Plasma levels of endothelin-1, angiotensin II, nitric oxide and prostaglandin E2 in the venous and cavernosal blood of patients with erectile dysfunction

Authors


Nagla Taha El Melegy, Medical Biochemistry, Faculty of Medicine, Assiut, Egypt.
e-mail: elmelegynagla@yahoo.com

Abstract

OBJECTIVES

To determine the alterations in the plasma levels of endothelin-1, angiotensin II, nitric oxide (NO) and prostaglandin E2 (PGE2) in the venous and cavernosal blood of patients with organic and psychogenic erectile dysfunction (ED).

PATIENTS, SUBJECTS AND METHODS

The study included 32 patients complaining of ED; they were subdivided into two equal groups with either organic or psychogenic ED. Fifteen healthy potent age-matched male volunteers were enrolled as a control group. For each patient, venous and cavernosal blood samples were obtained, while venous blood was obtained from the controls.

RESULTS

There were significantly greater mean plasma levels of endothelin-1 and angiotensin II, and significantly lower mean plasma levels of NO and PGE2, in the venous blood of patients with ED than in the controls. Patients with organic ED had significantly higher levels of endothelin-1 and significantly lower levels of NO in both venous and cavernosal blood than had those with psychogenic ED. There were significant positive correlations in both venous and cavernosal blood between endothelin-1 and angiotensin II, and between NO and PGE2 in all patients with ED and the two subgroups. There were significant negative correlations between venous and cavernosal endothelin-1 and NO, endothelin-1 and PGE2, angiotensin II and NO, and between angiotensin II and PGE2.

CONCLUSION

The present results suggest that endothelin-1 could be a clinical marker of diffuse endothelial disease manifested by ED. As angiotensin-converting enzyme (ACE) activity controls angiotensin II there might be a rationale for the use of ACE inhibitors to prevent or treat ED. NO and PGE2 may provide new strategies for the pharmacological treatment of ED.

Abbreviations
ED

erectile dysfunction

PG

prostaglandin

NO

nitric oxide

(i)(e)(n)NOS

(inducible), (endothelial) (neuronal) NO synthase

PPDU

pharmacological penile duplex ultrasonography

ACE

angiotensin-converting enzyme

NANC

nonadrenergic, noncholinergic

PDE

phosphodiesterase.

INTRODUCTION

Erectile dysfunction (ED) is defined as the persistent inability to attain and/or maintain a penile erection sufficient for sexual intercourse. The National Institutes of Health Consensus Development Conference [1] recommended the use of the term ‘ED’ rather than ‘impotence’ because it more accurately defines the problem and has fewer disparaging connotations. It affects millions of men to various degrees; most cases have an organic cause, most commonly vascular disease that decreases the blood flow into the penis [2,3]. The commonest organic causes of ED are diabetes and atherosclerosis, which can damage the arteries, veins, nerves and smooth muscle of the penis [4]. The process of penile erection is largely a haemodynamic event involving relaxation of the smooth muscles of the corpora cavernosa and arterioles. The resultant increase in blood flow into the trabecular spaces of the corpora cavernosa leads to erection [5]. Normal erectile physiology is heavily dependent on a delicate balance between the effects of endogenous vasoconstricting and vasorelaxing neurotransmitters on the tone of the corporal smooth muscle [6]. Several investigators have suggested that endothelins may contribute to the maintenance of corporal smooth muscle tone [7]. Endothelin-1 is a 31-amino-acid peptide with potent and prolonged vasoconstrictor activity. It exerts its biological effect through the activation of specific endothelin receptors (endothelinA and -B). Endothelin receptors have been found in human corporal smooth muscle membranes [8]. ED was suggested to be the result of impaired endothelium-dependent smooth muscle relaxation due to increased release of endothelin-1 [9].

In the mammalian body the most important physiological effect of angiotensin II, which is secreted by the paracrine vascular cells, is the induction of vascular smooth contractions [10]. Angiotensin II is the main active metabolite of the renin-angiotensin cascade. Renin, which is secreted by the juxtaglomerular cells of the kidney or in various tissues of the human male and female reproductive tract, catalyses the cleavage of the decapeptide angiotensin I from the precursor angiotensinogen produced in the liver. Angiotensin I is converted to the bioactive octapeptide angiotensin II by angiotensin-converting enzyme (ACE) [11]. The human corpus cavernosum produces and secretes physiologically relevant amounts of angiotensin II and angiotensin II receptors have been characterized in the mammalian cavernosal compartment [12]. The intracavernosal injection of angiotensin II has been reported to terminate penile erection in anaesthetized dogs and the application of an angiotensin II-receptor antagonist resulted in penile erection [10].

The relaxation of the corpus cavernosal smooth muscle is mediated by nonadrenergic, noncholinergic (NANC) neurotransmission. The responsible NANC neurotransmitter has been shown to be nitric oxide (NO); a labile substance synthesized from l-arginine by the enzyme NO synthase (NOS) [13]. Three distinct forms of NOS have been identified and designated as inducible (iNOS), endothelial (eNOS) and neuronal (nNOS) [14]. Both nNOS and eNOS are thought to be important sources of NO in the corpus cavernosum, as relaxation and erectile responses were reduced after removing or destroying the endothelium [15]. Indeed, animal models in which there is a deficiency in NOS have been associated with ED [16].

Human corpus cavernosum tissue can synthesise various prostanoids and metabolize them locally. The production of prostanoids can be modulated by oxygen tension and suppressed by hypoxia. Prostaglandins (PGs) have a moderate effect on the tone of penile muscles and may have a functional role in the mechanism of penile erection [17]. Prostaglandins are locally acting hormones derived from arachidonic acid by the action of cyclooxygenases [7]. There are five primary active prostanoid metabolites: PGD2, PGE2, PGF2-α, PGI2 and TXA2. PGE2 is involved in the relaxation of erectile tissues through stimulation of PG receptors and initiating an increase in the intracellular concentration of cAMP [18]. PGE2 has vasodilatory effects similar to PGE1, which has been studied extensively as an intracavernosal injection therapy [19]. Intraurethral PGE2 cream was used by Wolfson et al.[20] as a possible alternative treatment for ED. Regardless of the primary cause, ED can compromise self-esteem, quality of life and interpersonal relationships. Thus in the present study we determines the plasma levels of endothelin-1, angiotensin II (as vasoconstrictors), NO and PGE2 (as smooth muscle relaxants) in the venous and cavernosal blood of patients with organic and psychogenic ED, and compared the levels with those in healthy men, to investigate the cause of ED and to provide new strategies for treatment.

PATIENTS, SUBJECTS AND METHODS

The present study included 32 patients (mean age 43.7 years, sem 2.07) complaining of ED for >1 year among those presenting to the andrology outpatient clinic. In addition, 15 healthy age-matched male volunteers with no ED were enrolled as a control group (mean age 44.4 years, sem 2.9). Informed written consent to participate in the study was obtained from each patient and subject. The local ethics committee approved the study.

All patients had: a complete detailed and careful history taken, with special attention to the sexual history, including details to differentiate between psychogenic and organic ED; a complete physical examination, including genital and neurological examination; blood glucose assay, urine analysis, complete blood assessment, and kidney and liver function; hormonal assays of testosterone, prolactin and thyroxin; combined intracavernosal injection and stimulation with a standard dose of 1 mL papaverine HCl (30 mg); and in those not responding to combined intracavernosal injection, pharmacological penile duplex ultrasonography (PPDU) of the corpora cavernosa, with pharmaco-cavernosometry and -cavernosography.

Patients excluded were smokers or those with any special habits (alcohol and drug users), and those with hormonal disturbances, liver or renal diseases. The patients were classified by their history and investigations into two groups according to the main underlying cause of ED. The first group comprised men with organic ED (16, mean age 48.1 years, sem 2.77) characterized by gradual onset, progressive course, prolonged duration of ED and absence of a good rigid erection. They had an abnormal response to intracavernosal injection, and on investigation three patients were diagnosed as having arteriogenic ED, where the peak systolic velocity by PPDU was <20 cm/s after 30 min of intracavernosal injection, seven as having venogenic ED, with an end-diastolic velocity of >5 cm/s at 30 min after intracavernosal injection assessed by PPDU, and confirmed by cavernosometry and cavernosography, and six with mixed arteriogenic and venogenic ED. Of these 16 patients, six had diabetes, three had hypertension, two had both, one had ischaemic heart disease and one had Peyronie's disease; the remaining three men had no systemic diseases or local lesions (and had only changes in PPDU which indicated venous leakage, confirmed by cavernosometry and cavernosography).

Group 2 comprised 16 men with psychogenic ED (mean age 43 years, sem 3.03), characterized by sudden onset, intermittent course, short duration and abnormal findings in the psychosexual evaluation, but with no signs of neurological disease. They had a normal response to intracavernosal injection with no pathological findings on PPDU.

Patients were readmitted to the outpatient clinic 2 weeks after the first evaluation and 10 mL of blood was obtained from an antecubital vein after ≥ 10 min of supine rest; 10 mL of venous blood was also obtained from the controls. Cavernosal blood samples were obtained from the cavernosal body during an erection induced by an intracavernosal injection with 30 mg of papaverine HCl [21] (as disappointing results were reported by Francavilla et al.[9] when trying to obtain cavernosal blood samples from the flaccid penis of patients with ED). At 10 min after intracavernosal injection, 10 mL of blood was collected from the left cavernosal body with an insulin-syringe needle. Because the procedure was invasive, no cavernosal blood samples were taken from the control group. All blood samples were collected into EDTA-containing tubes treated with aprotinin (500 kallikrein-inhibiting units/mL blood) and immediately placed on ice. Blood was separated by centrifugation for 10 min at 3000 rpm at 4°C. The plasma was separated, collected into aliquots and stored at − 70°C until assayed for endothelin-1, angiotensin II, NO and PGE2.

Plasma endothelin-1 levels were determined using an enzymatic immunoassay kit (cat. no. 6901, Peninsula Labs, CA, USA) according to the method described by Aubin et al.[22]. Plasma levels of angiotensin II were measured with an enzyme immunoassay kit (cat no. 94071; Peninsula, San Carlos, Canada) according to the method described by Porstmann and Kiessig [23]. Levels of plasma NO were determined spectrophotometrically as nitrite and nitrate concentrations by the method of Van Bezooijen et al.[24]. Plasma PGE2 levels were measured by an enzyme immunoassay kit (cat no. 900–001; Assay Designs Inc. Ann Arbor, USA) according to the method of Tijssen [25].

The results were evaluated statistically as the mean (sem), with the difference between groups assessed using a paired and unpaired Student's t-test, and considered significant at P < 0.05. Correlations between the studied variables were evaluated using Spearman's rank correlation coefficient.

RESULTS

The plasma levels of endothelin-1, angiotensin II, NO and PGE2 in controls and patients with ED are shown in Table 1; there were significantly greater venous levels of endothelin-1 and angiotensin II (P < 0.001 and P < 0.01, respectively) in patients than in controls, and levels of NO and PGE2 were significantly lower in patients than controls (P < 0.01 and <0.001, respectively). There were significantly higher levels of angiotensin II and PGE2 in the cavernosal than venous blood samples in the patients (P < 0.001). Table 1 also shows the plasma levels of endothelin-1 and angiotensin II in controls and in patients with organic and psychogenic ED. Venous endothelin-1 levels were significantly greater in men with organic and psychogenic ED (both P < 0.001) than in controls, but patients with organic ED had significantly greater levels of venous and cavernosal endothelin-1 than men with psychogenic ED (both P < 0.01).

Table 1. 
Plasma levels of endothelin-1, angiotensin II, NO and PGE2 in controls and patients with ED, and for organic and psychogenic ED
Mean (sem, range) analyteControlsPatients with ED
Allorganicpsychogenic
  • *

    vs controls;

  • †between venous and cavernosal levels of patients;

  • ‡between organic and pyschogenic

N 15321616
Endothelin-1, pg/mL
Venous  1.25 (0.10, 0.7–2.0)11.20 (0.69, 5–20)13.09 (1.04, 7.5–20)9.30 (0.62, 5–13)
<0.001*<0.001*<0.001*, <0.01
Cavernosal11.63 (0.70, 5.5–22)13.39 (1.04, 7.5–22)9.86 (0.74, 5.5–14.5)
P >0.05>0.05<0.01, >0.05
Angiotensin II, pg/mL
Venous 18.13 (1.25, 12–28)22.84 (0.97, 13–34)24.19 (1.39, 15–34)21.5 (1.33, 13–30)
<0.01*<0.01*>0.05*, >0.05
Cavernosal28.63 (1.18, 16–40)30.56 (1.53, 18–40)26.69 (1.71, 16–38)
P <0.001<0.001<0.001, >0.05
NO, nmol/mL
Venous 20.43 (1.03, 15–28)15.70 (1.07, 6–27)13.47 (1.44, 6–25)17.94 (1.40, 10–27)
<0.01*<0.001*>0.05*, <0.05
Cavernosal15.87 (1.07, 7–28)13.56 (1.38, 7–27)18.19 (1.47, 10–28)
P >0.05>0.05>0.05, <0.05
PGE2, pg/mL
Venous161.0 (11.97, 80–225)89.06 (4.53, 40–150)87.5 (5.55, 50–100)90.63 (7.32, 40–150)
<0.001*<0.001*<0.001*, >0.05
Cavernosal105.16 (5.00, 50–160)103.13 (6.10, 60–160)107.19 (8.09, 50–160)
P <0.001<0.001<0.001, >0.05

Venous angiotensin II levels were significantly greater in patients with organic ED than in controls (P < 0.01), but the levels were not significantly different from those in men with psychogenic ED. There was no significant difference between angiotensin II levels in the two groups with ED, but cavernosal angiotensin II levels were higher than in venous blood in men with organic and psychogenic ED (P < 0.001).

Table 1 also shows the plasma levels of NO and PGE2 in controls and in patients with organic and psychogenic ED. There were significantly lower venous NO levels in patients with organic ED (P < 0.001) than in controls, and lower levels of NO in men with organic ED than in those with psychogenic ED (P < 0.05). Cavernosal NO levels were significantly lower in men with organic ED than in those with psychogenic ED (P < 0.05).

Venous PGE2 levels were significantly lower in both groups with ED than in the controls (both P < 0.001), with no significant difference between the levels of venous and cavernosal PGE2 in both groups with ED. However, there were significantly higher levels of plasma PGE2 in cavernosal blood in both groups with ED than in venous blood (both P < 0.001).

The correlations between the analytes in all patients with ED are shown in Table 2. There were significant positive correlations between endothelin-1 and angiotensin II, and between NO and PGE2 in both venous and cavernosal blood for all patients, and for those with organic and psychogenic ED, and significant negative correlations between endothelin-1 and NO, endothelin-1 and PGE2, angiotensin II and NO, and between angiotensin II and PGE2 in both venous and cavernosal blood in all patients, and those with psychogenic ED. Patients with organic ED had the same significant negative correlations, except that between endothelin-1 and PGE2, which was not significant. Figure 1(a–d) shows the individual plasma levels of the studied analytes in venous and cavernosal blood of the patients and controls.

Table 2. 
Correlation coefficient between the studied analytes among each group
CorrelationEndothelin-1 andAngiotensin II andNO and
angiotensin IINOPGE2NOPEG2PGE2
  1. Correlation coefficient, *P < 0.05, †< 0.01, ‡< 0.001.

Organic ED (n = 16)
 Venous0.7665−0.5038*−0.3775−0.6275−0.65420.5608*
 Cavernosal0.7069−0.4878*−0.3569−0.5723−0.63030.5072*
Psychogenic ED (n = 16)
 Venous0.8262−0.8445−0.6757−0.7754−0.62990.8480
 Cavernosal0.6498−0.7284−0.5882−0.7015−0.60670.7062
All patients with ED (n = 32)
 Venous0.7694−0.6794−0.4403−0.7206−0.62720.6836
 Cavernosal0.6993−0.6510−0.4304−0.6802−0.60690.5989
Figure 1.


The individual plasma levels of the studied bioindices in venous and cavernosal blood of the patients with ED and the controls.

DISCUSSION

Penile erection requires a change in the tone of the corpus cavernosum and penile vascular smooth muscle cells, from a state of contraction to relaxation. The required degree of relaxation can be achieved either by eliminating the contraction influences or by enhancing the effects of relaxant factors; probably both factors are required [7].

The role of endothelium in erectile function has attracted considerable interest since the discovery that the vasodilator response to acetylcholine requires the presence of an intact endothelium [26]. Endothelial cells lining sinusoids of the cavernosal tissue affect the tone of adjacent smooth muscle cells through the release of vasoconstrictive agents (e.g. endothelin-1 and angiotensin-II) and relaxing factors (e.g. NO and PGE2) [10,27–29]. Even a subtle alteration in the balance between the factors contributing to corporal relaxation and favouring contraction may result in ED [30].

In the present study, the mean venous endothelin-1 levels were significantly greater in patients with ED than in controls, with significantly higher venous and cavernosal levels in patients with organic than psychogenic ED, and no significant difference between venous and cavernosal levels (Table 1). These results are in agreement with those of previous reports [9,31,32]. About half of the present patients with organic ED were diabetic. The higher endothelin-1 levels in venous and cavernosal blood of diabetic men with ED suggests that impaired endothelium-dependent smooth muscle relaxation in corpus cavernosum [31] and in the forearm vessel resistance of diabetic men [33] could be due to the same endothelial dysfunction and to increased endothelin-1 release [9]. Elevated plasma levels of endothelin-1 were reported in patients with atherosclerosis and might be related to diffuse endothelial cell dysfunction [34]. Also, plasma and tissue endothelin-1 immunoreactivity was enhanced in early atherosclerosis induced by hypercholesterolaemia in pigs and coexisted with abnormal endothelium-dependent vasodilatation [35]. Properzi et al.[36] reported greater immunoreactivity for endothelin-1 in cutaneous vessels in men with diabetes for <10 years than in controls. All of these findings may reflect the local overproduction of endothelin-1 from damaged endothelial cells, with plasma leakage [9].

Although as altered neuro-dependent relaxation of corpus cavernosum cannot be excluded in nondiabetic patients, the finding of the increased endothelin-1 levels in venous and cavernosal blood of nondiabetic men with ED, and that endothelin-1 levels in cavernosal blood were similar to those in venous blood, indicates endothelial dysfunction in nondiabetic men with ED [9]. Increased endothelin-1 release from the altered endothelial cells in cavernosal tissue might contribute to ED through a direct contracting effect on smooth muscle cells [28]. The endothelin-1 released from corporal endothelial or smooth muscle cells might act as a paracrine/autocrine factor which could alter tissue contractility by two distinct mechanisms: (i) endothelin-1 could achieve the effective local concentrations required for activation of the putative endothelinA receptor subtype and thus elicit a direct vasoconstrictor effect on the smooth muscle cells; or (ii), at lower concentrations, with correspondingly lower receptor occupancy, endothelin-1 could also augment the actions of other vasoactive agents or neurotransmitters [6]. Also, because of its slow dissociation from specific binding sites (resulting in prolonged contractile responses) and its pleiotropic actions, it is possible that endothelin-1 could be involved in the long-term regulation of corporal tone (maintenance of flaccidity). Thus organic ED may be the product of the altered regulation of endothelin-1-induced vasoconstriction, resulting in heightened corporal vascular tone [6]. The results of the present study suggest that the endothelin-1 level in peripheral venous blood is a clinical marker of diffuse endothelial disease manifested by ED [9].

The renin-angiotensin system might be closely related to the regulation of male reproductive function [37]. It was speculated that disturbances in systemic or local secretion or degradation of angiotensin II may contribute to the manifestation of ED. In the present study venous angiotensin II levels were greater in both the organic and psychogenic groups, but were significantly higher only in the patients with organic ED. There were significantly higher levels of cavernosal angiotensin II in both groups of patients than venous levels. These findings are in agreement with those of others [10,11,37,38]. These results provide evidence for the functional importance of angiotensin II in terminating penile erection [11]. Kifore et al.[10] showed that intracavernosal injection with angiotensin II causes contraction of the corpus cavernosum and terminates spontaneous erections in the dog, and applying an angiotensin II receptor antagonist results in penile erection. Becker et al.[37] reported that in healthy men and in patients with ED, the mean angiotensin II levels in cavernosal blood during all penile conditions were higher than those detected in blood samples taken from the cubital vein. This clearly supports the conclusion of Kifore et al.[10] that the human corpus cavernosum is an angiotensin II-producing paracrine system. Tissue levels of angiotensin II and the different modes of action of this peptide on cavernosal smooth muscle are regulated by a ACE, which converts angiotensin I to the active angiotensin II [39]. Diseases of the cardiovascular system, e.g. arterial hypertension, are commonly associated with ED [40]. This might be due to a deletion polymorphism in the gene encoding ACE, the so-called DD genotype, which is considered a marker of general vascular disease and was reported to be more common in men with a diagnosis of organic ED [12]. The administration of antihypertensive drugs is associated with impotence, decreased libido, impaired ejaculation and gynaecomastia in 15–20% of men [41,42]. The inhibition of ACE activity results in a reduction of angiotensin II release. Angiotensin II receptor antagonists such as losartan do not affect erectile and ejaculatory function in rats [43]. These findings are in accordance with the observation that the therapy of arterial hypertension with ACE inhibitors and angiotensin II receptor antagonists has no adverse effects on the general well-being and sexual function of men [41]. Thus patients presenting with ED and concomitant vascular diseases may benefit from taking ACE inhibitors.

Much evidence supports the concept that NO derives from the autonomic innervation of the penis and locally operates as a post-ganglionic neurotransmitter of NANC-mediated penile erection [44]. The premise that NO could modulate corporeal smooth muscle function arises from its original description as an endothelium-derived relaxing factor. This substance was discovered in vasculature as deriving from endothelial cells, and it induced vascular smooth muscle relaxation [27].

There were significantly lower mean plasma levels of NO in venous blood from patients with ED and organic ED than in controls, but the difference in men with psychogenic ED was not significant (Table 1). There were also significantly lower levels of NO in both venous and cavernosal blood from men with organic than in those with psychogenic ED, but no significant difference between venous and cavernosal levels of NO in both groups. The results of the present study are in agreement with those of others [31,32,45,46].

NO is important for erectile function [7,44]; the constitutively active synthesizing enzyme, NOS has been identified and three types designated (nNOS, eNOS and iNOS). Both nNOS and eNOS are thought to be important sources of NO in the corpus cavernous, as relaxation and erectile responses were reduced after removing or destroying the endothelium [15]. Supporting the pivotal role of NO in mediating relaxation of the human corpus cavernosum to electric-field stimulation, inhibiting NOS completely blocked smooth muscle relaxation [47].

The relaxant actions of NO on the corpus cavernosum are caused by activation of the soluble adenylate cyclase and subsequent production of cGMP, which acts as a second messenger, resulting in a decrease in intracellular calcium and thus smooth muscle relaxation [48]. The activity of cGMP is regulated by cyclic nucleotide phosphodiesterase enzymes (PDEs). The major cGMP metabolizing PDE in human corpus cavernosum is PDE-5 [49]. Sildenafil (a selective PDE-5 inhibitor), by inhibiting this enzyme, prevents the breakdown of cGMP, resulting in enhanced relaxation of the corpus cavernosum and penile erection [50].

The significantly lower levels of NO in patients with organic ED could be attributed to the impairment of NO synthesis, which was reported in patients with risk factors for ED, including diabetes mellitus and atherosclerosis [32,51]. Vernet et al.[52] reported that penile NOS activity and content were reduced in rat models of type 1 and type 2 diabetes with ED. In humans, diabetic ED was suggested to be related to the effects of advanced glycation end-products on NO formation [53].

Previous results have supported the view that arachidonate cascade products may take part in the control of penile erection [54]. Human corpus cavernosum tissue can synthesise various prostanoids and metabolize them locally. The production of prostanoids can be modulated by oxygen tension and suppressed by hypoxia [7]. Prostanoids may be involved in the contraction of erectile tissue by PGF2 and thromboxane A2, and in relaxation by PGE1 and PGE2[18].

In the present study there were significantly lower mean plasma levels of PGE2 in venous blood from patients with ED and in both subgroups than in controls. However, there was no significant difference in PGE2 levels in venous and cavernosal blood between the organic and psychogenic ED groups. These results support the view reported by previous studies, that various prostanoids are important for regulating tone in penile vessels and erectile tissue in man [19,54–57]. PGE2-induced relaxation of human corporal smooth muscle occurs via stimulation of EP receptors. PGE2 modulates the enzyme adenyl cyclase leading to an increase in cAMP, which leads to a decrease in free calcium concentration and subsequently to relaxation of the cavernosal smooth muscle [19,55].

PGs were also reported to modulate the presynaptic noradrenaline release via PGE1 and PGE2 receptors, leading to inhibition of noradrenaline output [58]. Therefore PGE1 and PGE2 not only have a direct relaxant effect on the cavernosal smooth muscles via the cAMP pathway, but also inhibit the influence of sympathetic activity, which (as a logical consequence of these two synergistic effects) explains their better erectile potency than other vasoactive drugs [19]. PGE1 and PGE2 have important therapeutic roles for treating ED, where injection with PGE1 intracavernosally or administration intraurethrally are widely used for treating ED [56]. Also, PGE2 in the form of intraurethral cream has been reported to be a possible alternative treatment for ED [20].

In the present study there were significantly higher levels of plasma PGE2 in cavernosal blood of both groups with ED than in venous blood; this difference could be attributed to human corpus cavernosum tissue being able to synthesise various prostanoids and to metabolize them locally [7].

There were significant correlations among the analytes assessed (Table 2), with significant positive correlations between plasma levels of endothelin-1 and angiotensin II in venous and cavernosal blood of patients with ED. These findings could be attributed the calcium accumulated in intracellular pools being critical in the production, release and effects of these paracrine regulators. Angiotensin II increases cytosolic calcium, which promotes contraction of smooth muscle and facilitates the secretion of endothelin-1 [59].

There were also significant negative correlations between plasma levels of endothelin-1 and NO in both venous and cavernosal blood from patients with ED. Endothelin-1 and NO have not only contrasting and antagonistic actions but also may regulate each other's synthesis. NO decreases endothelin release from cultured endothelial cells and may terminate endothelin-1 signalling by dissociating the peptide from its receptor. Also, endothelin-1, being a potent vasoactive mediator, can rapidly stimulate NO generation, which may constitute an important negative feedback mechanism to modulate vascular tone [60].

There were also significant negative correlations between plasma levels of endothelin-1 and PGE2 in venous and cavernosal blood of patients with ED. These results could be attributable to the findings of Christ et al.[6], that the contractions induced by endothelin-1 might depend on both transmembrane calcium flux and on the mobilization of inositol triphosphate-sensitive intracellular calcium stores. On the contrary, PGE2 decreases intracellular calcium and thus muscles are relaxed [19].

There were significant negative correlations between angiotensin II and NO plasma levels in venous and cavernosal blood of patients with ED. Kifore et al.[10] reported that, although high cytosolic calcium levels activate eNOS and increase NO production, activation of proteins kinases by angiotensin II suppresses NO production. Also NO inhibits the effect of angiotensin II on smooth muscles.

There were significant negative correlations between plasma levels of angiotensin II and PGE2 in the venous and cavernosal blood of patients with ED; this can be attributed to the decrease in calcium influx by PGE2, which substantially decreases angiotensin II secretion in the corpus cavernosum [55]. The rapid effect of PGE2 on angiotensin II secretion is consistent with its role in modulating cytosolic calcium, leading to inhibition of angiotensin II secretion.

There were significant positive correlations between plasma levels of NO and PGE2 in both the venous and cavernosal blood from patients with ED. NO and PGE2 induce relaxation of human corpus cavernosum by the same mechanism. They increase cGMP, resulting in a decrease in intracellular calcium and thus relaxing smooth muscle [19,48].

In conclusion, the balance between vasoconstrictors and vasodilators might be important in the pathogenesis of ED. The present results suggest that endothelin-1 levels in venous blood could be used as a clinical marker of diffuse endothelial disease manifested by ED. Angiotensin II is an important modulator of erectile function, so the control of angiotensin II production or its numerous effects by using ACE inhibitors or angiotensin II receptor antagonists may provide a more specific way to prevent and/or correct the regulatory imbalance that leads to ED. Understanding the importance of NO in mediating penile erection in animals and humans is the key to identifying new medical therapies for ED. We conclude that PGE has an effect on the tone of penile muscles and is important in the mechanism of penile erection. However, we recommend future studies of the interactions between different transmitters/modulators, as this could help to reveal new combined therapies for ED.

CONFLICT OF INTEREST

None declared. Source of funding: Faculty of Medicine, Assiut University, Egypt.

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