Effect of incubation with L-citrulline ex vivo on relaxation dysfunction of corpus cavernosum from type 2 diabetic rats

Abstract

Objective To explore the role of the changes in endogenous L-citrulline levels and arginases expression in erectile dysfunction of diabetic rats, and investigate the effect of L-citrulline incubation ex vivo on the erectile dysfunction. Methods Type 2 diabetic rats were induced by high fat diet plus a single intraperitoneal injection of small dose streptozotocin (30 mg/kg), followed by high fat diet for 8 weeks. The erectile function of rats was reflected by the relaxation response to acetylcholine (ACh) of corpus cavernosum detected by isometric tension measurement. Levels of endogenous nitric oxide inhibitor asymmetric dimethylarginine (ADMA) and cyclic guanosine monophosphate (cGMP) were measured by enzyme-linked immunosorbent assay (ELISA). The content of L-citrulline, nitric oxide (NO) and the activity of nitric oxide synthase (NOS) were determined with colorimetric method. Western blotting was applied to detect the protein expression of NOS, arginases and phosphodiesterase 5 (PED5). Results The relaxation response to ACh of corpus cavernosum from diabetic rats was significantly lower than that of control rats (P<0.01), indicating the erectile dysfunction of diabetic rats. This erectile dysfunction was companied by the decreases of L-citrulline levels in serum and corpus cavernosum as well as up-regulation of arginaseⅠ& Ⅱ expression in the corpus cavernosum of diabetic rats. Incubation with L-citrulline (3 mmol/L) for 45 min ex vivo im-proved the relaxation function of corpus cavernosum from diabetic rats (P<0.05), while incubation with PED5 inhibitor silaenafil (3 μmol/L) had no significant effect. Furthermore, the ADMA level was increased, while the NOS activity, cGMP and NO contents were decreased in corpus cavernosum of diabetic rats compared to control rats. Down-regulation of endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) protein expression as well as up-regulation of inducible nitric oxide synthase (iNOS) and PDE5 protein expression were also observed in the corpus cavernosum of diabetic rats. Conclusions The decreased endogenous L-citrulline levels and upregulated arginases expression was closely related to erectile dysfunction of diabetic rats, and the mechanism is associated to the up-regulation of arginase activity and decrease of NO synthesis in corpus cavernosum. L-citrulline incubation ex vivo exerted therapeutic effect on the relaxation dysfunction of corpus cavernosum from diabetic rats. Key words: L-citrulline; Diabetes mellitus; Erectile dysfunction; Arginase; Nitric oxide