Significance of nitric oxide and its modulation mechanisms by endogenous nitric oxide synthase inhibitors and arginase in the micturition disorders and erectile dysfunction

Evidence indicates that nitric oxide (NO) deficiency contributes to micturition disorders, especially in the afferent pathway and erectile dysfunction (ED). Two possible causes of NO deficiency are substrate (L-arginine) limitation and increased levels of endogenous inhibitors of NO synthase (particularly asymmetric dimethylarginine: ADMA) in plasma and tissues. Elevated tissues of ADMA and N(G)-monomethyl-L-arginine (L-NMMA) have been reported to be associated with impaired NO-mediated urethral, trigonal and cavernosal relaxations by pelvic ischemia. Also, plasma ADMA may help to identify underlying cardiovascular disease in men with ED. Decreased l-arginine availability to NO synthase is due to the shunting of L-arginine into other pathways such as arginase. Interaction between NO synthase and arginase has been reported to be involved in NO-mediated urethral and prostatic relaxations. Also, increased arginase activity in cavernosal tissues likely contributes to the ED that accompanies diabetes mellitus and aging. Therefore, arginase inhibition has been reported to enhance the NO-dependent physiological process for erectile function.