We have demonstrated that adenosine has potent relaxant activity on the rabbit corpus cavernosum, acting through the A2a subtype receptor for adenosine. We now report studies on the identification and functional characterization of adenosine receptors in human penile vessels. To identify A2 receptors in human corpora cavernosa (HCC) we performed binding studies using the selective radioligand [125I]PAPA-APEC in membranes from HCC. We found the presence of a single class of high affinity (Kd= 0.23 +/- 0.06 nM), low capacity (Bmax=134 +/- 37 fmoles/mg protein) binding sites. Adenosine and CGS 21680 completely displaced [125I]PAPA-APEC binding (Kd= 146.7 +/- 64 microM and 51.52 +/- 27 nM, respectively). Accordingly, in functional studies adenosine relaxed phenylephrine precontracted HCC with an IC50=2.28 +/- 0.17 mM. The effect of adenosine was independent from nitric oxide (NO), and was counteracted by the A2 antagonist CGS 15943. In order to evaluate the in vivo effect of adenosine, increasing concentrations (6, 60, 600 microg) of adenosine or prostaglandin E1 (PGE1) (10 microg) were injected into the corpora cavernosa of four healthy volunteers. Blood flow and erectile response were evaluated at different times by duplex sonography and visual inspection, respectively. It was found that adenosine increased cavernosal peak blood flow velocity in a time- and dose-dependent manner. The highest concentrations of injected adenosine elicited a response that was not statistically different from that of PGE1 (10 microg). However, in contrast to PGE1, a full or partial erection was never obtained. To further investigate the lack of effect of adenosine on penile tumescence (despite the substantial increase in cavernosal blood flow), in vitro experiments were performed on human deep dorsal penile veins (DDPV) obtained from surgical ligation for impotence. Adenosine did not affect basal tone, but it induced almost complete relaxation in noradrenaline-precontracted vein strips with an IC50=1.6 +/- 0.22 mM. Conversely, PGE1 stimulated a sustained increase in basal tone. Therefore, the lack of effect of adenosine on penile tumescence could be due to a simultaneous relaxing activity on penile corpora cavernosa and veins. In conclusion, our study indicates that adenosine relaxes HCC as well as penile veins without affecting erection, at least at the concentrations we have used. Conversely, PGE1 relaxes corpora cavernosa as well as adenosine but strongly stimulates vein contraction, allowing penile tumescence.