The role of nitric oxide in obstructive nephropathy

Purpose: Ureteral obstruction leads to tubulointerstitial fibrosis and loss of renal function. Nitric oxide production ameliorates fibrosis due to obs tructive uropathy. However, nitric oxide is produced by 3 isoforms of the e nzyme, nitric oxide synthase. We evaluated the role of inducible nitric oxi de synthase in obstructive uropathy using nitric oxide synthase knockout mi ce, and determined whether the administration of L-arginine to promote nitr ic oxide synthesis by alternative nitric oxide synthase isoforms modulates renal fibrosis in these animals. Materials and Methods: Complete unilateral ureteral obstruction was created in wild-type C57 and inducible nitric oxide synthase knockout mice. Contro l animals of each strain underwent sham surgery. Throughout the experiment mice had free access to untreated tap water or water supplemented with 10 g m./l. L-arginine. Animals were sacrificed 1 and 2 weeks, respectively, afte r creation of unilateral ureteral obstruction. We obtained serum as well as bladder and obstructed renal pelvic urine, and determined the nitrite leve l in each fluid. Renal cortical thickness was measured in the normal and ob structed kidneys. The degree of tubulointerstitial fibrosis was evaluated b y trichrome staining and type I collagen deposition in kidney tissue specim ens. Results: Nitrite was significantly decreased in the serum, bladder and rena l pelvic urine of inducible nitric oxide synthase knockout mice with unilat eral ureteral obstruction compared with that in wild-type C57 mice at 1 and 2 weeks (p <0.05). In knockout mice with unilateral ureteral obstruction 1 week in duration that drank tap or L-arginine supplemented water nitrite i n serum and each urine sample was higher than in sham operated knockout con trols. The level returned to baseline after 2 weeks of obstruction (p <0.05 ). After 2 weeks of obstruction there was significantly greater cortical th inning in knockout than in C57 mice ip <0.05). Moreover, knockout mice give n L-arginine supplemented water for 2 weeks had even greater cortical thinn ing than after 1 week or than mice given tap water for 1 to 2 weeks ip <0.0 5). Decreased renal cortical thickness in knockout mice after 2 weeks of ob struction was associated with less intense trichrome staining and a virtual absence of type I collagen deposition compared with findings in the wild-t ype C57 strain. Conclusions: Inducible nitric oxide synthase knockout mice with unilateral ureteral obstruction have significantly lower nitrite in serum and urine th an wild-type C57 mice. Knockout mice also have more severe renal cortical t hinning than C57 animals after creation of unilateral ureteral obstruction. Providing L-arginine supplemented water to inducible nitric oxide synthase knockout mice exacerbates the loss of cortical thickness. Alterations in c ortical thinning that we observed in knockout mice were associated with dec reased tubulointerstitial fibrosis and a decreased net renal extracellular matrix accumulation. These data indicate that endothelial or neuronal nitri c oxide synthase may he more important than inducible nitric oxide synthase for modulating renal fibrosis in obstructive uropathy.