We identify whether tachyphylaxis and priapism effects of sildenafil are related to regulation of phosphodiesterase-5 (PDE-5) expression.

Cavernous smooth muscle cells (CSMCs) were isolated from young rats and treated with 0, 1, 10 and 25 μM sildenafil with or without 100 μM of sodium nitroprusside for 3 and 7 days. The cells were subjected to reverse transcriptase-polymerase chain reaction and Western blot analysis for PDE-5 expression. Plasmid constructs carrying PDE-5A1 and PDE-5A2 promoters were transfected into COS-7 cells, treated with 25 μM sildenafil and analyzed for promoter activities. To simulate priapism, CSMCs were cultured under anoxia or hypoxia and then analyzed for PDE-5 expression. Furthermore, rats underwent bilateral pudendal arterial ligation for 1 day to 14 weeks, and corpus cavernous tissues were subjected to reverse transcriptase-polymerase chain reaction analysis for PDE-5 expression.

Up-regulation of PDE-5 was noted in CSMCs treated with 25 μM sildenafil for 7 days. PDE-5 messenger RNA and protein levels were significantly increased in the 7-day sildenafil treated cultures. Sodium nitroprusside appeared to down-regulate PDE-5 expression. Sildenafil significantly increased the activities of PDE-5A1 promoter. PDE-5 expression was significantly reduced under anoxia and hypoxia. The corpus cavernous tissue showed a gradual decrease in PDE-5 expression under ischemia.

Repeated treatment with sildenafil at high concentrations was needed to simulate tachyphylaxis in our cell culture system. Adequate oxygenation was important for PDE-5 expression. Thus, sicklemic patients may express PDE-5 at abnormally low levels, predisposing them at risk of stuttering priapism.