In the penis, nitric oxide (NO) can be formed by both neuronal NO synthase and endothelial NOS (eNOS). eNOS is activated by viscous drag/shear stress in blood vessels to produce NO continuously, a process mediated by the phosphatidylinositol 3-kinase (PI3kinase)/Akt pathway. Here we show that PI3-kinase/Akt physiologically mediates erection. Both electrical stimulation of the cavernous nerve and direct intracavernosal injection of the vasorelaxant drug papaverine cause rapid increases in phosphorylated (activated) Akt and eNOS. Phosphorylation is diminished by wortmannin and LY294002, inhibitors of PI3-kinase, the upstream activator of Akt. The two drugs also reduce erection. Penile erection elicited by papaverine is reduced profoundly in mice with targeted deletion of eNOS. Our findings support a model in which rapid, brief activation of neuronal NOS initiates the erectile process, whereas PI3-kinase/Akt-dependent phosphorylation and activation of eNOS leads to sustained NO production and maximal erection.

The molecular mechanism for priapism is not well characterized. Although the nitric oxide (NO) pathway is known to mediate penile erection under normal conditions, we hypothesized that the mechanism of priapism rests in aberrant downstream signaling of this pathway based on our previous findings that mice lacking the gene for endothelial nitric oxide synthase (eNOS^—/—) and mice lacking both neuronal NOS (nNOS) and eNOS (nNOS^—/—, eNOS^—/—) have a tendency for priapic activity. We investigated the role of downstream guanylate cyclase and phosphodiesterase type 5 (PDE5A) expression and function in mediating these responses in eNOS^—/—and nNOS^—/—, eNOS^—/—mice. Erectile responses to both cavernous nerve stimulation and intracavernosal injection of the NO donor diethylamine-NONOate were augmented in eNOS^—/—and nNOS^—/—, eNOS^—/—mice but not in WT or nNOS^—/—mice. PDE5A protein expression and activity and cGMP levels were significantly lower in eNOS^—/—and nNOS^—/—, eNOS^—/—mice, and this effect was reproduced in WT corpus cavernosum exposed to NOS inhibitors. Moreover, cavernous nerve stimulation was associated with a marked augmentation of cavernosal cGMP levels, suggesting that, although lower at baseline, the production of cGMP is unchecked in eNOS^—/—and nNOS^—/—, eNOS^—/—mice upon neurostimulation. Transfection of eNOS^—/—mice with an adenovirus encoding eNOS resulted in a normalization of PDE5A protein and activity as well as a correction of priapic activity. Coupled with the observation that sickle cell disease mice (which show a priapism phenotype) evince dysregulated PDE5A expression/activity, these data suggest that PDE5A dysregulation is a fundamental mechanism for priapism.

In conjunction with the assignment to update the Guidelines for Management of Clinically Localized Prostate Cancer, the American Urological Association Prostate Cancer Guideline Update Panel performed a side analysis of the reporting of erectile function outcomes in this clinical context as published in the medical literature. Four National Library of Medicine PubMed(R) Services literature searches targeting articles published from 1991 through early 2004 were done to derive outcome reporting (efficacy or side effects) for the treatment of clinical stage T1 or T2 N0M0 prostate cancer. A database was constructed containing descriptions relating to erectile function as well as numerical frequency rates of complete erectile dysfunction, and partial and intact erectile function for various treatments. A literature review was also done, consisting of a PubMed Services search of current measures and protocols used for assessing erectile function outcomes and a survey of consensus opinion sources on the management of male sexual dysfunctions. Based on inclusion criteria 436 articles were selected. Of these articles database extraction from 100 pertaining to radical prostatectomy garnered various characterizations of erectile function, including qualitative descriptions, generic terminology and rating systems. Database extraction from 31 articles, in which results for at least 50 patients were reported, yielded ranges of rates for complete erectile dysfunction, partial erectile function and intact erectile function that were 26% to 100%, 16% to 48% and 9% to 86% for radical prostatectomy, 8% to 85%, 21% to 47% and 36% to 63% for external beam radiation, and 14% to 61%, 21% and 18% for interstitial radiation, respectively. The literature review showed an evolution in standards for studying and reporting erectile function outcomes. Clinical studies reporting erectile function outcomes after localized prostate cancer treatment often demonstrate poorly interpretable and inconsistent manners of assessment as well as widely disparate rates of erectile dysfunction and erectile function. Future studies must apply scientifically rigorous methodology and standard outcomes measures to advance this field of study.