Functional characterization of endothelin receptors in the bovine retractor penis muscle and penile artery

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.

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Erection induced by Tx2-6 toxin of Phoneutria nigriventer spider: expression profile of genes in the nitric oxide pathway of penile tissue of mice

The peptides Tx2-5 and Tx2-6, isolated from the whole venom of "armed-spider"Phoneutria nigriventer venom, are directly linked with the induction of persistent and painful erection in the penis of mammals. The erection induced by Tx2-6 has been associated with the activation of nitric oxide synthases. There is a scarcity of studies focusing on the outcome of Tx2-6 at the molecular level, by this reason we evaluated the gene profile activity of this toxin at the nitric oxide (NO) pathway. After microarray analyses on cavernous tissue of mice inoculated with Tx2-6 we found that only 10.4% (10/96) of these genes were differentially expressed, showing a limited effect of the toxin on the NO pathway. We found the genes sparc, ednrb, junb, cdkn1a, bcl2, ccl5, abcc1 over-expressed and the genes sod1, s100a10 and fth1 under-expressed after inoculation of Tx2-6. The differential expressions of sparc and ednrb genes were further confirmed using real-time PCR. Interestingly, ednrb activates the L-arginine/NO/cGMP pathway that is involved in the relaxation of the cavernous body. Therefore the priapism induced by Tx2-6 is a consequence of a highly specific interference of this neurotoxin with the NO pathway.

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.

DOCA-salt treatment enhances responses to endothelin-1 in murine corpus cavernosum

The penis is kept in the flaccid state mainly via a tonic activity of norepinephrine and endothelins (ETs). ET-1 is important in salt-sensitive forms of hypertension. We hypothesized that cavernosal responses to ET-1 are enhanced in deoxycorticosterone acetate (DOCA)-salt mice and that blockade of ETA receptors prevents abnormal responses of the corpus cavernosum in DOCA-salt hypertension. Male C57BL/6 mice were unilaterally nephrectomized and treated for 5 weeks with both DOCA and water containing 1% NaCl and 0.2% KCl. Control mice were uninephrectomized and received tap water with no added salt. Animals received either the ETA antagonist atrasentan (5 mg x day(-1) x kg(-1) body weight) or vehicle. DOCA-salt mice displayed increased systolic blood pressure (SBP), and treatment with atrasentan decreased SBP in DOCA-salt mice. Contractile responses in cavernosal strips from DOCA-salt mice were enhanced by ET-1, phenylephrine, and electrical field stimulation (EFS) of adrenergic nerves, whereas relaxations were not altered by IRL-1620 (an ETB agonist), acetylcholine, sodium nitroprusside, and EFS of nonadrenergic noncholinergic nerves. PD59089 (an ERK1/2 inhibitor), but not Y-27632 (a Rho-kinase inhibitor), abolished enhanced contractions to ET-1 in cavernosum from DOCA-salt mice. Treatment of DOCA-salt mice with atrasentan did not normalize cavernosal responses. In summary, DOCA-salt treatment in mice enhances cavernosal reactivity to contractile, but not to relaxant, stimuli, via ET-1/ETA receptor-independent mechanisms.

Erectile physiological and pathophysiological pathways involved in erectile dysfunction

PURPOSE

The importance of signaling pathways in penile smooth muscles involved in normal erection and erectile dysfunction (ED) is discussed based on a review of the literature.

MATERIALS AND METHODS

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents but also by visual, olfactory and imaginary stimuli. The generated nervous signals will influence the balance between the contractant and relaxant factors, which control the degree of contraction of penile smooth muscles and, thus, determine the functional state of the penis. The different steps involved in neurotransmission, impulse propagation and intracellular transduction of neural signals may be changed in different types of erectile dysfunction.

RESULTS

Recent findings have suggested an important role for RhoA/Rho kinase in the regulation of cavernosal smooth muscle tone and that changes in this pathway may contribute to ED in various patient subgroups, eg diabetes and vascular disease. Neurogenic nitric oxide is still considered the most important factor for immediate relaxation of penile vessels and corpus cavernosum. However, endothelially generated nitric oxide seems essential for maintaining erection. Endothelial dysfunction can contribute to ED in several patient subgroups. In addition, in conditions associated with reduced function of nerves and endothelium, such as aging, hypertension, smoking, hypercholesterolemia and diabetes, circulatory and structural changes in the penile tissues can result in arterial insufficiency and defect muscle relaxation.

CONCLUSIONS

Different types of ED often have overlapping pathophysiologies but may also have common pathways contributing to ED. Such pathways may be potential treatment targets.

No colocalization of immunoreactivities for VIP and neuronal NOS, and a differential relation to cGMP-immunoreactivity in bovine penile smooth muscle

The distribution of immunoreactivity (IR) for the neuropeptide vasoactive intestinal polypeptide (VIP) and neuronal nitric oxide synthase (nNOS) in the bovine retractor penis muscle (RP) and penile artery (PA) was studied by using two different methods. The distribution of these immunoreactivities was also compared with that of the immunoreactivity for cyclic guanosine monophosphate (cGMP). In both tissues the nerve fibers and terminals immunoreactive for VIP had a distribution that was completely different from that of the nerve fibers and terminals immunoreactive for nNOS. This contrasts with the previous observations in penile smooth muscle of other species. In the RP, as well as in the PA, many of the VIP-IR fibers were also immunoreactive for neurofilaments (NF), whereas the nNOS-IR fibers were consistently devoid of NF-IR. Stimulation with sodium nitroprusside, a nitric oxide donor, considerably increased cGMP-IR in the smooth muscle cells in both RP and PA, and in several nerve fibers in PA. Many of these cGMP-IR nerve fibers exhibited nNOS-IR, whereas none of them was immunoreactive for VIP. Our results suggest that the degree of coexistence of VIP-IR and nNOS-IR in the nerve fibers and terminals innervating penile smooth muscle show wide species differences. They also suggest that the mechanisms by which VIP could be involved in neurogenic penile erection may vary between species.

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.