Role of the L-arginine/nitric oxide pathway in relaxation of isolated human penile cavernous tissue and circumflex veins

Physiological regulation of penile arteries and veins

Recent experimental evidence suggests that arterial insufficiency precedes the structural and functional changes in corpora cavernosa (CC) leading to organic erectile dysfunction (ED). The present review gives an overview of the physiological factors involved in the regulation of penile vasculature. Sympathetic nerves maintain flaccidity and tonically released noradrenaline induces vasoconstriction of both arteries and veins through alpha(1)- and alpha(2)-postsynaptic receptors and downregulates its own release and that of nitric oxide (NO) through alpha(2)-presynaptic receptors. The sympathetic cotransmitter neuropeptide Y (NPY) modulates noradrenergic vasoconstriction in penile small arteries by both enhancing and depressing noradrenaline contractions through Y(1)- and Y(2)-postsynaptic and a NO-independent atypical endothelial receptor, respectively. Activation of alpha(1)-adrenoceptors involves both Ca(2+) influx through L-type and receptor-operated Ca(2+) channels (ROC) and Ca(2+) sensitization mechanisms mediated by protein kinase C (PKC), tyrosine kinases (TKs) and Rho kinase (RhoK). In addition, RhoK can regulate Ca(2+) entry in penile arteries upon receptor stimulation. Vasodilatation of penile arteries and large veins during erection is mediated by neurally released NO. The subsequent increased arterial inflow to the cavernosal sinoids and shear stress on the endothelium lining penile arteries activates endothelial NO production through Akt phosphorylation of endothelial NO synthase (eNOS). NO stimulates guanylate cyclase and increased cyclic guanin 3'-monophosphate (cGMP) levels in turn activate protein kinase G (PKG), which enhances K(+) efflux through Ca(2+)-activated (K(Ca)) and voltage-dependent Ca(2+) (K(v)) channels in penile arteries and veins, respectively. PKG-mediated decrease in Ca(2+) sensitivity and its regulation by RhoK remains to be clarified in penile vasculature. Phosphodiesterase type 5 (PDE5) inhibitors are potent vasodilators of penile resistance arteries and increase the content and effects of basally released endothelial NO. Endothelium-dependent relaxations of penile small arteries also include an endothelium-derived hyperpolarizing factor (EDHF)-type response, which is impaired in diabetes and hypertension-associated ED. Locally produced contractile and relaxant prostanoids regulate penile venous and arterial tone, respectively. The latter activates prostaglandin I (IP) and prostaglandin E (EP) receptors coupled to adenylate cyclase and to the increase of cyclic adenosine monophosphate (cAMP) levels, which in turn stimulates K(+) efflux through ATP-sensitive K(+) (K(ATP)) channels. There is a crosstalk between the cGMP and cAMP signaling pathways in penile small arteries. Relevant issues such as the mechanisms underlying the excitation-secretion coupling of the endothelial cells, as well as those involved in cell proliferation and vascular remodeling of the penile vasculature remain to be elucidated. In addition, only few studies have investigated the changes in structure and function of penile arteries in cardiovascular risk situations leading to ED.

Nitric oxide-cyclic GMP signaling pathway in the regulation of rabbit clitoral cavernosum tone

We investigated the role of nitric oxide (NO)-guanosine 3',5'-cyclic monophosphate (cGMP) signaling in the regulation of rabbit clitoral cavernosum (CC) tone. Tension measurements, reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting, and NADPH-diaphorase staining were performed in CC. In the precontracted CC strips with phenylephrine (10(-5) M), acetylcholine (ACh) relaxed, dependent on dosage. Pretreatment with atropine, N(omega) nitro-L-arginine-methyl ester (NAME) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), guanylate cyclase inhibitor abolished the ACh-induced relaxations, but tetrodotoxin (TTX) did not. Sodium nitroprusside relaxed the strips in the presence of atropine and NAME, but not in the presence of ODQ. Electrical field stimulation (EFS) relaxed the strips dependent on stimulus strength. Pretreatment with TTX, NAME, or ODQ abolished the EFS-induced relaxation, but atropine did not. L-Arginine partially restored the inhibited response to ACh and EFS. The inducible NO synthase (iNOS) and neuronal NOS (nNOS) mRNAs and iNOS and endothelial NOS (eNOS) proteins were identified in the CC. NADPH-diaphorase staining revealed the positivity on the nerve trunks and fine nerve fibers in the CC. Finally, results demonstrate that the nNOS, ENOS, and the NO-cGMP signaling pathway are involved in the regulation of clitoral tumescence.

The effect of cyclic GMP on rabbit corporal smooth muscle tone and its modulation by cyclo-oxygenase products

Corporal smooth muscle (CSM) tone is maintained by a finite balance between relaxant and contractile neurotransmitters. The aim of these experiments was to ascertain the degree to which cyclic GMP is involved in these interactions. We also sought to elucidate the pharmacological mechanism of action of MB in rabbit corpus cavernosum (RCC), an important tool in nitric oxide research. Using an organ chamber technique, strips of RCC were treated with the guanylate cyclase inhibitors Methylene Blue (MB) and LY83583; 100 microM MB led to increases in resting tension which were antagonized by indomethacin, nifedipine, phentolamine, but not superoxide dismutase (SOD). Contractile responses to noradrenaline (NA) were increased and relaxation to ACh was impaired by both MB and LY83583 and reversed with indomethacin, but not SOD. Pyrogallol had no effect on agonist-induced responses. The pharmacological action of MB in RCC does not depend on the generation of superoxide anions. Endothelium-dependent relaxation in RCC results in activation of soluble guanylate cyclase and release of a stable endothelium derived contracting factor(s), which is likely to be a constrictor prostanoid(s). Tonic production of cGMP in RCC inhibits the presynaptic release and contractile effects of NA and can be modulated by cyclo-oxygenase inhibition, demonstrating the important interaction and functional antagonism between cGMP and prostaglandins in the control of CSM tone.

Innervation of the fibro-elastic type of the penis: an immunohistochemical study in the male pig

The occurrence and colocalization of several biologically active neuropeptides, catecholamine-, acetylcholine- or nitric oxide-synthesizing enzymes-tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (D beta H), choline acetyl-transferase (ChAT) and nitric oxide synthase (NOS I), respectively, as well as the vesicular acetylcholine transporter (VAChT) were investigated in the penile glans (GP), corpus and crura (CP), as well as in the retractor penis muscle (RPM) of juvenile and adult boars. Immunohistochemistry revealed that nerves immunoreactive (IR) to TH, D beta H, vasoactive intestinal polypeptide (VIP) and somatostatin (SOM) were the most numerous, followed (in decreasing order of density) by nerves IR to NOS, neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP), galanin (GAL), Leu5-enkephalin (LENK) and ChAT/VAChT. The CP contained the largest number of nerve fibres followed by the RPM, GP and corpus. Enzyme/peptide-containing nerves were associated with both the vascular and non-vascular penile structures. However, differences existed for their density and intrapenile distribution. Nerve terminals IR for different combinations of VIP, GAL or SOM were more frequent than those IR for NOS or CGRP in the non-vascular penile structures while the vasculature and the RPM received a prominent TH/D beta H-, VIP-, SOM- or NOS-IR nerve input. The present data indicate that the porcine penis receives nerve fibres that exhibit diverse chemical codes and that differences in the chemical coding of the nerve fibres may depend on their penile target-structure.

Neurogenic contraction and relaxation of human penile deep dorsal vein

1. The aim of the present study was to characterize neurogenic and pharmacological responses of human penile deep dorsal vein and to determine whether the responses are mediated by nitric oxide from neural or endothelial origin. 2. Ring segments of human penile deep dorsal vein were obtained from 22 multiorgan donors during procurement of organs for transplantation. The rings were suspended in organ bath chambers for isometric recording of tension. We then studied the contractile and relaxant responses to electrical field stimulation and to vasoactive agents. 3. Electrical field stimulation (0.5-2 Hz) and noradrenaline (3 x 10(-10)-3 x 10(-5) M) caused frequency- and concentration-dependent contractions that were of greater magnitude in veins denuded of endothelium. The inhibitor of nitric oxide synthesis NG-nitro-L-arginine methyl ester hydrochloride (L-NAME, l0(-4) M) increased the adrenergic responses only in rings with endothelium. 4. In preparations contracted with noradrenaline in the presence of guanethidine (10(-6) M) and atropine (10(-6) M), electrical stimulation induced frequency-dependent relaxations. This neurogenic relaxation was prevented by L-NAME, methylene blue (3 x 10(-5) M) and tetrodotoxin (10(-6) M), but was unaffected by removal of endothelium. 5. Acetylcholine (10(-8)-3 x 10(-5) M) and substance P (3 x 10(-11) -3 x 10(-7) M) induced endothelium-dependent relaxations. In contrast, sodium nitroprusside (10(-9)-3 x 10(-5) M) and papaverine (10(-8) 3 x 10(-5) M) caused endothelium-independent relaxations. 6. The results provide functional evidence that the human penile deep dorsal vein is an active component of the penile vascular resistance through the release of nitric oxide from both neural and endothelial origin. Dysfunction in any of these sources of nitric oxide should be considered in some forms of impotence.

Contribution of K+ channels and ouabain-sensitive mechanisms to the endothelium-dependent relaxations of horse penile small arteries

1. Penile small arteries (effective internal lumen diameter of 300 600 microm) were isolated from the horse corpus cavernosum and mounted in microvascular myographs in order to investigate the mechanisms underlying the endothelium-dependent relaxations to acetylcholine (ACh) and bradykinin (BK). 2. In arteries preconstricted with the thromboxane analogue U46619 (3-30 nM), ACh and BK elicited concentration-dependent relaxations, pD2 and maximal responses being 7.71+/-0.09 and 91+/-1 % (n=23), and 8.80+/-0.07 and 89+/-2% (n=24) for ACh and BK, respectively. These relaxations were abolished by mechanical endothelial cell removal, attenuated by the nitric oxide (NO) synthase (NOS) inhibitor, NG-nitro-L-arginine (L-NOARG, 100 microM) and unchanged by indomethacin (3 microM). However, raising extracellular K+ to concentrations of 20-30 mM significantly inhibited the ACh and BK relaxant responses to 63+/-4% (P<0.01, n=7) and to 59+/-4% (P<0.01, n=6), respectively. ACh- and BK-elicited relaxations were abolished in arteries preconstricted with K+ in the presence of 100 microM L-NOARG. 3. In contrast to the inhibitor of ATP-sensitive K channels, the blockers of Ca2+-activated K+ (K(Ca)) channels, charybdotoxin (30 nM) and apamin (0.3 microM), each induced slight but significant rightward shifts of the relaxations to ACh and BK without affecting the maximal responses. Combination of charybdotoxin and apamin did not cause further inhibition of the relaxations compared to either toxin alone. In the presence of L-NOARG (100 microM), combined application of the two toxins resulted in the most effective inhibition of the relaxations to both ACh and BK. Thus, pD2 and maximal responses for ACh and BK were 7.65+/-0.08 and 98+/-1%, and 9.17+/-0.09 and 100+/-0%, respectively, in controls, and 5.87+/-0.09 (P<0.05, n=6) and 38+/-11% (P<0.05, n=6), and 8.09+/-0.14 (P<0.01, n=6) and 98+/-1% (n=6), respectively, after combined application of charybdotoxin plus apamin and L-NOARG. 4. The selective inhibitor of guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 5 microM) did not alter the maximal responses to either ACh or BK, but slightly decreased the sensitivity to both agonists, deltapD2 being 0.25+/-0.07 (P<0.05, n=6) and 0.62+/-0.12 (P< 0.01, n=6) for ACh and BK, respectively. Combined application of ODQ and charybdotoxin plus apamin produced further inhibition of the sensitivity to both ACh (deltapD2=1.39+/-0.09, P<0.01, n=6) and BK (1.29+/-0.11, P<0.01, n=6), compared to either ODQ or charybdotoxin plus apamin alone. 5. Exogenous nitric oxide (NO) present in acidified solutions of sodium nitrite (NaNO2) and S-nitrosocysteine (SNC) both concentration-dependently relaxed penile resistance arteries, pD2 and maximal responses being 4.84+/-0.06 and 82+/-3% (n=12), and 6.72+/-0.07 and 85+/-4% (n=19), respectively. Charybdotoxin displaced to the right the dose-relaxation curves for both NO (deltapD2 0.38+/-0.06, P<0.01, n=6) and SNC (deltapD2 0.50+/-0.10, P<0.01, n=5), whereas apamin only reduced sensitivity (deltapD2=0.35+/-0.12, P<0.05, n=5) and maximum response (65+/-9%, P<0.05, n=6) to SNC. ODQ shifted to the right the dose-relaxation curves to both NO and SNC. The relaxant responses to either NO or SNC were not further inhibited by a combination of ODQ and charybdotoxin or ODQ and charybdotoxin plus apamin, respectively, compared to either blocker alone. 6. In the presence of 3 microM phentolamine, 5 microM ouabain contracted penile resistance arteries by 50+/-6% (n=17) of K-PSS, but did not significantly change the relaxant responses to either ACh, BK or NO. However, in the presence of L-NOARG ouabain reduced the ACh- and BK-elicited relaxation from 94+/-3% to 16+/-5% (P<0.0001, n=6), and from 98+/-2% to 13+/-3% (P<0.0001, n=5), respectively. Combined application of ODQ and ouabain inhibited the relaxations to NO from 92+/-2% to 26+/-3% (P<0.0001, n=6). 7. The present results demonstrate that the endothelium-dependent relaxations of penile small arteries involve the release of NO and a non-NO non-prostanoid factor(s) which probably hyperpolarize(s) smooth muscle by two different mechanisms: an increased charybdotoxin and apamin-sensitive K+ conductance and an activation of the Na+-K+ATPase. These two mechanisms appear to be independent of guanylate cyclase stimulation, although NO itself can also activate charybdotoxin-sensitive K+ channels and the Na+-K+ pump through both cyclic GMP-dependent and independent mechanisms, respectively.

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.

Neural and endothelial nitric oxide synthase activity in rat penile erectile tissue

NADPH-diaphorase (NADPH-D) activity and immunoreactivity for neural and endothelial nitric oxide synthase (nNOS and eNOS, respectively) were used to investigate nitric oxide (NO) regulation of penile vasculature. Both the histochemical and immunohistochemical techniques for NOS showed that all smooth muscles regions of the penis (dorsal penile artery and vein, deep penile vessels, and cavernosal muscles) were richly innervated. The endothelium of penile arteries, deep dorsal penile vein, and select veins in the crura and shaft were also stained for NADPH-D and eNOS. However, the endothelium of cavernous sinuses was unstained by both techniques. Fewer fibers were seen in the glans penis, those present being associated with small blood vessels and large nerve bundles near the trabecular walls. All penile neurons in the pelvic plexus, located by retrograde transport of a dye placed in the corpora cavernosa penis, were stained by the NADPH-D method. Essentially similar results were obtained with an antibody to nNOS. These data suggest that penile parasympathetic neurons comprise a uniform population, as all seem capable of forming nitric oxide. However, in contrast to the endothelium of penile vessels, the endothelium lining the cavernosal spaces may not be capable of nitric oxide synthesis.

Nitric oxide control of lower genitourinary tract functions: a review

It is apparent that evolving concepts of the regulatory basis for functions in the pelvis must take into account the role exerted by nitric oxide. A recently characterized messenger molecule, nitric oxide has been associated with numerous physiologic processes. Intense investigations of this molecule have extended its importance to several genitourinary functions. Penile erection, micturition, peristalsis of the male excurrent duct system, contractile properties of the prostate, and lumbosacral spinal cord neurotransmission are all functions that may transpire under some degree of control by nitric oxide. Impotence, urinary obstruction, or ejaculatory problems, in turn, may represent alterations of nitric oxide production or action. The strategic manipulation of nitric oxide or its mechanism of action, possibly by pharmacologic means, may restore or produce desired functional effects. These possibilities, therefore, suggest that the advancing knowledge of nitric oxide in the genitourinary tract may be of enormous clinical value in the future.