Prostaglandin E1
Interesting quote in letter about possible reasons men stop responding to sildenafil
http://www.jurology.com/article/S0022-5347(05)64885-9/abstract
"Whereas intracavernous prostaglandin E1 characteristically allows erection despite a man's minimal (or even absent) sexual arousal, sildenafil's efficacy is dependent on the maintenance of arousal as opposed to the maintenance of stimulaiton (the latter may well be present, but ineffective).