Renal proximal tubular reabsorption is reduced in adult spontaneously hypertensive rats: roles of superoxide and NHE3

Proximal tubule reabsorption is regulated by systemic and intrinsic mechanisms, including locally produced autacoids. Superoxide (O2−), produced by NADPH oxidase (NOX) enhances NaCl transport in the loop of Henle and the collecting duct, but its role in the PT is unclear. We measured PT fluid reabsorption (Jv) in WKY rats and compared that to Jv in SHR, a model of enhanced renal O2− generation. Rats were treated with the NOX inhibitor apocynin (Apo), or with small interfering RNA (siRNA) for p22phox, which is the critical subunit of NOX. Jv was lower in SHR compared to WKY (WKY: 2.4±0.3 vs SHR: 1.1±0.2 nl/min/mm, n=9–11, p<0.001). Apo and siRNA to p22phox normalized Jv in SHR yet had no effect in WKY. Jv was reduced in proximal tubules perfused with S-1611, a highly selective inhibitor of the Na+/H+ exchanger 3 (NHE3), the major Na+ uptake pathway in the proximal tubule, in WKY but not in SHR. Pretreatment with Apo restored an effect of S-1611 to reduce Jv in the SHR (SHR+Apo: 2.9±0.4 vs SHR+Apo+S-1611: 1.0±0.3 nl/min/mm, p<0.001). However, since expression of NHE3 was similar between SHR and WKY, this suggests that O2− affects NHE3 activity. Direct microperfusion of tempol or apocynin into the PT also restored Jv in SHR. In conclusion, O2− generated by NOX, inhibits proximal tubule fluid reabsorption in SHR. This finding implies that PT fluid reabsorption is regulated by redox balance, which may have profound effects on ion and fluid homeostasis in the hypertensive kidney. Keywords: Proximal reabsorption, superoxide, tempol, apocynin