A simplified in vitro preparation of the corpus cavernosum as a tool for investigating erectile pharmacology in the rat

Restorative effects of red onion (Allium cepa L.) juice on erectile function after-treatment with 5α-reductase inhibitor in rats

Benign prostatic hyperplasia (BPH) is one of the most prevalent conditions among aged men. The use of 5α-reductase inhibitors (5-ARIs) to treat BPH was linked to erectile dysfunction (ED). Many medicinal plants and secondary metabolites are used in the management of ED. Onion (Allium cepa L.) is an economically affordable vegetable with vital phytochemicals and biological functions. The study aimed to identify the beneficial effects of onion juice on dutasteride (a 5-ARI)-induced ED. Rats were divided into two groups (n = 5 per group): control and dutasteride-treated rats (0.5 mg/kg/day). Dutasteride was administered in drinking water for 12 weeks. Experiments were performed at the end of the 12th week. In vivo erectile responses were measured before and after intracavernosal injection of onion. Relaxant responses to onion juice were examined in the corpus cavernosum (CC). Acetylcholine (ACh)-, electrical field stimulation (EFS)-, sodium nitroprusside (SNP)-induced relaxation responses in CC tissues were evaluated in the absence and presence of onion juice. Total intracavernosal pressure (ICP) and ICP/ mean arterial pressure were significantly reduced in dutasteride-treated rats (1881.14 ± 249.72 mmHg, P < 0.001;0.26 ± 0.03, P < 0.01) as compared to control rats (4542.60 ± 429.19 mmHg, 0.51 ± 0.05), which was normalized after the intracavernous administration of onion (3288.60 ± 185.45 mmHg, 0.58 ± 0.04). Onion markedly induced relaxant responses in control (72.5 ± 4.7) and dutasteride-treated (66.5 ± 2.7) groups after precontraction with phenylephrine. Relaxation responses to onion were partially inhibited after precontraction with KCl (32.5 ± 3.1, P < 0.001). The relaxant responses to ACh (14.9 ± 4.2, P < 0.01) were diminished in dutasteride-treated CC) compared to control CC (59.8 ± 3.4), which was enhanced after the incubation with onion (36.6 ± 4.8). There were no differences in relaxation response to SNP among all groups. However, relaxation response to SNP was reduced in dutasteride-treated CC at 1 μM (P < 0.05) and 10 μM dosages (P < 0.001), which was partially increased after the incubation with onion at 10 μM dosage (P < 0.01). The presence of onion did not change the reduction in EFS-caused relaxation in the dutasteride-treated group. The current data suggest that red onion juice has a restorative effect on erectile function and endothelium-dependent relaxation response following the treatment of dutasteride.

Antioxidant Potential and Ability of Phloroglucinol to Decrease Formation of Advanced Glycation End Products Increase Efficacy of Sildenafil in Diabetes-Induced Sexual Dysfunction of Rats

INTRODUCTION

Diabetes-induced sexual dysfunction is associated with an increase in oxidative stress. Scavengers of reactive oxygen species (ROS) have been shown to reduce oxidative stress and aid in the management of sexual dysfunction in diabetes.

AIM

The aim of the study was to test the hypothesis that antioxidant, which scavenge ROS and reduce formation of advanced glycation end products (AGEs), can potentiate efficacy of phosphodiesterase type 5 inhibitors in diabetes-induced sexual dysfunction that is associated with oxidative stress.

MATERIALS AND METHODS

Effect of phloroglucinol and sildenafil on serum glucose level, sexual function, penile smooth muscle : collagen ratio, and phenylephrine precontracted corpus cavernosum smooth muscle (CCSM) was studied. The ability of phloroglucinol to reduce the formation of AGEs and its ability to scavenge 2,2-diphenyl-1-picrylhydrazyl (DPPH) and nitric oxide (NO) was also evaluated.

MAIN OUTCOME MEASURES

Antioxidant potential of phloroglucinol was studied in addition to its effect on diabetes-induced sexual dysfunction in presence and absence of sildenafil.

RESULTS

Phloroglucinol (50 mg/kg, p.o.) significantly decreased serum glucose level and increased sexual function in streptozotocin-induced diabetic rats when compared with diabetic control rats. Sildenafil (5 mg/kg, p.o.) had no effect on glycemia but significantly increased sexual function of diabetic rats. Coadministration of phloroglucinol increased the efficacy of sildenafil by improving sexual function. Treatment of diabetic rats with phloroglucinol + sildenafil maintained smooth muscle : collagen levels similar to that of normal rat penile tissue. Phloroglucinol decreased formation of AGEs and significantly scavenged DPPH radical activity in vitro. Sildenafil relaxed isolated CCSM of normal rat and diabetic rat significantly, but phloroglucinol did not show any significant effect. Phloroglucinol also inhibited human CYP3A4 enzyme activity in vitro.

CONCLUSION

Phloroglucinol coadministration increases efficacy of sildenafil in diabetes-induced sexual dysfunction. However, further studies are required to ascertain the benefits of phloroglucinol owing to its undesirable CYP3A4 inhibition activity.

Efficacy of ellagic acid and sildenafil in diabetes-induced sexual dysfunction

BACKGROUND

Diabetes induced sexual dysfunction is a leading cause of male sexual disorder and an early indicator of cardiovascular complication. Reactive oxygen species generated in body during diabetes is a main causative factor for erectile dysfunction, a sexual dysfunction. Adjuvant antioxidant therapy along with phosphodiesterases type 5 enzyme inhibitor (PDE5i) is more effective than PDE5i alone.

OBJECTIVE

The aim of the study was to investigate efficacy of ellagic acid a known antioxidant and sildenafil in diabetes induced erectile dysfunction.

MATERIALS AND METHODS

Type 1 diabetes was induced in male rats and rats were treated with ellagic acid (50 mg/kg, p.o.) and a combination of ellagic acid (50 mg/kg, p.o.) and sildenafil (5 mg/kg, p.o.), a PDE5i for 28 days. Sexual function was observed in diabetic rat and compared with those of treatment group and normal rats. Effect of ellagic acid was studied on advanced glycation end products (AGE) and isolated rat corpus cavernosum in vitro.

RESULTS

Sexual function of diabetic rats was found to be reduced and ellegic acid treatment could preserve sexual function of diabetic rats to some extent. Ellagic acid + sildenafil treatment was more efficient in management of diabetes induced sexual dysfunction. Ellagic acid inhibited (AGE) in vitro implying its role in reducing oxidative stress in diabetes. The polyphenol could not increase sexual function in normal rats and relax isolated rat corpus cavernosum smooth muscle significantly.

CONCLUSION

The study proves usefulness of adjuvant antioxidant therapy in the management of erectile dysfunction in diabetes.

Treatment with hypotensive agents affects the impaired relaxation of the penile corpus cavernosum in hypertensive rats

Treatment of erectile dysfunction (ED) in hypertensive subjects remains to be formally established. There is currently no standardized treatment for ED in hypertensive subjects. In this study, we tested our hypothesis that hypotensive drugs would improve impaired relaxation in the corpus cavernosum of spontaneously hypertensive rats (SHR). Ten-week-old SHR was treated with amlodipine, imidapril or hydralazine for 4 weeks. Although all three drugs achieved an equivalent decrease in systolic blood pressure (SBP), only amlodipine and imidapril induced an increase in relaxation in response to electrical field stimulation (EFS) of the corpus cavernosum. In the case of amlodipine, this effect was dose- and SBP-dependent. Nitric oxide (NO)-dependent relaxation was increased by amlodipine over a wide range of EFS frequencies, was increased by imidapril at low EFS frequencies, and was decreased by hydralazine. Carbon monoxide (CO)-dependent relaxation was only increased by hydralazine, and this increase occurred over a wide range of frequencies. The NOx and cGMP levels in the EFS-stimulated corpus cavernosum were increased by amlodipine. Amlodipine did not affect the thiobarbituric acid-reacting substance levels in the serum and the corpus cavernosum, but did decrease superoxide dismutase activity in the tissue. Imidapril and hydralazine inhibited the acetylcholine-induced relaxation in the corpus cavernosum. Sodium nitroprusside-induced relaxation in the tissue was increased by amlodipine. All three agents similarly inhibited the phenylephrine-induced contraction. These results suggest that impaired neurogenic relaxation in the corpus cavernosum of SHR is improved by amlodipine and imidapril through an increase in the synthesis and/or release of neuronal NO, but not CO, and presumably the inhibited detumescence of erection, which is induced by norepinephrine being released from sympathetic neuron. These findings indicate that amlodipine and imidapril may ameliorate the decreased relaxation of cavernous smooth muscle in the setting of hypertension.

Erectile dysfunction in hypertensive rats results from impairment of the relaxation evoked by neurogenic carbon monoxide and nitric oxide

Erectile dysfunction (ED) with aging and diabetes mellitus is caused by impairment of the relaxation evoked by nitric oxide (NO) of penile cavernous smooth muscles and arterioles. However, the mechanism of ED in hypertension is unknown. Carbon monoxide (CO), which is produced by heme oxygenase (HO)-2 in the neuronal system is a neurotransmitter and a vasodilator. We examined the neurogenic role of CO in penile erection and the neurogenic mechanisms of ED in hypertension, using spontaneously hypertensive rats (SHR) or Wistar-Kyoto rats (WKY). The isometric tension of corpus cavernosum tissues from both strains was recorded after guanethidine and atropine treatment. Relaxation in response to electrical field stimulation (EFS) in WKY was suppressed dose-dependently by HO inhibitors both in the absence and presence of an NO synthase (NOS) inhibitor. Reverse transcription-polymerase chain reaction (RT-PCR) showed that the HO-2 gene was expressed in the corpus cavernosum. CO-saturated solution induced a concentration-dependent relaxation in WKY. The neurogenic relaxation to EFS in SHR was impaired as compared with that in WKY after the age of 5 weeks, when blood pressure began to be elevated, due to the attenuated relaxation in response to neurogenic NO and CO. In the corpus cavernosum of SHR, expression of the HO-2 and nNOS genes was similar, and NOx levels after EFS were similar to those of WKY. cGMP levels after EFS and the relaxation evoked by the NO donor was lower in SHR than WKY. Thiobarbituric acid-reacting substance (TBARS) levels were increased, and superoxide dismutase (SOD) activity was suppressed in SHR, as compared with those in WKY, suggesting that the increasing oxidative stress partially causes the impairment of NO-dependent relaxation. These findings suggest that CO regulates the relaxation evoked by EFS in the rat corpus cavernosum, and that ED in hypertension in rats results from an impairment of the relaxation induced by neurogenic CO and NO.

Effects of Diabetes on Nitric Oxide-Mediated Relaxations in Male Rat Corpus cavernosum Smooth Muscle

INTRODUCTION

We evaluated the effects of diabetes on nitric oxide-mediated relaxations and nitric oxide synthase activity in male rat corpus cavernosum smooth muscles.

METHODS

Eight-week-old male rats were assigned to three groups: control (injected with the vehicle), DM (diabetes mellitus, induced by injection with 65 mg/kg streptozotocin), and TES (testosterone, testosterone supplemented after induction of diabetes). After 8 weeks, corpus cavernosum smooth muscle strips were mounted in an organ bath for isometric tension recordings. Electrical field stimulation (EFS, 2-ms pulse duration, 0.3-20 Hz and 3 s train) was applied to the strips precontracted with 30 microM phenylephrine. The microdialysis probe was inserted into the strip, and Krebs-Henseleit solution was perfused into the probe. The dialysate during EFS was collected, and the amount of NO(-)(2)/NO(-)(3) (NOx) released in the dialysate was measured by the Greiss method. Sodium nitroprusside (0.1 nM to 10 mM) and carbachol (1 nM to 10 mM) were cumulatively added to the strips precontracted with 30 microM phenylephrine.

RESULTS

EFS caused frequency-dependent relaxations and NOx releases of the strips. Pretreatment with N(omega)-nitro-L-arginine (100 microM) and tetrodotoxin (1 microM) completely inhibited the relaxations and NOx releases. The maximum relaxation was significantly greater in the DM group than in the control or TES group. The release of NOx was significantly greater in the DM group than in the control or TES group. Sodium nitroprusside, the endothelium-independent vasodilator, relaxed the tissues in all three groups. There were no significant differences among control, DM and TES groups in the maximum relaxation to sodium nitroprusside.

CONCLUSION

The present data suggest that diabetes enhances nitric oxide synthase activity and nitric oxide-mediated relaxations in the male rat corpus cavernosum by the reduced testosterone level in the diabetic animals.

A role for the melanocortin 4 receptor in sexual function

By using a combination of genetic, pharmacological, and anatomical approaches, we show that the melanocortin 4 receptor (MC4R), implicated in the control of food intake and energy expenditure, also modulates erectile function and sexual behavior. Evidence supporting this notion is based on several findings: (i) a highly selective non-peptide MC4R agonist augments erectile activity initiated by electrical stimulation of the cavernous nerve in wild-type but not Mc4r-null mice; (ii) copulatory behavior is enhanced by administration of a selective MC4R agonist and is diminished in mice lacking Mc4r; (iii) reverse transcription (RT)-PCR and non-PCR based methods demonstrate MC4R expression in rat and human penis, and rat spinal cord, hypothalamus, brainstem, pelvic ganglion (major autonomic relay center to the penis), but not in rat primary corpus smooth muscle cavernosum cells; and (iv) in situ hybridization of glans tissue from the human and rat penis reveal MC4R expression in nerve fibers and mechanoreceptors in the glans of the penis. Collectively, these data implicate the MC4R in the modulation of penile erectile function and provide evidence that MC4R-mediated proerectile responses may be activated through neuronal circuitry in spinal cord erectile centers and somatosensory afferent nerve terminals of the penis. Our results provide a basis for the existence of MC4R-controlled neuronal pathways that control sexual function.

General use of animal models for investigation of the physiology of erection

In review, animal models have accounted significantly for the amazing strides made in the field of sexual dysfunction research. Fundamentally, they have offered a unique experimental approach to test many hypotheses regarding sexual function. Since their early use for sexual physiology research, there has been increasing sophistication using animals involving techniques for stimulating and monitoring sexual responses. One specific area that has been advanced is the use of conscious animal models to obtain a better sense of the natural contexts for sexual physiology and to avoid pharmacological interference associated with anesthetics. Another area of interest is the increasing use of simple but valid techniques to record and assess sexual responses. Efforts to develop and evaluate animal models that replicate disorders of sexual function have also been most advantageous. In the future, animal models will remain useful. The expanded applications of animal models include the study of predisposing disease states associated with sexual dysfunction and the study of all aspects of sexual dysfunction, in both male and female subjects. Continued judgment must be applied, understanding the advantages of one or another animal model, to explore questions and provide answers that are most scientifically relevant to the human condition. The promise of advancing therapies in this field indicates the additional prominent role for animals for the purposes of drug development.

Impairment of corpus cavernosal smooth muscle relaxation by glycosylated human haemoglobin

OBJECTIVE

To examine the effect of HbA1c, an isoform of glycosylated haemoglobin (GHb, a product of non-enzymatic reactions between elevated blood glucose and haemoglobin), on nitric oxide-mediated corpus cavernosal smooth muscle relaxation, and to categorize the mechanisms involved.

MATERIALS AND METHODS

Corpus cavernosal tissue from Wistar rats (300-350 g body weight) was prepared for the measurement of isometric tension. After equilibration in Krebs solution gassed with 95% O2/5% CO2 at 37 degrees C for 90 min, optimal resting tension was applied. Tissue was precontracted with 1 micromol/L noradrenaline (NAd) and either relaxed with incremental doses of acetylcholine (ACh) or sodium nitroprusside (SNP). After washout, strips were again precontracted with NAd and then incubated with pyrogallol (100 micromol/L), 100 microL of haemoglobin or 100 microL of GHb in the presence of either L-arginine (100 micromol/L), indomethacin (10 micromol/L), allopurinol (100 micromol/L), deferoxamine (100 micromol/L), catalase (600 IU/mL), or superoxide dismutase (SOD) (120 IU/mL) before ACh- or SNP-induced relaxation responses were repeated.

RESULTS

Haemoglobin and GHb significantly impaired the relaxation of rat corpus cavernosum to ACh in a dose-dependent manner. L-arginine reversed the impairment caused by Hb, but not GHb. A donor of superoxide anions, pyrogallol, mimicked this impairment to ACh when added to control strips. Catalase, deferoxamine, indomethacin and allopurinol had no significant effect on the impaired relaxation response to ACh, whilst L-arginine partially reversed it. SOD completely reversed the GHb-induced impaired relaxation; GHb did not alter the relaxation response to SNP.

CONCLUSION

GHb significantly impairs endothelial NO-mediated corpus cavernosal relaxation in the rat, in vitro. This effect is caused partly by the generation of superoxide anions and the extracellular inactivation of NO.

The effects of diabetes on nitric oxide-mediated responses in rat corpus cavernosum

Nitric oxide (NO)-mediated responses were investigated in corpora cavernosa isolated from 8-week diabetic rats. Relaxations to field stimulation were abolished by N(G)-nitro-L-arginine (NOARG, 100 microM). Responses to stimulation and sodium nitroprusside were reduced in tissues from diabetic rats compared to control rats, when data were expressed as g tension, but not when expressed as g/g tissue. The endothelium-dependent vasodilator, acetylcholine, failed to relax tissues. Stimulation-induced contractions were smaller in the diabetic group compared to the control group when data were expressed as g tension, but not g/g tissue. Contractions were enhanced by NOARG, and inhibited by acetylcholine (300 microM), by a similar degree in both groups. NOARG reduced the inhibitory effect of acetylcholine in tissues from control, but not diabetic rats. The results suggest diabetes caused a general impairment in responsiveness of rat corpus cavernosum, which may be a consequence of tissue weight change. A role for endothelium-dependent NO could not be identified; however, NO-mediated modulation of noradrenergic transmission by acetylcholine, may be defective in diabetes.

NO synthase in cholinergic nerves and NO-induced relaxation in the rat isolated corpus cavernosum

1. In the rat corpus cavernosum (CC), the distribution of immunoreactivity for neuronal and endothelial NO synthase (nNOS and eNOS), and the pattern of NOS-immunoreactive (-IR) nerves in relation to some other nerve populations, were investigated. Cholinergic nerves were specifically immunolabelled with antibodies to the vesicular acetylcholine transporter protein (VAChT). 2. In the smooth muscle septa surrounding the cavernous spaces, and around the central and helicine arteries, the numbers of PGP- and tyrosine hydroxylase (TH)-IR terminals were large, whereas neuropeptide Y (NPY)-, VAChT-, nNOS-, and vasoactive intestinal polypeptide (VIP)-IR terminals were found in few to moderate numbers. 3. Double immunolabelling revealed that VAChT- and nNOS-IR terminals, VAChT- and VIP-IR terminals, nNOS-IR and VIP-IR terminals, and TH- and NPY-IR terminals showed coinciding profiles, and co-existence was verified by confocal laser scanning microscopy. TH immunoreactivity was not found in VAChT-, nNOS-, or VIP-IR nerve fibres or terminals. 4. An isolated strip preparation of the rat CC was developed, and characterized. In this preparation, cumulative addition of NO to noradrenaline (NA)-contracted strips, produced concentration-dependent, rapid, and almost complete relaxations. Electrical field stimulation of endothelin-1-contracted preparations produced frequency-dependent responses: a contractile twitch followed by a fast relaxant response. After cessation of stimulation, there was a slow relaxant phase. Inhibition of NO synthesis, or blockade of guanylate cyclase, abolished the first relaxant phase, whereas the second relaxation was unaffected. 5. The results suggest that in the rat CC, nNOS, VAChT- and VIP-immunoreactivities can be found in the same parasympathetic cholinergic neurons. Inhibitory neurotransmission involves activation of the NO-system, and the release of other, as yet unknown, transmitters.