(2002) 14, Suppl 1, S17 S21 ß 2002 Nature Publishing Group All rights reserved 0955-9930/02 $25.00 www.nature.com/ijir and erectile dysfunction 1 * 1 Department of Urology, C.H.U. de Charleroi, Charleroi, Belgium Penile erection is a vascular phenomenon that results from smooth muscle relaxation, arterial dilation and venous restriction. The atherosclerosis of the penis that occurs with aging causes a decrease in penile oxygen tension. A reduction of smooth muscle cells has been demonstrated in relation with this change in oxygen tension. Changes in the ratio of penile collagen have also been observed and could explain the decrease in penile elasticity and compliance. Chronic ischemia is, therefore, associated with fibrosis but also with nitric oxide (NO)-cyclic guanosine monophosphate. The sensitivity of the a-adrenoceptors on the smooth muscle cells increases with aging. All those modifications can explain the prevalence of erectile dysfunction with aging. Low oxygen tension in prostanoid production may also play a role in the mechanism of ischemia-induced cavernosal fibrosis; however, intracavernous injections of prostaglandin E 1 do not seem to modify the intracavernous structures by reducing muscular atrophy. The effects of androgen on libido and sexual behavior are well established, but their role in the human erectile mechanism remains unclear. Several studies performed on animals have demonstrated impacts directly on both the physiological function and the trabecular structure of the corpora cavernosa in rats, dogs and rabbits. However, in humans, no study seems to demonstrate a role of testosterone on muscular atrophy or penile neurologic control. Testosterone treatment alters the human behavior but not penile physiologic processes. Further studies are necessary to explain the real role of testosterone not only on the peripheral mechanism of erection but also on the central control. (2002) 14, Suppl 1, S17 S21. DOI: 10.1038= sj=ijir=3900792 Keywords: erectile dysfunction; smooth muscle pathology; aging; androgen; testosterone Introduction *Correspondence:, Department of Urology, C.H.U. de Charleroi, Boulevard Zoë Drion 1, 6000 Charleroi, Belgium. E-mail: dr.wespes@skynet.be Penile erection is a complex neurovascular phenomenon. It involves the coordination of three hemodynamic events: increased arterial inflow, sinusoidal smooth muscle relaxation and decreased venous outflow. It also implies the interaction of the brain, nerves, neurotransmitters and smooth and striated muscles. An alteration in any of these components may affect the response of the erectile tissue and cause erectile dysfunction. 1 The Massachusetts Male Aging Study has recently provided a comprehensive epidemiological report on erectile dysfunction, demonstrating the determinant role of age on the physiopathological mechanism of erection. Men between the ages of 40 and 70 y were asked to categorize their erectile function as potent or either totally, moderately, or minimally impotent. 2 In all, 52% of the sample reported some erectile dysfunction. Erectile dysfunction is an age-dependent disorder: between 40 and 70 y of age the probability of complete impotence tripled from 5.1 to 15%, the probability of moderate impotence doubled from 17 to 34%, whereas the probability of minimal impotence remained constant at 17%. By the age of 70 y, only 32% portrayed themselves as being free of erectile dysfunction. It is estimated that in 1995 there were more than 152 million men worldwide who experienced erectile dysfunction; the projection for 2025 shows a prevalence of 322 million men with erectile dysfunction. 2 The cause of age-related dysfunction is poorly understood and is likely to be multifactorial in origin. 3 Can erectile dysfunction be explained by local structural modifications of the penis? Using computerized image analysis, the percentage of smooth muscle cells was measured in patients of different ages with normal erections. At younger than 40 y, the percentage was 46%, between 41 and 60 y it was 40%, and at older than 60 y it was 35%; this decrease in smooth muscle content may be responsible for the decline in erection in older men. 4
S18 Penile corporal veno-occlusive function is determined, in part, by the ability of elastic and easily expandable cavernosal tissues to provide adequate compressive and stretching forces on subtunical draining venules to cause venous outflow resistance. Aging and vascular risk factors, such as diabetes and hypercholesterolemia, interfere with the histology of penile erectile tissue leading to corporal venoocclusive dysfunction. 5,6 The essential factor that determines the ability to achieve normal penile corporal veno-occlusion is the percentage of corporal smooth muscle content, whereas the number of elastic fibers or endothelial cells does not seem to correlate with the importance of venous leakage. 7 An association has been observed between increased severity of penile erectile dysfunction and altered cavernosal smooth muscle content, and in patients with veno-occlusive dysfunction, values of erectile flow rates have been shown to correlate with percentage of cavernosal smooth muscle content. 5,7 Patients with a low percentage of smooth muscle content exhibited higher flows to maintain the erection. The flaccid penis is deformable, extensible and elastic in its longitudinal axis. The length changes of the flaccid penis provoked by a maximum manual stretching of the glands have been correlated with age and show a significant decrease. 8 Changes of elastic fibers or collagen types can provoke mechanical alterations of the penis, which reduce its elasticity and compliance. The collagen in the corpus cavernosum tissue is predominantly types I, III and IV. Type I collagen, which forms stiff bands of fibrils, has been shown to be less compliant than type III collagen, which is found predominantly in distensible elastic tissue and is essential for normal tensile strength. The endothelial cells are believed to be responsible for the secretion of type IV collagen, which forms the basement membrane of blood vessels. In the penis, there is an equal abundance of types I and IV collagen with concomitant diminution of type III. 9,10 Although one study reported a decreased ratio of type III collagen with senescence, 9 another study did not note a similar significant difference in the ratios with aging. 10 These alterations that are observed with aging in collagen configuration may also be related in another way to advanced glycosylation products. 11 The elastic fibers are reduced in the aging penis. 12 All these modifications have a role on penile hemodynamic changes. A change in arterial flow velocity in patients with a normal response to pharmacological injection was evaluated. A statistically significant decreasing tendency of peak systolic velocity with age was revealed. 13 The greatest decrease was observed between patients in the third and fourth decades of life. These data demonstrate that cavernous arterial flow during pharmacological erection decreases, and the response time of the cavernous artery or tissue to a vasoactive drug becomes longer with age. These hemodynamic alterations can explain the modifications observed during nocturnal erections in patients older than 60 y. 14 What is responsible for these alterations? Role of ischemic factors Postmortem studies have revealed that aging is associated with increasing degrees of atherosclerotic vascular alteration in the arterial bed of the penis. 15 The exact pathophysiological mechanism of ischemia-induced fibrosis of the corpus cavernosum is not clearly understood but it is likely to be caused by hypoxia-induced overexpression of transforming growth factor beta 1 (TGF-b1). 16 TGF-b1 is a pleotrophic cytokine that has been shown to increase collagen synthesis in corpus cavernosum smooth muscle cells in vitro. Under ischemia conditions, TGF-b1 induces its own messenger RNA (mrna), leading to a further increase in TGF-b1 synthesis that reinforces the development of severe fibrosis. 16 Measuring the differential mrna expression for various growth factors in young and aging rat penile tissues, it has been demonstrated that TGF-b1 is higher in older rats compared with young rats and seems to confirm the role of TGF-b1 and the atrophy of cavernous smooth muscle replaced by fibrosis. 17 mrna expression of nerve growth factor is reduced in older rat penile tissues. Therefore, age-related neuronal atrophy may be caused by the reduced synthesis or availability of target-derived neurotrophic factors. In man, a correlation between oxygen tension in the penis has been demonstrated with the percentage of smooth muscle fibers. 18 The number of muscular fibers is therefore dependent on good oxygenation of the penis. It seems that the histological alterations start distally in the very small penile arteries, which produces replacement of smooth muscle cells by fibrosis. 6 The patient develops a corporeal venoocclusive dysfunction, whereas the cavernous arteries do not demonstrate any pathological alteration at the Doppler examination. Later, the pathophysiological mechanism progress provokes severe arterial disease. At this stage, important fibrosis of the corpus cavernosum is observed. Another important role of cavernosal oxygen tension appears to be the regulation of prostanoid production in the corpus cavernosum. It has been shown that low oxygen tension decreases basal and acetylcholine-stimulated production of prostacyclin, thromboxane A 2, prostaglandin F 2a and prostaglandin E 2 by inhibiting the activity of prostaglandin H synthase. 19 Low
oxygen tension in prostanoid production may also play a role in the mechanism of ischemia-induced cavernosal fibrosis, because decreased levels of prostaglandin E 1 correlated with the increased expression of TGF-b1 mrna in human corpus cavernosum smooth muscle cells. 19 Oxygen tension appears also to be an important regulator of nitric oxide (NO) synthesis. Because NO is one of the major neurotransmitters in erection, low oxygen tension could explain the decrease in relaxation of smooth muscle fibers as it has been demonstrated in in vitro studies. 20 In rats, staining for vasoactive intestinal polypeptide showed no difference among the age groups, whereas a difference was observed for NO synthase (NOS)-containing nerve fibers. The number of NOS fibers was reduced by half in old rats. These findings emphasize the role played by NO rather than vasoactive intestinal polypeptide in erectile physiology, and a reduction of NOS nerve fibers may be the most important neurological factor of age-related changes. 21,22 Role of hormonal factors: testosterone Beside the role of oxygen on erectile function, the concept that androgens affect the male erectile response is still under investigation and incompletely defined. Possible regulatory roles for androgens in penile erection physiology have been mostly studied in animals, with focus given to the hemodynamic characteristics and NO neurotransmission involved in erectile responses. However, generalizations regarding androgen control in the erection physiology of humans remain uncertain. More importantly, the exact molecular mechanism of androgen action in erectile function has not been described. Using different animals, several investigators evaluated the consequences of androgen deprivation (castration) and androgen replacement (testosterone administration following castration) on a range of regulatory features concerning penile erection, intracavernosal hemodynamics in vivo and a 1 -adrenoceptor expression, neuronal NOS (nnos) expression and activity, phosphodiesterase 5 activity, and smooth muscle amount in isolated corpus cavernosal tissue. Androgen receptors have been identified in the cavernosal tissue of most mammalians. 23,24 In the rat, cavernosal androgen receptor activity varies widely with age, with maximal levels at puberty and markedly lower levels thereafter. This agerelated reduction in the number of androgen receptors is androgen mediated and is not reversible. The age-dependent decline in androgen receptor coincides with, and appears to be responsible for, the cessation of penile growth. 25 Many investigators believe that androgens act primarily to stimulate or maintain the activity of the enzyme NOS. Their conclusion is supported by several investigations. The intracavernosal pressure increase during erection in castrated rats is not enhanced by systemic administration of nitroglycerin whereas pressure is increased in testosterone replacement rats given the same dose of nitroglycerin. 26 NADPH diaphorase staining decreases by more than half in cavernosal nerves 10 days after castration but returns to near normal values following testosterone replacement. 27 Based on the conversion of arginine to citrulline to measure NOS activity, there is decreased NOS activity in castrate as compared to intact or testosterone replacement animals. 28 Measurements of NOS enzyme protein showed that in castrated animals there is less than half the quantity of NOS protein as in testosterone replacement rats. 29 In adult rats, the decrease in NOS activity that followed castration is restored with androgen replacement, whereas no additional NOS activity is observed when intact animals were given additional androgens. 30 In dogs, castration leads to a decline in the ratio of intracavernosal pressure to peak systolic pressure and there is also evidence of higher outflow rates in the castrate animals. 31 A decline in the magnitude of the induced intracavernosal pressure rise during erection is observed but the investigators attributed the lower intracavernosal pressure to be accompanying reduction in mean arterial pressure. 31,32 In rabbits, with strips of cavernosal tissue precontracted with a-adrenergic agonists, electrical field stimulation caused a greater degree of relaxation in strips from castrate than from intact animals. 33 The contraction in response to norepinephrine may also be hormonally dependent, with strips from intact rabbits showing a greater degree of contraction than tissues from castrates in response to the same concentration of the agonist. 34 Relaxation in response to field stimulation was greater in castrated than in intact animals, although relaxation in response to NO donor drugs was not different in the two groups, suggesting that basal NOS may be adequate to mediate the erectile response. An important difference between the rabbit and rat models is the report that nonadrenergic noncholinergic (NANC) fibers are reduced in castrated rats but are increased in castrated rabbits. 27 Androgen dependence was shown for nervestimulated intracavernosal pressures, a 1 -adrenoceptor content in corpus cavernosal tissue, and corpus cavernosal smooth muscle integrity. Androgen levels had no particular effect on nnos biochemical measurements in the corpus cavernosum. Androgen replacement produced an increased biochemical measurement of phosphodiesterase 5 in the corpus cavernosum. The sum of these findings indicates S19
S20 that androgen deprivation causes functional and=or structural changes in the corpus cavernosum unrelated to adrenergic or nnos-based mechanisms, at least in the rabbit, that affect penile erection. 35 In normal men, testosterone treatment does not increase the frequency of nocturnal penile tumescence (NPT) episodes but does increase the rigidity of their erections, suggesting that testosterone acts on the cavernosal arterioles to control blood flow into the sinuses. 36 The episodes of NPT and the quality of spontaneous erections are diminished in older men and hypogonadal men and treatment with gonadotropin or testosterone improves the quality of erection and NPT frequency. Testosterone replacement is important for sleep-associated erections but not for erection in response to fantasy or visual erotic stimulation. 37 Although androgens are known to be essential for libido, further investigations are still needed to resolve their role in the regulation of human penile erection. Can the intracavernous structures be improved or restored? Hypoxia in patients with impotence of vascular origin induces the expression of TGF-b1, which produces collagen synthesis in the corpus cavernosum and, therefore, fibrosis of the penis. 16 Prostaglandins seem to be able to suppress this collagen synthesis in primary cultures of human corpus cavernosum smooth muscle cells and may have the potential to prevent fibrotic lesions associated with vasculogenic impotence. Intracavernous prostaglandin E 1 (PGE 1 ) injection would result in oxygenation of the corpora cavernosa and perhaps remodeling of the penile connective tissue. However, no increase in the percentage of smooth cells quantified on biopsies performed during treatment has been observed in patients treated by intracavernous PGE 1 injection. 38 It is well known that what is observed in vitro cannot always be applied in vivo. Improvement in the erectile quality in patients treated with intracavernous PGE 1 injection would be due to psychogenic effects rather than modifications of the intracavernous structures. Gene therapy may be applied to human erectile tissues and several approaches have been proposed. 39,40 Conclusions Organic erectile dysfunction results from alterations of the intracavernous structures, mainly smooth muscle atrophy, and could be explained by atherosclerotic lesions and ischemic factors. The role of testosterone on the human mechanism of erection has to be established. The actual treatments at our disposal can improve erectile rigidity but do not cure erectile impotence. New, potentially useful therapeutic modalities need to be developed and the application of gene therapy might produce a new option for the treatment of erectile dysfunction. References 1 Lue TF, Tanagho EA. Physiology of erection and pharmacological management of impotence. J Urol 1987; 137: 829 833. 2 Feldman HA et al. Impotence and its medical and psychosocial correlates: results of the Massachussetts Male Aging Study. J Urol 1994; 151: 54 61. 3 Aytaç IA, McKinlay JB, Krane RJ. The likely worldwide increase in erectile dysfunction between 1995 and 2025 and some possible policy consequences. Br J Urol Int 1999; 84: 50 56. 4 Wespes E, Moreira de Goes P, Schulman CC. Age-related changes in the qualification of the intracavernous smooth muscles in potent men. J Urol 1998; 159: 379 (abstract). 5 Nehra A et al. Mechanisms of venous leakage: a prospective clinicopathological correlation of corporeal function and structure. J Urol 1996; 156: 1320 1329. 6 Wespes E et al. Corporeal veno-occlusive dysfunction: a distal arterial pathology? J Urol 1998; 160: 2054 2057. 7 Wespes E et al. Corporeal veno-occlusive dysfunction predominantly intracavernous muscular pathology. J Urol 1997; 157: 1678 1680. 8 Bondil P et al. Clinical study of the longitudinal deformation of the flaccid penis and of its variations with aging. Eur Urol 1992; 21: 284 286. 9 Padma-Nathan H et al. The effects of aging, diabetes, and vascular ischemia on the biochemical composition of collagen found in the corpora and tunica of potent and impotent men. Int J Impot Res 1990; 2: 75 76. 10 Luangkhot R et al. Collagen alterations in the corpus cavernosum of men with sexual dysfunction. J Urol 1992; 148: 467 471. 11 Jiaan DB et al. Age-related increase in an advanced glycation end product in penile tissue. World J Urol 1995; 13: 369 375. 12 Akkus E et al. Structural alterations in the tunica albuginea of the penis: impact of Peyronie s disease, ageing and impotence. Br J Urol Int 1997; 79: 47 53. 13 Chung WS, Park YY, Kwon SW. The impact of aging on penile hemodynamics in normal responders to pharmacological injection: a Doppler sonographic study. J Urol 1997; 157: 2129 2131. 14 Zimmern P, Leach GE, Yao J, Wolde-Tsadik G. Critères de normalité de Rigiscan 1 chez l homme âgé. Prog Urol 1999; 9: 37 44. 15 Bossart MI, Spjut HJ, Scott FB. Ultrastructural analysis of human penile corpus cavernosum. Urology 1980; 15: 448 456. 16 Moreland RB. Is there a role of hypoxemia in penile fibrosis: a viewpoint presented to the Society for the Study of Impotence. Int J Impot Res 1998; 10: 113 120. 17 Dahiya R et al. Differential gene expression of growth factors in young and old rat penile tissues is associated with erectile dysfunction. Int J Impot Res 1999; 11: 201 206. 18 Sattar AA et al. Cavernous oxygen tension and smooth muscle fibers: relation and function. J Urol 1995; 154: 1736 1739. 19 Moreland RB et al. PGE 1 suppresses the induction of collagen synthesis by transforming growth factor-b 1 in human corpus cavernosum smooth muscle. J Urol 1995; 153: 826 834. 20 Kim N et al. Oxygen tension regulates the nitric oxide pathway: physiological role in penile erection. J Clin Invest 1993; 91: 437 442.
21 Garban H et al. Effect of aging on nitric oxide-mediated penile erection in rats. Am J Physiol 1995; 268: 467 475. 22 Carrier S et al. Age decreases nitric oxide synthase-containing nerve fibers in the rat penis. J Urol 1997; 157: 1088 1092. 23 Horwitz KB, Horwitz LD. Canne vascular tissues are targets for androgens, estrogens, progestins, and glucocorticoids. J Clin Invest 1982; 69: 750 758. 24 Takane KK, Wilson JD, McPhaul MJ. Decreased levels of the androgen receptor in the mature rat phallus are associated with decreased levels of androgen receptor messenger ribonucleic acid. Endocrinology 1991; 129: 1093 1100. 25 Takane KK, George FW, Wilson JD. Androgen receptor of rat penis is down-regulated by androgen. Am J Physiol 1990; 258: E46 E50. 26 Mills TM, Wiedmeier VT, Stopper VS. Androgen maintenance of erectile function in the rat penis. Biol Reprod 1992; 46: 342 348. 27 Zvara P et al. Nitric oxide mediated erectile activity is a testosterone dependent event: a rat erection model. Int J Impot Res 1995; 7: 209 219. 28 Lugg J, Rajfer J, Gonzalez-Cadavid NF. Dihydrotestosterone is the active androgen in the maintenance of nitric oxidemediated penile erection in the rat. Endocrinology 1995; 136: 1495 1501. 29 Chammes SL et al. The effect of androgen on nitric oxide synthase in the male reproductive tract of the rat. Fertil Steril 1995; 63: 1101 1107. 30 Garban H et al. Restoration of normal adult penile erectile response in aged rats by long-term treatment with androgens. Biol Reprod 1995; 53: 1365 1372. 31 Muller SC, Hsieh JT, Lue TF, Tanagho EA. Castration and erection: an animal study. Eur Urol 1988; 15: 118 124. 32 Lin SN et al. Castration may not affect the penile erection ability in terms of peripheral neurocavernous mechanism in dogs. J Urol 1990; 143: 172 174. 33 Holmquist F, Persson K, Bodker A, Anderson KE. Some preand post-junctional effects of castration in rabbit isolated corpus cavernosum and urethra. J Urol 1994; 152: 1011 1016. 34 Baba K. Effects of testosterone on smooth muscle in the isolated rabbit corpus cavernosum penis. Jpn J Urol 1993; 84: 1783 1790. 35 Traish AM et al. Effects of castration and androgen replacement on erectile function in a rabbit model. Endocrinology 1999; 140: 1861 1868. 36 Carani C, Scuteri A, Marrama P, Bancroft J. The effects of testosterone administration and visual erotic stimuli on nocturnal penile tumescence in normal men. Horm Behav 1990; 24: 435 441. 37 Zini D et al. Sexual behavior of men with isolated hypogonadotropic hypogonadism or prepubertal anterior panhypopituitarism. Horm Behav 1990; 24: 174 185. 38 Wespes E, Sattar AA, Noël JC, Schulman CC. Does prostaglandin E1 therapy modify the intracavernous musculature? J Urol 2000; 163: 464 466. 39 Champion HC et al. Gene transfer of endothelial nitric oxide synthase to the penis augments erectile responses in the aged rat. Proc Natl Acad Sci USA 1999; 96: 11648 11652. 40 Wessells H, Williams SK. Endothelial cell transplantation into the corpus cavernosum: moving towards cell-based gene therapy. J Urol 1999; 162: 2162 2164. S21 Appendix Open discussion following Dr Wespes presentation Dr Porst: In your relatively small study you found that androgen depletion did not have an impact on the concentration of smooth muscle cells. On the other hand, there are publications that have proven that with castration we see an apoptosis of smooth muscle cells. Thus, these findings are contradictory to your finding. Dr Nehra: And that castration was done in very young rats. Dr Wespes: There are several studies concerning testosterone. We have very few receptors of testosterone inside the human penis. When you do all the in vitro studies and when you inject testosterone, there is no modification of the answer. So it seems that in the in vitro study testosterone does not influence the answer. I ve shown, of course, in a small study of patients with hypogonadism, a resultant decrease of percentage of smooth muscle cells. Dr Porst: But these patients who have withdrawal of androgens due to prostate cancer have a considerable shrinkage of the penis within 2 y. You can measure it. So the shrinkage is due to a decrease of the smooth muscle cells. Otherwise, you can objectively measure the shrinkage of the penis when you make a depletion of androgens in prostate cancer. Dr Hatzichristou: When you say that testosterone seems not to modify penile erection in man, you are talking about its action on the corpora cavernosa. You re not talking about erection. When we say erection, we do know that we need testosterone for the whole erectile process. The question I have is the following: Did you notice the decrease in the amount of smooth muscle at the site of the corpora used for self-injections? Dr Wespes: In the corpora, we expect to have less and less smooth muscle over time. Dr Hatzichristou: How can you explain that we have patients who have been in injection therapy for more than 10 y who have not needed their dosages of the vasoactive drugs increased? Dr Wespes: Most of the time the patient does the injection at the same level on the same site, even if you ask him to change. So, in fact, you have only a local restriction of the smooth muscle.