Accumulated endogenous nitric oxide synthase inhibitors in inhibiting urethral relaxation following estrogen supplementation in ovariectomized rabbits.

PURPOSE: We investigated the possible role of the endogenous nitric oxide (NO) synthase (NOS) inhibitors N-monomethyl-L-arginine (L-NMMA) and asymmetrical N, N-dimethyl-L-arginine (ADMA) in inhibiting urethral relaxation following estrogen supplementation in ovariectomized rabbits. MATERIALS AND METHODS: A total of 16 mature Japanese White female rabbits were divided into 2 groups. In the control group rabbits were sacrificed 2 weeks after bilateral ovariectomy. In the estrogen group estradiol was administered subcutaneously for 2 weeks with the aid of sustained release pellet from 2 weeks after ovariectomy until sacrifice. Isolated urethra was cut into transverse strips for functional study and processed to determine endogenous NOS inhibitors, NOS activity, dimethylarginine dimethylaminohydrolase (DDAH) activity as a metabolizing enzyme of endogenous NOS inhibitors and cyclic guanosine monophosphate production. RESULTS: Electrical field stimulation produced NO mediated and neurogenic relaxation of the urethral strip in the presence of guanethidine and atropine under contraction with phenylephrine. Relaxation was significantly decreased in the estrogen group and accompanied by decreased cyclic guanosine monophosphate production. Sodium nitroprusside induced relaxation was not different between the 2 groups. The content of L-NMMA plus ADMA in the urethra was significantly increased in the estrogen group. Ca dependent NOS activity in the urethra remained unaffected. DDAH activity was significantly lower in the estrogen group. CONCLUSIONS: Estrogen supplementation leads to decreased NO mediated and neurogenic urethral relaxation through the accumulation of L-NMMA and ADMA in the urethra. The accumulation of NOS inhibitors is possibly brought about by impaired DDAH activity.

Involvement of increased arginase activity in impaired cavernous relaxation with aging in the rabbit.

PURPOSE: Arginase shares L-arginine as a common substrate with nitric oxide (NO) synthase (NOS). We examined whether increased arginase activity is involved in impaired cavernous relaxation with aging in the rabbit. MATERIALS AND METHODS: Young adult (3 to 5 months old) and aged (36 to 48 months old) rabbits were used for the current experiments. Cavernous tissues obtained from the 2 groups were processed for isometric tension experiments, cyclic guanosine monophosphate determination, measurements of NOS and arginase activities, endogenous methylarginines and L-arginine. RESULTS: Carbachol (CCh) produced an endothelium dependent and NO mediated relaxation that was significantly impaired in aged cavernous specimens without change in sodium nitroprusside induced relaxation. Stimulated cyclic guanosine monophosphate production with CCh was significantly decreased in aged cavernous specimens. Ca dependent NOS was predominant in rabbit cavernous specimens. Ca dependent and independent NOS activities remained unchanged in the 2 groups. The tissue contents of N-monomethyl-L-arginine and asymmetric N,N-dimethyl-L-arginine as endogenous NOS inhibitors, symmetrical N,N'-dimethyl-L-arginine and L-arginine as a substrate of NOS were decreased in aged cavernous specimens. Arginase activity was significantly higher in aged cavernous specimens. Impaired CCh induced relaxation in aged cavernous specimens was normalized in the presence of N-hydroxy-L-arginine as an arginase inhibitor or by the supplementation of excess L-arginine. CONCLUSIONS: These results strongly suggest that impaired endothelium dependent and NO mediated cavernous relaxation with aging is due to decreased NO production, which would result from increased arginase activity and probably from decreased L-arginine content.