Abstract
Nitric oxide prevents hypertension yet enhances proximal tubule Na+ reabsorption. Nitric oxide synthase is inhibited by asymmetric dimethylarginine (ADMA) that is metabolized by dimethylarginine dimethylaminohydrolase (DDAH) whose type 1 isoform is expressed abundantly in the proximal tubule (PT). We hypothesize that ADMA metabolized by DDAH-1 inhibits fluid reabsorbtion (Jv) by the proximal tubule. S2 segments of the PT were microperfused between blocks in vivo to assess Jv in anesthetized rats. Compared with vehicle, microperfusion of ADMA or Nω-nitro-l-arginine methyl ester (l-NAME) in the proximal tubule reduced Jv dose dependently. At 10-4 mol/l both reduced Jv by ~40% (vehicle: 3.2 ± 0.7 vs. ADMA: 2.1 ± 0.5, P < 0.01 vs. l-NAME: 1.9 ± 0.4 nl·min-1·mm-1, P < 0.01; n = 10). Selective inhibition of DDAH-1 in rats with intravenous L-257 (60 mg/kg) given 2 h before and L-257 (10-5 mol/l) perfused in the proximal tubule for 5 min reduced Jv by 32 ± 4% (vehicle: 3.2 ± 0.5 vs. L-257: 2.2 ± 0.5 nl·min-1·mm-1; P < 0.01) and increased plasma ADMA by ≈50% (vehicle: 0.46 ± 0.03 vs. L-257: 0.67 ± 0.03 µmol/l, P < 0.0001) without changing plasma symmetric dimethylarginine. Compared with nontargeted control small-interference RNA, knock down of DDAH-1 in mice by 60% with targeted small-interference RNAs (siRNA) reduced Jv by 29 ± 5% (nontargeted siRNA: 2.8 ± 0.20 vs. DDAH-1 knockdown: 1.9 ± 0.31 nl·min-1·mm-1, P < 0.05). In conclusion, fluid reabsorption in the proximal tubule is reduced by tubular ADMA or by blocking its metabolism by DDAH-1. L-257 is a novel regulator of proximal tubule fluid reabsorption.