Objectives: We hypothesized that advanced glycation end product (AGE) formation contributes to erectile dysfunction (ED) by quenching nitric oxide. Our first goal was to identify the specific AGE pentosidine in the diabetic human penis. Because AGE-mediated effects may involve inducible nitric oxide synthase (iNOS), we performed immunohistochemical and Western blot analysis of diabetic and nondiabetic human penile tissue for iNOS. Finally, because AGEs may act intracellularly to affect proteins, we set out to identify endothelial NOS (eNOS) in the human penis as an initial step in examining a possible intracellular interaction between eNOS and AGEs.
Methods: We performed high-performance liquid chromatographic analysis of diabetic human penile corpus cavernosum and serum for pentosidine and performed immunohistochemical, electron microscopic (EM), and Western blot analysis of the diabetic and nondiabetic penile corpus cavernosum and tunica for pyrraline, iNOS, and eNOS (and neural NOS [nNOS] for comparative purposes) via standard methods.
Results: We found a significant elevation of pentosidine in the penile tissue but not the serum of diabetic patients (average age 55.6 +/- 2.3 years) compared with that of nondiabetic patients (average age 61.8 +/- 3.6 years). Pentosidine was 117.06 +/- 9.19 pmol/mg collagen in the diabetic tunica versus 77.58 +/- 5.5 pmol/mg collagen in the nondiabetic tunica (P < 0.01) and 74.58 +/- 8.49 pmol/mg collagen in the diabetic corpus cavernosum versus 46.59 +/- 2.53 pmol/mg collagen in the nondiabetic corpus cavernosum (P < 0.01), suggesting a tissue-specific effect of the AGEs. We localized the site of deposition of the specific AGE pyrraline to the human penile tunica and the penile corpus cavernosum collagen. Immunohistochemical and EM analysis localized eNOS and iNOS to the cavernosal endothelium and smooth muscle. Western blot analysis in 6 patients revealed the following: iNOS, but no eNOS, in penile tissue from 1 insulin-dependent diabetic man; eNOS only in 1 man after radical prostatectomy; both eNOS and iNOS in 2 men with Peyronie's disease, as well as in 2 other men with impotence and hypertension. Finally, the specific iNOS inhibitor PNU-19451A significantly augmented relaxation of precontracted human cavernosal tissue, from 64.7% +/- 5.58 to 80.03% +/- 4.55 at 10 microM acetylcholine and 65.06% +/- 2.84 to 86.16% +/- 3.96 at 0.1 mM acetylcholine (n = 4, P < 0.002 and P < 0.02, respectively).
Conclusions: AGEs are elevated in diabetic human penile tissue, but not in serum, and are localized to the collagen of the penile tunica and corpus cavernosum. We identified eNOS and iNOS in the human penile cavernosal smooth muscle and endothelium. The augmentation of cavernosal relaxation with a specific iNOS inhibitor, combined with the identification of iNOS protein, but not eNOS, in a patient with severe diabetes and ED, allows for speculation of a pathophysiologic mechanism for AGE-mediated ED via upregulation of iNOS and downregulation of eNOS. These data provide further insight into the mechanisms of advanced glycation end product-mediated ED and provide a foundation for further study.