Phospholipase C activation by endothelin-1 and noradrenaline in isolated penile erectile tissue from rabbit

Erectile Dysfunction in Men on the Rise: Is There a Link with Endocrine Disrupting Chemicals?

Erectile dysfunction (ED) is one of the most prevalent chronic conditions affecting men. ED can arise from disruptions during development, affecting the patterning of erectile tissues in the penis and/or disruptions in adulthood that impact sexual stimuli, neural pathways, molecular changes, and endocrine signalling that are required to drive erection. Sexual stimulation activates the parasympathetic system which causes nerve terminals in the penis to release nitric oxide (NO). As a result, the penile blood vessels dilate, allowing the penis to engorge with blood. This expansion subsequently compresses the veins surrounding the erectile tissue, restricting venous outflow. As a result, the blood pressure localised in the penis increases dramatically to produce a rigid erection, a process known as tumescence. The sympathetic pathway releases noradrenaline (NA) which causes detumescence: the reversion of the penis to the flaccid state. Androgen signalling is critical for erectile function through its role in penis development and in regulating the physiological processes driving erection in the adult. Interestingly, estrogen signalling is also implicated in penis development and potentially in processes which regulate erectile function during adulthood. Given that endocrine signalling has a prominent role in erectile function, it is likely that exposure to endocrine disrupting chemicals (EDCs) is a risk factor for ED, although this is an under-researched field. Thus, our review provides a detailed description of the underlying biology of erectile function with a focus on the role of endocrine signalling, exploring the potential link between EDCs and ED based on animal and human studies.

The promise of inhibition of smooth muscle tone as a treatment for erectile dysfunction: where are we now?

Ten years ago, the inhibition of Rho kinase by intracavernosal injection of Y-27632 was found to induce an erectile response. This effect did not require activation of nitric oxide-mediated signaling, introducing a novel target pathway for the treatment of erectile dysfunction (ED), with potential added benefit in cases where nitric oxide bioavailability is attenuated (and thus phosphodiesterase type 5 (PDE5) inhibitors are less efficacious). Rho-kinase antagonists are currently being developed and tested for a wide range of potential uses. The inhibition of this calcium-sensitizing pathway results in blood vessel relaxation. It is also possible that blockade of additional smooth muscle contractile signaling mechanisms may have the same effect. In this review, we conducted an extensive search of pertinent literature using PUBMED. We have outlined the various pathways involved in the maintenance of penile smooth muscle tone and discussed the current potential benefit for the pharmacological inhibition of these targets for the treatment of ED.

Murine and rat cavernosal responses to endothelin-1 and urotensin-II Vasoactive Peptide Symposium

BACKGROUND: Endothelin-1 (ET-1) and urotensin-II (U-II) are the most potent constrictors of human vessels. Although the cavernosal tissue is higly responsive to ET-1, no information exists on the effects of U-II on cavernosal function. The aim of this study was to characterize ET-1 and U-II responses in corpora cavernosa from rats and mice. METHODS AND RESULTS: Male Wistar rats and C57/BL6 mice were used at 13 weeks. Cumulative concentration-response curves to ET-1, U-II and IRL-1620, an ET(B) agonist, were performed. ET-1 increased force generation in cavernosal strips from mice and rats, but no response to U-II was observed in the presence or absence of L-NAME, or in strips pre-stimulated with 20mM KCl. IRL-1620 did not induce cavernosal contraction even in presence of L-NAME, but induced a cavernosal relaxation which was greater in rats than mice. No relaxation responses to U-II were observed in cavernosal strips pre-contracted with phenylephrine. mRNA expression of ET-1, ET(A), ET(B) and U-II receptors, but not U-II was observed in cavernosal strips. CONCLUSION: ET-1, via ET(A) receptors activation, causes contractile responses in cavernosal strips from rats and mice whereas ET(B) receptor activation produces relaxation. Although the cavernosal tissue expresses U-II receptors, U-II does not induce contractile responses in corpora cavernosa from mice or rats.

Paullinia pinnata Extracts Rich in Polyphenols Promote Vascular Relaxation via Endothelium-dependent Mechanisms

Paullinia pinnata L. (Sapindaceae) is an African tropical plant whose roots and leaves are used in traditional medicine for many purposes, especially for erectile dysfunction, but its action mechanism is unknown. P. pinnata root and leaf methanolic extracts are rich in phenolic compounds. This study shows that both extracts are highly antioxidative and induce a slight transcriptional activity of peroxisome proliferator activated receptor-alpha. They also increased and decreased endothelial nitric oxide synthase and endothelin-1 mRNA levels in bovine aortic endothelial cells, respectively. In this study P. pinnata methanolic extracts in cumulative doses elicited in a dose-dependent manner the relaxation of phenylephrine precontracted isolated rat aortic rings. N-nitro-L-arginine methyl ester significantly attenuated the capacity of both extracts to induce arterial relaxation, indicating that this arterial relaxation was mediated by endothelial nitric oxide release. It could be suggested that the arterial relaxation induced by both extracts could be mainly linked to their capacities to inhibit nitric oxide oxidation through their antioxidant properties.

RhoA-Rho kinase mediates synergistic ET-1 and phenylephrine contraction of rat corpus cavernosum

Maintenance of the detumescent state of the penis is believed to involve the actions of several vasoconstrictors. However, our mechanistic understanding of any synergistic vasoconstrictor influences is extremely limited. We tested the hypothesis that a vasoconstrictor combination of endothelin (ET-1) and phenylephrine (PE) augments the constrictor responses in rat corporal cavernosal tissues by a mechanism involving the RhoA-Rho kinase pathway. Independently, ET-1 (1 nM-30 microM) and PE (100 nM-100 microM) both caused dose-dependent contractions of isolated rat cavernosal tissues. In combination, ET-1 (30 nM) augmented the contractile effect of PE and shifted the calculated EC50 for PE (90 +/- 12 to 45 +/- 5 microM). The active stress generated by cavernosal strips during the ET-1 + PE combined stimulation (4.9 +/- 0.2 mN/mm2) was greater than the combined stress generated with ET-1 (0.4 +/- 0.1 mN/mm2) or PE (3.3 +/- 0.2 mN/mm2) stimulations alone. Blockade of ETA receptors (30 nM; A-127722) reversed the augmented stress generation and the Rho-kinase inhibitor Y-27632 differentially and dose-dependently relaxed the tissue. The combined constrictor effect was associated with a fourfold increase of RhoA in the membrane faction of the tissue homogenates. We conclude that the ET-1 + PE combination potentiate vasoconstriction through mutual activation of the RhoA-Rho kinase pathway. The interactions of these agonists likely play important roles in the maintenance of the flaccid state and contribute to some forms of erectile dysfunction.

Endothelin-1-induced vasoconstriction is inhibited during erection in rats

Recent evidence indicates that endothelin-1 (ET-1) might be a principal vasoconstrictor in the penis. We report that ET-1 injection into the cavernous sinuses before erection sharply reduced the magnitude of subsequent erections. Corpus cavernosum pressure-to-mean arterial pressure ratios (CCP/MAP), with maximal ganglionic stimulation, were 0.62 +/- 0.05 before ET-1 injection and 0.31 +/- 0.05 after, indicating that ET-1 acted as a vasoconstrictor. When ET-1 was injected during a maximal neurally induced erection, the ability of ET-1 to attenuate subsequent erections was diminished (CCP/MAP 0.75 +/- 0.02 before ET-1, 0.61 +/- 0.03 after). At submaximal stimulation voltages, injection of ET-1 during erection also attenuated its vasoconstrictive effect. Similarly, when ET-1 was injected during erection induced by intracavernosal injection of the nitric oxide (NO) donor NOR-1, subsequent erections were not significantly suppressed (CCP/MAP 0.53 +/- 0.04 before ET-1, 0.45 +/- 0.04 after). These findings that ET-1-induced vasoconstriction is attenuated during erection are consistent with the hypothesis that NO mediates erection both by initiating pathways that cause smooth muscle relaxation and by inhibiting the vasoconstrictive actions of ET-1.

Correlation of calcium-activated ATPase activity, lipid peroxidation, and the contractile response of rabbit corporal smooth muscle treated with in vitro ischemia

Oxygen and glucose are critical to support the survival and integrity of all smooth muscles. Hypoxia alone has been demonstrated to suppress the contractile response of corporal smooth muscle, and one might expect simultaneous deprivation of oxygen and glucose (in vitro model of ischemia) to exert more serious damage to corporal smooth muscle contraction. The effect of in vitro ischemia on the pharmacological responses of isolated rabbit corporal smooth muscle was correlated with the level of tissue lipid peroxidation. The effects of in vitro ischemia were as follows: (1) In vitro ischemia resulted in an 85% reduction in the contractile response to phenylephrine; (2) more than a 50% reduction in the activity of thapsigargin-sensitive calcium-activated ATPase activity of the microsomes (sarcoplasmic reticulum [SR]); (3) more than a fourfold increase in the tissue concentration of thiobarbituric acid reactive substances (TBARS) (level of lipid peroxidation). In conclusion, stimulation of lipid peroxidation in part may be responsible for the decrease in thapsigargin-sensitive calcium-activated ATPase activity of the SR (SERCA), and the correlated decrease in the contractile response to phenylephrine in response to ischemia.

Renin angiotensin system in rabbit corpus cavernosum: functional characterization of angiotensin II receptors

PURPOSE

The regulation of the corporal smooth muscle tone is important in the process of penile erection. Although specific angiotensin (ANG) II binding to and effects of ANG II on some reproductive structures have been studied, the presence of the renin-angiotensin system has not yet been defined in the corpus cavernosum. ANG II is formed from ANG I by angiotensin I-converting enzyme (ACE). ANG II and ANG I produce contractions in vascular smooth muscles. Two subtypes of ANG II receptors (AT1 and AT2) have been characterized. The purpose of the present experiments was to determine whether the renin-angiotensin system regulates rabbit corpus cavernosum smooth muscle tone.

MATERIALS AND METHODS

A strip of rabbit corpus cavernosum was mounted in an organ chamber to measure the isometric tension. The specific binding for 125I-ANG II was characterized by in vitro autoradiography.

RESULTS

ANG II and ANG I, precursor of ANG II, contracted corpus cavernosum smooth muscle dose-dependently, but the response of smooth muscle to ANG I was 10-fold less than that to ANG II. Contractile responses of smooth muscle to ANG II and ANG I were blocked by Dup 753, a specific inhibitor of ANG II type 1 receptor, but not by PD 123,319, a specific inhibitor of ANG II type 2 receptor. The effect of ANG I was attenuated by captopril, an inhibitor of ACE. Specific binding sites for 125I-ANG II were found in the corpus cavernosum. The dissociation constant (Kd) was 5.32 +/- 1.65 nM. and maximum binding capacity (Bmax) was 305.72 +/- 85.24 amol/mm. Specific binding of 125I-ANG II was displaced by Dup 753 (10(-6) M) but not by PD 123,319 (10(-5) M). The inhibitory constant (Ki) for Dup 753 was 8.09 +/- 2.51 nM.

CONCLUSION

The present results suggest that the renin-angiotensin system is involved in the regulation of corpus cavernosum smooth muscle tone of rabbit and the ANG II receptor subtype AT1 is important in the regulation of penile erection.

Neurotransmission and the contraction and relaxation of penile erectile tissues

The balance between contractant and relaxant factors controls the smooth muscle of the corpus cavernosum and determines the functional state of the penis (detumescence and flaccidity versus tumescence and erection). Noradrenaline contracts both the corpus cavernosum and penile vessels, mainly via stimulation of alpha(1)-adrenoceptors. Recent investigations have demonstrated the presence of several subtypes of alpha 1-adrenoceptors (alpha(1A), alpha(1B), and alpha(1D)) in the human corpus cavernosum and also that the noradrenaline-induced contraction in this tissue is probably mediated by two or, possibly, three receptor subtypes. Even if much of the available in vitro information suggests that endothelins (ETs) may be of importance for mechanisms of detumescence and flaccidity, the role of the peptides in the control of penile smooth-muscle tone in vivo is unclear, as is the question as to whether they can contribute to erectile dysfunction. For further evaluation of the clinical importance of ETs in penile physiology and pathophysiology, clinical studies on ET-receptor antagonists would be of interest. Neurogenic nitric oxide (NO) has been considered the most important factor for relaxation of penile vessels and the corpus cavernosum, but recent studies in mice lacking neurogenic NO synthase (NOS) have shown these animals to have normal erections. This focuses interest on the role of endothelial NOS and on other agents released from nerves or endothelium. For the time being the most effective means of inducing penile erection in men involves the intracavernous administration of prostaglandin E1 (PGE1). PGE1 may act partly by increasing intracellular concentrations of cyclic adenosine monophosphate (cAMP). Recent results obtained with the adenylate cyclase stimulator forskolin suggest that penile smooth-muscle relaxation leading to penile erection can be achieved through the cAMP pathway. Thus, transmitters and agents acting through this second-messenger system may significantly contribute to relaxation of penile smooth muscle and to erection.

Suppression by prolactin of the electrically induced erectile response through its direct effect on the corpus cavernosum penis in the dog

PURPOSE

Men become impotent when exposed to hyperprolactinemia. To clarify its mechanisms the effects of intracorporal infusion of prolactin on electrically induced penile erection were evaluated in 12 male dogs.

MATERIALS AND METHODS

Prolactin (10 micrograms./ml.) or control saline was directly infused into the corpus cavernosum penis 5 minutes before electrical pulse stimulation of the pelvic nerve and the intracorporal pressure was monitored.

RESULTS

In 8 dogs erection was markedly suppressed or completely abolished by prolactin. In the remaining 4, this effect of prolactin became manifest only when the ipsilateral internal pudenal artery was ligated. Saline infusion was without effect.

CONCLUSIONS

An excess of prolactin directly inhibited the smooth muscle relaxation of corpus cavernosum penis.

Diminished vascular response to inhibition of endothelium-derived nitric oxide and enhanced vasoconstriction to exogenously administered endothelin-1 in clinically healthy smokers

BACKGROUND

Smoking is a major risk factor for the development of atherosclerosis. Because endothelial dysfunction may be a marker for future atherosclerosis, we investigated the effects of smoking on endothelium-dependent control of vascular tone.

METHODS AND RESULTS

The effects of brachial arterial infusions of NG-monomethyl-L-arginine (L-NMMA), a nitric oxide synthesis inhibitor; sodium nitroprusside; endothelin-1; and norepinephrine on forearm blood flow (strain-gauge plethysmography) were compared in 29 long-term smokers and 16 nonsmokers. The acute effects of smoking on systemic hemodynamics, plasma catecholamines, and forearm vascular responses to these compounds were investigated in smokers only. Smokers did not differ from nonsmokers (n = 16) regarding the vascular effects of sodium nitroprusside (n = 13) or vasoconstriction due to norepinephrine and endothelin-1 (n = 16). Low-dose endothelin-1-induced vasodilation, believed to reflect endothelial prostacyclin or nitric oxide release, was absent in smokers (n = 16), and their increase of forearm vascular resistance (FVR) after L-NMMA (n = 13) was impaired (35.6 +/- 27.9% versus 118.8 +/- 43.2%, P < .001). Short-term smoking (n = 11) increased blood pressure, heart rate, and plasma epinephrine concentrations (P < .05 or less); enhanced endothelin-1-induced vasoconstriction (delta FVR, 457 +/- 192% versus 254 +/- 143%, P < .01); and decreased norepinephrine-induced vasoconstriction (P < .05), but had no effect on the other interventions.

CONCLUSIONS

Long-term smoking is associated with a diminished nitric oxide-dependent component of basal vascular tone and an impaired endothelium-dependent vasodilator response to low-dose endothelin-1 and short-term smoking enhances endothelin-1-induced vasoconstriction. Impaired endothelial control of vascular tone might reflect impairment of normal antiatherosclerotic endothelial functions in smokers, but the relevance of smoking-induced enhancement of endothelin-1 vasoconstriction remains to be determined.