin to block the effects of Ang II. In the presence of apocynin, Ang II did not significantly change O2− production by tubules from either fructose‐fed (Δ 27 ± 19 RLU/mg/s) or control (Δ 29 ± 39 RLU/mg/s) rats. We next tested whether the fructose‐dependent increase was due to enhanced NOX4 activity by examining the effects of the NOX1 and 4 inhibitor GKT136901 on O2− production and NOX4 expression by Western blot. In the presence of GKT136901, Ang II did not significantly change O2− production by tubules from either fructose‐fed (Δ 36 ± 20 RLU/mg/s) or control (Δ 82 ± 78 RLU/mg/s) animals. Fructose feeding increased NOX4 expression in proximal tubules by 55 ± 13% compared to controls (p < 0.01, n=6). Together these data indicate that a diet including 20% fructose in the drinking water enhances the ability to increase O2− production by proximal tubules in response to Ang II due to activation of NOX4. Given that O2− is a potent oxidant, we studied whether the primary antioxidant in proximal tubule cells, reduced glutathione was decreased under basal conditions or in response to Ang II. We found no significant differences in either reduced or total glutathione in proximal tubules from fructose‐fed and control rats. However, Ang II decreased the ratio of reduced to total glutathione by 10 ± 3 % (p<0.03; n=19) in tubules from fructose‐fed rats and by only 1 ± 3 % in in tubules from controls. We conclude that elevated dietary fructose enhances O2− production by proximal tubules in response to Ang II. This increase is likely mediated by enhanced activity and expression of NOX4. The elevated oxidative stress caused by augmented O2− production results in diminished antioxidant defenses. In total these effects increase proximal nephron Na reabsorption and contribute to salt‐sensitive hypertension.Support or Funding InformationThis work was supported in part by a grant from the Heart, Lung and Blood Institute of the National Institutes of Health to J.L.G. (HL128053).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.